diff --git a/Documentation/config-gerrit.txt b/Documentation/config-gerrit.txt
index fc4b00a..82cb3f9 100644
--- a/Documentation/config-gerrit.txt
+++ b/Documentation/config-gerrit.txt
@@ -1171,15 +1171,6 @@
 +
 Default is `ALL`.
 
-[[change.api.excludeMergeableInChangeInfo]]change.api.excludeMergeableInChangeInfo::
-+
-If true, the mergeability bit in
-link:rest-api-changes.html#change-info[ChangeInfo] will never be set. It can
-be requested separately through the
-link:rest-api-changes.html#get-mergeable[get-mergeable] endpoint.
-+
-Default is false.
-
 [[change.cacheAutomerge]]change.cacheAutomerge::
 +
 When reviewing diff commits, the left-hand side shows the output of the
@@ -1195,6 +1186,15 @@
 +
 Default is true.
 
+[[change.commentSizeLimit]]change.commentSizeLimit::
++
+Maximum allowed size in characters of a regular (non-robot) comment. Comments
+which exceed this size will be rejected. Size computation is approximate and may
+be off by roughly 1%. Common unit suffixes of 'k', 'm', or 'g' are supported.
+The value must be positive.
++
+The default limit is 16kB.
+
 [[change.disablePrivateChanges]]change.disablePrivateChanges::
 +
 If set to true, users are not allowed to create private changes.
@@ -1212,6 +1212,13 @@
 +
 By default 500.
 
+[[change.maxComments]]change.maxComments::
++
+Maximum number of comments (regular plus robot) allowed per change. Additional
+comments are rejected.
++
+By default 5,000.
+
 [[change.maxUpdates]]change.maxUpdates::
 +
 Maximum number of updates to a change. Counts only updates to the main NoteDb
@@ -1232,6 +1239,34 @@
 
 By default 1000.
 
+[[change.mergeabilityComputationBehavior]]change.mergeabilityComputationBehavior::
++
+This setting determines when Gerrit computes if a change is mergeable or not.
+This computation is expensive, especially when the repository is large or when
+there are many open changes.
++
+This config can have the following states:
++
+* `API_REF_UPDATED_AND_CHANGE_REINDEX`: Gerrit indexes `mergeability` enabling
+  the `is:mergeable` predicate in change search and allowing fast retrieval of
+  this bit in query responses. Gerrit will always serve `mergeable` in
+  link:rest-api-changes.html#change-info[ChangeInfo] objects.
+  Gerrit will reindex all open changes when the target ref advances (expensive).
+* `REF_UPDATED_AND_CHANGE_REINDEX`: Gerrit indexes `mergeability` enabling the
+  `is:mergeable` predicate in change search and allowing fast retrieval of this
+  bit in query responses. Gerrit will never serve `mergeable` in
+  link:rest-api-changes.html#change-info[ChangeInfo]
+  objects. This state can be a final state for instances that only want to
+  optimize the read path, but not the write path for speed or serve as an
+  intermediary step for instances that want to optimize both and need to migrate
+  callers of their API.
+  Gerrit will reindex all open changes when the target ref advances (expensive).
+* `NEVER`: Gerrit does not index `mergeable`, so `is:mergeable` is disabled as
+  query operator. Gerrit does not serve `mergeable` in
+  link:rest-api-changes.html#change-info[ChangeInfo].
+
+Default is `REF_UPDATED_AND_CHANGE_REINDEX`.
+
 [[change.move]]change.move::
 +
 Whether the link:rest-api-changes.html#move-change[Move Change] REST
@@ -1266,11 +1301,10 @@
 
 [[change.robotCommentSizeLimit]]change.robotCommentSizeLimit::
 +
-Maximum allowed size of a robot comment that will be accepted. Robot comments
-which exceed the indicated size will be rejected on addition. The specified
-value is interpreted as the maximum size in bytes of the JSON representation of
-the robot comment. Common unit suffixes of 'k', 'm', or 'g' are supported.
-Zero or negative values allow robot comments of unlimited size.
+Maximum allowed size in characters of a robot comment. Robot comments which
+exceed this size will be rejected on addition. Size computation is approximate
+and may be off by roughly 1%. Common unit suffixes of 'k', 'm', or 'g' are
+supported. Zero or negative values allow robot comments of unlimited size.
 +
 The default limit is 1024kB.
 
@@ -1400,9 +1434,10 @@
 +
 By default `true`, meaning mergeable changes are auto-abandoned.
 +
-If link:#index.change.indexMergeable[`index.change.indexMergeable`]
-is disabled, setting this option to `false` has no effect and it
-behaves as though it were set to `true`.
+If
+link:#change.mergeabilityComputationBehavior[`change.mergeabilityComputationBehavior`]
+is set to `NEVER`, setting this option to `false` has no effect and it behaves
+as though it were set to `true`.
 
 [[changeCleanup.cleanupAccountPatchReview]]changeCleanup.cleanupAccountPatchReview::
 +
@@ -2776,22 +2811,6 @@
 If not set or set to a zero, defaults to the number of logical CPUs as returned
 by the JVM. If set to a negative value, defaults to a direct executor.
 
-[[index.change.indexMergeable]]index.change.indexMergeable::
-+
-Specifies if `mergeable` should be index or not. Indexing this field enables
-queries that contain the mergeability operator (`is:mergeable`). If enabled,
-Gerrit will check if the change is mergeable into the target branch when
-reindexing a change. This is an expensive operation.
-+
-If true, Gerrit will reindex all open changes when the target ref advances.
-Depending on the frequency of updates to the ref and the number of open changes,
-this can be very expensive.
-+
-When this setting is changed from `false` to `true`, all changes need to be
-reindexed.
-+
-Defaults to true.
-
 [[index.onlineUpgrade]]index.onlineUpgrade::
 +
 Whether to upgrade to new index schema versions while the server is
@@ -3102,7 +3121,7 @@
 events.
 +
 This can be set to the set of minimal options that consumers of Gerrit's
-events need. A minimal set would be (`SKIP_MERGEABLE`,`SKIP_DIFFSTAT`).
+events need. A minimal set would be (`SKIP_DIFFSTAT`).
 +
 Every option that gets added here will have a performance impact. The
 general recommendation is therefore to set this to a minimal set of
diff --git a/Documentation/dev-bazel.txt b/Documentation/dev-bazel.txt
index 116ac76..037c859 100644
--- a/Documentation/dev-bazel.txt
+++ b/Documentation/dev-bazel.txt
@@ -22,12 +22,22 @@
 * Python 2 or 3
 * link:https://github.com/nodesource/distributions/blob/master/README.md[Node.js (including npm),role=external,window=_blank]
 * Bower (`sudo npm install -g bower`)
-* link:https://docs.bazel.build/versions/master/install.html[Bazel,role=external,window=_blank] directly
-or through link:https://github.com/bazelbuild/bazelisk[Bazelisk,role=external,window=_blank]
+* link:https://docs.bazel.build/versions/master/install.html[Bazel,role=external,window=_blank] -launched with
+link:https://github.com/bazelbuild/bazelisk[Bazelisk,role=external,window=_blank]
 * Maven
 * zip, unzip
 * gcc
 
+[[bazel]]
+=== Bazel
+
+link:https://github.com/bazelbuild/bazelisk[Bazelisk,role=external,window=_blank] includes a
+link:https://bazel.build/[Bazel,role=external,window=_blank] version check and downloads the correct
+`bazel` version for the git project/repository. Bazelisk is the recommended
+`bazel` launcher for Gerrit. Once Bazelisk is installed locally, a `bazel`
+symlink can be created towards it. This is so that every `bazel` command
+seamlessly uses Bazelisk, which then runs the proper `bazel` binary version.
+
 [[java]]
 === Java
 
diff --git a/Documentation/dev-crafting-changes.txt b/Documentation/dev-crafting-changes.txt
index 2364fcc..ac8ca25 100644
--- a/Documentation/dev-crafting-changes.txt
+++ b/Documentation/dev-crafting-changes.txt
@@ -116,7 +116,10 @@
 link:https://github.com/google/google-java-format[`google-java-format`,role=external,window=_blank]
 tool (version 1.7), and to format Bazel BUILD, WORKSPACE and .bzl files the
 link:https://github.com/bazelbuild/buildtools/tree/master/buildifier[`buildifier`,role=external,window=_blank]
-tool (version 0.29.0).
+tool (version 0.29.0). Unused dependencies are found and removed using the
+link:https://github.com/bazelbuild/buildtools/tree/master/unused_deps[`unused_deps`,role=external,window=_blank]
+build tool, a sibling of `buildifier`.
+
 These tools automatically apply format according to the style guides; this
 streamlines code review by reducing the need for time-consuming, tedious,
 and contentious discussions about trivial issues like whitespace.
diff --git a/Documentation/dev-e2e-tests.txt b/Documentation/dev-e2e-tests.txt
index 5e39687..946dcc1 100644
--- a/Documentation/dev-e2e-tests.txt
+++ b/Documentation/dev-e2e-tests.txt
@@ -20,32 +20,35 @@
 
 === How to run the load tests
 
-==== Prerequisites
-
-* link:https://www.scala-lang.org/download/[`Scala 2.12`]
-
 ==== How to build
 
+An link:https://www.scala-sbt.org/download.html[sbt-based installation] of
+link:https://www.scala-lang.org/download/[Scala] is required.
+
+The `scalaVersion` used by `sbt` once installed is defined in the `build.sbt` file.
+That specific version of Scala is automatically used by `sbt` while building:
+
 ----
 sbt compile
 ----
 
 ==== Setup
 
-If you are running SSH commands the private keys of the users used for testing need to go in `/tmp/ssh-keys`.
-The keys need to be generated this way (JSch won't validate them [otherwise,role=external,window=_blank](https://stackoverflow.com/questions/53134212/invalid-privatekey-when-using-jsch):
+If you are running SSH commands, the private keys of the users used for testing need to go in
+`/tmp/ssh-keys`. The keys need to be generated this way (JSch won't validate them
+link:https://stackoverflow.com/questions/53134212/invalid-privatekey-when-using-jsch[otherwise,role=external,window=_blank]):
 
 ----
 ssh-keygen -m PEM -t rsa -C "test@mail.com" -f /tmp/ssh-keys/id_rsa
 ----
 
-*NOTE*: Don't forget to add the public keys for the testing user(s) to your git server
+*NOTE*: Don't forget to add the public keys for the testing user(s) to your git server.
 
 ==== Input file
 
-The ReplayRecordsScenario is fed by the data coming from the [src/test/resources/data/requests.json](/src/test/resources/data/requests.json) file.
-Such file contains the commands and repo used during the load test.
-Below an example:
+The `ReplayRecordsFromFeederScenario` is fed with the data coming from the
+`src/test/resources/data/requests.json` file. Such a file contains the commands and repo used
+during the load test. Example below:
 
 ----
 [
@@ -61,10 +64,11 @@
 ----
 
 Valid commands are:
-* fetch
-* pull
-* push
-* clone
+
+* `fetch`
+* `pull`
+* `push`
+* `clone`
 
 ==== How to use the framework
 
diff --git a/Documentation/dev-readme.txt b/Documentation/dev-readme.txt
index 5f72f37..5974aca 100644
--- a/Documentation/dev-readme.txt
+++ b/Documentation/dev-readme.txt
@@ -2,7 +2,7 @@
 = Gerrit Code Review: Developer Setup
 
 To build a developer instance, you'll need link:https://bazel.build/[Bazel,role=external,window=_blank] to
-compile the code.
+compile the code, preferably launched with link:https://github.com/bazelbuild/bazelisk[Bazelisk,role=external,window=_blank].
 
 == Git Setup
 
@@ -62,6 +62,12 @@
 For instructions on running the acceptance tests with Bazel,
 see <<dev-bazel#tests,Running Unit Tests with Bazel>>.
 
+[[e2e]]
+=== End-to-end tests
+
+<<dev-e2e-tests#,This document>> describes how load test scenarios are
+implemented using link:https://gatling.io/[`Gatling`].
+
 
 == Local server
 
diff --git a/Documentation/metrics.txt b/Documentation/metrics.txt
index 236792b..00e5b97 100644
--- a/Documentation/metrics.txt
+++ b/Documentation/metrics.txt
@@ -152,6 +152,10 @@
 * `notedb/external_id_update_count`: Total number of external ID updates.
 * `notedb/read_all_external_ids_latency`: Latency for reading all
 external ID's from NoteDb.
+* `notedb/read_single_account_config_latency`: Latency for reading a single
+account config from NoteDb.
+* `notedb/read_single_external_id_latency`: Latency for reading a single
+external ID from NoteDb.
 
 === Permissions
 
diff --git a/Documentation/rest-api-accounts.txt b/Documentation/rest-api-accounts.txt
index e3e9009..bb83d95 100644
--- a/Documentation/rest-api-accounts.txt
+++ b/Documentation/rest-api-accounts.txt
@@ -2428,8 +2428,8 @@
 |Field Name                 |Description
 |`change`                   |
 link:rest-api-changes.html#change-info[ChangeInfo] entity describing the change
-on which one or more comments was deleted. Populated with only the
-link:rest-api-changes.html#skip_mergeable[SKIP_MERGEABLE] option.
+on which one or more comments was deleted. Populated with no change list
+options.
 |`deleted`                  |
 List of link:rest-api-changes.html#comment-info[CommentInfo] entities for each
 comment that was deleted.
diff --git a/Documentation/rest-api-changes.txt b/Documentation/rest-api-changes.txt
index 6da455d..ca0e188 100644
--- a/Documentation/rest-api-changes.txt
+++ b/Documentation/rest-api-changes.txt
@@ -306,26 +306,6 @@
     from the change owner, i.e. this change would show up in the results of
     link:user-search.html#reviewedby[reviewedby:self].
 --
-
-[[skip_mergeable]]
---
-* `SKIP_MERGEABLE`: skip the `mergeable` field in
-link:#change-info[ChangeInfo]. For fast moving projects, this field must
-be recomputed often, which is slow for projects with big trees.
-+
-When link:config-gerrit.html#change.api.excludeMergeableInChangeInfo[
-`change.api.excludeMergeableInChangeInfo`] is set in the `gerrit.config`,
-the `mergeable` field will always be omitted and `SKIP_MERGEABLE` has no
-effect.
-+
-When link:config-gerrit.html#index.change.indexMergeable[
-`index.change.indexMergeable`] is set to `false` in the `gerrit.config`,
-the `mergeable` field will always be omitted when querying changes and
-`SKIP_MERGEABLE` has no effect.
-+
-A change's mergeability can be requested separately by calling the
-link:#get-mergeable[get-mergeable] endpoint.
---
 [[skip_diffstat]]
 --
 * `SKIP_DIFFSTAT`: skip the 'insertions' and 'deletions' field in link:#change-info[ChangeInfo].
@@ -2098,8 +2078,8 @@
           "name": "Code Analyzer",
           "email": "code.analyzer@example.com"
         },
-        "robotId": "importChecker",
-        "robotRunId": "76b1375aa8626ea7149792831fe2ed85e80d9e04"
+        "robot_id": "importChecker",
+        "robot_run_id": "76b1375aa8626ea7149792831fe2ed85e80d9e04"
       },
       {
         "id": "TveXwFiA",
@@ -2111,8 +2091,8 @@
           "name": "Code Analyzer",
           "email": "code.analyzer@example.com"
         },
-        "robotId": "styleChecker",
-        "robotRunId": "5c606c425dd45184484f9d0a2ffd725a7607839b"
+        "robot_id": "styleChecker",
+        "robot_run_id": "5c606c425dd45184484f9d0a2ffd725a7607839b"
       }
     ]
   }
@@ -4900,8 +4880,8 @@
           "name": "Code Analyzer",
           "email": "code.analyzer@example.com"
         },
-        "robotId": "importChecker",
-        "robotRunId": "76b1375aa8626ea7149792831fe2ed85e80d9e04"
+        "robot_id": "importChecker",
+        "robot_run_id": "76b1375aa8626ea7149792831fe2ed85e80d9e04"
       },
       {
         "id": "TveXwFiA",
@@ -4913,8 +4893,8 @@
           "name": "Code Analyzer",
           "email": "code.analyzer@example.com"
         },
-        "robotId": "styleChecker",
-        "robotRunId": "5c606c425dd45184484f9d0a2ffd725a7607839b"
+        "robot_id": "styleChecker",
+        "robot_run_id": "5c606c425dd45184484f9d0a2ffd725a7607839b"
       }
     ]
   }
@@ -4954,8 +4934,8 @@
       "name": "Code Analyzer",
       "email": "code.analyzer@example.com"
     },
-    "robotId": "importChecker",
-    "robotRunId": "76b1375aa8626ea7149792831fe2ed85e80d9e04"
+    "robot_id": "importChecker",
+    "robot_run_id": "76b1375aa8626ea7149792831fe2ed85e80d9e04"
   }
 ----
 
@@ -5964,10 +5944,9 @@
 Not set for merged changes.
 |`mergeable`          |optional|
 Whether the change is mergeable. +
-Not set for merged changes, if the change has not yet been tested, or
-if the link:#skip_mergeable[skip_mergeable] option is set or when
-link:config-gerrit.html#change.api.excludeMergeableInChangeInfo[change.api.excludeMergeableInChangeInfo]
-is set.
+Only set for open changes if
+link:config-gerrit.html#change.mergeabilityComputationBehavior[change.mergeabilityComputationBehavior]
+is `API_REF_UPDATED_AND_CHANGE_REINDEX`.
 |`submittable`        |optional|
 Whether the change has been approved by the project submit rules. +
 Only set if link:#submittable[requested].
@@ -6063,6 +6042,10 @@
 If the change that triggered the submission also has a topic, it will be
 "<id>-<topic>" of the change that triggered the submission.
 The callers must not rely on the format of the submission ID.
+|`cherry_pick_of_change`             |optional|
+The numeric Change-Id of the change that this change was cherry-picked from.
+|`cherry_pick_of_patch_set`          |optional|
+The patchset number of the change that this change was cherry-picked from.
 |==================================
 
 [[change-input]]
@@ -6801,16 +6784,16 @@
 Submit type used for this change, can be `MERGE_IF_NECESSARY`,
 `FAST_FORWARD_ONLY`, `REBASE_IF_NECESSARY`, `REBASE_ALWAYS`, `MERGE_ALWAYS` or
 `CHERRY_PICK`.
-|`strategy`     |optional|
+|`strategy`      |optional|
 The strategy of the merge, can be `recursive`, `resolve`,
 `simple-two-way-in-core`, `ours` or `theirs`.
 |`mergeable`     ||
 `true` if this change is cleanly mergeable, `false` otherwise
-|`commit_merged`     |optional|
+|`commit_merged` |optional|
 `true` if this change is already merged, `false` otherwise
-|`content_merged`     |optional|
+|`content_merged`|optional|
 `true` if the content of this change is already merged, `false` otherwise
-|`conflicts`|optional|
+|`conflicts`     |optional|
 A list of paths with conflicts
 |`mergeable_into`|optional|
 A list of other branch names where this change could merge cleanly
@@ -6823,16 +6806,16 @@
 [options="header",cols="1,^1,5"]
 |============================
 |Field Name      ||Description
-|`source`   ||
+|`source`        ||
 The source to merge from, e.g. a complete or abbreviated commit SHA-1,
 a complete reference name, a short reference name under `refs/heads`, `refs/tags`,
 or `refs/remotes` namespace, etc.
-|`source_branch`  |optional|
+|`source_branch` |optional|
 A branch from which `source` is reachable. If specified,
 `source` is checked for visibility and reachability against only this
 branch. This speeds up the operation, especially for large repos with
 many branches.
-|`strategy`     |optional|
+|`strategy`      |optional|
 The strategy of the merge, can be `recursive`, `resolve`,
 `simple-two-way-in-core`, `ours` or `theirs`, default will use project settings.
 |============================
@@ -6848,7 +6831,7 @@
 |`subject`            |optional|
 The new subject for the change, if not specified, will reuse the current patch
 set's subject
-|`inherit_parent`      |optional, default to `false`|
+|`inherit_parent`     |optional, default to `false`|
 Use the current patch set's first parent as the merge tip when set to `true`.
 |`base_change`        |optional|
 A link:#change-id[\{change-id\}] that identifies a change. When `inherit_parent`
diff --git a/Documentation/rest-api-config.txt b/Documentation/rest-api-config.txt
index e9e3b94..828674f 100644
--- a/Documentation/rest-api-config.txt
+++ b/Documentation/rest-api-config.txt
@@ -1566,10 +1566,11 @@
 the whole topic is submitted].
 |`disable_private_changes` |not set if `false`|
 Returns true if private changes are disabled.
-|`exclude_mergeable_in_change_info` |not set if `false`|
-Value of the link:config-gerrit.html#change.api.excludeMergeableInChangeInfo[
+|`mergeability_computation_behavior` ||
+Value of the link:config-gerrit.html#change.mergeabilityComputationBehavior[
 configuration parameter] that controls whether the mergeability bit in
-link:rest-api-changes.html#change-info[ChangeInfo] will never be set.
+link:rest-api-changes.html#change-info[ChangeInfo] will never be set and if the
+bit is indexed.
 |=============================
 
 [[change-index-config-info]]
diff --git a/Jenkinsfile b/Jenkinsfile
index 7417bb3..edd4234 100644
--- a/Jenkinsfile
+++ b/Jenkinsfile
@@ -25,15 +25,6 @@
     static final String gerritRepositoryNameSha1Suffix = "-a6a0e4682515f3521897c5f950d1394f4619d928"
 }
 
-class Change {
-    static String sha1 = ""
-    static String number = ""
-    static String branch = ""
-    static String ref = ""
-    static String patchNum = ""
-    static String url = ""
-}
-
 class Build {
     String url
     String result
@@ -83,36 +74,17 @@
     gerritCheck(checks: [ "${check.uuid}" : "${check.getCheckResultFromBuild()}" ], url: "${check.consoleUrl}")
 }
 
-def queryChangedFiles(url, changeNum, sha1) {
-    def queryUrl = "${url}changes/${Change.number}/revisions/${Change.sha1}/files/"
+def queryChangedFiles(url) {
+    def queryUrl = "${url}changes/${env.GERRIT_CHANGE_NUMBER}/revisions/${env.GERRIT_PATCHSET_REVISION}/files/"
     def response = httpRequest queryUrl
     def files = response.getContent().substring(5)
     def filesJson = new JsonSlurper().parseText(files)
     return filesJson.keySet().findAll { it != "/COMMIT_MSG" }
 }
 
-def queryChange(){
-    def requestedChangeId = env.BRANCH_NAME.split('/')[1]
-    def queryUrl = "${Globals.gerritUrl}changes/${requestedChangeId}/?pp=0&O=3"
-    def response = httpRequest queryUrl
-    def jsonSlurper = new JsonSlurper()
-    return jsonSlurper.parseText(response.getContent().substring(5))
-}
-
-def getChangeMetaData(){
-    def changeJson = queryChange()
-    Change.sha1 = changeJson.current_revision
-    Change.number = changeJson._number
-    Change.branch = changeJson.branch
-    def revision = changeJson.revisions.get(Change.sha1)
-    Change.ref = revision.ref
-    Change.patchNum = revision._number
-    Change.url = Globals.gerritUrl + "#/c/" + Change.number + "/" + Change.patchNum
-}
-
 def collectBuildModes() {
     Builds.modes = ["notedb"]
-    def changedFiles = queryChangedFiles(Globals.gerritUrl, Change.number, Change.sha1)
+    def changedFiles = queryChangedFiles(Globals.gerritUrl)
     def polygerritFiles = changedFiles.findAll { it.startsWith("polygerrit-ui") ||
         it.startsWith("lib/js") }
 
@@ -137,11 +109,11 @@
             for (int i = 1; i <= retryTimes; i++) {
                 try {
                     slaveBuild = build job: "${buildName}", parameters: [
-                        string(name: 'REFSPEC', value: Change.ref),
-                        string(name: 'BRANCH', value: Change.sha1),
-                        string(name: 'CHANGE_URL', value: Change.url),
+                        string(name: 'REFSPEC', value: "refs/changes/${env.BRANCH_NAME}"),
+                        string(name: 'BRANCH', value: env.GERRIT_PATCHSET_REVISION),
+                        string(name: 'CHANGE_URL', value: "${Globals.gerritUrl}c/${env.GERRIT_PROJECT}/+/${env.GERRIT_CHANGE_NUMBER}"),
                         string(name: 'MODE', value: mode),
-                        string(name: 'TARGET_BRANCH', value: Change.branch)
+                        string(name: 'TARGET_BRANCH', value: env.GERRIT_BRANCH)
                     ], propagate: false
                 } finally {
                     if (buildName == "Gerrit-codestyle"){
@@ -243,8 +215,6 @@
     if (hasChangeNumber()) {
         stage('Preparing'){
             gerritReview labels: ['Verified': 0, 'Code-Style': 0]
-
-            getChangeMetaData()
             collectBuildModes()
         }
     }
diff --git a/WORKSPACE b/WORKSPACE
index 6e9fb38..1619c45 100644
--- a/WORKSPACE
+++ b/WORKSPACE
@@ -24,13 +24,6 @@
 rbe_autoconfig(name = "rbe_default")
 
 http_archive(
-    name = "bazel_skylib",
-    sha256 = "2ea8a5ed2b448baf4a6855d3ce049c4c452a6470b1efd1504fdb7c1c134d220a",
-    strip_prefix = "bazel-skylib-0.8.0",
-    urls = ["https://github.com/bazelbuild/bazel-skylib/archive/0.8.0.tar.gz"],
-)
-
-http_archive(
     name = "io_bazel_rules_closure",
     sha256 = "03c3b16f205085817fd89cfdcb2220a0138647ee7992be9cef291b069dd90301",
     strip_prefix = "rules_closure-196a45f0ede2faec11dcc6c60fbc5e7471f4bd58",
@@ -47,15 +40,6 @@
     urls = ["https://raw.githubusercontent.com/google/closure-compiler/35d2b3340ff23a69441f10fa3bc820691c2942f2/contrib/externs/polymer-1.0.js"],
 )
 
-# Check Bazel version when invoked by Bazel directly
-load("//tools/bzl:bazelisk_version.bzl", "bazelisk_version")
-
-bazelisk_version(name = "bazelisk_version")
-
-load("@bazelisk_version//:check.bzl", "check_bazel_version")
-
-check_bazel_version()
-
 load("@io_bazel_rules_closure//closure:repositories.bzl", "rules_closure_dependencies", "rules_closure_toolchains")
 
 # Prevent redundant loading of dependencies.
@@ -64,7 +48,6 @@
 # https://github.com/google/closure-templates/pull/155
 rules_closure_dependencies(
     omit_aopalliance = True,
-    omit_bazel_skylib = True,
     omit_javax_inject = True,
     omit_rules_cc = True,
 )
diff --git a/java/com/google/gerrit/acceptance/BUILD b/java/com/google/gerrit/acceptance/BUILD
index 135a80e..89d1f3d 100644
--- a/java/com/google/gerrit/acceptance/BUILD
+++ b/java/com/google/gerrit/acceptance/BUILD
@@ -1,5 +1,4 @@
 load("@rules_java//java:defs.bzl", "java_binary", "java_library")
-load("//tools/bzl:java.bzl", "java_library2")
 load("//tools/bzl:javadoc.bzl", "java_doc")
 
 FUNCTION_SRCS = [
@@ -11,13 +10,9 @@
     "//java/com/google/gerrit/exceptions",
     "//java/com/google/gerrit/gpg",
     "//java/com/google/gerrit/git",
-    "//java/com/google/gerrit/httpd/auth/openid",
     "//java/com/google/gerrit/index:query_exception",
     "//java/com/google/gerrit/launcher",
     "//java/com/google/gerrit/lifecycle",
-    "//java/com/google/gerrit/pgm",
-    "//java/com/google/gerrit/pgm/http/jetty",
-    "//java/com/google/gerrit/pgm/util",
     "//java/com/google/gerrit/common:annotations",
     "//java/com/google/gerrit/common:server",
     "//java/com/google/gerrit/entities",
@@ -51,7 +46,6 @@
     "//lib/guice",
     "//lib/guice:guice-assistedinject",
     "//lib/guice:guice-servlet",
-    "//lib/jetty:servlet",
     "//lib/mail",
     "//lib/mina:sshd",
     "//lib/log:impl-log4j",
@@ -63,6 +57,10 @@
 ]
 
 TEST_DEPS = [
+    "//java/com/google/gerrit/httpd/auth/openid",
+    "//java/com/google/gerrit/pgm",
+    "//java/com/google/gerrit/pgm/http/jetty",
+    "//java/com/google/gerrit/pgm/util",
     "//java/com/google/gerrit/truth",
     "//java/com/google/gerrit/acceptance/config",
     "//java/com/google/gerrit/acceptance/testsuite/project",
@@ -76,6 +74,13 @@
     "//java/com/google/gerrit/git/testing",
 ]
 
+PGM_DEPLOY_ENV = [
+    "//lib:caffeine",
+    "//lib:caffeine-guava",
+    "//lib/jackson:jackson-core",
+    "//lib/prolog:cafeteria",
+]
+
 java_library(
     name = "lib",
     testonly = True,
@@ -100,31 +105,33 @@
     name = "framework-deploy-env",
     testonly = True,
     main_class = "Dummy",
-    runtime_deps = DEPLOY_ENV,
+    runtime_deps = DEPLOY_ENV + PGM_DEPLOY_ENV,
 )
 
-java_library2(
+exported_deps = [
+    ":function",
+    "//lib:jgit-junit",
+    "//lib:jimfs",
+    "//lib:servlet-api",
+    "//lib/httpcomponents:fluent-hc",
+    "//lib/httpcomponents:httpclient",
+    "//lib/httpcomponents:httpcore",
+    "//lib/mockito",
+    "//lib/truth",
+    "//lib/truth:truth-java8-extension",
+    "//lib/greenmail",
+] + TEST_DEPS
+
+java_library(
     name = "framework-lib",
     testonly = True,
     srcs = glob(
         ["**/*.java"],
         exclude = FUNCTION_SRCS,
     ),
-    exported_deps = [
-        ":function",
-        "//lib:jgit-junit",
-        "//lib:jimfs",
-        "//lib:servlet-api",
-        "//lib/httpcomponents:fluent-hc",
-        "//lib/httpcomponents:httpclient",
-        "//lib/httpcomponents:httpcore",
-        "//lib/mockito",
-        "//lib/truth",
-        "//lib/truth:truth-java8-extension",
-        "//lib/greenmail",
-    ] + TEST_DEPS,
     visibility = ["//visibility:public"],
-    deps = DEPLOY_ENV,
+    exports = exported_deps,
+    deps = DEPLOY_ENV + exported_deps,
 )
 
 java_library(
diff --git a/java/com/google/gerrit/acceptance/GerritServer.java b/java/com/google/gerrit/acceptance/GerritServer.java
index f90df67..95afad7 100644
--- a/java/com/google/gerrit/acceptance/GerritServer.java
+++ b/java/com/google/gerrit/acceptance/GerritServer.java
@@ -571,8 +571,8 @@
     cfg.setInt("sshd", null, "commandStartThreads", 1);
     cfg.setInt("receive", null, "threadPoolSize", 1);
     cfg.setInt("index", null, "threads", 1);
-    if (cfg.getString("index", "change", "indexMergeable") == null) {
-      cfg.setBoolean("index", "change", "indexMergeable", false);
+    if (cfg.getString("index", null, "mergeabilityComputationBehavior") == null) {
+      cfg.setString("index", null, "mergeabilityComputationBehavior", "NEVER");
     }
   }
 
diff --git a/java/com/google/gerrit/acceptance/InProcessProtocol.java b/java/com/google/gerrit/acceptance/InProcessProtocol.java
index 8a46d57..8fcc1e7 100644
--- a/java/com/google/gerrit/acceptance/InProcessProtocol.java
+++ b/java/com/google/gerrit/acceptance/InProcessProtocol.java
@@ -232,7 +232,7 @@
       try {
         perm.check(ProjectPermission.RUN_UPLOAD_PACK);
       } catch (AuthException e) {
-        throw new ServiceNotAuthorizedException();
+        throw new ServiceNotAuthorizedException(e.getMessage(), e);
       } catch (PermissionBackendException e) {
         throw new RuntimeException(e);
       }
@@ -307,7 +307,7 @@
             .project(req.project)
             .check(ProjectPermission.RUN_RECEIVE_PACK);
       } catch (AuthException e) {
-        throw new ServiceNotAuthorizedException();
+        throw new ServiceNotAuthorizedException(e.getMessage(), e);
       } catch (PermissionBackendException e) {
         throw new RuntimeException(e);
       }
diff --git a/java/com/google/gerrit/acceptance/StandaloneSiteTest.java b/java/com/google/gerrit/acceptance/StandaloneSiteTest.java
index 1d6f51f..da93610 100644
--- a/java/com/google/gerrit/acceptance/StandaloneSiteTest.java
+++ b/java/com/google/gerrit/acceptance/StandaloneSiteTest.java
@@ -220,8 +220,11 @@
     try {
       status = p.waitFor();
     } catch (InterruptedException e) {
-      throw new InterruptedIOException(
-          "interrupted waiting for: " + Joiner.on(' ').join(pb.command()));
+      InterruptedIOException iioe =
+          new InterruptedIOException(
+              "interrupted waiting for: " + Joiner.on(' ').join(pb.command()));
+      iioe.initCause(e);
+      throw iioe;
     }
 
     String result = new String(out, UTF_8);
diff --git a/java/com/google/gerrit/common/data/CommentDetail.java b/java/com/google/gerrit/common/data/CommentDetail.java
index d69f0bb..55e0143 100644
--- a/java/com/google/gerrit/common/data/CommentDetail.java
+++ b/java/com/google/gerrit/common/data/CommentDetail.java
@@ -18,10 +18,7 @@
 import com.google.gerrit.entities.Comment;
 import com.google.gerrit.entities.PatchSet;
 import java.util.ArrayList;
-import java.util.Collections;
-import java.util.HashMap;
 import java.util.List;
-import java.util.Map;
 
 public class CommentDetail {
   protected List<Comment> a;
@@ -29,8 +26,6 @@
 
   private transient PatchSet.Id idA;
   private transient PatchSet.Id idB;
-  private transient Map<Integer, List<Comment>> forA;
-  private transient Map<Integer, List<Comment>> forB;
 
   public CommentDetail(PatchSet.Id idA, PatchSet.Id idB) {
     this.a = new ArrayList<>();
@@ -67,88 +62,4 @@
   public boolean isEmpty() {
     return a.isEmpty() && b.isEmpty();
   }
-
-  public List<Comment> getForA(int lineNbr) {
-    if (forA == null) {
-      forA = index(a);
-    }
-    return get(forA, lineNbr);
-  }
-
-  public List<Comment> getForB(int lineNbr) {
-    if (forB == null) {
-      forB = index(b);
-    }
-    return get(forB, lineNbr);
-  }
-
-  private static List<Comment> get(Map<Integer, List<Comment>> m, int i) {
-    List<Comment> r = m.get(i);
-    return r != null ? orderComments(r) : Collections.emptyList();
-  }
-
-  /**
-   * Order the comments based on their parent_uuid parent. It is possible to do this by iterating
-   * over the list only once but it's probably overkill since the number of comments on a given line
-   * will be small most of the time.
-   *
-   * @param comments The list of comments for a given line.
-   * @return The comments sorted as they should appear in the UI
-   */
-  private static List<Comment> orderComments(List<Comment> comments) {
-    // Map of comments keyed by their parent. The values are lists of comments since it is
-    // possible for several comments to have the same parent (this can happen if two reviewers
-    // click Reply on the same comment at the same time). Such comments will be displayed under
-    // their correct parent in chronological order.
-    Map<String, List<Comment>> parentMap = new HashMap<>();
-
-    // It's possible to have more than one root comment if two reviewers create a comment on the
-    // same line at the same time
-    List<Comment> rootComments = new ArrayList<>();
-
-    // Store all the comments in parentMap, keyed by their parent
-    for (Comment c : comments) {
-      String parentUuid = c.parentUuid;
-      List<Comment> l = parentMap.get(parentUuid);
-      if (l == null) {
-        l = new ArrayList<>();
-        parentMap.put(parentUuid, l);
-      }
-      l.add(c);
-      if (parentUuid == null) {
-        rootComments.add(c);
-      }
-    }
-
-    // Add the comments in the list, starting with the head and then going through all the
-    // comments that have it as a parent, and so on
-    List<Comment> result = new ArrayList<>();
-    addChildren(parentMap, rootComments, result);
-
-    return result;
-  }
-
-  /** Add the comments to {@code outResult}, depth first */
-  private static void addChildren(
-      Map<String, List<Comment>> parentMap, List<Comment> children, List<Comment> outResult) {
-    if (children != null) {
-      for (Comment c : children) {
-        outResult.add(c);
-        addChildren(parentMap, parentMap.get(c.key.uuid), outResult);
-      }
-    }
-  }
-
-  private Map<Integer, List<Comment>> index(List<Comment> in) {
-    HashMap<Integer, List<Comment>> r = new HashMap<>();
-    for (Comment p : in) {
-      List<Comment> l = r.get(p.lineNbr);
-      if (l == null) {
-        l = new ArrayList<>();
-        r.put(p.lineNbr, l);
-      }
-      l.add(p);
-    }
-    return r;
-  }
 }
diff --git a/java/com/google/gerrit/elasticsearch/ElasticChangeIndex.java b/java/com/google/gerrit/elasticsearch/ElasticChangeIndex.java
index 084c2ec..f659bca 100644
--- a/java/com/google/gerrit/elasticsearch/ElasticChangeIndex.java
+++ b/java/com/google/gerrit/elasticsearch/ElasticChangeIndex.java
@@ -49,6 +49,7 @@
 import com.google.gerrit.server.ReviewerByEmailSet;
 import com.google.gerrit.server.ReviewerSet;
 import com.google.gerrit.server.StarredChangesUtil;
+import com.google.gerrit.server.change.MergeabilityComputationBehavior;
 import com.google.gerrit.server.config.GerritServerConfig;
 import com.google.gerrit.server.config.SitePaths;
 import com.google.gerrit.server.index.IndexUtils;
@@ -111,7 +112,7 @@
     this.idField =
         this.schema.useLegacyNumericFields() ? ChangeField.LEGACY_ID : ChangeField.LEGACY_ID_STR;
     this.skipFields =
-        gerritConfig.getBoolean("index", "change", "indexMergeable", true)
+        MergeabilityComputationBehavior.fromConfig(gerritConfig).includeInIndex()
             ? ImmutableSet.of()
             : ImmutableSet.of(ChangeField.MERGEABLE.getName());
   }
diff --git a/java/com/google/gerrit/entities/Comment.java b/java/com/google/gerrit/entities/Comment.java
index 55d739a..9c58fef 100644
--- a/java/com/google/gerrit/entities/Comment.java
+++ b/java/com/google/gerrit/entities/Comment.java
@@ -29,6 +29,8 @@
  *
  * <p>Changing fields in this class changes the storage format of inline comments in NoteDb and may
  * require a corresponding data migration (adding new optional fields is generally okay).
+ *
+ * <p>Consider updating {@link #getApproximateSize()} when adding/changing fields.
  */
 public class Comment {
   public enum Status {
@@ -56,7 +58,7 @@
     }
   }
 
-  public static class Key {
+  public static final class Key {
     public String uuid;
     public String filename;
     public int patchSetId;
@@ -97,7 +99,7 @@
     }
   }
 
-  public static class Identity {
+  public static final class Identity {
     int id;
 
     public Identity(Account.Id id) {
@@ -147,7 +149,7 @@
    *       </ul>
    * </ul>
    */
-  public static class Range implements Comparable<Range> {
+  public static final class Range implements Comparable<Range> {
     private static final Comparator<Range> RANGE_COMPARATOR =
         Comparator.<Range>comparingInt(range -> range.startLine)
             .thenComparingInt(range -> range.startChar)
@@ -220,10 +222,12 @@
   public Range range;
   public String tag;
 
-  // Hex commit SHA1 of the commit of the patchset to which this comment applies. Other classes call
-  // this "commitId", but this class uses the old ReviewDb term "revId", and this field name is
-  // serialized into JSON in NoteDb, so it can't easily be changed. Callers do not access this field
-  // directly, and instead use the public getter/setter that wraps an ObjectId.
+  /**
+   * Hex commit SHA1 of the commit of the patchset to which this comment applies. Other classes call
+   * this "commitId", but this class uses the old ReviewDb term "revId", and this field name is
+   * serialized into JSON in NoteDb, so it can't easily be changed. Callers do not access this field
+   * directly, and instead use the public getter/setter that wraps an ObjectId.
+   */
   private String revId;
 
   public String serverId;
@@ -293,6 +297,23 @@
     return realAuthor != null ? realAuthor : author;
   }
 
+  /**
+   * Returns the comment's approximate size. This is used to enforce size limits and should
+   * therefore include all unbounded fields (e.g. String-s).
+   */
+  public int getApproximateSize() {
+    return nullableLength(message, parentUuid, tag, revId, serverId)
+        + (key != null ? nullableLength(key.filename, key.uuid) : 0);
+  }
+
+  static int nullableLength(String... strings) {
+    int length = 0;
+    for (String s : strings) {
+      length += s == null ? 0 : s.length();
+    }
+    return length;
+  }
+
   @Override
   public boolean equals(Object o) {
     if (!(o instanceof Comment)) {
diff --git a/java/com/google/gerrit/entities/FixReplacement.java b/java/com/google/gerrit/entities/FixReplacement.java
index 046300e..fbbf746 100644
--- a/java/com/google/gerrit/entities/FixReplacement.java
+++ b/java/com/google/gerrit/entities/FixReplacement.java
@@ -14,10 +14,10 @@
 
 package com.google.gerrit.entities;
 
-public class FixReplacement {
-  public String path;
-  public Comment.Range range;
-  public String replacement;
+public final class FixReplacement {
+  public final String path;
+  public final Comment.Range range;
+  public final String replacement;
 
   public FixReplacement(String path, Comment.Range range, String replacement) {
     this.path = path;
@@ -38,4 +38,9 @@
         + '\''
         + '}';
   }
+
+  /** Returns this instance's approximate size in bytes for the purpose of applying size limits. */
+  int getApproximateSize() {
+    return path.length() + replacement.length();
+  }
 }
diff --git a/java/com/google/gerrit/entities/FixSuggestion.java b/java/com/google/gerrit/entities/FixSuggestion.java
index ac4e720..c88afe7 100644
--- a/java/com/google/gerrit/entities/FixSuggestion.java
+++ b/java/com/google/gerrit/entities/FixSuggestion.java
@@ -16,10 +16,10 @@
 
 import java.util.List;
 
-public class FixSuggestion {
-  public String fixId;
-  public String description;
-  public List<FixReplacement> replacements;
+public final class FixSuggestion {
+  public final String fixId;
+  public final String description;
+  public final List<FixReplacement> replacements;
 
   public FixSuggestion(String fixId, String description, List<FixReplacement> replacements) {
     this.fixId = fixId;
@@ -40,4 +40,11 @@
         + replacements
         + '}';
   }
+
+  /** Returns this instance's approximate size in bytes for the purpose of applying size limits. */
+  int getApproximateSize() {
+    return fixId.length()
+        + description.length()
+        + replacements.stream().map(FixReplacement::getApproximateSize).reduce(0, Integer::sum);
+  }
 }
diff --git a/java/com/google/gerrit/entities/KeyUtil.java b/java/com/google/gerrit/entities/KeyUtil.java
index d000b31..40fb757 100644
--- a/java/com/google/gerrit/entities/KeyUtil.java
+++ b/java/com/google/gerrit/entities/KeyUtil.java
@@ -97,7 +97,7 @@
         }
       }
     } catch (ArrayIndexOutOfBoundsException err) {
-      throw new IllegalArgumentException("Bad encoding: " + e);
+      throw new IllegalArgumentException("Bad encoding" + e, err);
     }
     try {
       return new String(b, 0, bPtr, "UTF-8");
diff --git a/java/com/google/gerrit/entities/RobotComment.java b/java/com/google/gerrit/entities/RobotComment.java
index a7951ad..4a17566 100644
--- a/java/com/google/gerrit/entities/RobotComment.java
+++ b/java/com/google/gerrit/entities/RobotComment.java
@@ -19,7 +19,7 @@
 import java.util.Map;
 import java.util.Objects;
 
-public class RobotComment extends Comment {
+public final class RobotComment extends Comment {
   public String robotId;
   public String robotRunId;
   public String url;
@@ -41,6 +41,22 @@
   }
 
   @Override
+  public int getApproximateSize() {
+    int approximateSize = super.getApproximateSize() + nullableLength(robotId, robotRunId, url);
+    approximateSize +=
+        properties != null
+            ? properties.entrySet().stream()
+                .map(entry -> nullableLength(entry.getKey(), entry.getValue()))
+                .reduce(0, Integer::sum)
+            : 0;
+    approximateSize +=
+        fixSuggestions != null
+            ? fixSuggestions.stream().map(FixSuggestion::getApproximateSize).reduce(0, Integer::sum)
+            : 0;
+    return approximateSize;
+  }
+
+  @Override
   public String toString() {
     return toStringHelper()
         .add("robotId", robotId)
diff --git a/java/com/google/gerrit/exceptions/InvalidSshKeyException.java b/java/com/google/gerrit/exceptions/InvalidSshKeyException.java
index 8baba20..dda80cf 100644
--- a/java/com/google/gerrit/exceptions/InvalidSshKeyException.java
+++ b/java/com/google/gerrit/exceptions/InvalidSshKeyException.java
@@ -23,4 +23,8 @@
   public InvalidSshKeyException() {
     super(MESSAGE);
   }
+
+  public InvalidSshKeyException(Throwable cause) {
+    super(MESSAGE, cause);
+  }
 }
diff --git a/java/com/google/gerrit/extensions/api/changes/ChangeApi.java b/java/com/google/gerrit/extensions/api/changes/ChangeApi.java
index 119d941..96455a6 100644
--- a/java/com/google/gerrit/extensions/api/changes/ChangeApi.java
+++ b/java/com/google/gerrit/extensions/api/changes/ChangeApi.java
@@ -259,17 +259,12 @@
    *
    * <ul>
    *   <li>{@code CHECK} is omitted, to skip consistency checks.
-   *   <li>{@code SKIP_MERGEABLE} is omitted, so the {@code mergeable} bit <em>is</em> set.
    *   <li>{@code SKIP_DIFFSTAT} is omitted to ensure diffstat calculations.
    * </ul>
    */
   default ChangeInfo get() throws RestApiException {
     return get(
-        EnumSet.complementOf(
-            EnumSet.of(
-                ListChangesOption.CHECK,
-                ListChangesOption.SKIP_MERGEABLE,
-                ListChangesOption.SKIP_DIFFSTAT)));
+        EnumSet.complementOf(EnumSet.of(ListChangesOption.CHECK, ListChangesOption.SKIP_DIFFSTAT)));
   }
 
   /** {@link #get(ListChangesOption...)} with no options included. */
diff --git a/java/com/google/gerrit/extensions/client/ListChangesOption.java b/java/com/google/gerrit/extensions/client/ListChangesOption.java
index 425265b..6071cc7 100644
--- a/java/com/google/gerrit/extensions/client/ListChangesOption.java
+++ b/java/com/google/gerrit/extensions/client/ListChangesOption.java
@@ -74,7 +74,11 @@
   /** If tracking Ids are included, include detailed tracking Ids info. */
   TRACKING_IDS(21),
 
-  /** Skip mergeability data */
+  /**
+   * Use {@code gerrit.config} instead to turn this off for your instance. See {@code
+   * change.mergeabilityComputationBehavior}.
+   */
+  @Deprecated
   SKIP_MERGEABLE(22),
 
   /**
diff --git a/java/com/google/gerrit/extensions/common/ChangeConfigInfo.java b/java/com/google/gerrit/extensions/common/ChangeConfigInfo.java
index 67d0b0d9..66a2dd5 100644
--- a/java/com/google/gerrit/extensions/common/ChangeConfigInfo.java
+++ b/java/com/google/gerrit/extensions/common/ChangeConfigInfo.java
@@ -24,5 +24,5 @@
   public String replyTooltip;
   public int updateDelay;
   public Boolean submitWholeTopic;
-  public Boolean excludeMergeableInChangeInfo;
+  public String mergeabilityComputationBehavior;
 }
diff --git a/java/com/google/gerrit/extensions/common/ChangeIndexConfigInfo.java b/java/com/google/gerrit/extensions/common/ChangeIndexConfigInfo.java
deleted file mode 100644
index 0cce106..0000000
--- a/java/com/google/gerrit/extensions/common/ChangeIndexConfigInfo.java
+++ /dev/null
@@ -1,22 +0,0 @@
-// Copyright (C) 2019 The Android Open Source Project
-//
-// Licensed under the Apache License, Version 2.0 (the "License");
-// you may not use this file except in compliance with the License.
-// You may obtain a copy of the License at
-//
-// http://www.apache.org/licenses/LICENSE-2.0
-//
-// Unless required by applicable law or agreed to in writing, software
-// distributed under the License is distributed on an "AS IS" BASIS,
-// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-// See the License for the specific language governing permissions and
-// limitations under the License.
-
-package com.google.gerrit.extensions.common;
-
-/**
- * API response containing values from the {@code index.change} section of {@code gerrit.config}.
- */
-public class ChangeIndexConfigInfo {
-  public Boolean indexMergeable;
-}
diff --git a/java/com/google/gerrit/extensions/common/IndexConfigInfo.java b/java/com/google/gerrit/extensions/common/IndexConfigInfo.java
deleted file mode 100644
index f3f510b..0000000
--- a/java/com/google/gerrit/extensions/common/IndexConfigInfo.java
+++ /dev/null
@@ -1,20 +0,0 @@
-// Copyright (C) 2019 The Android Open Source Project
-//
-// Licensed under the Apache License, Version 2.0 (the "License");
-// you may not use this file except in compliance with the License.
-// You may obtain a copy of the License at
-//
-// http://www.apache.org/licenses/LICENSE-2.0
-//
-// Unless required by applicable law or agreed to in writing, software
-// distributed under the License is distributed on an "AS IS" BASIS,
-// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-// See the License for the specific language governing permissions and
-// limitations under the License.
-
-package com.google.gerrit.extensions.common;
-
-/** API response containing values from the {@code index} section of {@code gerrit.config}. */
-public class IndexConfigInfo {
-  public ChangeIndexConfigInfo change;
-}
diff --git a/java/com/google/gerrit/extensions/common/ServerInfo.java b/java/com/google/gerrit/extensions/common/ServerInfo.java
index 555c363..bc7fcfd 100644
--- a/java/com/google/gerrit/extensions/common/ServerInfo.java
+++ b/java/com/google/gerrit/extensions/common/ServerInfo.java
@@ -21,7 +21,6 @@
   public ChangeConfigInfo change;
   public DownloadInfo download;
   public GerritInfo gerrit;
-  public IndexConfigInfo index;
   public Boolean noteDbEnabled;
   public PluginConfigInfo plugin;
   public SshdInfo sshd;
diff --git a/java/com/google/gerrit/extensions/restapi/MethodNotAllowedException.java b/java/com/google/gerrit/extensions/restapi/MethodNotAllowedException.java
index 2d59f27..8422b61 100644
--- a/java/com/google/gerrit/extensions/restapi/MethodNotAllowedException.java
+++ b/java/com/google/gerrit/extensions/restapi/MethodNotAllowedException.java
@@ -22,4 +22,12 @@
   public MethodNotAllowedException(String msg) {
     super(msg);
   }
+
+  /**
+   * @param msg error text for client describing why the method is not allowed.
+   * @param cause reason for the method not being allowed.
+   */
+  public MethodNotAllowedException(String msg, Throwable cause) {
+    super(msg, cause);
+  }
 }
diff --git a/java/com/google/gerrit/extensions/restapi/NotImplementedException.java b/java/com/google/gerrit/extensions/restapi/NotImplementedException.java
index 566159d..d74986a7 100644
--- a/java/com/google/gerrit/extensions/restapi/NotImplementedException.java
+++ b/java/com/google/gerrit/extensions/restapi/NotImplementedException.java
@@ -25,4 +25,8 @@
   public NotImplementedException(String message) {
     super(message);
   }
+
+  public NotImplementedException(String message, Throwable cause) {
+    super(message, cause);
+  }
 }
diff --git a/java/com/google/gerrit/extensions/restapi/ResourceNotFoundException.java b/java/com/google/gerrit/extensions/restapi/ResourceNotFoundException.java
index 75cf713..270a040 100644
--- a/java/com/google/gerrit/extensions/restapi/ResourceNotFoundException.java
+++ b/java/com/google/gerrit/extensions/restapi/ResourceNotFoundException.java
@@ -34,6 +34,10 @@
     super("Not found: " + id.get());
   }
 
+  public ResourceNotFoundException(IdString id, Throwable cause) {
+    super("Not found: " + id.get(), cause);
+  }
+
   public ResourceNotFoundException caching(CacheControl c) {
     setCaching(c);
     return this;
diff --git a/java/com/google/gerrit/extensions/validators/CommentForValidation.java b/java/com/google/gerrit/extensions/validators/CommentForValidation.java
index 51ae5ae..9b91dff 100644
--- a/java/com/google/gerrit/extensions/validators/CommentForValidation.java
+++ b/java/com/google/gerrit/extensions/validators/CommentForValidation.java
@@ -17,13 +17,21 @@
 import com.google.auto.value.AutoValue;
 
 /**
- * Holds a comment's text and {@link CommentType} in order to pass it to a validation plugin.
+ * Holds a comment's text and some metadata in order to pass it to a validation plugin.
  *
  * @see CommentValidator
  */
 @AutoValue
 public abstract class CommentForValidation {
 
+  /** The creator of the comment. */
+  public enum CommentSource {
+    /** A regular user comment. */
+    HUMAN,
+    /** A robot comment. */
+    ROBOT
+  }
+
   /** The type of comment. */
   public enum CommentType {
     /** A regular (inline) comment. */
@@ -34,14 +42,27 @@
     CHANGE_MESSAGE
   }
 
-  public static CommentForValidation create(CommentType type, String text) {
-    return new AutoValue_CommentForValidation(type, text);
+  public static CommentForValidation create(
+      CommentSource source, CommentType type, String text, long size) {
+    return new AutoValue_CommentForValidation(source, type, text, size);
   }
 
+  public abstract CommentSource getSource();
+
   public abstract CommentType getType();
 
+  /**
+   * Returns the comment text. Note that especially for robot comments the total size may be
+   * significantly larger and should be determined by using {@link #getApproximateSize()}.
+   */
   public abstract String getText();
 
+  /**
+   * Returns this instance's approximate size in bytes for the purpose of applying size limits. For
+   * robot comments this may be significantly larger than the size of the comment text.
+   */
+  public abstract long getApproximateSize();
+
   public CommentValidationFailure failValidation(String message) {
     return CommentValidationFailure.create(this, message);
   }
diff --git a/java/com/google/gerrit/extensions/validators/CommentValidationContext.java b/java/com/google/gerrit/extensions/validators/CommentValidationContext.java
index 1cb00e3..db08058 100644
--- a/java/com/google/gerrit/extensions/validators/CommentValidationContext.java
+++ b/java/com/google/gerrit/extensions/validators/CommentValidationContext.java
@@ -34,16 +34,7 @@
   /** Returns the project the comment is being added to. */
   public abstract String getProject();
 
-  public static Builder builder() {
-    return new AutoValue_CommentValidationContext.Builder();
-  }
-
-  @AutoValue.Builder
-  public abstract static class Builder {
-    public abstract Builder changeId(int value);
-
-    public abstract Builder project(String value);
-
-    public abstract CommentValidationContext build();
+  public static CommentValidationContext create(int changeId, String project) {
+    return new AutoValue_CommentValidationContext(changeId, project);
   }
 }
diff --git a/java/com/google/gerrit/extensions/validators/CommentValidator.java b/java/com/google/gerrit/extensions/validators/CommentValidator.java
index ba73e46..2dfd6e2 100644
--- a/java/com/google/gerrit/extensions/validators/CommentValidator.java
+++ b/java/com/google/gerrit/extensions/validators/CommentValidator.java
@@ -25,7 +25,10 @@
 public interface CommentValidator {
 
   /**
-   * Validate the specified comments.
+   * Validate the specified comments. This method will be called with all new comments that need to
+   * be validated. This allows validators to statelessly count the new comments. Note that the
+   * method may be called separately for texts that are not comments, but similar in nature and also
+   * subject to size limits, such as a change message.
    *
    * @return An empty list if all comments are valid, or else a list of validation failures.
    */
diff --git a/java/com/google/gerrit/httpd/HtmlDomUtil.java b/java/com/google/gerrit/httpd/HtmlDomUtil.java
index 25ae71c..57f2664 100644
--- a/java/com/google/gerrit/httpd/HtmlDomUtil.java
+++ b/java/com/google/gerrit/httpd/HtmlDomUtil.java
@@ -131,7 +131,7 @@
     try {
       d = newBuilder().newDocument();
     } catch (ParserConfigurationException e) {
-      throw new IOException("Cannot clone document");
+      throw new IOException("Cannot clone document", e);
     }
     Node n = d.importNode(doc.getDocumentElement(), true);
     d.appendChild(n);
diff --git a/java/com/google/gerrit/httpd/ProjectOAuthFilter.java b/java/com/google/gerrit/httpd/ProjectOAuthFilter.java
index 693232f..dab36c4 100644
--- a/java/com/google/gerrit/httpd/ProjectOAuthFilter.java
+++ b/java/com/google/gerrit/httpd/ProjectOAuthFilter.java
@@ -195,7 +195,7 @@
       defaultAuthPlugin = loginProvider.getPluginName();
       defaultAuthProvider = loginProvider.getExportName();
     } catch (NoSuchElementException e) {
-      throw new ServletException("No OAuth login provider installed");
+      throw new ServletException("No OAuth login provider installed", e);
     } catch (IllegalArgumentException e) {
       // multiple providers found => do not pick any
     }
diff --git a/java/com/google/gerrit/httpd/plugins/PluginServletContext.java b/java/com/google/gerrit/httpd/plugins/PluginServletContext.java
index 6a8ef32..40083e4 100644
--- a/java/com/google/gerrit/httpd/plugins/PluginServletContext.java
+++ b/java/com/google/gerrit/httpd/plugins/PluginServletContext.java
@@ -61,9 +61,11 @@
       try {
         handler = API.class.getDeclaredMethod(method.getName(), method.getParameterTypes());
       } catch (NoSuchMethodException e) {
-        throw new NoSuchMethodError(
+        String msg =
             String.format(
-                "%s does not implement %s", PluginServletContext.class, method.toGenericString()));
+                "%s does not implement %s", PluginServletContext.class, method.toGenericString());
+        logger.atSevere().withCause(e).log(msg);
+        throw new NoSuchMethodError(msg);
       }
       return handler.invoke(this, args);
     }
diff --git a/java/com/google/gerrit/httpd/raw/BazelBuild.java b/java/com/google/gerrit/httpd/raw/BazelBuild.java
index a13078d..7677e97 100644
--- a/java/com/google/gerrit/httpd/raw/BazelBuild.java
+++ b/java/com/google/gerrit/httpd/raw/BazelBuild.java
@@ -63,8 +63,9 @@
     try {
       status = rebuild.waitFor();
     } catch (InterruptedException e) {
-      throw new InterruptedIOException(
-          "interrupted waiting for: " + Joiner.on(' ').join(proc.command()));
+      String msg = "interrupted waiting for: " + Joiner.on(' ').join(proc.command());
+      logger.atSevere().withCause(e).log(msg);
+      throw new InterruptedIOException(msg);
     }
     if (status != 0) {
       logger.atWarning().log("build failed: %s", new String(out, UTF_8));
diff --git a/java/com/google/gerrit/httpd/restapi/RestApiServlet.java b/java/com/google/gerrit/httpd/restapi/RestApiServlet.java
index 722955c..1fd0f82 100644
--- a/java/com/google/gerrit/httpd/restapi/RestApiServlet.java
+++ b/java/com/google/gerrit/httpd/restapi/RestApiServlet.java
@@ -697,8 +697,9 @@
 
           if (status.isPresent()) {
             responseBytes = reply(req, res, e, status.get(), getUserMessages(traceContext, e));
+          } else {
+            responseBytes = replyInternalServerError(req, res, e, getUserMessages(traceContext, e));
           }
-          responseBytes = replyInternalServerError(req, res, e, getUserMessages(traceContext, e));
         }
       } finally {
         String metric = getViewName(viewData);
@@ -1182,7 +1183,7 @@
           try {
             first = json.peek();
           } catch (EOFException e) {
-            throw new BadRequestException("Expected JSON object");
+            throw new BadRequestException("Expected JSON object", e);
           }
           if (first == JsonToken.STRING) {
             return parseString(json.nextString(), type);
diff --git a/java/com/google/gerrit/lucene/LuceneChangeIndex.java b/java/com/google/gerrit/lucene/LuceneChangeIndex.java
index 3b277dd..375967b 100644
--- a/java/com/google/gerrit/lucene/LuceneChangeIndex.java
+++ b/java/com/google/gerrit/lucene/LuceneChangeIndex.java
@@ -55,6 +55,7 @@
 import com.google.gerrit.index.query.ResultSet;
 import com.google.gerrit.proto.Protos;
 import com.google.gerrit.server.StarredChangesUtil;
+import com.google.gerrit.server.change.MergeabilityComputationBehavior;
 import com.google.gerrit.server.config.GerritServerConfig;
 import com.google.gerrit.server.config.SitePaths;
 import com.google.gerrit.server.index.IndexExecutor;
@@ -182,7 +183,7 @@
     this.changeDataFactory = changeDataFactory;
     this.schema = schema;
     this.skipFields =
-        cfg.getBoolean("index", "change", "indexMergeable", true)
+        MergeabilityComputationBehavior.fromConfig(cfg).includeInIndex()
             ? ImmutableSet.of()
             : ImmutableSet.of(ChangeField.MERGEABLE.getName());
 
diff --git a/java/com/google/gerrit/lucene/QueryBuilder.java b/java/com/google/gerrit/lucene/QueryBuilder.java
index e8ef95f..7d82bf5 100644
--- a/java/com/google/gerrit/lucene/QueryBuilder.java
+++ b/java/com/google/gerrit/lucene/QueryBuilder.java
@@ -208,7 +208,7 @@
       // subclasses of OperatorPredicate.
       value = Integer.parseInt(p.getValue());
     } catch (NumberFormatException e) {
-      throw new QueryParseException("not an integer: " + p.getValue());
+      throw new QueryParseException("not an integer: " + p.getValue(), e);
     }
     return intTermQuery.get(p.getField().getName(), value);
   }
diff --git a/java/com/google/gerrit/pgm/BUILD b/java/com/google/gerrit/pgm/BUILD
index 03b7c3d..aa4df82 100644
--- a/java/com/google/gerrit/pgm/BUILD
+++ b/java/com/google/gerrit/pgm/BUILD
@@ -23,7 +23,7 @@
         "//java/com/google/gerrit/lifecycle",
         "//java/com/google/gerrit/lucene",
         "//java/com/google/gerrit/metrics/dropwizard",
-        "//java/com/google/gerrit/pgm/http",
+        "//java/com/google/gerrit/pgm/http/jetty",
         "//java/com/google/gerrit/pgm/init",
         "//java/com/google/gerrit/pgm/init/api",
         "//java/com/google/gerrit/pgm/util",
diff --git a/java/com/google/gerrit/pgm/DeleteZombieDrafts.java b/java/com/google/gerrit/pgm/DeleteZombieDrafts.java
new file mode 100644
index 0000000..90a60c1
--- /dev/null
+++ b/java/com/google/gerrit/pgm/DeleteZombieDrafts.java
@@ -0,0 +1,78 @@
+// Copyright (C) 2020 The Android Open Source Project
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package com.google.gerrit.pgm;
+
+import com.google.gerrit.pgm.init.api.ConsoleUI;
+import com.google.gerrit.pgm.util.SiteProgram;
+import com.google.gerrit.server.config.GerritServerConfigModule;
+import com.google.gerrit.server.config.SitePath;
+import com.google.gerrit.server.notedb.DeleteZombieCommentsRefs;
+import com.google.gerrit.server.notedb.DeleteZombieCommentsRefs.Factory;
+import com.google.gerrit.server.schema.SchemaModule;
+import com.google.gerrit.server.securestore.SecureStoreClassName;
+import com.google.inject.AbstractModule;
+import com.google.inject.Guice;
+import com.google.inject.Injector;
+import com.google.inject.Module;
+import com.google.inject.assistedinject.FactoryModuleBuilder;
+import com.google.inject.util.Providers;
+import java.io.IOException;
+import java.nio.file.Path;
+import java.util.ArrayList;
+import java.util.List;
+import org.kohsuke.args4j.Option;
+
+/**
+ * A pgm which can be used to clean zombie draft comments refs More context in
+ * https://gerrit-review.googlesource.com/c/gerrit/+/246233
+ *
+ * <p>The implementation is in {@link DeleteZombieCommentsRefs}
+ */
+public class DeleteZombieDrafts extends SiteProgram {
+  @Option(
+      name = "--cleanup-percentage",
+      aliases = {"-c"},
+      usage = "Clean a % of zombie drafts (default is 100%)")
+  private Integer cleanupPercentage = 100;
+
+  @Override
+  public int run() throws IOException {
+    mustHaveValidSite();
+    Injector sysInjector = getSysInjector();
+    DeleteZombieCommentsRefs cleanup =
+        sysInjector.getInstance(Factory.class).create(cleanupPercentage);
+    cleanup.execute();
+    return 0;
+  }
+
+  private Injector getSysInjector() {
+    List<Module> modules = new ArrayList<>();
+    modules.add(
+        new AbstractModule() {
+          @Override
+          protected void configure() {
+            bind(Path.class).annotatedWith(SitePath.class).toInstance(getSitePath());
+            bind(ConsoleUI.class).toInstance(ConsoleUI.getInstance(false));
+            bind(String.class)
+                .annotatedWith(SecureStoreClassName.class)
+                .toProvider(Providers.of(getConfiguredSecureStoreClass()));
+            install(new FactoryModuleBuilder().build(DeleteZombieCommentsRefs.Factory.class));
+          }
+        });
+    modules.add(new GerritServerConfigModule());
+    modules.add(new SchemaModule());
+    return Guice.createInjector(modules);
+  }
+}
diff --git a/java/com/google/gerrit/pgm/http/BUILD b/java/com/google/gerrit/pgm/http/BUILD
deleted file mode 100644
index 34115ae..0000000
--- a/java/com/google/gerrit/pgm/http/BUILD
+++ /dev/null
@@ -1,7 +0,0 @@
-load("@rules_java//java:defs.bzl", "java_library")
-
-java_library(
-    name = "http",
-    visibility = ["//visibility:public"],
-    exports = ["//java/com/google/gerrit/pgm/http/jetty"],
-)
diff --git a/java/com/google/gerrit/pgm/init/BaseInit.java b/java/com/google/gerrit/pgm/init/BaseInit.java
index 7fbf2d7..62ff66a 100644
--- a/java/com/google/gerrit/pgm/init/BaseInit.java
+++ b/java/com/google/gerrit/pgm/init/BaseInit.java
@@ -332,7 +332,7 @@
       IoUtil.loadJARs(secureStoreLib);
       return new SecureStoreInitData(secureStoreLib, secureStores.get(0));
     } catch (IOException e) {
-      throw new InvalidSecureStoreException(String.format("%s is not a valid jar", secureStore));
+      throw new InvalidSecureStoreException(String.format("%s is not a valid jar", secureStore), e);
     }
   }
 
diff --git a/java/com/google/gerrit/pgm/init/InitHttpd.java b/java/com/google/gerrit/pgm/init/InitHttpd.java
index 027a3ac..b9fe874 100644
--- a/java/com/google/gerrit/pgm/init/InitHttpd.java
+++ b/java/com/google/gerrit/pgm/init/InitHttpd.java
@@ -169,7 +169,7 @@
         uri = new URI(s + "://" + uri.getHost() + uri.getPath());
       }
     } catch (URISyntaxException e) {
-      throw die("invalid httpd.listenUrl");
+      throw die("invalid httpd.listenUrl", e);
     }
     httpd.set("listenUrl", urlbuf.toString());
     gerrit.string("Canonical URL", "canonicalWebUrl", uri.toString());
diff --git a/java/com/google/gerrit/server/ApprovalsUtil.java b/java/com/google/gerrit/server/ApprovalsUtil.java
index 29a5748..58b601f 100644
--- a/java/com/google/gerrit/server/ApprovalsUtil.java
+++ b/java/com/google/gerrit/server/ApprovalsUtil.java
@@ -331,7 +331,7 @@
         forChange.check(new LabelPermission.WithValue(name, value));
       } catch (AuthException e) {
         throw new AuthException(
-            String.format("applying label \"%s\": %d is restricted", name, value));
+            String.format("applying label \"%s\": %d is restricted", name, value), e);
       }
     }
   }
diff --git a/java/com/google/gerrit/server/PublishCommentsOp.java b/java/com/google/gerrit/server/PublishCommentsOp.java
index 57d50fb..df57629 100644
--- a/java/com/google/gerrit/server/PublishCommentsOp.java
+++ b/java/com/google/gerrit/server/PublishCommentsOp.java
@@ -14,7 +14,7 @@
 
 package com.google.gerrit.server;
 
-import com.google.common.flogger.FluentLogger;
+import com.google.common.collect.ImmutableMap;
 import com.google.gerrit.entities.ChangeMessage;
 import com.google.gerrit.entities.Comment;
 import com.google.gerrit.entities.PatchSet;
@@ -45,8 +45,6 @@
  * necessary event for this.
  */
 public class PublishCommentsOp implements BatchUpdateOp {
-  private static final FluentLogger logger = FluentLogger.forEnclosingClass();
-
   private final PatchSetUtil psUtil;
   private final ChangeNotes.Factory changeNotesFactory;
   private final ChangeMessagesUtil cmUtil;
@@ -118,18 +116,14 @@
     if (notify.shouldNotify()) {
       email.create(notify, changeNotes, ps, user, message, comments, null, labelDelta).sendAsync();
     }
-    try {
-      commentAdded.fire(
-          changeNotes.getChange(),
-          ps,
-          ctx.getAccount(),
-          message.getMessage(),
-          null,
-          null,
-          ctx.getWhen());
-    } catch (Exception e) {
-      logger.atWarning().withCause(e).log("comment-added event invocation failed");
-    }
+    commentAdded.fire(
+        changeNotes.getChange(),
+        ps,
+        ctx.getAccount(),
+        message.getMessage(),
+        ImmutableMap.of(),
+        ImmutableMap.of(),
+        ctx.getWhen());
   }
 
   private boolean insertMessage(ChangeContext ctx, ChangeUpdate changeUpdate) {
diff --git a/java/com/google/gerrit/server/account/AccountManager.java b/java/com/google/gerrit/server/account/AccountManager.java
index 8137b92..6a16a53 100644
--- a/java/com/google/gerrit/server/account/AccountManager.java
+++ b/java/com/google/gerrit/server/account/AccountManager.java
@@ -391,7 +391,7 @@
     try {
       groupsUpdate.updateGroup(groupUuid, groupUpdate);
     } catch (NoSuchGroupException e) {
-      throw new AccountException(String.format("Group %s not found", groupUuid));
+      throw new AccountException(String.format("Group %s not found", groupUuid), e);
     }
   }
 
diff --git a/java/com/google/gerrit/server/account/Accounts.java b/java/com/google/gerrit/server/account/Accounts.java
index 8136631..7a6eb7a 100644
--- a/java/com/google/gerrit/server/account/Accounts.java
+++ b/java/com/google/gerrit/server/account/Accounts.java
@@ -21,6 +21,9 @@
 import com.google.common.flogger.FluentLogger;
 import com.google.gerrit.entities.Account;
 import com.google.gerrit.entities.RefNames;
+import com.google.gerrit.metrics.Description;
+import com.google.gerrit.metrics.MetricMaker;
+import com.google.gerrit.metrics.Timer0;
 import com.google.gerrit.server.account.externalids.ExternalIds;
 import com.google.gerrit.server.config.AllUsersName;
 import com.google.gerrit.server.git.GitRepositoryManager;
@@ -45,12 +48,23 @@
   private final GitRepositoryManager repoManager;
   private final AllUsersName allUsersName;
   private final ExternalIds externalIds;
+  private final Timer0 readSingleLatency;
 
   @Inject
-  Accounts(GitRepositoryManager repoManager, AllUsersName allUsersName, ExternalIds externalIds) {
+  Accounts(
+      GitRepositoryManager repoManager,
+      AllUsersName allUsersName,
+      ExternalIds externalIds,
+      MetricMaker metricMaker) {
     this.repoManager = repoManager;
     this.allUsersName = allUsersName;
     this.externalIds = externalIds;
+    this.readSingleLatency =
+        metricMaker.newTimer(
+            "notedb/read_single_account_config_latency",
+            new Description("Latency for reading a single account config.")
+                .setCumulative()
+                .setUnit(Description.Units.MILLISECONDS));
   }
 
   public Optional<AccountState> get(Account.Id accountId)
@@ -133,8 +147,11 @@
 
   private Optional<AccountState> read(Repository allUsersRepository, Account.Id accountId)
       throws IOException, ConfigInvalidException {
-    return AccountState.fromAccountConfig(
-        externalIds, new AccountConfig(accountId, allUsersName, allUsersRepository).load());
+    AccountConfig cfg;
+    try (Timer0.Context ignored = readSingleLatency.start()) {
+      cfg = new AccountConfig(accountId, allUsersName, allUsersRepository).load();
+    }
+    return AccountState.fromAccountConfig(externalIds, cfg);
   }
 
   public static Stream<Account.Id> readUserRefs(Repository repo) throws IOException {
diff --git a/java/com/google/gerrit/server/account/externalids/ExternalId.java b/java/com/google/gerrit/server/account/externalids/ExternalId.java
index b18b27b..2d501ad 100644
--- a/java/com/google/gerrit/server/account/externalids/ExternalId.java
+++ b/java/com/google/gerrit/server/account/externalids/ExternalId.java
@@ -24,6 +24,7 @@
 import com.google.common.base.Strings;
 import com.google.common.collect.ImmutableSet;
 import com.google.common.collect.Iterables;
+import com.google.common.flogger.FluentLogger;
 import com.google.common.hash.Hashing;
 import com.google.gerrit.common.Nullable;
 import com.google.gerrit.entities.Account;
@@ -43,6 +44,8 @@
 
 @AutoValue
 public abstract class ExternalId implements Serializable {
+  private static final FluentLogger logger = FluentLogger.forEnclosingClass();
+
   // If these regular expressions are modified the same modifications should be done to the
   // corresponding regular expressions in the
   // com.google.gerrit.client.account.UsernameField class.
@@ -391,11 +394,12 @@
       }
       return accountId;
     } catch (IllegalArgumentException e) {
-      throw invalidConfig(
-          noteId,
+      String msg =
           String.format(
               "Value %s for '%s.%s.%s' is invalid, expected account ID",
-              accountIdStr, EXTERNAL_ID_SECTION, externalIdKeyStr, ACCOUNT_ID_KEY));
+              accountIdStr, EXTERNAL_ID_SECTION, externalIdKeyStr, ACCOUNT_ID_KEY);
+      logger.atSevere().withCause(e).log(msg);
+      throw invalidConfig(noteId, msg);
     }
   }
 
diff --git a/java/com/google/gerrit/server/account/externalids/ExternalIdReader.java b/java/com/google/gerrit/server/account/externalids/ExternalIdReader.java
index 6334265..c055313 100644
--- a/java/com/google/gerrit/server/account/externalids/ExternalIdReader.java
+++ b/java/com/google/gerrit/server/account/externalids/ExternalIdReader.java
@@ -68,6 +68,7 @@
   private final AllUsersName allUsersName;
   private boolean failOnLoad = false;
   private final Timer0 readAllLatency;
+  private final Timer0 readSingleLatency;
 
   @Inject
   ExternalIdReader(
@@ -80,6 +81,12 @@
             new Description("Latency for reading all external IDs from NoteDb.")
                 .setCumulative()
                 .setUnit(Units.MILLISECONDS));
+    this.readSingleLatency =
+        metricMaker.newTimer(
+            "notedb/read_single_external_id_latency",
+            new Description("Latency for reading a single external ID from NoteDb.")
+                .setCumulative()
+                .setUnit(Units.MILLISECONDS));
   }
 
   @VisibleForTesting
@@ -126,7 +133,8 @@
   Optional<ExternalId> get(ExternalId.Key key) throws IOException, ConfigInvalidException {
     checkReadEnabled();
 
-    try (Repository repo = repoManager.openRepository(allUsersName)) {
+    try (Timer0.Context ctx = readSingleLatency.start();
+        Repository repo = repoManager.openRepository(allUsersName)) {
       return ExternalIdNotes.loadReadOnly(allUsersName, repo).get(key);
     }
   }
@@ -136,7 +144,8 @@
       throws IOException, ConfigInvalidException {
     checkReadEnabled();
 
-    try (Repository repo = repoManager.openRepository(allUsersName)) {
+    try (Timer0.Context ctx = readSingleLatency.start();
+        Repository repo = repoManager.openRepository(allUsersName)) {
       return ExternalIdNotes.loadReadOnly(allUsersName, repo, rev).get(key);
     }
   }
diff --git a/java/com/google/gerrit/server/args4j/AccountIdHandler.java b/java/com/google/gerrit/server/args4j/AccountIdHandler.java
index 4d0af53..73a970b 100644
--- a/java/com/google/gerrit/server/args4j/AccountIdHandler.java
+++ b/java/com/google/gerrit/server/args4j/AccountIdHandler.java
@@ -16,6 +16,7 @@
 
 import static com.google.gerrit.util.cli.Localizable.localizable;
 
+import com.google.common.flogger.FluentLogger;
 import com.google.gerrit.entities.Account;
 import com.google.gerrit.exceptions.StorageException;
 import com.google.gerrit.extensions.client.AuthType;
@@ -38,6 +39,8 @@
 import org.kohsuke.args4j.spi.Setter;
 
 public class AccountIdHandler extends OptionHandler<Account.Id> {
+  private static final FluentLogger logger = FluentLogger.forEnclosingClass();
+
   private final AccountResolver accountResolver;
   private final AccountManager accountManager;
   private final AuthType authType;
@@ -78,11 +81,15 @@
           case OPENID:
           case OPENID_SSO:
           default:
-            throw new CmdLineException(owner, localizable("user \"%s\" not found"), token);
+            String msg = "user \"%s\" not found";
+            logger.atSevere().withCause(e).log(msg, token);
+            throw new CmdLineException(owner, localizable(msg), token);
         }
       }
     } catch (StorageException e) {
-      throw new CmdLineException(owner, localizable("database is down"));
+      String msg = "database is down";
+      logger.atSevere().withCause(e).log(msg);
+      throw new CmdLineException(owner, localizable(msg));
     } catch (IOException e) {
       throw new CmdLineException(owner, "Failed to load account", e);
     } catch (ConfigInvalidException e) {
@@ -102,7 +109,9 @@
       req.setSkipAuthentication(true);
       return accountManager.authenticate(req).getAccountId();
     } catch (AccountException e) {
-      throw new CmdLineException(owner, localizable("user \"%s\" not found"), user);
+      String msg = "user \"%s\" not found";
+      logger.atSevere().withCause(e).log(msg, user);
+      throw new CmdLineException(owner, localizable(msg), user);
     }
   }
 
diff --git a/java/com/google/gerrit/server/args4j/ChangeIdHandler.java b/java/com/google/gerrit/server/args4j/ChangeIdHandler.java
index 9c14aca..ddcd4db 100644
--- a/java/com/google/gerrit/server/args4j/ChangeIdHandler.java
+++ b/java/com/google/gerrit/server/args4j/ChangeIdHandler.java
@@ -66,7 +66,7 @@
         return 1;
       }
     } catch (IllegalArgumentException e) {
-      throw new CmdLineException(owner, localizable("Change-Id is not valid"));
+      throw new CmdLineException(owner, localizable("Change-Id is not valid: %s"), e.getMessage());
     } catch (StorageException e) {
       throw new CmdLineException(owner, localizable("Database error: %s"), e.getMessage());
     }
diff --git a/java/com/google/gerrit/server/args4j/PatchSetIdHandler.java b/java/com/google/gerrit/server/args4j/PatchSetIdHandler.java
index 5016363..8b7cbd6 100644
--- a/java/com/google/gerrit/server/args4j/PatchSetIdHandler.java
+++ b/java/com/google/gerrit/server/args4j/PatchSetIdHandler.java
@@ -43,7 +43,8 @@
     try {
       id = PatchSet.Id.parse(token);
     } catch (IllegalArgumentException e) {
-      throw new CmdLineException(owner, localizable("\"%s\" is not a valid patch set"), token);
+      throw new CmdLineException(
+          owner, localizable("\"%s\" is not a valid patch set: %s"), token, e.getMessage());
     }
 
     setter.addValue(id);
diff --git a/java/com/google/gerrit/server/args4j/ProjectHandler.java b/java/com/google/gerrit/server/args4j/ProjectHandler.java
index abb6a1f..61dbd2c 100644
--- a/java/com/google/gerrit/server/args4j/ProjectHandler.java
+++ b/java/com/google/gerrit/server/args4j/ProjectHandler.java
@@ -89,7 +89,7 @@
       permissionBackend.currentUser().project(nameKey).check(permissionToCheck);
     } catch (AuthException e) {
       throw new CmdLineException(
-          owner, localizable(new NoSuchProjectException(nameKey).getMessage()));
+          owner, localizable(new NoSuchProjectException(nameKey, e).getMessage()));
     } catch (PermissionBackendException | IOException e) {
       logger.atWarning().withCause(e).log("Cannot load project %s", nameWithoutSuffix);
       throw new CmdLineException(
diff --git a/java/com/google/gerrit/server/cache/serialize/IntegerCacheSerializer.java b/java/com/google/gerrit/server/cache/serialize/IntegerCacheSerializer.java
index 4494454..aca9b05 100644
--- a/java/com/google/gerrit/server/cache/serialize/IntegerCacheSerializer.java
+++ b/java/com/google/gerrit/server/cache/serialize/IntegerCacheSerializer.java
@@ -37,7 +37,7 @@
       cout.writeInt32NoTag(requireNonNull(object));
       cout.flush();
     } catch (IOException e) {
-      throw new IllegalStateException("Failed to serialize int");
+      throw new IllegalStateException("Failed to serialize int", e);
     }
     int n = cout.getTotalBytesWritten();
     return n == buf.length ? buf : Arrays.copyOfRange(buf, 0, n);
@@ -50,7 +50,7 @@
     try {
       ret = cin.readRawVarint32();
     } catch (IOException e) {
-      throw new IllegalArgumentException("Failed to deserialize int");
+      throw new IllegalArgumentException("Failed to deserialize int", e);
     }
     int n = cin.getTotalBytesRead();
     if (n != in.length) {
diff --git a/java/com/google/gerrit/server/change/ChangeJson.java b/java/com/google/gerrit/server/change/ChangeJson.java
index 974cb47..70e7967 100644
--- a/java/com/google/gerrit/server/change/ChangeJson.java
+++ b/java/com/google/gerrit/server/change/ChangeJson.java
@@ -29,7 +29,6 @@
 import static com.google.gerrit.extensions.client.ListChangesOption.REVIEWED;
 import static com.google.gerrit.extensions.client.ListChangesOption.REVIEWER_UPDATES;
 import static com.google.gerrit.extensions.client.ListChangesOption.SKIP_DIFFSTAT;
-import static com.google.gerrit.extensions.client.ListChangesOption.SKIP_MERGEABLE;
 import static com.google.gerrit.extensions.client.ListChangesOption.SUBMITTABLE;
 import static com.google.gerrit.extensions.client.ListChangesOption.TRACKING_IDS;
 import static com.google.gerrit.server.ChangeMessagesUtil.createChangeMessageInfo;
@@ -219,7 +218,7 @@
   private final Metrics metrics;
   private final RevisionJson revisionJson;
   private final Optional<PluginDefinedAttributesFactory> pluginDefinedAttributesFactory;
-  private final boolean excludeMergeableInChangeInfo;
+  private final boolean includeMergeable;
   private final boolean lazyLoad;
 
   private AccountLoader accountLoader;
@@ -257,8 +256,7 @@
     this.metrics = metrics;
     this.revisionJson = revisionJsonFactory.create(options);
     this.options = Sets.immutableEnumSet(options);
-    this.excludeMergeableInChangeInfo =
-        cfg.getBoolean("change", "api", "excludeMergeableInChangeInfo", false);
+    this.includeMergeable = MergeabilityComputationBehavior.fromConfig(cfg).includeInApi();
     this.lazyLoad = containsAnyOf(this.options, REQUIRE_LAZY_LOAD);
     this.pluginDefinedAttributesFactory = pluginDefinedAttributesFactory;
 
@@ -525,7 +523,7 @@
       if (str.isOk()) {
         out.submitType = str.type;
       }
-      if (!excludeMergeableInChangeInfo && !has(SKIP_MERGEABLE)) {
+      if (includeMergeable) {
         out.mergeable = cd.isMergeable();
       }
       if (has(SUBMITTABLE)) {
diff --git a/java/com/google/gerrit/server/change/MergeabilityComputationBehavior.java b/java/com/google/gerrit/server/change/MergeabilityComputationBehavior.java
new file mode 100644
index 0000000..26e7a89
--- /dev/null
+++ b/java/com/google/gerrit/server/change/MergeabilityComputationBehavior.java
@@ -0,0 +1,58 @@
+// Copyright (C) 2020 The Android Open Source Project
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package com.google.gerrit.server.change;
+
+import org.eclipse.jgit.lib.Config;
+
+/**
+ * State that is used to decide if {@code mergeable} should be included in the REST API or the
+ * change index.
+ *
+ * <p>Computing mergeability of a change is an expensive operation - especially for Gerrit
+ * installations with a large number of open changes or large repositories. Therefore, we want to
+ * control when this computation is performed.
+ */
+public enum MergeabilityComputationBehavior {
+  NEVER(false, false),
+  REF_UPDATED_AND_CHANGE_REINDEX(true, false),
+  API_REF_UPDATED_AND_CHANGE_REINDEX(true, true);
+
+  private final boolean includeInIndex;
+  private final boolean includeInApi;
+
+  MergeabilityComputationBehavior(boolean includeInIndex, boolean includeInApi) {
+    this.includeInIndex = includeInIndex;
+    this.includeInApi = includeInApi;
+  }
+
+  /** Returns a {@link MergeabilityComputationBehavior} constructed from a Gerrit server config. */
+  public static MergeabilityComputationBehavior fromConfig(Config cfg) {
+    return cfg.getEnum(
+        "change",
+        null,
+        "mergeabilityComputationBehavior",
+        MergeabilityComputationBehavior.API_REF_UPDATED_AND_CHANGE_REINDEX);
+  }
+
+  /** Whether {@code mergeable} should be included in the change API. */
+  public boolean includeInApi() {
+    return includeInApi;
+  }
+
+  /** Whether {@code mergeable} should be included in the change index. */
+  public boolean includeInIndex() {
+    return includeInIndex;
+  }
+}
diff --git a/java/com/google/gerrit/server/change/PureRevert.java b/java/com/google/gerrit/server/change/PureRevert.java
index f66d9bc..cb632dc 100644
--- a/java/com/google/gerrit/server/change/PureRevert.java
+++ b/java/com/google/gerrit/server/change/PureRevert.java
@@ -50,7 +50,7 @@
     try {
       claimedOriginalObjectId = ObjectId.fromString(claimedOriginal.get());
     } catch (InvalidObjectIdException e) {
-      throw new BadRequestException("invalid object ID");
+      throw new BadRequestException("invalid object ID", e);
     }
 
     return pureRevertCache.isPureRevert(
diff --git a/java/com/google/gerrit/server/config/ChangeCleanupConfig.java b/java/com/google/gerrit/server/config/ChangeCleanupConfig.java
index afd159c..bd51fc7 100644
--- a/java/com/google/gerrit/server/config/ChangeCleanupConfig.java
+++ b/java/com/google/gerrit/server/config/ChangeCleanupConfig.java
@@ -17,6 +17,7 @@
 import com.google.common.base.Strings;
 import com.google.common.flogger.FluentLogger;
 import com.google.gerrit.extensions.registration.DynamicItem;
+import com.google.gerrit.server.change.MergeabilityComputationBehavior;
 import com.google.gerrit.server.config.ScheduleConfig.Schedule;
 import com.google.inject.Inject;
 import com.google.inject.Singleton;
@@ -51,7 +52,7 @@
     this.urlFormatter = urlFormatter;
     schedule = ScheduleConfig.createSchedule(cfg, SECTION);
     abandonAfter = readAbandonAfter(cfg);
-    boolean indexMergeable = cfg.getBoolean("index", "change", "indexMergeable", true);
+    boolean indexMergeable = MergeabilityComputationBehavior.fromConfig(cfg).includeInIndex();
     if (!indexMergeable) {
       if (!readAbandonIfMergeable(cfg)) {
         logger.atWarning().log(
diff --git a/java/com/google/gerrit/server/config/ConfigUtil.java b/java/com/google/gerrit/server/config/ConfigUtil.java
index f476adf..43c05e0 100644
--- a/java/com/google/gerrit/server/config/ConfigUtil.java
+++ b/java/com/google/gerrit/server/config/ConfigUtil.java
@@ -30,7 +30,6 @@
 import org.eclipse.jgit.lib.Config;
 
 public class ConfigUtil {
-
   @SuppressWarnings("unchecked")
   private static <T> T[] allValuesOf(T defaultValue) {
     try {
@@ -182,7 +181,7 @@
     try {
       return getTimeUnit(s, defaultValue, wantUnit);
     } catch (IllegalArgumentException notTime) {
-      throw notTimeUnit(section, subsection, setting, valueString);
+      throw notTimeUnit(section, subsection, setting, valueString, notTime);
     }
   }
 
@@ -250,7 +249,7 @@
     try {
       return wantUnit.convert(Long.parseLong(digits) * inputMul, inputUnit);
     } catch (NumberFormatException nfe) {
-      throw notTimeUnit(valueString);
+      throw notTimeUnit(valueString, nfe);
     }
   }
 
@@ -421,13 +420,21 @@
   }
 
   private static IllegalArgumentException notTimeUnit(
-      final String section,
-      final String subsection,
-      final String setting,
-      final String valueString) {
-    return new IllegalArgumentException(
-        "Invalid time unit value: "
-            + section
+      String section, String subsection, String setting, String valueString, Throwable why) {
+    return notTimeUnit(
+        section
+            + (subsection != null ? "." + subsection : "")
+            + "."
+            + setting
+            + " = "
+            + valueString,
+        why);
+  }
+
+  private static IllegalArgumentException notTimeUnit(
+      String section, String subsection, String setting, String valueString) {
+    return notTimeUnit(
+        section
             + (subsection != null ? "." + subsection : "")
             + "."
             + setting
@@ -439,5 +446,9 @@
     return new IllegalArgumentException("Invalid time unit value: " + val);
   }
 
+  private static IllegalArgumentException notTimeUnit(String val, Throwable why) {
+    return new IllegalArgumentException("Invalid time unit value: " + val, why);
+  }
+
   private ConfigUtil() {}
 }
diff --git a/java/com/google/gerrit/server/config/GerritGlobalModule.java b/java/com/google/gerrit/server/config/GerritGlobalModule.java
index 7df3902..012e4cd 100644
--- a/java/com/google/gerrit/server/config/GerritGlobalModule.java
+++ b/java/com/google/gerrit/server/config/GerritGlobalModule.java
@@ -128,7 +128,8 @@
 import com.google.gerrit.server.git.TagCache;
 import com.google.gerrit.server.git.TransferConfig;
 import com.google.gerrit.server.git.receive.ReceiveCommitsModule;
-import com.google.gerrit.server.git.validators.CommentLimitsValidator;
+import com.google.gerrit.server.git.validators.CommentCountValidator;
+import com.google.gerrit.server.git.validators.CommentSizeValidator;
 import com.google.gerrit.server.git.validators.CommitValidationListener;
 import com.google.gerrit.server.git.validators.MergeValidationListener;
 import com.google.gerrit.server.git.validators.MergeValidators;
@@ -410,8 +411,11 @@
     DynamicSet.setOf(binder(), UploadValidationListener.class);
 
     bind(CommentValidator.class)
-        .annotatedWith(Exports.named(CommentLimitsValidator.class.getSimpleName()))
-        .to(CommentLimitsValidator.class);
+        .annotatedWith(Exports.named(CommentCountValidator.class.getSimpleName()))
+        .to(CommentCountValidator.class);
+    bind(CommentValidator.class)
+        .annotatedWith(Exports.named(CommentSizeValidator.class.getSimpleName()))
+        .to(CommentSizeValidator.class);
 
     DynamicMap.mapOf(binder(), DynamicOptions.DynamicBean.class);
     DynamicMap.mapOf(binder(), ChangeQueryBuilder.ChangeOperatorFactory.class);
diff --git a/java/com/google/gerrit/server/fixes/FixCalculator.java b/java/com/google/gerrit/server/fixes/FixCalculator.java
index 5d9d2c2..9ea628e 100644
--- a/java/com/google/gerrit/server/fixes/FixCalculator.java
+++ b/java/com/google/gerrit/server/fixes/FixCalculator.java
@@ -88,7 +88,68 @@
     return String.format(
         "(%s:%s - %s:%s)", range.startLine, range.startChar, range.endLine, range.endChar);
   }
+  /*
+  Algorithm:
+  Input:
+    Original text (aka srcText)
+    Sorted list of replacements in ascending order, where each replacement has:
+        srcRange - part of the original text to be
+                   replaced, inserted or deleted (see {@link Comment.Range} for details)
+        replacement - text to be set instead of srcRange
+    Replacement ranges must not intersect.
 
+  Output:
+    Final text (aka finalText)
+    List of Edit, where each Edit is an instance of {@link ReplaceEdit}
+      Each ReplaceEdit cover one or more lines in the original text
+      Each ReplaceEdit contains one or more Edit for intraline edits
+    See {@link ReplaceEdit} and {@link Edit} for details.
+  *
+  Note: The algorithm is implemented in this way to avoid string.replace operations.
+  It has complexity O(len(replacements) + max(len(originalText), len(finalText)) )
+
+  Main steps:
+  - set srcPos to start of the original text. It is like a cursor position in the original text.
+  - set dstPos to start of the final text.  It is like a cursor position in the final text.
+  - the finalText initially empty
+
+  - for each replacement:
+       - append text between a previous and a current replacement to the finalText
+           (because replacements were sorted, this part of text can't be changed by
+             following replacements). I.e. append substring of srcText between srcPos
+             and replacement.srcRange.start to the finalText
+           Update srcPos and dstPos - set them at the end of appended text
+           (i.e. srcPos points to the position before replacement.srcRange.start,
+            dstPos points to the position where replacement.text should be inserted)
+       - set dstReplacementStart = dstPos
+       - append replacement.text to the finalText.
+           Update srcPos and dstPos accordingly (i.e. srcPos points to the position after
+           replacement.srcRange, dstPos points to the position in the finalText after
+           the appended replacement.text).
+       - set dstReplacementEnd = dstPos
+       - dstRange = (dstReplacementStart, dstReplacementEnd) - is the range in the finalText.
+       - srcRange = (replacement.Start, replacement.End) -  is the range in the original text *
+
+       - If previously created ReplaceEdit ends on the same or previous line as srcRange.startLine,
+           then intraline edit is added to it (and ReplaceEdit endLine must be updated if needed);
+           srcRange and dstRange together is used to calculate intraline Edit
+         otherwise
+          create new ReplaceEdit and add intraline Edit to it
+          srcRange and dstRange together is used to calculate intraline Edit
+
+  - append text after the last replacements,
+      i.e. add part of srcText after srcPos to the finalText
+
+  - Return the finalText and all created ReplaceEdits
+
+  Implementation notes:
+  1) The intraline Edits inside ReplaceEdit stores positions relative to ReplaceEdit start.
+  2) srcPos and dstPos tracks current position as 3 numbers:
+  - line number
+  - column number
+  - textPos - absolute position from the start of the text. The textPos is used to calculate
+  relative positions of Edit inside ReplaceEdit
+     */
   private static class ContentBuilder {
     private static class FixRegion {
       int startSrcLine;
diff --git a/java/com/google/gerrit/server/git/MergeUtil.java b/java/com/google/gerrit/server/git/MergeUtil.java
index 1dbf029..18394b5 100644
--- a/java/com/google/gerrit/server/git/MergeUtil.java
+++ b/java/com/google/gerrit/server/git/MergeUtil.java
@@ -462,7 +462,7 @@
   }
 
   public static String createConflictMessage(List<String> conflicts) {
-    StringBuilder sb = new StringBuilder("merge conflict(s)");
+    StringBuilder sb = new StringBuilder("merge conflict(s):");
     for (String c : conflicts) {
       sb.append('\n').append(c);
     }
@@ -760,6 +760,7 @@
       try {
         failed(rw, mergeTip, n, getCommitMergeStatus(e.getReason()));
       } catch (IOException e2) {
+        logger.atSevere().withCause(e2).log("Failed to set merge failure status for " + n.name());
         throw new IntegrationException("Cannot merge " + n.name(), e);
       }
     } catch (IOException e) {
diff --git a/java/com/google/gerrit/server/git/PermissionAwareReadOnlyRefDatabase.java b/java/com/google/gerrit/server/git/PermissionAwareReadOnlyRefDatabase.java
index 1bd8b45..b7dc2b3 100644
--- a/java/com/google/gerrit/server/git/PermissionAwareReadOnlyRefDatabase.java
+++ b/java/com/google/gerrit/server/git/PermissionAwareReadOnlyRefDatabase.java
@@ -106,7 +106,7 @@
     try {
       result = forProject.filter(refs, getDelegate(), RefFilterOptions.defaults());
     } catch (PermissionBackendException e) {
-      throw new IOException("");
+      throw new IOException("", e);
     }
     return buildPrefixRefMap(prefix, result);
   }
diff --git a/java/com/google/gerrit/server/git/PureRevertCache.java b/java/com/google/gerrit/server/git/PureRevertCache.java
index b21cb5c..9f9530c 100644
--- a/java/com/google/gerrit/server/git/PureRevertCache.java
+++ b/java/com/google/gerrit/server/git/PureRevertCache.java
@@ -166,7 +166,7 @@
           try {
             claimedOriginalCommit = rw.parseCommit(original);
           } catch (InvalidObjectIdException | MissingObjectException e) {
-            throw new BadRequestException("invalid object ID");
+            throw new BadRequestException("invalid object ID", e);
           }
           if (claimedOriginalCommit.getParentCount() == 0) {
             throw new BadRequestException("can't check against initial commit");
diff --git a/java/com/google/gerrit/server/git/meta/VersionedMetaData.java b/java/com/google/gerrit/server/git/meta/VersionedMetaData.java
index 66482a9..c52bbbc 100644
--- a/java/com/google/gerrit/server/git/meta/VersionedMetaData.java
+++ b/java/com/google/gerrit/server/git/meta/VersionedMetaData.java
@@ -436,7 +436,7 @@
 
   private String errorMsg(RefUpdate ru, File location) {
     return String.format(
-        "Cannot update %s in %s : %s (%s)",
+        "Cannot update %s in %s: %s (%s)",
         ru.getName(), location, ru.getResult(), ru.getRefLogMessage());
   }
 
diff --git a/java/com/google/gerrit/server/git/receive/ReceiveCommits.java b/java/com/google/gerrit/server/git/receive/ReceiveCommits.java
index d8aa054..4b6a4cd 100644
--- a/java/com/google/gerrit/server/git/receive/ReceiveCommits.java
+++ b/java/com/google/gerrit/server/git/receive/ReceiveCommits.java
@@ -92,6 +92,7 @@
 import com.google.gerrit.extensions.restapi.RestApiException;
 import com.google.gerrit.extensions.restapi.UnprocessableEntityException;
 import com.google.gerrit.extensions.validators.CommentForValidation;
+import com.google.gerrit.extensions.validators.CommentForValidation.CommentSource;
 import com.google.gerrit.extensions.validators.CommentForValidation.CommentType;
 import com.google.gerrit.extensions.validators.CommentValidationContext;
 import com.google.gerrit.extensions.validators.CommentValidationFailure;
@@ -126,6 +127,8 @@
 import com.google.gerrit.server.git.ReceivePackInitializer;
 import com.google.gerrit.server.git.TagCache;
 import com.google.gerrit.server.git.ValidationError;
+import com.google.gerrit.server.git.validators.CommentCountValidator;
+import com.google.gerrit.server.git.validators.CommentSizeValidator;
 import com.google.gerrit.server.git.validators.CommitValidationMessage;
 import com.google.gerrit.server.git.validators.RefOperationValidationException;
 import com.google.gerrit.server.git.validators.RefOperationValidators;
@@ -1441,11 +1444,18 @@
     final ReceiveCommand cmd;
     final LabelTypes labelTypes;
     /**
-     * Result of running {@link CommentValidator}-s on drafts that are published with the commit
-     * (which happens iff {@code --publish-comments} is set). Remains {@code true} if none are
-     * installed.
+     * Draft comments are published with the commit iff {@code --publish-comments} is set. All
+     * drafts are withheld (overriding the option) if at least one of the following conditions are
+     * met:
+     *
+     * <ul>
+     *   <li>Installed {@link CommentValidator} plugins reject one or more draft comments.
+     *   <li>One or more comments exceed the maximum comment size (see {@link
+     *       CommentSizeValidator}).
+     *   <li>The maximum number of comments would be exceeded (see {@link CommentCountValidator}).
+     * </ul>
      */
-    private boolean commentsValid = true;
+    private boolean withholdComments = false;
 
     BranchNameKey dest;
     PermissionBackend.ForRef perm;
@@ -1642,18 +1652,19 @@
           .collect(toImmutableSet());
     }
 
-    void setCommentsValid(boolean commentsValid) {
-      this.commentsValid = commentsValid;
+    void setWithholdComments(boolean withholdComments) {
+      this.withholdComments = withholdComments;
     }
 
     boolean shouldPublishComments() {
-      if (!commentsValid) {
+      if (withholdComments) {
         // Validation messages of type WARNING have already been added, now withhold the comments.
         return false;
       }
       if (publishComments) {
         return true;
-      } else if (noPublishComments) {
+      }
+      if (noPublishComments) {
         return false;
       }
       return defaultPublishComments;
@@ -2010,19 +2021,18 @@
                 .map(
                     comment ->
                         CommentForValidation.create(
+                            CommentSource.HUMAN,
                             comment.lineNbr > 0
                                 ? CommentType.INLINE_COMMENT
                                 : CommentType.FILE_COMMENT,
-                            comment.message))
+                            comment.message,
+                            comment.message.length()))
                 .collect(toImmutableList());
         CommentValidationContext ctx =
-            CommentValidationContext.builder()
-                .changeId(change.getChangeId())
-                .project(change.getProject().get())
-                .build();
+            CommentValidationContext.create(change.getChangeId(), change.getProject().get());
         ImmutableList<CommentValidationFailure> commentValidationFailures =
             PublishCommentUtil.findInvalidComments(ctx, commentValidators, draftsForValidation);
-        magicBranch.setCommentsValid(commentValidationFailures.isEmpty());
+        magicBranch.setWithholdComments(!commentValidationFailures.isEmpty());
         commentValidationFailures.forEach(
             failure ->
                 addMessage(
diff --git a/java/com/google/gerrit/server/git/validators/CommentCountValidator.java b/java/com/google/gerrit/server/git/validators/CommentCountValidator.java
new file mode 100644
index 0000000..2c939c8
--- /dev/null
+++ b/java/com/google/gerrit/server/git/validators/CommentCountValidator.java
@@ -0,0 +1,61 @@
+// Copyright (C) 2020 The Android Open Source Project
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package com.google.gerrit.server.git.validators;
+
+import com.google.common.collect.ImmutableList;
+import com.google.common.collect.Iterables;
+import com.google.gerrit.entities.Change;
+import com.google.gerrit.entities.Project;
+import com.google.gerrit.extensions.validators.CommentForValidation;
+import com.google.gerrit.extensions.validators.CommentValidationContext;
+import com.google.gerrit.extensions.validators.CommentValidationFailure;
+import com.google.gerrit.extensions.validators.CommentValidator;
+import com.google.gerrit.server.config.GerritServerConfig;
+import com.google.gerrit.server.notedb.ChangeNotes;
+import com.google.inject.Inject;
+import org.eclipse.jgit.lib.Config;
+
+/** Limits number of comments to prevent space/time complexity issues. */
+public class CommentCountValidator implements CommentValidator {
+  private final int maxComments;
+  private final ChangeNotes.Factory notesFactory;
+
+  @Inject
+  CommentCountValidator(@GerritServerConfig Config serverConfig, ChangeNotes.Factory notesFactory) {
+    this.notesFactory = notesFactory;
+    maxComments = serverConfig.getInt("change", "maxComments", 5_000);
+  }
+
+  @Override
+  public ImmutableList<CommentValidationFailure> validateComments(
+      CommentValidationContext ctx, ImmutableList<CommentForValidation> comments) {
+    ImmutableList.Builder<CommentValidationFailure> failures = ImmutableList.builder();
+    ChangeNotes notes =
+        notesFactory.createChecked(Project.nameKey(ctx.getProject()), Change.id(ctx.getChangeId()));
+    int numExistingComments = notes.getComments().size() + notes.getRobotComments().size();
+    if (!comments.isEmpty() && numExistingComments + comments.size() > maxComments) {
+      // This warning really applies to the set of all comments, but we need to pick one to attach
+      // the message to.
+      CommentForValidation commentForFailureMessage = Iterables.getLast(comments);
+
+      failures.add(
+          commentForFailureMessage.failValidation(
+              String.format(
+                  "Exceeding maximum number of comments: %d (existing) + %d (new) > %d",
+                  numExistingComments, comments.size(), maxComments)));
+    }
+    return failures.build();
+  }
+}
diff --git a/java/com/google/gerrit/server/git/validators/CommentLimitsValidator.java b/java/com/google/gerrit/server/git/validators/CommentLimitsValidator.java
deleted file mode 100644
index 3a8bcac..0000000
--- a/java/com/google/gerrit/server/git/validators/CommentLimitsValidator.java
+++ /dev/null
@@ -1,47 +0,0 @@
-// Copyright (C) 2020 The Android Open Source Project
-//
-// Licensed under the Apache License, Version 2.0 (the "License");
-// you may not use this file except in compliance with the License.
-// You may obtain a copy of the License at
-//
-// http://www.apache.org/licenses/LICENSE-2.0
-//
-// Unless required by applicable law or agreed to in writing, software
-// distributed under the License is distributed on an "AS IS" BASIS,
-// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-// See the License for the specific language governing permissions and
-// limitations under the License.
-
-package com.google.gerrit.server.git.validators;
-
-import com.google.common.collect.ImmutableList;
-import com.google.gerrit.extensions.validators.CommentForValidation;
-import com.google.gerrit.extensions.validators.CommentValidationContext;
-import com.google.gerrit.extensions.validators.CommentValidationFailure;
-import com.google.gerrit.extensions.validators.CommentValidator;
-import com.google.gerrit.server.config.GerritServerConfig;
-import com.google.inject.Inject;
-import org.eclipse.jgit.lib.Config;
-
-/** Limits the size of comments to prevent large comments from causing issues. */
-public class CommentLimitsValidator implements CommentValidator {
-  private final int maxCommentLength;
-
-  @Inject
-  CommentLimitsValidator(@GerritServerConfig Config serverConfig) {
-    maxCommentLength = serverConfig.getInt("change", null, "maxCommentLength", 16 << 10);
-  }
-
-  @Override
-  public ImmutableList<CommentValidationFailure> validateComments(
-      CommentValidationContext ctx, ImmutableList<CommentForValidation> comments) {
-    return comments.stream()
-        .filter(c -> c.getText().length() > maxCommentLength)
-        .map(
-            c ->
-                c.failValidation(
-                    String.format(
-                        "Comment too large (%d > %d)", c.getText().length(), maxCommentLength)))
-        .collect(ImmutableList.toImmutableList());
-  }
-}
diff --git a/java/com/google/gerrit/server/git/validators/CommentSizeValidator.java b/java/com/google/gerrit/server/git/validators/CommentSizeValidator.java
new file mode 100644
index 0000000..58b0cb1
--- /dev/null
+++ b/java/com/google/gerrit/server/git/validators/CommentSizeValidator.java
@@ -0,0 +1,71 @@
+// Copyright (C) 2020 The Android Open Source Project
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package com.google.gerrit.server.git.validators;
+
+import com.google.common.collect.ImmutableList;
+import com.google.gerrit.extensions.validators.CommentForValidation;
+import com.google.gerrit.extensions.validators.CommentValidationContext;
+import com.google.gerrit.extensions.validators.CommentValidationFailure;
+import com.google.gerrit.extensions.validators.CommentValidator;
+import com.google.gerrit.server.config.GerritServerConfig;
+import com.google.inject.Inject;
+import org.eclipse.jgit.lib.Config;
+
+/** Limits the size of comments to prevent space/time complexity issues. */
+public class CommentSizeValidator implements CommentValidator {
+  private final int commentSizeLimit;
+  private final int robotCommentSizeLimit;
+
+  @Inject
+  CommentSizeValidator(@GerritServerConfig Config serverConfig) {
+    commentSizeLimit = serverConfig.getInt("change", "commentSizeLimit", 16 << 10);
+    robotCommentSizeLimit = serverConfig.getInt("change", "robotCommentSizeLimit", 1 << 20);
+  }
+
+  @Override
+  public ImmutableList<CommentValidationFailure> validateComments(
+      CommentValidationContext ctx, ImmutableList<CommentForValidation> comments) {
+    return comments.stream()
+        .filter(this::exceedsSizeLimit)
+        .map(c -> c.failValidation(buildErrorMessage(c)))
+        .collect(ImmutableList.toImmutableList());
+  }
+
+  private boolean exceedsSizeLimit(CommentForValidation comment) {
+    switch (comment.getSource()) {
+      case HUMAN:
+        return comment.getApproximateSize() > commentSizeLimit;
+      case ROBOT:
+        return robotCommentSizeLimit > 0 && comment.getApproximateSize() > robotCommentSizeLimit;
+    }
+    throw new RuntimeException(
+        "Unknown comment source (should not have compiled): " + comment.getSource());
+  }
+
+  private String buildErrorMessage(CommentForValidation comment) {
+    switch (comment.getSource()) {
+      case HUMAN:
+        return String.format(
+            "Comment size exceeds limit (%d > %d)", comment.getApproximateSize(), commentSizeLimit);
+
+      case ROBOT:
+        return String.format(
+            "Size %d (bytes) of robot comment is greater than limit %d (bytes)",
+            comment.getApproximateSize(), robotCommentSizeLimit);
+    }
+    throw new RuntimeException(
+        "Unknown comment source (should not have compiled): " + comment.getSource());
+  }
+}
diff --git a/java/com/google/gerrit/server/git/validators/CommitValidationException.java b/java/com/google/gerrit/server/git/validators/CommitValidationException.java
index 4dd2951..220fc12 100644
--- a/java/com/google/gerrit/server/git/validators/CommitValidationException.java
+++ b/java/com/google/gerrit/server/git/validators/CommitValidationException.java
@@ -32,6 +32,11 @@
     this.messages = ImmutableList.of(message);
   }
 
+  public CommitValidationException(String reason, CommitValidationMessage message, Throwable why) {
+    super(reason, why);
+    this.messages = ImmutableList.of(message);
+  }
+
   public CommitValidationException(String reason, List<CommitValidationMessage> messages) {
     super(reason);
     this.messages = ImmutableList.copyOf(messages);
diff --git a/java/com/google/gerrit/server/git/validators/CommitValidators.java b/java/com/google/gerrit/server/git/validators/CommitValidators.java
index 4159ebb..4d2dbef 100644
--- a/java/com/google/gerrit/server/git/validators/CommitValidators.java
+++ b/java/com/google/gerrit/server/git/validators/CommitValidators.java
@@ -151,6 +151,7 @@
               new ProjectStateValidationListener(projectState),
               new AmendedGerritMergeCommitValidationListener(perm, gerritIdent),
               new AuthorUploaderValidator(user, perm, urlFormatter.get()),
+              new FileCountValidator(patchListCache, config),
               new CommitterUploaderValidator(user, perm, urlFormatter.get()),
               new SignedOffByValidator(user, perm, projectState),
               new ChangeIdValidator(
@@ -414,7 +415,11 @@
                   diffSummary.getPaths().size(), maxFileCount));
         }
       } catch (PatchListNotAvailableException e) {
-        logger.atWarning().withCause(e).log("Failed to validate file count");
+        // This happens e.g. for cherrypicks.
+        if (!receiveEvent.command.getRefName().startsWith(REFS_CHANGES)) {
+          logger.atWarning().withCause(e).log(
+              "Failed to validate file count for commit: %s", receiveEvent.commit.toString());
+        }
       }
       return Collections.emptyList();
     }
@@ -500,7 +505,7 @@
         perm.check(RefPermission.MERGE);
         return Collections.emptyList();
       } catch (AuthException e) {
-        throw new CommitValidationException("you are not allowed to upload merges");
+        throw new CommitValidationException("you are not allowed to upload merges", e);
       } catch (PermissionBackendException e) {
         logger.atSevere().withCause(e).log("cannot check MERGE");
         throw new CommitValidationException("internal auth error");
@@ -597,7 +602,7 @@
           perm.check(RefPermission.FORGE_COMMITTER);
         } catch (AuthException denied) {
           throw new CommitValidationException(
-              "not Signed-off-by author/committer/uploader in message footer");
+              "not Signed-off-by author/committer/uploader in message footer", denied);
         } catch (PermissionBackendException e) {
           logger.atSevere().withCause(e).log("cannot check FORGE_COMMITTER");
           throw new CommitValidationException("internal auth error");
@@ -632,7 +637,7 @@
         return Collections.emptyList();
       } catch (AuthException e) {
         throw new CommitValidationException(
-            "invalid author", invalidEmail("author", author, user, urlFormatter));
+            "invalid author", invalidEmail("author", author, user, urlFormatter), e);
       } catch (PermissionBackendException e) {
         logger.atSevere().withCause(e).log("cannot check FORGE_AUTHOR");
         throw new CommitValidationException("internal auth error");
@@ -665,7 +670,7 @@
         return Collections.emptyList();
       } catch (AuthException e) {
         throw new CommitValidationException(
-            "invalid committer", invalidEmail("committer", committer, user, urlFormatter));
+            "invalid committer", invalidEmail("committer", committer, user, urlFormatter), e);
       } catch (PermissionBackendException e) {
         logger.atSevere().withCause(e).log("cannot check FORGE_COMMITTER");
         throw new CommitValidationException("internal auth error");
@@ -704,7 +709,8 @@
                   "pushing merge commit %s by %s requires '%s' permission",
                   receiveEvent.commit.getId(),
                   gerritIdent.getEmailAddress(),
-                  RefPermission.FORGE_SERVER.name()));
+                  RefPermission.FORGE_SERVER.name()),
+              denied);
         } catch (PermissionBackendException e) {
           logger.atSevere().withCause(e).log("cannot check FORGE_SERVER");
           throw new CommitValidationException("internal auth error");
diff --git a/java/com/google/gerrit/server/git/validators/MergeValidationException.java b/java/com/google/gerrit/server/git/validators/MergeValidationException.java
index 3624fe0..2b7803d 100644
--- a/java/com/google/gerrit/server/git/validators/MergeValidationException.java
+++ b/java/com/google/gerrit/server/git/validators/MergeValidationException.java
@@ -28,4 +28,8 @@
   public MergeValidationException(String msg) {
     super(msg);
   }
+
+  public MergeValidationException(String msg, Throwable why) {
+    super(msg, why);
+  }
 }
diff --git a/java/com/google/gerrit/server/git/validators/MergeValidators.java b/java/com/google/gerrit/server/git/validators/MergeValidators.java
index 8abe5c6..9c557a7 100644
--- a/java/com/google/gerrit/server/git/validators/MergeValidators.java
+++ b/java/com/google/gerrit/server/git/validators/MergeValidators.java
@@ -192,7 +192,7 @@
                 try {
                   permissionBackend.user(caller).check(GlobalPermission.ADMINISTRATE_SERVER);
                 } catch (AuthException e) {
-                  throw new MergeValidationException(SET_BY_ADMIN);
+                  throw new MergeValidationException(SET_BY_ADMIN, e);
                 } catch (PermissionBackendException e) {
                   logger.atWarning().withCause(e).log("Cannot check ADMINISTRATE_SERVER");
                   throw new MergeValidationException("validation unavailable");
@@ -204,7 +204,7 @@
                       .project(destProject.getNameKey())
                       .check(ProjectPermission.WRITE_CONFIG);
                 } catch (AuthException e) {
-                  throw new MergeValidationException(SET_BY_OWNER);
+                  throw new MergeValidationException(SET_BY_OWNER, e);
                 } catch (PermissionBackendException e) {
                   logger.atWarning().withCause(e).log("Cannot check WRITE_CONFIG");
                   throw new MergeValidationException("validation unavailable");
@@ -245,7 +245,7 @@
             }
           }
         } catch (ConfigInvalidException | IOException e) {
-          throw new MergeValidationException(INVALID_CONFIG);
+          throw new MergeValidationException(INVALID_CONFIG, e);
         }
       }
     }
diff --git a/java/com/google/gerrit/server/git/validators/RefOperationValidators.java b/java/com/google/gerrit/server/git/validators/RefOperationValidators.java
index f3d8f4a..f3b6983 100644
--- a/java/com/google/gerrit/server/git/validators/RefOperationValidators.java
+++ b/java/com/google/gerrit/server/git/validators/RefOperationValidators.java
@@ -134,7 +134,7 @@
             try {
               perm.check(GlobalPermission.ACCESS_DATABASE);
             } catch (AuthException | PermissionBackendException e) {
-              throw new ValidationException("Not allowed to create user branch.");
+              throw new ValidationException("Not allowed to create user branch.", e);
             }
             if (Account.Id.fromRef(refEvent.command.getRefName()) == null) {
               throw new ValidationException(
@@ -145,7 +145,7 @@
             try {
               perm.check(GlobalPermission.ACCESS_DATABASE);
             } catch (AuthException | PermissionBackendException e) {
-              throw new ValidationException("Not allowed to delete user branch.");
+              throw new ValidationException("Not allowed to delete user branch.", e);
             }
           }
         }
diff --git a/java/com/google/gerrit/server/index/change/ReindexAfterRefUpdate.java b/java/com/google/gerrit/server/index/change/ReindexAfterRefUpdate.java
index 4e27859..19bb423 100644
--- a/java/com/google/gerrit/server/index/change/ReindexAfterRefUpdate.java
+++ b/java/com/google/gerrit/server/index/change/ReindexAfterRefUpdate.java
@@ -28,6 +28,7 @@
 import com.google.gerrit.entities.RefNames;
 import com.google.gerrit.extensions.events.GitReferenceUpdatedListener;
 import com.google.gerrit.server.account.AccountCache;
+import com.google.gerrit.server.change.MergeabilityComputationBehavior;
 import com.google.gerrit.server.config.AllUsersName;
 import com.google.gerrit.server.config.GerritServerConfig;
 import com.google.gerrit.server.git.QueueProvider.QueueType;
@@ -85,7 +86,7 @@
     this.accountCache = accountCache;
     this.indexer = indexer;
     this.executor = executor;
-    this.enabled = cfg.getBoolean("index", "change", "indexMergeable", true);
+    this.enabled = MergeabilityComputationBehavior.fromConfig(cfg).includeInIndex();
   }
 
   @Override
diff --git a/java/com/google/gerrit/server/mail/receive/MailProcessor.java b/java/com/google/gerrit/server/mail/receive/MailProcessor.java
index e79696a..9c3dd02 100644
--- a/java/com/google/gerrit/server/mail/receive/MailProcessor.java
+++ b/java/com/google/gerrit/server/mail/receive/MailProcessor.java
@@ -19,6 +19,7 @@
 import com.google.common.base.Strings;
 import com.google.common.collect.ImmutableList;
 import com.google.common.collect.ImmutableMap;
+import com.google.common.collect.Iterables;
 import com.google.common.flogger.FluentLogger;
 import com.google.gerrit.entities.Account;
 import com.google.gerrit.entities.Change;
@@ -243,7 +244,7 @@
         sendRejectionEmail(message, InboundEmailRejectionSender.Error.INTERNAL_EXCEPTION);
         return;
       }
-      ChangeData cd = changeDataList.get(0);
+      ChangeData cd = Iterables.getOnlyElement(changeDataList);
       if (existingMessageIds(cd).contains(message.id())) {
         logger.atInfo().log("Message %s was already processed. Will delete message.", message.id());
         return;
@@ -285,14 +286,14 @@
               .map(
                   comment ->
                       CommentForValidation.create(
+                          CommentForValidation.CommentSource.HUMAN,
                           MAIL_COMMENT_TYPE_TO_VALIDATION_TYPE.get(comment.getType()),
-                          comment.getMessage()))
+                          comment.getMessage(),
+                          comment.getMessage().length()))
               .collect(ImmutableList.toImmutableList());
       CommentValidationContext commentValidationCtx =
-          CommentValidationContext.builder()
-              .changeId(cd.change().getChangeId())
-              .project(cd.change().getProject().get())
-              .build();
+          CommentValidationContext.create(
+              cd.change().getChangeId(), cd.change().getProject().get());
       ImmutableList<CommentValidationFailure> commentValidationFailures =
           PublishCommentUtil.findInvalidComments(
               commentValidationCtx, commentValidators, parsedCommentsForValidation);
diff --git a/java/com/google/gerrit/server/mail/send/ChangeEmail.java b/java/com/google/gerrit/server/mail/send/ChangeEmail.java
index d7f09d2..9e53c65 100644
--- a/java/com/google/gerrit/server/mail/send/ChangeEmail.java
+++ b/java/com/google/gerrit/server/mail/send/ChangeEmail.java
@@ -293,7 +293,7 @@
       try {
         ps = args.patchSetUtil.get(changeData.notes(), PatchSet.id(change.getId(), patchSetId));
       } catch (StorageException e) {
-        throw new PatchListNotAvailableException("Failed to get patchSet");
+        throw new PatchListNotAvailableException("Failed to get patchSet", e);
       }
     }
     return args.patchListCache.get(change, ps);
diff --git a/java/com/google/gerrit/server/notedb/ChangeNotesParser.java b/java/com/google/gerrit/server/notedb/ChangeNotesParser.java
index 5468e23..428df16 100644
--- a/java/com/google/gerrit/server/notedb/ChangeNotesParser.java
+++ b/java/com/google/gerrit/server/notedb/ChangeNotesParser.java
@@ -907,7 +907,9 @@
     try {
       adr = Address.parse(line);
     } catch (IllegalArgumentException e) {
-      throw invalidFooter(state.getByEmailFooterKey(), line);
+      ConfigInvalidException cie = invalidFooter(state.getByEmailFooterKey(), line);
+      cie.initCause(e);
+      throw cie;
     }
     if (!reviewersByEmail.containsRow(adr)) {
       reviewersByEmail.put(adr, state, ts);
diff --git a/java/com/google/gerrit/server/notedb/DeleteZombieCommentsRefs.java b/java/com/google/gerrit/server/notedb/DeleteZombieCommentsRefs.java
new file mode 100644
index 0000000..9ec1d69
--- /dev/null
+++ b/java/com/google/gerrit/server/notedb/DeleteZombieCommentsRefs.java
@@ -0,0 +1,130 @@
+// Copyright (C) 2020 The Android Open Source Project
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package com.google.gerrit.server.notedb;
+
+import static com.google.common.collect.ImmutableList.toImmutableList;
+
+import com.google.common.collect.Iterables;
+import com.google.gerrit.entities.Change;
+import com.google.gerrit.git.RefUpdateUtil;
+import com.google.gerrit.server.config.AllUsersName;
+import com.google.gerrit.server.git.GitRepositoryManager;
+import com.google.inject.Inject;
+import com.google.inject.assistedinject.Assisted;
+import java.io.IOException;
+import java.util.ArrayList;
+import java.util.List;
+import org.eclipse.jgit.lib.BatchRefUpdate;
+import org.eclipse.jgit.lib.ObjectId;
+import org.eclipse.jgit.lib.Ref;
+import org.eclipse.jgit.lib.Repository;
+import org.eclipse.jgit.transport.ReceiveCommand;
+
+/**
+ * This class can be used to clean zombie draft comments refs. More context in <a
+ * href="https://gerrit-review.googlesource.com/c/gerrit/+/246233">
+ * https://gerrit-review.googlesource.com/c/gerrit/+/246233 </a>
+ *
+ * <p>An earlier bug in the deletion of draft comments {@code
+ * refs/draft-comments/$change_id_short/$change_id/$user_id} caused some draft refs to remain in Git
+ * and not get deleted. These refs point to an empty tree.
+ */
+public class DeleteZombieCommentsRefs {
+  private final String EMPTY_TREE_ID = "4b825dc642cb6eb9a060e54bf8d69288fbee4904";
+  private final String DRAFT_REFS_PREFIX = "refs/draft-comments";
+  private final int CHUNK_SIZE = 100; // log progress after deleting every CHUNK_SIZE refs
+  private final GitRepositoryManager repoManager;
+  private final AllUsersName allUsers;
+  private final int cleanupPercentage;
+  private Repository allUsersRepo;
+
+  public interface Factory {
+    DeleteZombieCommentsRefs create(int cleanupPercentage);
+  }
+
+  @Inject
+  public DeleteZombieCommentsRefs(
+      AllUsersName allUsers,
+      GitRepositoryManager repoManager,
+      @Assisted Integer cleanupPercentage) {
+    this.allUsers = allUsers;
+    this.repoManager = repoManager;
+    this.cleanupPercentage = (cleanupPercentage == null) ? 100 : cleanupPercentage;
+  }
+
+  public void execute() throws IOException {
+    allUsersRepo = repoManager.openRepository(allUsers);
+
+    List<Ref> draftRefs = allUsersRepo.getRefDatabase().getRefsByPrefix(DRAFT_REFS_PREFIX);
+    List<Ref> zombieRefs = filterZombieRefs(draftRefs);
+
+    System.out.println(
+        String.format(
+            "Found a total of %d zombie draft refs in %s repo.",
+            zombieRefs.size(), allUsers.get()));
+
+    System.out.println(String.format("Cleanup percentage = %d", cleanupPercentage));
+    zombieRefs =
+        zombieRefs.stream()
+            .filter(ref -> Change.Id.fromAllUsersRef(ref.getName()).get() % 100 < cleanupPercentage)
+            .collect(toImmutableList());
+    System.out.println(
+        String.format("Number of zombie refs to be cleaned = %d", zombieRefs.size()));
+
+    long zombieRefsCnt = zombieRefs.size();
+    long deletedRefsCnt = 0;
+    long startTime = System.currentTimeMillis();
+
+    for (List<Ref> refsBatch : Iterables.partition(zombieRefs, CHUNK_SIZE)) {
+      deleteBatchZombieRefs(refsBatch);
+      long elapsed = (System.currentTimeMillis() - startTime) / 1000;
+      deletedRefsCnt += refsBatch.size();
+      logProgress(deletedRefsCnt, zombieRefsCnt, elapsed);
+    }
+  }
+
+  private void deleteBatchZombieRefs(List<Ref> refsBatch) throws IOException {
+    List<ReceiveCommand> deleteCommands =
+        refsBatch.stream()
+            .map(
+                zombieRef ->
+                    new ReceiveCommand(
+                        zombieRef.getObjectId(), ObjectId.zeroId(), zombieRef.getName()))
+            .collect(toImmutableList());
+    BatchRefUpdate bru = allUsersRepo.getRefDatabase().newBatchUpdate();
+    bru.setAtomic(true);
+    bru.addCommand(deleteCommands);
+    RefUpdateUtil.executeChecked(bru, allUsersRepo);
+  }
+
+  private List<Ref> filterZombieRefs(List<Ref> allDraftRefs) throws IOException {
+    List<Ref> zombieRefs = new ArrayList<>((int) (allDraftRefs.size() * 0.5));
+    for (Ref ref : allDraftRefs) {
+      if (isZombieRef(ref)) {
+        zombieRefs.add(ref);
+      }
+    }
+    return zombieRefs;
+  }
+
+  private boolean isZombieRef(Ref ref) throws IOException {
+    return allUsersRepo.parseCommit(ref.getObjectId()).getTree().getName().equals(EMPTY_TREE_ID);
+  }
+
+  private void logProgress(long deletedRefsCount, long allRefsCount, long elapsed) {
+    System.out.format(
+        "Deleted %d/%d zombie draft refs (%d seconds)\n", deletedRefsCount, allRefsCount, elapsed);
+  }
+}
diff --git a/java/com/google/gerrit/server/notedb/LimitExceededException.java b/java/com/google/gerrit/server/notedb/LimitExceededException.java
index 69f9241..0a57d96 100644
--- a/java/com/google/gerrit/server/notedb/LimitExceededException.java
+++ b/java/com/google/gerrit/server/notedb/LimitExceededException.java
@@ -23,12 +23,13 @@
  * <ul>
  *   <li>The number of NoteDb updates per change.
  *   <li>The number of patch sets per change.
+ *   <li>The number of files per change.
  * </ul>
  */
 public class LimitExceededException extends StorageException {
   private static final long serialVersionUID = 1L;
 
-  LimitExceededException(String message) {
+  public LimitExceededException(String message) {
     super(message);
   }
 }
diff --git a/java/com/google/gerrit/server/patch/PatchScriptFactory.java b/java/com/google/gerrit/server/patch/PatchScriptFactory.java
index afc1ab1..b7b3d81 100644
--- a/java/com/google/gerrit/server/patch/PatchScriptFactory.java
+++ b/java/com/google/gerrit/server/patch/PatchScriptFactory.java
@@ -193,7 +193,7 @@
     try {
       permissionBackend.currentUser().change(notes).check(ChangePermission.READ);
     } catch (AuthException e) {
-      throw new NoSuchChangeException(changeId);
+      throw new NoSuchChangeException(changeId, e);
     }
 
     if (!projectCache.checkedGet(notes.getProjectName()).statePermitsRead()) {
diff --git a/java/com/google/gerrit/server/patch/PatchScriptFactoryForAutoFix.java b/java/com/google/gerrit/server/patch/PatchScriptFactoryForAutoFix.java
index 572aa9b..2f5458c 100644
--- a/java/com/google/gerrit/server/patch/PatchScriptFactoryForAutoFix.java
+++ b/java/com/google/gerrit/server/patch/PatchScriptFactoryForAutoFix.java
@@ -99,7 +99,7 @@
     try {
       permissionBackend.currentUser().change(notes).check(ChangePermission.READ);
     } catch (AuthException e) {
-      throw new NoSuchChangeException(changeId);
+      throw new NoSuchChangeException(changeId, e);
     }
 
     if (!projectCache.checkedGet(notes.getProjectName()).statePermitsRead()) {
diff --git a/java/com/google/gerrit/server/project/ProjectCreator.java b/java/com/google/gerrit/server/project/ProjectCreator.java
index ba44142..c4c466e 100644
--- a/java/com/google/gerrit/server/project/ProjectCreator.java
+++ b/java/com/google/gerrit/server/project/ProjectCreator.java
@@ -132,9 +132,10 @@
               + nameKey.get()
               + " because the name is already occupied by another project."
               + " The other project has the same name, only spelled in a"
-              + " different case.");
+              + " different case.",
+          e);
     } catch (RepositoryNotFoundException badName) {
-      throw new BadRequestException("invalid project name: " + nameKey);
+      throw new BadRequestException("invalid project name: " + nameKey, badName);
     } catch (ConfigInvalidException e) {
       String msg = "Cannot create " + nameKey;
       logger.atSevere().withCause(e).log(msg);
diff --git a/java/com/google/gerrit/server/project/RefUtil.java b/java/com/google/gerrit/server/project/RefUtil.java
index 5d6379a..dc8cdc7 100644
--- a/java/com/google/gerrit/server/project/RefUtil.java
+++ b/java/com/google/gerrit/server/project/RefUtil.java
@@ -55,7 +55,7 @@
           "Cannot resolve \"%s\" in project \"%s\"", baseRevision, projectName.get());
       throw new InvalidRevisionException(baseRevision);
     } catch (RevisionSyntaxException err) {
-      throw new InvalidRevisionException(baseRevision);
+      throw new InvalidRevisionException(baseRevision, err);
     }
   }
 
@@ -66,7 +66,7 @@
       try {
         rw.markStart(rw.parseCommit(revid));
       } catch (IncorrectObjectTypeException err) {
-        throw new InvalidRevisionException(revid.name());
+        throw new InvalidRevisionException(revid.name(), err);
       }
       RefDatabase refDb = repo.getRefDatabase();
       Iterable<Ref> refs =
@@ -86,7 +86,7 @@
       rw.checkConnectivity();
       return rw;
     } catch (IncorrectObjectTypeException | MissingObjectException err) {
-      throw new InvalidRevisionException(revid.name());
+      throw new InvalidRevisionException(revid.name(), err);
     } catch (IOException err) {
       logger.atSevere().withCause(err).log(
           "Repository \"%s\" may be corrupt; suggest running git fsck", repo.getDirectory());
@@ -128,5 +128,9 @@
     InvalidRevisionException(@Nullable String invalidRevision) {
       super(MESSAGE + ": " + invalidRevision);
     }
+
+    InvalidRevisionException(@Nullable String invalidRevision, Throwable why) {
+      super(MESSAGE + ": " + invalidRevision, why);
+    }
   }
 }
diff --git a/java/com/google/gerrit/server/query/account/AccountQueryBuilder.java b/java/com/google/gerrit/server/query/account/AccountQueryBuilder.java
index 664df70..42a8310 100644
--- a/java/com/google/gerrit/server/query/account/AccountQueryBuilder.java
+++ b/java/com/google/gerrit/server/query/account/AccountQueryBuilder.java
@@ -123,7 +123,9 @@
     try {
       args.permissionBackend.user(args.getUser()).change(changeNotes).check(ChangePermission.READ);
     } catch (AuthException e) {
-      throw error(String.format("change %s not found", change));
+      String msg = String.format("change %s not found", change);
+      logger.atSevere().withCause(e).log(msg);
+      throw error(msg);
     }
 
     return AccountPredicates.cansee(args, changeNotes);
diff --git a/java/com/google/gerrit/server/query/change/ChangeQueryBuilder.java b/java/com/google/gerrit/server/query/change/ChangeQueryBuilder.java
index 7747998..327c21f 100644
--- a/java/com/google/gerrit/server/query/change/ChangeQueryBuilder.java
+++ b/java/com/google/gerrit/server/query/change/ChangeQueryBuilder.java
@@ -63,6 +63,7 @@
 import com.google.gerrit.server.account.VersionedAccountDestinations;
 import com.google.gerrit.server.account.VersionedAccountQueries;
 import com.google.gerrit.server.change.ChangeTriplet;
+import com.google.gerrit.server.change.MergeabilityComputationBehavior;
 import com.google.gerrit.server.config.AllProjectsName;
 import com.google.gerrit.server.config.AllUsersName;
 import com.google.gerrit.server.config.GerritServerConfig;
@@ -293,7 +294,7 @@
           groupMembers,
           anonymousUserProvider,
           operatorAliasConfig,
-          gerritConfig.getBoolean("index", "change", "indexMergeable", true));
+          MergeabilityComputationBehavior.fromConfig(gerritConfig).includeInIndex());
     }
 
     private Arguments(
@@ -1288,7 +1289,8 @@
         throw new QueryParseException(
             "'"
                 + value
-                + "' is not a valid input. It must be in the 'ChangeNumber[,PatchsetNumber]' format.");
+                + "' is not a valid input. It must be in the 'ChangeNumber[,PatchsetNumber]' format.",
+            e);
       }
     }
     throw new QueryParseException(
diff --git a/java/com/google/gerrit/server/query/project/ProjectQueryBuilder.java b/java/com/google/gerrit/server/query/project/ProjectQueryBuilder.java
index 2992869..d234546 100644
--- a/java/com/google/gerrit/server/query/project/ProjectQueryBuilder.java
+++ b/java/com/google/gerrit/server/query/project/ProjectQueryBuilder.java
@@ -75,7 +75,7 @@
     try {
       parsedState = ProjectState.valueOf(state.replace('-', '_').toUpperCase());
     } catch (IllegalArgumentException e) {
-      throw error("state operator must be either 'active' or 'read-only'");
+      throw error("state operator must be either 'active' or 'read-only'", e);
     }
     if (parsedState == ProjectState.HIDDEN) {
       throw error("state operator must be either 'active' or 'read-only'");
diff --git a/java/com/google/gerrit/server/restapi/account/Capabilities.java b/java/com/google/gerrit/server/restapi/account/Capabilities.java
index 07b1214..3d719ff9 100644
--- a/java/com/google/gerrit/server/restapi/account/Capabilities.java
+++ b/java/com/google/gerrit/server/restapi/account/Capabilities.java
@@ -75,7 +75,7 @@
       permissionBackend.absentUser(target.getAccountId()).check(perm);
       return new AccountResource.Capability(target, globalOrPluginPermissionName(perm));
     } catch (AuthException e) {
-      throw new ResourceNotFoundException(id);
+      throw new ResourceNotFoundException(id, e);
     }
   }
 
diff --git a/java/com/google/gerrit/server/restapi/account/CreateAccount.java b/java/com/google/gerrit/server/restapi/account/CreateAccount.java
index 8f525ef..907dd18 100644
--- a/java/com/google/gerrit/server/restapi/account/CreateAccount.java
+++ b/java/com/google/gerrit/server/restapi/account/CreateAccount.java
@@ -163,7 +163,7 @@
       try {
         addGroupMember(groupUuid, accountId);
       } catch (NoSuchGroupException e) {
-        throw new UnprocessableEntityException(String.format("Group %s not found", groupUuid));
+        throw new UnprocessableEntityException(String.format("Group %s not found", groupUuid), e);
       }
     }
 
diff --git a/java/com/google/gerrit/server/restapi/account/DeleteDraftComments.java b/java/com/google/gerrit/server/restapi/account/DeleteDraftComments.java
index a6fbb10..4b505c6 100644
--- a/java/com/google/gerrit/server/restapi/account/DeleteDraftComments.java
+++ b/java/com/google/gerrit/server/restapi/account/DeleteDraftComments.java
@@ -27,7 +27,6 @@
 import com.google.gerrit.entities.Project;
 import com.google.gerrit.extensions.api.accounts.DeleteDraftCommentsInput;
 import com.google.gerrit.extensions.api.accounts.DeletedDraftCommentInfo;
-import com.google.gerrit.extensions.client.ListChangesOption;
 import com.google.gerrit.extensions.common.CommentInfo;
 import com.google.gerrit.extensions.restapi.AuthException;
 import com.google.gerrit.extensions.restapi.BadRequestException;
@@ -190,9 +189,7 @@
       if (dirty) {
         result = new DeletedDraftCommentInfo();
         result.change =
-            changeJsonFactory
-                .create(ListChangesOption.SKIP_MERGEABLE)
-                .format(changeDataFactory.create(ctx.getNotes()));
+            changeJsonFactory.noOptions().format(changeDataFactory.create(ctx.getNotes()));
         result.deleted = comments.build();
       }
       return dirty;
diff --git a/java/com/google/gerrit/server/restapi/account/PutAgreement.java b/java/com/google/gerrit/server/restapi/account/PutAgreement.java
index 459ff26..42504a0 100644
--- a/java/com/google/gerrit/server/restapi/account/PutAgreement.java
+++ b/java/com/google/gerrit/server/restapi/account/PutAgreement.java
@@ -100,7 +100,7 @@
     try {
       addMembers.addMembers(uuid, ImmutableSet.of(accountState.account().id()));
     } catch (NoSuchGroupException e) {
-      throw new ResourceConflictException("autoverify group not found");
+      throw new ResourceConflictException("autoverify group not found", e);
     }
     agreementSignup.fire(accountState, agreementName);
 
diff --git a/java/com/google/gerrit/server/restapi/account/PutUsername.java b/java/com/google/gerrit/server/restapi/account/PutUsername.java
index dabfaa5..dc841b8 100644
--- a/java/com/google/gerrit/server/restapi/account/PutUsername.java
+++ b/java/com/google/gerrit/server/restapi/account/PutUsername.java
@@ -122,7 +122,7 @@
       }
 
       // Otherwise, someone else has this identity.
-      throw new ResourceConflictException("username already used");
+      throw new ResourceConflictException("username already used", dupeErr);
     }
 
     sshKeyCache.evict(input.username);
diff --git a/java/com/google/gerrit/server/restapi/account/SshKeys.java b/java/com/google/gerrit/server/restapi/account/SshKeys.java
index 6e3f905..a6c4d80 100644
--- a/java/com/google/gerrit/server/restapi/account/SshKeys.java
+++ b/java/com/google/gerrit/server/restapi/account/SshKeys.java
@@ -70,7 +70,7 @@
         permissionBackend.currentUser().check(GlobalPermission.MODIFY_ACCOUNT);
       } catch (AuthException e) {
         // If lacking MODIFY_ACCOUNT claim the resource does not exist.
-        throw new ResourceNotFoundException();
+        throw new ResourceNotFoundException(id, e);
       }
     }
     return parse(rsrc.getUser(), id);
@@ -86,7 +86,7 @@
       }
       return new AccountResource.SshKey(user, sshKey);
     } catch (NumberFormatException e) {
-      throw new ResourceNotFoundException(id);
+      throw new ResourceNotFoundException(id, e);
     }
   }
 
diff --git a/java/com/google/gerrit/server/restapi/account/StarredChanges.java b/java/com/google/gerrit/server/restapi/account/StarredChanges.java
index 3c14ad3..c108dcb 100644
--- a/java/com/google/gerrit/server/restapi/account/StarredChanges.java
+++ b/java/com/google/gerrit/server/restapi/account/StarredChanges.java
@@ -123,10 +123,10 @@
       try {
         change = changes.parse(TopLevelResource.INSTANCE, id);
       } catch (ResourceNotFoundException e) {
-        throw new UnprocessableEntityException(String.format("change %s not found", id.get()));
+        throw new UnprocessableEntityException(String.format("change %s not found", id.get()), e);
       } catch (StorageException | PermissionBackendException | IOException e) {
         logger.atSevere().withCause(e).log("cannot resolve change");
-        throw new UnprocessableEntityException("internal server error");
+        throw new UnprocessableEntityException("internal server error", e);
       }
 
       try {
diff --git a/java/com/google/gerrit/server/restapi/change/CherryPickChange.java b/java/com/google/gerrit/server/restapi/change/CherryPickChange.java
index 3496491..ac81b45 100644
--- a/java/com/google/gerrit/server/restapi/change/CherryPickChange.java
+++ b/java/com/google/gerrit/server/restapi/change/CherryPickChange.java
@@ -422,7 +422,8 @@
     try {
       baseObjectId = ObjectId.fromString(base);
     } catch (InvalidObjectIdException e) {
-      throw new BadRequestException(String.format("Base %s doesn't represent a valid SHA-1", base));
+      throw new BadRequestException(
+          String.format("Base %s doesn't represent a valid SHA-1", base), e);
     }
 
     RevCommit baseCommit;
diff --git a/java/com/google/gerrit/server/restapi/change/CreateMergePatchSet.java b/java/com/google/gerrit/server/restapi/change/CreateMergePatchSet.java
index 8d50b86..cfb4c77 100644
--- a/java/com/google/gerrit/server/restapi/change/CreateMergePatchSet.java
+++ b/java/com/google/gerrit/server/restapi/change/CreateMergePatchSet.java
@@ -21,6 +21,7 @@
 import com.google.gerrit.entities.Change;
 import com.google.gerrit.entities.PatchSet;
 import com.google.gerrit.entities.Project;
+import com.google.gerrit.exceptions.InvalidMergeStrategyException;
 import com.google.gerrit.extensions.client.ListChangesOption;
 import com.google.gerrit.extensions.common.ChangeInfo;
 import com.google.gerrit.extensions.common.MergeInput;
@@ -192,6 +193,8 @@
 
       ChangeJson json = jsonFactory.create(ListChangesOption.CURRENT_REVISION);
       return Response.ok(json.format(psInserter.getChange()));
+    } catch (InvalidMergeStrategyException e) {
+      throw new BadRequestException(e.getMessage());
     }
   }
 
@@ -205,7 +208,7 @@
     try {
       permissionBackend.currentUser().change(change).check(ChangePermission.READ);
     } catch (AuthException e) {
-      throw new UnprocessableEntityException("Read not permitted for " + baseChange);
+      throw new UnprocessableEntityException("Read not permitted for " + baseChange, e);
     }
     return psUtil.current(change);
   }
diff --git a/java/com/google/gerrit/server/restapi/change/GetDiff.java b/java/com/google/gerrit/server/restapi/change/GetDiff.java
index c1d286a..c3deb79 100644
--- a/java/com/google/gerrit/server/restapi/change/GetDiff.java
+++ b/java/com/google/gerrit/server/restapi/change/GetDiff.java
@@ -293,6 +293,7 @@
             throw new NumberFormatException();
           }
         } catch (NumberFormatException e) {
+          logger.atFine().withCause(e).log("invalid numeric value");
           throw new CmdLineException(
               owner,
               localizable("\"%s\" is not a valid value for \"%s\""),
diff --git a/java/com/google/gerrit/server/restapi/change/PostReview.java b/java/com/google/gerrit/server/restapi/change/PostReview.java
index 324069d..129df59 100644
--- a/java/com/google/gerrit/server/restapi/change/PostReview.java
+++ b/java/com/google/gerrit/server/restapi/change/PostReview.java
@@ -80,7 +80,6 @@
 import com.google.gerrit.extensions.validators.CommentValidationContext;
 import com.google.gerrit.extensions.validators.CommentValidationFailure;
 import com.google.gerrit.extensions.validators.CommentValidator;
-import com.google.gerrit.json.OutputFormat;
 import com.google.gerrit.mail.Address;
 import com.google.gerrit.server.ApprovalsUtil;
 import com.google.gerrit.server.ChangeMessagesUtil;
@@ -125,11 +124,9 @@
 import com.google.gerrit.server.update.UpdateException;
 import com.google.gerrit.server.util.LabelVote;
 import com.google.gerrit.server.util.time.TimeUtil;
-import com.google.gson.Gson;
 import com.google.inject.Inject;
 import com.google.inject.Singleton;
 import java.io.IOException;
-import java.nio.charset.StandardCharsets;
 import java.sql.Timestamp;
 import java.util.ArrayList;
 import java.util.Collection;
@@ -141,7 +138,6 @@
 import java.util.Map;
 import java.util.Objects;
 import java.util.Optional;
-import java.util.OptionalInt;
 import java.util.Set;
 import java.util.stream.Collectors;
 import java.util.stream.Stream;
@@ -159,9 +155,6 @@
 
   public static final String START_REVIEW_MESSAGE = "This change is ready for review.";
 
-  private static final Gson GSON = OutputFormat.JSON_COMPACT.newGson();
-  private static final int DEFAULT_ROBOT_COMMENT_SIZE_LIMIT_IN_BYTES = 1024 * 1024;
-
   private final BatchUpdate.Factory updateFactory;
   private final ChangeResource.Factory changeResourceFactory;
   private final ChangeData.Factory changeDataFactory;
@@ -177,7 +170,6 @@
   private final ReviewerAdder reviewerAdder;
   private final AddReviewersEmail addReviewersEmail;
   private final NotifyResolver notifyResolver;
-  private final Config gerritConfig;
   private final WorkInProgressOp.Factory workInProgressOpFactory;
   private final ProjectCache projectCache;
   private final PermissionBackend permissionBackend;
@@ -221,7 +213,6 @@
     this.reviewerAdder = reviewerAdder;
     this.addReviewersEmail = addReviewersEmail;
     this.notifyResolver = notifyResolver;
-    this.gerritConfig = gerritConfig;
     this.workInProgressOpFactory = workInProgressOpFactory;
     this.projectCache = projectCache;
     this.permissionBackend = permissionBackend;
@@ -514,7 +505,8 @@
           throw new AuthException(
               String.format(
                   "not permitted to modify label \"%s\" on behalf of \"%s\"",
-                  type.getName(), in.onBehalfOf));
+                  type.getName(), in.onBehalfOf),
+              e);
         }
       }
     }
@@ -530,7 +522,7 @@
       permissionBackend.user(reviewer).change(rev.getNotes()).check(ChangePermission.READ);
     } catch (AuthException e) {
       throw new UnprocessableEntityException(
-          String.format("on_behalf_of account %s cannot see change", reviewer.getAccountId()));
+          String.format("on_behalf_of account %s cannot see change", reviewer.getAccountId()), e);
     }
 
     return new RevisionResource(
@@ -580,7 +572,7 @@
         perm.check(new LabelPermission.WithValue(lt, val));
       } catch (AuthException e) {
         throw new AuthException(
-            String.format("Applying label \"%s\": %d is restricted", lt.getName(), val));
+            String.format("Applying label \"%s\": %d is restricted", lt.getName(), val), e);
       }
     }
   }
@@ -671,41 +663,15 @@
     for (Map.Entry<String, List<RobotCommentInput>> e : in.entrySet()) {
       String commentPath = e.getKey();
       for (RobotCommentInput c : e.getValue()) {
-        ensureSizeOfJsonInputIsWithinBounds(c);
         ensureRobotIdIsSet(c.robotId, commentPath);
         ensureRobotRunIdIsSet(c.robotRunId, commentPath);
         ensureFixSuggestionsAreAddable(c.fixSuggestions, commentPath);
+        // Size is validated later, in CommentLimitsValidator.
       }
     }
     checkComments(revision, in);
   }
 
-  private void ensureSizeOfJsonInputIsWithinBounds(RobotCommentInput robotCommentInput)
-      throws BadRequestException {
-    OptionalInt robotCommentSizeLimit = getRobotCommentSizeLimit();
-    if (robotCommentSizeLimit.isPresent()) {
-      int sizeLimit = robotCommentSizeLimit.getAsInt();
-      byte[] robotCommentBytes = GSON.toJson(robotCommentInput).getBytes(StandardCharsets.UTF_8);
-      int robotCommentSize = robotCommentBytes.length;
-      if (robotCommentSize > sizeLimit) {
-        throw new BadRequestException(
-            String.format(
-                "Size %d (bytes) of robot comment is greater than limit %d (bytes)",
-                robotCommentSize, sizeLimit));
-      }
-    }
-  }
-
-  private OptionalInt getRobotCommentSizeLimit() {
-    int robotCommentSizeLimit =
-        gerritConfig.getInt(
-            "change", "robotCommentSizeLimit", DEFAULT_ROBOT_COMMENT_SIZE_LIMIT_IN_BYTES);
-    if (robotCommentSizeLimit <= 0) {
-      return OptionalInt.empty();
-    }
-    return OptionalInt.of(robotCommentSizeLimit);
-  }
-
   private static void ensureRobotIdIsSet(String robotId, String commentPath)
       throws BadRequestException {
     if (robotId == null) {
@@ -909,8 +875,10 @@
       user = ctx.getIdentifiedUser();
       notes = ctx.getNotes();
       ps = psUtil.get(ctx.getNotes(), psId);
-      boolean dirty = insertComments(ctx);
-      dirty |= insertRobotComments(ctx);
+      List<RobotComment> newRobotComments =
+          in.robotComments == null ? ImmutableList.of() : getNewRobotComments(ctx);
+      boolean dirty = insertComments(ctx, newRobotComments);
+      dirty |= insertRobotComments(ctx, newRobotComments);
       dirty |= updateLabels(projectState, ctx);
       dirty |= insertMessage(ctx);
       return dirty;
@@ -937,7 +905,7 @@
           ctx.getWhen());
     }
 
-    private boolean insertComments(ChangeContext ctx)
+    private boolean insertComments(ChangeContext ctx, List<RobotComment> newRobotComments)
         throws UnprocessableEntityException, PatchListNotAvailableException,
             CommentsRejectedException {
       Map<String, List<CommentInput>> inputComments = in.comments;
@@ -1001,21 +969,21 @@
       }
 
       CommentValidationContext commentValidationCtx =
-          CommentValidationContext.builder()
-              .changeId(ctx.getChange().getChangeId())
-              .project(ctx.getChange().getProject().get())
-              .build();
+          CommentValidationContext.create(
+              ctx.getChange().getChangeId(), ctx.getChange().getProject().get());
       switch (in.drafts) {
         case PUBLISH:
         case PUBLISH_ALL_REVISIONS:
           validateComments(
-              commentValidationCtx, Streams.concat(drafts.values().stream(), toPublish.stream()));
+              commentValidationCtx,
+              Streams.concat(
+                  drafts.values().stream(), toPublish.stream(), newRobotComments.stream()));
           publishCommentUtil.publish(ctx, ctx.getUpdate(psId), drafts.values(), in.tag);
           comments.addAll(drafts.values());
           break;
         case KEEP:
-        default:
-          validateComments(commentValidationCtx, toPublish.stream());
+          validateComments(
+              commentValidationCtx, Stream.concat(toPublish.stream(), newRobotComments.stream()));
           break;
       }
       ChangeUpdate changeUpdate = ctx.getUpdate(psId);
@@ -1031,10 +999,14 @@
               .map(
                   comment ->
                       CommentForValidation.create(
+                          comment instanceof RobotComment
+                              ? CommentForValidation.CommentSource.ROBOT
+                              : CommentForValidation.CommentSource.HUMAN,
                           comment.lineNbr > 0
                               ? CommentForValidation.CommentType.INLINE_COMMENT
                               : CommentForValidation.CommentType.FILE_COMMENT,
-                          comment.message))
+                          comment.message,
+                          comment.getApproximateSize()))
               .collect(toImmutableList());
       ImmutableList<CommentValidationFailure> draftValidationFailures =
           PublishCommentUtil.findInvalidComments(ctx, commentValidators, draftsForValidation);
@@ -1043,12 +1015,10 @@
       }
     }
 
-    private boolean insertRobotComments(ChangeContext ctx) throws PatchListNotAvailableException {
+    private boolean insertRobotComments(ChangeContext ctx, List<RobotComment> newRobotComments) {
       if (in.robotComments == null) {
         return false;
       }
-
-      List<RobotComment> newRobotComments = getNewRobotComments(ctx);
       commentsUtil.putRobotComments(ctx.getUpdate(psId), newRobotComments);
       comments.addAll(newRobotComments);
       return !newRobotComments.isEmpty();
@@ -1424,17 +1394,18 @@
       }
       if (!msg.isEmpty()) {
         CommentValidationContext commentValidationCtx =
-            CommentValidationContext.builder()
-                .changeId(ctx.getChange().getChangeId())
-                .project(ctx.getChange().getProject().get())
-                .build();
+            CommentValidationContext.create(
+                ctx.getChange().getChangeId(), ctx.getChange().getProject().get());
         ImmutableList<CommentValidationFailure> messageValidationFailure =
             PublishCommentUtil.findInvalidComments(
                 commentValidationCtx,
                 commentValidators,
                 ImmutableList.of(
                     CommentForValidation.create(
-                        CommentForValidation.CommentType.CHANGE_MESSAGE, msg)));
+                        CommentForValidation.CommentSource.HUMAN,
+                        CommentForValidation.CommentType.CHANGE_MESSAGE,
+                        msg,
+                        msg.length())));
         if (!messageValidationFailure.isEmpty()) {
           throw new CommentsRejectedException(messageValidationFailure);
         }
diff --git a/java/com/google/gerrit/server/restapi/change/PreviewSubmit.java b/java/com/google/gerrit/server/restapi/change/PreviewSubmit.java
index ee1f2e3..e6a60d5 100644
--- a/java/com/google/gerrit/server/restapi/change/PreviewSubmit.java
+++ b/java/com/google/gerrit/server/restapi/change/PreviewSubmit.java
@@ -174,7 +174,7 @@
           archiveFormat.putEntry(aos, path, bos.toByteArray());
         }
       } catch (LimitExceededException e) {
-        throw new NotImplementedException("The bundle is too big to generate at the server");
+        throw new NotImplementedException("The bundle is too big to generate at the server", e);
       } catch (NoSuchProjectException e) {
         throw new IOException(e);
       }
diff --git a/java/com/google/gerrit/server/restapi/change/PutAssignee.java b/java/com/google/gerrit/server/restapi/change/PutAssignee.java
index dc1adfa..7b0d905 100644
--- a/java/com/google/gerrit/server/restapi/change/PutAssignee.java
+++ b/java/com/google/gerrit/server/restapi/change/PutAssignee.java
@@ -94,7 +94,7 @@
           .change(rsrc.getNotes())
           .check(ChangePermission.READ);
     } catch (AuthException e) {
-      throw new AuthException("read not permitted for " + input.assignee);
+      throw new AuthException("read not permitted for " + input.assignee, e);
     }
 
     try (BatchUpdate bu =
diff --git a/java/com/google/gerrit/server/restapi/change/QueryChanges.java b/java/com/google/gerrit/server/restapi/change/QueryChanges.java
index 544177f..878e714 100644
--- a/java/com/google/gerrit/server/restapi/change/QueryChanges.java
+++ b/java/com/google/gerrit/server/restapi/change/QueryChanges.java
@@ -129,7 +129,7 @@
     try {
       out = query();
     } catch (QueryRequiresAuthException e) {
-      throw new AuthException("Must be signed-in to use this operator");
+      throw new AuthException("Must be signed-in to use this operator", e);
     } catch (QueryParseException e) {
       logger.atFine().withCause(e).log("Reject change query with 400 Bad Request: %s", queries);
       throw new BadRequestException(e.getMessage(), e);
diff --git a/java/com/google/gerrit/server/restapi/change/RevertSubmission.java b/java/com/google/gerrit/server/restapi/change/RevertSubmission.java
index 8fc59a2..d67fc6a 100644
--- a/java/com/google/gerrit/server/restapi/change/RevertSubmission.java
+++ b/java/com/google/gerrit/server/restapi/change/RevertSubmission.java
@@ -15,7 +15,6 @@
 package com.google.gerrit.server.restapi.change;
 
 import static com.google.common.base.MoreObjects.firstNonNull;
-import static com.google.gerrit.extensions.conditions.BooleanCondition.and;
 import static com.google.gerrit.server.permissions.RefPermission.CREATE_CHANGE;
 import static java.util.Objects.requireNonNull;
 
@@ -39,7 +38,6 @@
 import com.google.gerrit.extensions.restapi.Response;
 import com.google.gerrit.extensions.restapi.RestApiException;
 import com.google.gerrit.extensions.restapi.RestModifyView;
-import com.google.gerrit.extensions.webui.UiAction;
 import com.google.gerrit.server.ChangeMessagesUtil;
 import com.google.gerrit.server.ChangeUtil;
 import com.google.gerrit.server.CurrentUser;
@@ -97,8 +95,7 @@
 import org.eclipse.jgit.revwalk.RevCommit;
 import org.eclipse.jgit.revwalk.RevWalk;
 
-public class RevertSubmission
-    implements RestModifyView<ChangeResource, RevertInput>, UiAction<ChangeResource> {
+public class RevertSubmission implements RestModifyView<ChangeResource, RevertInput> {
   private static final FluentLogger logger = FluentLogger.forEnclosingClass();
 
   private final Provider<InternalChangeQuery> queryProvider;
@@ -529,32 +526,6 @@
             potentialCommitToReturn.getName(), changeNotes.getChange().getChangeId()));
   }
 
-  @Override
-  public Description getDescription(ChangeResource rsrc) {
-    Change change = rsrc.getChange();
-    boolean projectStatePermitsWrite = false;
-    try {
-      projectStatePermitsWrite = projectCache.checkedGet(rsrc.getProject()).statePermitsWrite();
-    } catch (IOException e) {
-      logger.atSevere().withCause(e).log(
-          "Failed to check if project state permits write: %s", rsrc.getProject());
-    }
-    return new UiAction.Description()
-        .setLabel("Revert submission")
-        .setTitle(
-            "Revert this change and all changes that have been submitted together with this change")
-        .setVisible(
-            and(
-                change.isMerged()
-                    && change.getSubmissionId() != null
-                    && isChangePartOfSubmission(change.getSubmissionId())
-                    && projectStatePermitsWrite,
-                permissionBackend
-                    .user(rsrc.getUser())
-                    .ref(change.getDest())
-                    .testCond(CREATE_CHANGE)));
-  }
-
   /**
    * @param submissionId the submission id of the change.
    * @return True if the submission has more than one change, false otherwise.
diff --git a/java/com/google/gerrit/server/restapi/change/Submit.java b/java/com/google/gerrit/server/restapi/change/Submit.java
index 53e96f7..6fc3ece 100644
--- a/java/com/google/gerrit/server/restapi/change/Submit.java
+++ b/java/com/google/gerrit/server/restapi/change/Submit.java
@@ -438,7 +438,7 @@
       permissionBackend.user(submitter).change(rsrc.getNotes()).check(ChangePermission.READ);
     } catch (AuthException e) {
       throw new UnprocessableEntityException(
-          String.format("on_behalf_of account %s cannot see change", submitter.getAccountId()));
+          String.format("on_behalf_of account %s cannot see change", submitter.getAccountId()), e);
     }
     return submitter;
   }
diff --git a/java/com/google/gerrit/server/restapi/config/ConfirmEmail.java b/java/com/google/gerrit/server/restapi/config/ConfirmEmail.java
index 03715c9..b55562e 100644
--- a/java/com/google/gerrit/server/restapi/config/ConfirmEmail.java
+++ b/java/com/google/gerrit/server/restapi/config/ConfirmEmail.java
@@ -88,7 +88,7 @@
       }
       throw new UnprocessableEntityException("invalid token");
     } catch (EmailTokenVerifier.InvalidTokenException e) {
-      throw new UnprocessableEntityException("invalid token");
+      throw new UnprocessableEntityException("invalid token", e);
     } catch (AccountException e) {
       throw new UnprocessableEntityException(e.getMessage());
     }
diff --git a/java/com/google/gerrit/server/restapi/config/GetServerInfo.java b/java/com/google/gerrit/server/restapi/config/GetServerInfo.java
index 2d4bfe6..bbe38a3 100644
--- a/java/com/google/gerrit/server/restapi/config/GetServerInfo.java
+++ b/java/com/google/gerrit/server/restapi/config/GetServerInfo.java
@@ -23,11 +23,9 @@
 import com.google.gerrit.extensions.common.AccountsInfo;
 import com.google.gerrit.extensions.common.AuthInfo;
 import com.google.gerrit.extensions.common.ChangeConfigInfo;
-import com.google.gerrit.extensions.common.ChangeIndexConfigInfo;
 import com.google.gerrit.extensions.common.DownloadInfo;
 import com.google.gerrit.extensions.common.DownloadSchemeInfo;
 import com.google.gerrit.extensions.common.GerritInfo;
-import com.google.gerrit.extensions.common.IndexConfigInfo;
 import com.google.gerrit.extensions.common.PluginConfigInfo;
 import com.google.gerrit.extensions.common.ReceiveInfo;
 import com.google.gerrit.extensions.common.ServerInfo;
@@ -45,6 +43,7 @@
 import com.google.gerrit.server.account.Realm;
 import com.google.gerrit.server.avatar.AvatarProvider;
 import com.google.gerrit.server.change.ArchiveFormat;
+import com.google.gerrit.server.change.MergeabilityComputationBehavior;
 import com.google.gerrit.server.config.AllProjectsName;
 import com.google.gerrit.server.config.AllUsersName;
 import com.google.gerrit.server.config.AnonymousCowardName;
@@ -143,7 +142,6 @@
     info.change = getChangeInfo();
     info.download = getDownloadInfo();
     info.gerrit = getGerritInfo();
-    info.index = getIndexInfo();
     info.noteDbEnabled = true;
     info.plugin = getPluginInfo();
     info.defaultTheme = getDefaultTheme();
@@ -230,10 +228,10 @@
     info.updateDelay =
         (int) ConfigUtil.getTimeUnit(config, "change", null, "updateDelay", 300, TimeUnit.SECONDS);
     info.submitWholeTopic = toBoolean(MergeSuperSet.wholeTopicEnabled(config));
-    info.excludeMergeableInChangeInfo =
-        toBoolean(this.config.getBoolean("change", "api", "excludeMergeableInChangeInfo", false));
     info.disablePrivateChanges =
         toBoolean(this.config.getBoolean("change", null, "disablePrivateChanges", false));
+    info.mergeabilityComputationBehavior =
+        MergeabilityComputationBehavior.fromConfig(config).name();
     return info;
   }
 
@@ -299,14 +297,6 @@
     return info;
   }
 
-  private IndexConfigInfo getIndexInfo() {
-    ChangeIndexConfigInfo change = new ChangeIndexConfigInfo();
-    change.indexMergeable = toBoolean(config.getBoolean("index", "change", "indexMergeable", true));
-    IndexConfigInfo index = new IndexConfigInfo();
-    index.change = change;
-    return index;
-  }
-
   private String getDocUrl() {
     String docUrl = config.getString("gerrit", null, "docUrl");
     if (Strings.isNullOrEmpty(docUrl)) {
diff --git a/java/com/google/gerrit/server/restapi/config/TasksCollection.java b/java/com/google/gerrit/server/restapi/config/TasksCollection.java
index 289f76f..837d071 100644
--- a/java/com/google/gerrit/server/restapi/config/TasksCollection.java
+++ b/java/com/google/gerrit/server/restapi/config/TasksCollection.java
@@ -81,7 +81,7 @@
     try {
       taskId = (int) Long.parseLong(id.get(), 16);
     } catch (NumberFormatException e) {
-      throw new ResourceNotFoundException(id);
+      throw new ResourceNotFoundException(id, e);
     }
 
     Task<?> task = workQueue.getTask(taskId);
diff --git a/java/com/google/gerrit/server/restapi/group/AddMembers.java b/java/com/google/gerrit/server/restapi/group/AddMembers.java
index b60d78e..93d095d 100644
--- a/java/com/google/gerrit/server/restapi/group/AddMembers.java
+++ b/java/com/google/gerrit/server/restapi/group/AddMembers.java
@@ -138,7 +138,7 @@
     try {
       addMembers(groupUuid, newMemberIds);
     } catch (NoSuchGroupException e) {
-      throw new ResourceNotFoundException(String.format("Group %s not found", groupUuid));
+      throw new ResourceNotFoundException(String.format("Group %s not found", groupUuid), e);
     }
     return Response.ok(toAccountInfoList(newMemberIds));
   }
@@ -234,7 +234,7 @@
         }
         throw new IllegalStateException();
       } catch (UnprocessableEntityException e) {
-        throw new ResourceNotFoundException(id);
+        throw new ResourceNotFoundException(id, e);
       }
     }
   }
diff --git a/java/com/google/gerrit/server/restapi/group/AddSubgroups.java b/java/com/google/gerrit/server/restapi/group/AddSubgroups.java
index c969ace..3fd3f29 100644
--- a/java/com/google/gerrit/server/restapi/group/AddSubgroups.java
+++ b/java/com/google/gerrit/server/restapi/group/AddSubgroups.java
@@ -116,7 +116,7 @@
     try {
       addSubgroups(groupUuid, subgroupUuids);
     } catch (NoSuchGroupException e) {
-      throw new ResourceNotFoundException(String.format("Group %s not found", groupUuid));
+      throw new ResourceNotFoundException(String.format("Group %s not found", groupUuid), e);
     }
     return Response.ok(result);
   }
@@ -153,7 +153,7 @@
         }
         throw new IllegalStateException();
       } catch (UnprocessableEntityException e) {
-        throw new ResourceNotFoundException(id);
+        throw new ResourceNotFoundException(id, e);
       }
     }
   }
diff --git a/java/com/google/gerrit/server/restapi/group/CreateGroup.java b/java/com/google/gerrit/server/restapi/group/CreateGroup.java
index b80b0e4..531d350 100644
--- a/java/com/google/gerrit/server/restapi/group/CreateGroup.java
+++ b/java/com/google/gerrit/server/restapi/group/CreateGroup.java
@@ -220,7 +220,7 @@
       return groupsUpdateProvider.get().createGroup(groupCreation, groupUpdateBuilder.build());
     } catch (DuplicateKeyException e) {
       throw new ResourceConflictException(
-          "group '" + createGroupArgs.getGroupName() + "' already exists");
+          "group '" + createGroupArgs.getGroupName() + "' already exists", e);
     }
   }
 }
diff --git a/java/com/google/gerrit/server/restapi/group/DeleteMembers.java b/java/com/google/gerrit/server/restapi/group/DeleteMembers.java
index fa1c5c6..a7b2e2d 100644
--- a/java/com/google/gerrit/server/restapi/group/DeleteMembers.java
+++ b/java/com/google/gerrit/server/restapi/group/DeleteMembers.java
@@ -78,7 +78,7 @@
     try {
       removeGroupMembers(groupUuid, membersToRemove);
     } catch (NoSuchGroupException e) {
-      throw new ResourceNotFoundException(String.format("Group %s not found", groupUuid));
+      throw new ResourceNotFoundException(String.format("Group %s not found", groupUuid), e);
     }
 
     return Response.none();
diff --git a/java/com/google/gerrit/server/restapi/group/DeleteSubgroups.java b/java/com/google/gerrit/server/restapi/group/DeleteSubgroups.java
index ef8612e..b9d6ca8 100644
--- a/java/com/google/gerrit/server/restapi/group/DeleteSubgroups.java
+++ b/java/com/google/gerrit/server/restapi/group/DeleteSubgroups.java
@@ -77,7 +77,7 @@
     try {
       removeSubgroups(groupUuid, subgroupsToRemove);
     } catch (NoSuchGroupException e) {
-      throw new ResourceNotFoundException(String.format("Group %s not found", groupUuid));
+      throw new ResourceNotFoundException(String.format("Group %s not found", groupUuid), e);
     }
 
     return Response.none();
diff --git a/java/com/google/gerrit/server/restapi/group/GetOwner.java b/java/com/google/gerrit/server/restapi/group/GetOwner.java
index 30d5c16..e8bdfaa 100644
--- a/java/com/google/gerrit/server/restapi/group/GetOwner.java
+++ b/java/com/google/gerrit/server/restapi/group/GetOwner.java
@@ -47,7 +47,7 @@
       GroupControl c = controlFactory.validateFor(group.getOwnerGroupUUID());
       return Response.ok(json.format(c.getGroup()));
     } catch (NoSuchGroupException e) {
-      throw new ResourceNotFoundException();
+      throw new ResourceNotFoundException(group.getOwnerGroupUUID().get(), e);
     }
   }
 }
diff --git a/java/com/google/gerrit/server/restapi/group/PutDescription.java b/java/com/google/gerrit/server/restapi/group/PutDescription.java
index 98c773e..8fe4b20 100644
--- a/java/com/google/gerrit/server/restapi/group/PutDescription.java
+++ b/java/com/google/gerrit/server/restapi/group/PutDescription.java
@@ -66,7 +66,7 @@
       try {
         groupsUpdateProvider.get().updateGroup(groupUuid, groupUpdate);
       } catch (NoSuchGroupException e) {
-        throw new ResourceNotFoundException(String.format("Group %s not found", groupUuid));
+        throw new ResourceNotFoundException(String.format("Group %s not found", groupUuid), e);
       }
     }
 
diff --git a/java/com/google/gerrit/server/restapi/group/PutName.java b/java/com/google/gerrit/server/restapi/group/PutName.java
index 802956a..9a3c87d 100644
--- a/java/com/google/gerrit/server/restapi/group/PutName.java
+++ b/java/com/google/gerrit/server/restapi/group/PutName.java
@@ -79,9 +79,9 @@
     try {
       groupsUpdateProvider.get().updateGroup(groupUuid, groupUpdate);
     } catch (NoSuchGroupException e) {
-      throw new ResourceNotFoundException(String.format("Group %s not found", groupUuid));
+      throw new ResourceNotFoundException(String.format("Group %s not found", groupUuid), e);
     } catch (DuplicateKeyException e) {
-      throw new ResourceConflictException("group with name " + newName + " already exists");
+      throw new ResourceConflictException("group with name " + newName + " already exists", e);
     }
   }
 }
diff --git a/java/com/google/gerrit/server/restapi/group/PutOptions.java b/java/com/google/gerrit/server/restapi/group/PutOptions.java
index c96972f..53bf571 100644
--- a/java/com/google/gerrit/server/restapi/group/PutOptions.java
+++ b/java/com/google/gerrit/server/restapi/group/PutOptions.java
@@ -66,7 +66,7 @@
       try {
         groupsUpdateProvider.get().updateGroup(groupUuid, groupUpdate);
       } catch (NoSuchGroupException e) {
-        throw new ResourceNotFoundException(String.format("Group %s not found", groupUuid));
+        throw new ResourceNotFoundException(String.format("Group %s not found", groupUuid), e);
       }
     }
 
diff --git a/java/com/google/gerrit/server/restapi/group/PutOwner.java b/java/com/google/gerrit/server/restapi/group/PutOwner.java
index 709f7e3..04129af 100644
--- a/java/com/google/gerrit/server/restapi/group/PutOwner.java
+++ b/java/com/google/gerrit/server/restapi/group/PutOwner.java
@@ -77,7 +77,7 @@
       try {
         groupsUpdateProvider.get().updateGroup(groupUuid, groupUpdate);
       } catch (NoSuchGroupException e) {
-        throw new ResourceNotFoundException(String.format("Group %s not found", groupUuid));
+        throw new ResourceNotFoundException(String.format("Group %s not found", groupUuid), e);
       }
     }
     return Response.ok(json.format(owner));
diff --git a/java/com/google/gerrit/server/restapi/project/BranchesCollection.java b/java/com/google/gerrit/server/restapi/project/BranchesCollection.java
index fe2fb0f..2d78bb0 100644
--- a/java/com/google/gerrit/server/restapi/project/BranchesCollection.java
+++ b/java/com/google/gerrit/server/restapi/project/BranchesCollection.java
@@ -82,9 +82,9 @@
           .check(RefPermission.READ);
       return new BranchResource(parent.getProjectState(), parent.getUser(), ref);
     } catch (AuthException notAllowed) {
-      throw new ResourceNotFoundException(id);
+      throw new ResourceNotFoundException(id, notAllowed);
     } catch (RepositoryNotFoundException noRepo) {
-      throw new ResourceNotFoundException();
+      throw new ResourceNotFoundException(id, noRepo);
     }
   }
 
diff --git a/java/com/google/gerrit/server/restapi/project/CommitsCollection.java b/java/com/google/gerrit/server/restapi/project/CommitsCollection.java
index 757b650..21d7f0b 100644
--- a/java/com/google/gerrit/server/restapi/project/CommitsCollection.java
+++ b/java/com/google/gerrit/server/restapi/project/CommitsCollection.java
@@ -87,7 +87,7 @@
     try {
       objectId = ObjectId.fromString(id.get());
     } catch (IllegalArgumentException e) {
-      throw new ResourceNotFoundException(id);
+      throw new ResourceNotFoundException(id, e);
     }
 
     try (Repository repo = repoManager.openRepository(parent.getNameKey());
@@ -102,7 +102,7 @@
       }
       return new CommitResource(parent, commit);
     } catch (MissingObjectException | IncorrectObjectTypeException e) {
-      throw new ResourceNotFoundException(id);
+      throw new ResourceNotFoundException(id, e);
     }
   }
 
diff --git a/java/com/google/gerrit/server/restapi/project/CreateAccessChange.java b/java/com/google/gerrit/server/restapi/project/CreateAccessChange.java
index 76f1105..fe48301 100644
--- a/java/com/google/gerrit/server/restapi/project/CreateAccessChange.java
+++ b/java/com/google/gerrit/server/restapi/project/CreateAccessChange.java
@@ -102,7 +102,7 @@
       try {
         forProject.ref(RefNames.REFS_CONFIG).check(RefPermission.CREATE_CHANGE);
       } catch (AuthException denied) {
-        throw new AuthException("cannot create change for " + RefNames.REFS_CONFIG);
+        throw new AuthException("cannot create change for " + RefNames.REFS_CONFIG, denied);
       }
     }
     projectCache.checkedGet(rsrc.getNameKey()).checkStatePermitsWrite();
diff --git a/java/com/google/gerrit/server/restapi/project/CreateBranch.java b/java/com/google/gerrit/server/restapi/project/CreateBranch.java
index 67213c5..c15fdeb 100644
--- a/java/com/google/gerrit/server/restapi/project/CreateBranch.java
+++ b/java/com/google/gerrit/server/restapi/project/CreateBranch.java
@@ -127,7 +127,7 @@
         try {
           object = rw.parseCommit(object);
         } catch (IncorrectObjectTypeException notCommit) {
-          throw new BadRequestException("\"" + input.revision + "\" not a commit");
+          throw new BadRequestException("\"" + input.revision + "\" not a commit", notCommit);
         }
       }
 
@@ -197,7 +197,7 @@
         throw err;
       }
     } catch (RefUtil.InvalidRevisionException e) {
-      throw new BadRequestException("invalid revision \"" + input.revision + "\"");
+      throw new BadRequestException("invalid revision \"" + input.revision + "\"", e);
     }
   }
 }
diff --git a/java/com/google/gerrit/server/restapi/project/CreateTag.java b/java/com/google/gerrit/server/restapi/project/CreateTag.java
index 8fdf5e4..b087f15 100644
--- a/java/com/google/gerrit/server/restapi/project/CreateTag.java
+++ b/java/com/google/gerrit/server/restapi/project/CreateTag.java
@@ -149,7 +149,7 @@
         }
       }
     } catch (InvalidRevisionException e) {
-      throw new BadRequestException("Invalid base revision");
+      throw new BadRequestException("Invalid base revision", e);
     } catch (GitAPIException e) {
       logger.atSevere().withCause(e).log("Cannot create tag \"%s\"", ref);
       throw new IOException(e);
diff --git a/java/com/google/gerrit/server/restapi/project/DashboardsCollection.java b/java/com/google/gerrit/server/restapi/project/DashboardsCollection.java
index 606f3bef..ca48109 100644
--- a/java/com/google/gerrit/server/restapi/project/DashboardsCollection.java
+++ b/java/com/google/gerrit/server/restapi/project/DashboardsCollection.java
@@ -103,14 +103,14 @@
     try {
       info = newDashboardInfo(id.get());
     } catch (InvalidDashboardId e) {
-      throw new ResourceNotFoundException(id);
+      throw new ResourceNotFoundException(id, e);
     }
 
     for (ProjectState ps : parent.getProjectState().tree()) {
       try {
         return parse(ps, parent.getProjectState(), parent.getUser(), info);
       } catch (AmbiguousObjectException | ConfigInvalidException | IncorrectObjectTypeException e) {
-        throw new ResourceNotFoundException(id);
+        throw new ResourceNotFoundException(id, e);
       } catch (ResourceNotFoundException e) {
         continue;
       }
@@ -135,7 +135,7 @@
       permissionBackend.user(user).project(parent.getNameKey()).ref(ref).check(RefPermission.READ);
     } catch (AuthException e) {
       // Don't leak the project's existence
-      throw new ResourceNotFoundException(info.id);
+      throw new ResourceNotFoundException(info.id, e);
     }
     if (!Repository.isValidRefName(ref)) {
       throw new ResourceNotFoundException(info.id);
@@ -151,7 +151,7 @@
       BlobBasedConfig cfg = new BlobBasedConfig(null, git, objId);
       return new DashboardResource(current, user, ref, info.path, cfg, false);
     } catch (RepositoryNotFoundException e) {
-      throw new ResourceNotFoundException(info.id);
+      throw new ResourceNotFoundException(info.id, e);
     }
   }
 
diff --git a/java/com/google/gerrit/server/restapi/project/GetAccess.java b/java/com/google/gerrit/server/restapi/project/GetAccess.java
index 550c4c4..a9a9403 100644
--- a/java/com/google/gerrit/server/restapi/project/GetAccess.java
+++ b/java/com/google/gerrit/server/restapi/project/GetAccess.java
@@ -165,7 +165,7 @@
     } catch (ConfigInvalidException e) {
       throw new ResourceConflictException(e.getMessage());
     } catch (RepositoryNotFoundException e) {
-      throw new ResourceNotFoundException(rsrc.getName());
+      throw new ResourceNotFoundException(rsrc.getName(), e);
     }
 
     // The following implementation must match the ProjectAccessFactory JSON RPC endpoint.
diff --git a/java/com/google/gerrit/server/restapi/project/GetHead.java b/java/com/google/gerrit/server/restapi/project/GetHead.java
index 928db97..4e0a144 100644
--- a/java/com/google/gerrit/server/restapi/project/GetHead.java
+++ b/java/com/google/gerrit/server/restapi/project/GetHead.java
@@ -82,13 +82,13 @@
                 .project(rsrc.getNameKey())
                 .check(ProjectPermission.WRITE_CONFIG);
           } catch (AuthException ae) {
-            throw new AuthException("not allowed to see HEAD");
+            throw new AuthException("not allowed to see HEAD", ae);
           }
         }
       }
       throw new ResourceNotFoundException(Constants.HEAD);
     } catch (RepositoryNotFoundException e) {
-      throw new ResourceNotFoundException(rsrc.getName());
+      throw new ResourceNotFoundException(rsrc.getName(), e);
     }
   }
 }
diff --git a/java/com/google/gerrit/server/restapi/project/GetReflog.java b/java/com/google/gerrit/server/restapi/project/GetReflog.java
index a249f26..f9c6fd9 100644
--- a/java/com/google/gerrit/server/restapi/project/GetReflog.java
+++ b/java/com/google/gerrit/server/restapi/project/GetReflog.java
@@ -104,7 +104,7 @@
       } catch (UnsupportedOperationException e) {
         String msg = "reflog not supported on repo " + rsrc.getNameKey().get();
         logger.atSevere().log(msg);
-        throw new MethodNotAllowedException(msg);
+        throw new MethodNotAllowedException(msg, e);
       }
       if (r == null) {
         throw new ResourceNotFoundException(rsrc.getRef());
diff --git a/java/com/google/gerrit/server/restapi/project/GetStatistics.java b/java/com/google/gerrit/server/restapi/project/GetStatistics.java
index d68e0af..db97855 100644
--- a/java/com/google/gerrit/server/restapi/project/GetStatistics.java
+++ b/java/com/google/gerrit/server/restapi/project/GetStatistics.java
@@ -51,7 +51,7 @@
     } catch (GitAPIException | JGitInternalException e) {
       throw new ResourceConflictException(e.getMessage());
     } catch (IOException e) {
-      throw new ResourceNotFoundException(rsrc.getName());
+      throw new ResourceNotFoundException(rsrc.getName(), e);
     }
   }
 }
diff --git a/java/com/google/gerrit/server/restapi/project/ListBranches.java b/java/com/google/gerrit/server/restapi/project/ListBranches.java
index cd20820..fecdc8e 100644
--- a/java/com/google/gerrit/server/restapi/project/ListBranches.java
+++ b/java/com/google/gerrit/server/restapi/project/ListBranches.java
@@ -149,7 +149,7 @@
       }
       return toBranchInfo(rsrc, ImmutableList.of(r)).get(0);
     } catch (RepositoryNotFoundException noRepo) {
-      throw new ResourceNotFoundException();
+      throw new ResourceNotFoundException(rsrc.getNameKey().get(), noRepo);
     }
   }
 
@@ -165,7 +165,7 @@
               .exactRef(Constants.HEAD, RefNames.REFS_CONFIG, RefNames.REFS_USERS_DEFAULT)
               .values());
     } catch (RepositoryNotFoundException noGitRepository) {
-      throw new ResourceNotFoundException();
+      throw new ResourceNotFoundException(rsrc.getNameKey().get(), noGitRepository);
     }
     return toBranchInfo(rsrc, refs);
   }
diff --git a/java/com/google/gerrit/server/restapi/project/ListDashboards.java b/java/com/google/gerrit/server/restapi/project/ListDashboards.java
index 058083c..4406719 100644
--- a/java/com/google/gerrit/server/restapi/project/ListDashboards.java
+++ b/java/com/google/gerrit/server/restapi/project/ListDashboards.java
@@ -120,7 +120,7 @@
       }
       return all;
     } catch (RepositoryNotFoundException e) {
-      throw new ResourceNotFoundException();
+      throw new ResourceNotFoundException(project, e);
     }
   }
 
diff --git a/java/com/google/gerrit/server/restapi/project/ListTags.java b/java/com/google/gerrit/server/restapi/project/ListTags.java
index 8cea7f5..123c78a 100644
--- a/java/com/google/gerrit/server/restapi/project/ListTags.java
+++ b/java/com/google/gerrit/server/restapi/project/ListTags.java
@@ -219,7 +219,7 @@
     try {
       return repoManager.openRepository(project);
     } catch (RepositoryNotFoundException noGitRepository) {
-      throw new ResourceNotFoundException();
+      throw new ResourceNotFoundException(project.get(), noGitRepository);
     }
   }
 
diff --git a/java/com/google/gerrit/server/restapi/project/PutConfig.java b/java/com/google/gerrit/server/restapi/project/PutConfig.java
index a0badd7..9f9433b 100644
--- a/java/com/google/gerrit/server/restapi/project/PutConfig.java
+++ b/java/com/google/gerrit/server/restapi/project/PutConfig.java
@@ -190,7 +190,7 @@
           uiActions,
           views);
     } catch (RepositoryNotFoundException notFound) {
-      throw new ResourceNotFoundException(projectName.get());
+      throw new ResourceNotFoundException(projectName.get(), notFound);
     } catch (ConfigInvalidException err) {
       throw new ResourceConflictException("Cannot read project " + projectName, err);
     } catch (IOException err) {
diff --git a/java/com/google/gerrit/server/restapi/project/PutDescription.java b/java/com/google/gerrit/server/restapi/project/PutDescription.java
index 0281b8d..a0b9feb 100644
--- a/java/com/google/gerrit/server/restapi/project/PutDescription.java
+++ b/java/com/google/gerrit/server/restapi/project/PutDescription.java
@@ -92,7 +92,7 @@
           ? Response.none()
           : Response.ok(project.getDescription());
     } catch (RepositoryNotFoundException notFound) {
-      throw new ResourceNotFoundException(resource.getName());
+      throw new ResourceNotFoundException(resource.getName(), notFound);
     } catch (ConfigInvalidException e) {
       throw new ResourceConflictException(
           String.format("invalid project.config: %s", e.getMessage()));
diff --git a/java/com/google/gerrit/server/restapi/project/SetAccess.java b/java/com/google/gerrit/server/restapi/project/SetAccess.java
index f8c7234..02c1b54 100644
--- a/java/com/google/gerrit/server/restapi/project/SetAccess.java
+++ b/java/com/google/gerrit/server/restapi/project/SetAccess.java
@@ -130,7 +130,7 @@
     } catch (InvalidNameException e) {
       throw new BadRequestException(e.toString());
     } catch (ConfigInvalidException e) {
-      throw new ResourceConflictException(rsrc.getName());
+      throw new ResourceConflictException(rsrc.getName(), e);
     }
 
     return Response.ok(getAccess.apply(rsrc.getNameKey()));
diff --git a/java/com/google/gerrit/server/restapi/project/SetDefaultDashboard.java b/java/com/google/gerrit/server/restapi/project/SetDefaultDashboard.java
index 8879fae..9920be0 100644
--- a/java/com/google/gerrit/server/restapi/project/SetDefaultDashboard.java
+++ b/java/com/google/gerrit/server/restapi/project/SetDefaultDashboard.java
@@ -89,7 +89,7 @@
                 new ProjectResource(rsrc.getProjectState(), rsrc.getUser()),
                 IdString.fromUrl(input.id));
       } catch (ResourceNotFoundException e) {
-        throw new BadRequestException("dashboard " + input.id + " not found");
+        throw new BadRequestException("dashboard " + input.id + " not found", e);
       } catch (ConfigInvalidException e) {
         throw new ResourceConflictException(e.getMessage());
       }
@@ -125,7 +125,7 @@
       }
       return Response.none();
     } catch (RepositoryNotFoundException notFound) {
-      throw new ResourceNotFoundException(rsrc.getProjectState().getProject().getName());
+      throw new ResourceNotFoundException(rsrc.getProjectState().getProject().getName(), notFound);
     } catch (ConfigInvalidException e) {
       throw new ResourceConflictException(
           String.format("invalid project.config: %s", e.getMessage()));
diff --git a/java/com/google/gerrit/server/restapi/project/SetHead.java b/java/com/google/gerrit/server/restapi/project/SetHead.java
index 946695e..ae70e46 100644
--- a/java/com/google/gerrit/server/restapi/project/SetHead.java
+++ b/java/com/google/gerrit/server/restapi/project/SetHead.java
@@ -114,7 +114,7 @@
       }
       return Response.ok(ref);
     } catch (RepositoryNotFoundException e) {
-      throw new ResourceNotFoundException(rsrc.getName());
+      throw new ResourceNotFoundException(rsrc.getName(), e);
     }
   }
 
diff --git a/java/com/google/gerrit/server/restapi/project/SetParent.java b/java/com/google/gerrit/server/restapi/project/SetParent.java
index 7241fcf..4cf0182 100644
--- a/java/com/google/gerrit/server/restapi/project/SetParent.java
+++ b/java/com/google/gerrit/server/restapi/project/SetParent.java
@@ -121,7 +121,7 @@
       requireNonNull(parent);
       return parent.get();
     } catch (RepositoryNotFoundException notFound) {
-      throw new ResourceNotFoundException(rsrc.getName());
+      throw new ResourceNotFoundException(rsrc.getName(), notFound);
     } catch (ConfigInvalidException e) {
       throw new ResourceConflictException(
           String.format("invalid project.config: %s", e.getMessage()));
diff --git a/java/com/google/gerrit/server/rules/StoredValues.java b/java/com/google/gerrit/server/rules/StoredValues.java
index 085475e..1e08a24 100644
--- a/java/com/google/gerrit/server/rules/StoredValues.java
+++ b/java/com/google/gerrit/server/rules/StoredValues.java
@@ -57,7 +57,8 @@
     try {
       return cd.change();
     } catch (StorageException e) {
-      throw new SystemException("Cannot load change " + cd.getId());
+      throw new SystemException(
+          String.format("Cannot load change %s: %s", cd.getId(), e.getMessage()));
     }
   }
 
@@ -101,7 +102,7 @@
           try {
             patchList = plCache.get(plKey, project);
           } catch (PatchListNotAvailableException e) {
-            throw new SystemException("Cannot create " + plKey);
+            throw new SystemException(String.format("Cannot create %s: %s", plKey, e.getMessage()));
           }
           return patchList;
         }
diff --git a/java/com/google/gerrit/server/schema/NoteDbSchemaUpdater.java b/java/com/google/gerrit/server/schema/NoteDbSchemaUpdater.java
index 5dec581..0e22af9 100644
--- a/java/com/google/gerrit/server/schema/NoteDbSchemaUpdater.java
+++ b/java/com/google/gerrit/server/schema/NoteDbSchemaUpdater.java
@@ -104,7 +104,7 @@
     try {
       schemaCreator.ensureCreated();
     } catch (IOException | ConfigInvalidException e) {
-      throw new StorageException("Cannot initialize Gerrit site");
+      throw new StorageException("Cannot initialize Gerrit site", e);
     }
   }
 
diff --git a/java/com/google/gerrit/server/schema/NoteDbSchemaVersions.java b/java/com/google/gerrit/server/schema/NoteDbSchemaVersions.java
index 02250f2..b450a4b 100644
--- a/java/com/google/gerrit/server/schema/NoteDbSchemaVersions.java
+++ b/java/com/google/gerrit/server/schema/NoteDbSchemaVersions.java
@@ -28,7 +28,7 @@
 public class NoteDbSchemaVersions {
   static final ImmutableSortedMap<Integer, Class<? extends NoteDbSchemaVersion>> ALL =
       // List all supported NoteDb schema versions here.
-      Stream.of(Schema_180.class, Schema_181.class)
+      Stream.of(Schema_180.class, Schema_181.class, Schema_182.class)
           .collect(toImmutableSortedMap(naturalOrder(), v -> guessVersion(v).get(), v -> v));
 
   public static final int FIRST = ALL.firstKey();
diff --git a/java/com/google/gerrit/server/schema/Schema_182.java b/java/com/google/gerrit/server/schema/Schema_182.java
new file mode 100644
index 0000000..3928b2b
--- /dev/null
+++ b/java/com/google/gerrit/server/schema/Schema_182.java
@@ -0,0 +1,34 @@
+// Copyright (C) 2020 The Android Open Source Project
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package com.google.gerrit.server.schema;
+
+import com.google.gerrit.server.config.AllUsersName;
+import com.google.gerrit.server.git.GitRepositoryManager;
+import com.google.gerrit.server.notedb.DeleteZombieCommentsRefs;
+
+/**
+ * Schema 182 for Gerrit metadata.
+ *
+ * <p>Upgrading to this schema version cleans the zombie draft comment refs in NoteDb
+ */
+public class Schema_182 implements NoteDbSchemaVersion {
+  @Override
+  public void upgrade(Arguments args, UpdateUI ui) throws Exception {
+    AllUsersName allUsers = args.allUsers;
+    GitRepositoryManager gitRepoManager = args.repoManager;
+    DeleteZombieCommentsRefs cleanup = new DeleteZombieCommentsRefs(allUsers, gitRepoManager, 100);
+    cleanup.execute();
+  }
+}
diff --git a/java/com/google/gerrit/server/submit/RebaseSubmitStrategy.java b/java/com/google/gerrit/server/submit/RebaseSubmitStrategy.java
index a6e01af..65e18ad 100644
--- a/java/com/google/gerrit/server/submit/RebaseSubmitStrategy.java
+++ b/java/com/google/gerrit/server/submit/RebaseSubmitStrategy.java
@@ -161,7 +161,8 @@
         } catch (MergeConflictException mce) {
           // Unlike in Cherry-pick case, this should never happen.
           toMerge.setStatusCode(CommitMergeStatus.REBASE_MERGE_CONFLICT);
-          throw new IllegalStateException("MergeConflictException on message edit must not happen");
+          throw new IllegalStateException(
+              "MergeConflictException on message edit must not happen", mce);
         } catch (MergeIdenticalTreeException mie) {
           // this should not happen
           toMerge.setStatusCode(SKIPPED_IDENTICAL_TREE);
diff --git a/java/com/google/gerrit/server/submit/SubmitStrategyOp.java b/java/com/google/gerrit/server/submit/SubmitStrategyOp.java
index 6be4a91..cc40a30 100644
--- a/java/com/google/gerrit/server/submit/SubmitStrategyOp.java
+++ b/java/com/google/gerrit/server/submit/SubmitStrategyOp.java
@@ -550,7 +550,7 @@
       return args.submoduleOp.composeGitlinksCommit(args.destBranch, commit);
     } catch (SubmoduleException | IOException e) {
       throw new IntegrationException(
-          "cannot update gitlink for the commit at branch: " + args.destBranch);
+          "cannot update gitlink for the commit at branch: " + args.destBranch, e);
     }
   }
 }
diff --git a/java/com/google/gerrit/server/util/SocketUtil.java b/java/com/google/gerrit/server/util/SocketUtil.java
index afa2aee..e617650 100644
--- a/java/com/google/gerrit/server/util/SocketUtil.java
+++ b/java/com/google/gerrit/server/util/SocketUtil.java
@@ -96,7 +96,7 @@
       try {
         port = Integer.parseInt(portStr);
       } catch (NumberFormatException e) {
-        throw new IllegalArgumentException("invalid port: " + desc);
+        throw new IllegalArgumentException("invalid port: " + desc, e);
       }
     } else {
       port = defaultPort;
diff --git a/java/com/google/gerrit/sshd/BaseCommand.java b/java/com/google/gerrit/sshd/BaseCommand.java
index ac22dfc..5d7dba2 100644
--- a/java/com/google/gerrit/sshd/BaseCommand.java
+++ b/java/com/google/gerrit/sshd/BaseCommand.java
@@ -387,6 +387,10 @@
     return new UnloggedFailure(1, "fatal: " + msg);
   }
 
+  protected UnloggedFailure die(String msg, Throwable why) {
+    return new UnloggedFailure(1, "fatal: " + msg, why);
+  }
+
   protected UnloggedFailure die(Throwable why) {
     return new UnloggedFailure(1, "fatal: " + why.getMessage(), why);
   }
diff --git a/java/com/google/gerrit/sshd/ChangeArgumentParser.java b/java/com/google/gerrit/sshd/ChangeArgumentParser.java
index c273b4c..491bcb8 100644
--- a/java/com/google/gerrit/sshd/ChangeArgumentParser.java
+++ b/java/com/google/gerrit/sshd/ChangeArgumentParser.java
@@ -111,7 +111,7 @@
     try {
       changeResource = changesCollection.parse(cId);
     } catch (RestApiException e) {
-      throw new UnloggedFailure(1, "\"" + id + "\" no such change");
+      throw new UnloggedFailure(1, "\"" + id + "\" no such change", e);
     }
     changes.put(cId, changeResource);
   }
diff --git a/java/com/google/gerrit/sshd/SshKeyCreatorImpl.java b/java/com/google/gerrit/sshd/SshKeyCreatorImpl.java
index e6c3a40..02a9e48 100644
--- a/java/com/google/gerrit/sshd/SshKeyCreatorImpl.java
+++ b/java/com/google/gerrit/sshd/SshKeyCreatorImpl.java
@@ -34,11 +34,10 @@
       SshUtil.parse(key);
       return key;
     } catch (NoSuchAlgorithmException | InvalidKeySpecException e) {
-      throw new InvalidSshKeyException();
-
+      throw new InvalidSshKeyException(e);
     } catch (NoSuchProviderException e) {
       logger.atSevere().withCause(e).log("Cannot parse SSH key");
-      throw new InvalidSshKeyException();
+      throw new InvalidSshKeyException(e);
     }
   }
 }
diff --git a/java/com/google/gerrit/sshd/SuExec.java b/java/com/google/gerrit/sshd/SuExec.java
index f485bd7..ea163d5 100644
--- a/java/com/google/gerrit/sshd/SuExec.java
+++ b/java/com/google/gerrit/sshd/SuExec.java
@@ -127,9 +127,9 @@
       try {
         permissionBackend.user(caller).check(GlobalPermission.RUN_AS);
       } catch (AuthException e) {
-        throw die("suexec not permitted");
+        throw die("suexec not permitted", e);
       } catch (PermissionBackendException e) {
-        throw die("suexec not available: " + e);
+        throw die("suexec not available", e);
       }
     }
   }
diff --git a/java/com/google/gerrit/sshd/commands/LsUserRefs.java b/java/com/google/gerrit/sshd/commands/LsUserRefs.java
index b4a2b42..80aee01 100644
--- a/java/com/google/gerrit/sshd/commands/LsUserRefs.java
+++ b/java/com/google/gerrit/sshd/commands/LsUserRefs.java
@@ -104,9 +104,9 @@
         throw new Failure(1, "fatal: Error reading refs: '" + projectName, e);
       }
     } catch (RepositoryNotFoundException e) {
-      throw die("'" + projectName + "': not a git archive");
+      throw die("'" + projectName + "': not a git archive", e);
     } catch (IOException e) {
-      throw die("Error opening: '" + projectName);
+      throw die("Error opening: '" + projectName, e);
     }
   }
 }
diff --git a/java/com/google/gerrit/sshd/commands/PatchSetParser.java b/java/com/google/gerrit/sshd/commands/PatchSetParser.java
index e40ab87..f804c2d 100644
--- a/java/com/google/gerrit/sshd/commands/PatchSetParser.java
+++ b/java/com/google/gerrit/sshd/commands/PatchSetParser.java
@@ -99,7 +99,7 @@
       try {
         patchSetId = PatchSet.Id.parse(token);
       } catch (IllegalArgumentException e) {
-        throw error("\"" + token + "\" is not a valid patch set");
+        throw error("\"" + token + "\" is not a valid patch set", e);
       }
       ChangeNotes notes = getNotes(projectState, patchSetId.changeId());
       PatchSet patchSet = psUtil.get(notes, patchSetId);
@@ -130,7 +130,7 @@
       ChangeNotes notes = changeFinder.findOne(changeId);
       return notesFactory.create(notes.getProjectName(), changeId);
     } catch (NoSuchChangeException e) {
-      throw error("\"" + changeId + "\" no such change");
+      throw error("\"" + changeId + "\" no such change", e);
     }
   }
 
@@ -153,4 +153,8 @@
   public static UnloggedFailure error(String msg) {
     return new UnloggedFailure(1, msg);
   }
+
+  public static UnloggedFailure error(String msg, Throwable why) {
+    return new UnloggedFailure(1, msg, why);
+  }
 }
diff --git a/java/com/google/gerrit/sshd/commands/PluginInstallCommand.java b/java/com/google/gerrit/sshd/commands/PluginInstallCommand.java
index 8b045ec..cfb47f7 100644
--- a/java/com/google/gerrit/sshd/commands/PluginInstallCommand.java
+++ b/java/com/google/gerrit/sshd/commands/PluginInstallCommand.java
@@ -70,21 +70,21 @@
       try {
         data = Files.newInputStream(new File(source).toPath());
       } catch (IOException e) {
-        throw die("cannot read " + source);
+        throw die("cannot read " + source, e);
       }
     } else {
       try {
         data = new URL(source).openStream();
       } catch (MalformedURLException e) {
-        throw die("invalid url " + source);
+        throw die("invalid url " + source, e);
       } catch (IOException e) {
-        throw die("cannot read " + source);
+        throw die("cannot read " + source, e);
       }
     }
     try {
       loader.installPluginFromStream(name, data);
     } catch (IOException e) {
-      throw die("cannot install plugin");
+      throw die("cannot install plugin", e);
     } catch (PluginInstallException e) {
       e.printStackTrace(stderr);
       String msg = String.format("Plugin failed to install. Cause: %s", e.getMessage());
diff --git a/java/com/google/gerrit/sshd/commands/Receive.java b/java/com/google/gerrit/sshd/commands/Receive.java
index e2fb633..6791b70 100644
--- a/java/com/google/gerrit/sshd/commands/Receive.java
+++ b/java/com/google/gerrit/sshd/commands/Receive.java
@@ -78,7 +78,7 @@
           .project(project.getNameKey())
           .check(ProjectPermission.RUN_RECEIVE_PACK);
     } catch (AuthException e) {
-      throw new Failure(1, "fatal: receive-pack not permitted on this server");
+      throw new Failure(1, "fatal: receive-pack not permitted on this server", e);
     } catch (PermissionBackendException e) {
       throw new Failure(1, "fatal: unable to check permissions " + e);
     }
diff --git a/java/com/google/gerrit/sshd/commands/ReviewCommand.java b/java/com/google/gerrit/sshd/commands/ReviewCommand.java
index 886f21e..3a1bf6c 100644
--- a/java/com/google/gerrit/sshd/commands/ReviewCommand.java
+++ b/java/com/google/gerrit/sshd/commands/ReviewCommand.java
@@ -327,7 +327,7 @@
     try {
       allProjectsState = projectCache.checkedGet(allProjects);
     } catch (IOException e) {
-      throw die("missing " + allProjects.get());
+      throw die("missing " + allProjects.get(), e);
     }
 
     for (LabelType type : allProjectsState.getLabelTypes().getLabelTypes()) {
diff --git a/java/com/google/gerrit/sshd/commands/Upload.java b/java/com/google/gerrit/sshd/commands/Upload.java
index 87ae493..6c4e80a 100644
--- a/java/com/google/gerrit/sshd/commands/Upload.java
+++ b/java/com/google/gerrit/sshd/commands/Upload.java
@@ -61,9 +61,9 @@
     try {
       perm.check(ProjectPermission.RUN_UPLOAD_PACK);
     } catch (AuthException e) {
-      throw new Failure(1, "fatal: upload-pack not permitted on this server");
+      throw new Failure(1, "fatal: upload-pack not permitted on this server", e);
     } catch (PermissionBackendException e) {
-      throw new Failure(1, "fatal: unable to check permissions " + e);
+      throw new Failure(1, "fatal: unable to check permissions ", e);
     }
 
     Repository permissionAwareRepo = PermissionAwareRepositoryManager.wrap(repo, perm);
diff --git a/java/com/google/gerrit/testing/IndexConfig.java b/java/com/google/gerrit/testing/IndexConfig.java
index fb6c926..a8ed5be 100644
--- a/java/com/google/gerrit/testing/IndexConfig.java
+++ b/java/com/google/gerrit/testing/IndexConfig.java
@@ -14,6 +14,7 @@
 
 package com.google.gerrit.testing;
 
+import com.google.gerrit.server.change.MergeabilityComputationBehavior;
 import org.eclipse.jgit.lib.Config;
 
 public class IndexConfig {
@@ -23,9 +24,10 @@
 
   public static Config createFromExistingConfig(Config cfg) {
     cfg.setInt("index", null, "maxPages", 10);
-    // To avoid this flakiness indexMergeable is switched off for the tests as it incurs background
+    // To avoid this flakiness indexing mergeable is disabled for the tests as it incurs background
     // reindex calls.
-    cfg.setBoolean("index", "change", "indexMergeable", false);
+    cfg.setEnum(
+        "change", null, "mergeabilityComputationBehavior", MergeabilityComputationBehavior.NEVER);
     cfg.setString("trackingid", "query-bug", "footer", "Bug:");
     cfg.setString("trackingid", "query-bug", "match", "QUERY\\d{2,8}");
     cfg.setString("trackingid", "query-bug", "system", "querytests");
diff --git a/javatests/com/google/gerrit/acceptance/api/change/AbandonIT.java b/javatests/com/google/gerrit/acceptance/api/change/AbandonIT.java
index 7ecbe69..d04eebd 100644
--- a/javatests/com/google/gerrit/acceptance/api/change/AbandonIT.java
+++ b/javatests/com/google/gerrit/acceptance/api/change/AbandonIT.java
@@ -143,7 +143,9 @@
   @UseClockStep
   @GerritConfig(name = "changeCleanup.abandonAfter", value = "1w")
   @GerritConfig(name = "changeCleanup.abandonIfMergeable", value = "false")
-  @GerritConfig(name = "index.change.indexMergeable", value = "true")
+  @GerritConfig(
+      name = "change.mergeabilityComputationBehavior",
+      value = "API_REF_UPDATED_AND_CHANGE_REINDEX")
   public void notAbandonedIfMergeableWhenMergeableOperatorIsEnabled() throws Exception {
     ObjectId initial = repo().exactRef(HEAD).getLeaf().getObjectId();
 
@@ -180,14 +182,14 @@
   }
 
   /**
-   * When indexMergeable is disabled then the abandonIfMergeable option is ineffective and the auto
-   * abandon behaves as though it were set to its default value (true).
+   * When indexing mergeable is disabled then the abandonIfMergeable option is ineffective and the
+   * auto abandon behaves as though it were set to its default value (true).
    */
   @Test
   @UseClockStep
   @GerritConfig(name = "changeCleanup.abandonAfter", value = "1w")
   @GerritConfig(name = "changeCleanup.abandonIfMergeable", value = "false")
-  @GerritConfig(name = "index.change.indexMergeable", value = "false")
+  @GerritConfig(name = "change.mergeabilityComputationBehavior", value = "NEVER")
   public void abandonedIfMergeableWhenMergeableOperatorIsDisabled() throws Exception {
     ObjectId initial = repo().exactRef(HEAD).getLeaf().getObjectId();
 
diff --git a/javatests/com/google/gerrit/acceptance/api/change/ChangeIT.java b/javatests/com/google/gerrit/acceptance/api/change/ChangeIT.java
index 74f753d..c889984 100644
--- a/javatests/com/google/gerrit/acceptance/api/change/ChangeIT.java
+++ b/javatests/com/google/gerrit/acceptance/api/change/ChangeIT.java
@@ -295,19 +295,7 @@
   }
 
   @Test
-  public void skipMergeable() throws Exception {
-    PushOneCommit.Result r = createChange();
-    String triplet = project.get() + "~master~" + r.getChangeId();
-    ChangeInfo c =
-        gApi.changes().id(triplet).get(ImmutableList.of(ListChangesOption.SKIP_MERGEABLE));
-    assertThat(c.mergeable).isNull();
-
-    c = gApi.changes().id(triplet).get();
-    assertThat(c.mergeable).isTrue();
-  }
-
-  @Test
-  @GerritConfig(name = "change.api.excludeMergeableInChangeInfo", value = "true")
+  @GerritConfig(name = "change.mergeabilityComputationBehavior", value = "NEVER")
   public void excludeMergeableInChangeInfo() throws Exception {
     PushOneCommit.Result r = createChange();
     ChangeInfo c = gApi.changes().id(r.getChangeId()).get();
@@ -3196,20 +3184,19 @@
 
   @Test
   public void createMergePatchSet() throws Exception {
-    PushOneCommit.Result start = pushTo("refs/heads/master");
-    start.assertOkStatus();
-    // create a change for master
-    PushOneCommit.Result r = createChange();
-    r.assertOkStatus();
-    String changeId = r.getChangeId();
+    RevCommit initialHead = projectOperations.project(project).getHead("master");
+    createBranch("dev");
 
-    testRepo.reset(start.getCommit());
+    // create a change for master
+    String changeId = createChange().getChangeId();
+
+    testRepo.reset(initialHead);
     PushOneCommit.Result currentMaster = pushTo("refs/heads/master");
     currentMaster.assertOkStatus();
     String parent = currentMaster.getCommit().getName();
 
     // push a commit into dev branch
-    createBranch("dev");
+    testRepo.reset(initialHead);
     PushOneCommit.Result changeA =
         pushFactory
             .create(user.newIdent(), testRepo, "change A", "A.txt", "A content")
@@ -3230,22 +3217,57 @@
   }
 
   @Test
+  public void createMergePatchSet_Conflict() throws Exception {
+    RevCommit initialHead = projectOperations.project(project).getHead("master");
+    createBranch("dev");
+
+    // create a change for master
+    String changeId = createChange().getChangeId();
+
+    String fileName = "shared.txt";
+    testRepo.reset(initialHead);
+    PushOneCommit.Result currentMaster =
+        pushFactory
+            .create(admin.newIdent(), testRepo, "change 1", fileName, "content 1")
+            .to("refs/heads/master");
+    currentMaster.assertOkStatus();
+
+    // push a commit into dev branch
+    testRepo.reset(initialHead);
+    PushOneCommit.Result changeA =
+        pushFactory
+            .create(user.newIdent(), testRepo, "change 2", fileName, "content 2")
+            .to("refs/heads/dev");
+    changeA.assertOkStatus();
+    MergeInput mergeInput = new MergeInput();
+    mergeInput.source = "dev";
+    MergePatchSetInput in = new MergePatchSetInput();
+    in.merge = mergeInput;
+    in.subject = "update change by merge ps2";
+    ResourceConflictException thrown =
+        assertThrows(
+            ResourceConflictException.class,
+            () -> gApi.changes().id(changeId).createMergePatchSet(in));
+    assertThat(thrown).hasMessageThat().isEqualTo("merge conflict(s):\n" + fileName);
+  }
+
+  @Test
   public void createMergePatchSetInheritParent() throws Exception {
-    PushOneCommit.Result start = pushTo("refs/heads/master");
-    start.assertOkStatus();
+    RevCommit initialHead = projectOperations.project(project).getHead("master");
+    createBranch("dev");
+
     // create a change for master
     PushOneCommit.Result r = createChange();
-    r.assertOkStatus();
     String changeId = r.getChangeId();
     String parent = r.getCommit().getParent(0).getName();
 
     // advance master branch
-    testRepo.reset(start.getCommit());
+    testRepo.reset(initialHead);
     PushOneCommit.Result currentMaster = pushTo("refs/heads/master");
     currentMaster.assertOkStatus();
 
     // push a commit into dev branch
-    createBranch("dev");
+    testRepo.reset(initialHead);
     PushOneCommit.Result changeA =
         pushFactory
             .create(user.newIdent(), testRepo, "change A", "A.txt", "A content")
@@ -3327,6 +3349,46 @@
         .isEqualTo(expectedParent);
   }
 
+  @Test
+  public void createMergePatchSetWithUnupportedMergeStrategy() throws Exception {
+    RevCommit initialHead = projectOperations.project(project).getHead("master");
+    createBranch("dev");
+
+    // create a change for master
+    String changeId = createChange().getChangeId();
+
+    String fileName = "shared.txt";
+    String sourceSubject = "source change";
+    String sourceContent = "source content";
+    String targetSubject = "target change";
+    String targetContent = "target content";
+    testRepo.reset(initialHead);
+    PushOneCommit.Result currentMaster =
+        pushFactory
+            .create(admin.newIdent(), testRepo, targetSubject, fileName, targetContent)
+            .to("refs/heads/master");
+    currentMaster.assertOkStatus();
+
+    // push a commit into dev branch
+    testRepo.reset(initialHead);
+    PushOneCommit.Result changeA =
+        pushFactory
+            .create(user.newIdent(), testRepo, sourceSubject, fileName, sourceContent)
+            .to("refs/heads/dev");
+    changeA.assertOkStatus();
+    MergeInput mergeInput = new MergeInput();
+    mergeInput.source = "dev";
+    mergeInput.strategy = "unsupported-strategy";
+    MergePatchSetInput in = new MergePatchSetInput();
+    in.merge = mergeInput;
+    in.subject = "update change by merge ps2";
+
+    BadRequestException ex =
+        assertThrows(
+            BadRequestException.class, () -> gApi.changes().id(changeId).createMergePatchSet(in));
+    assertThat(ex).hasMessageThat().isEqualTo("invalid merge strategy: " + mergeInput.strategy);
+  }
+
   private MergePatchSetInput createMergePatchSetInput(String baseChange) {
     MergeInput mergeInput = new MergeInput();
     mergeInput.source = "foo";
@@ -4425,7 +4487,6 @@
             ListChangesOption.MESSAGES,
             ListChangesOption.SUBMITTABLE,
             ListChangesOption.WEB_LINKS,
-            ListChangesOption.SKIP_MERGEABLE,
             ListChangesOption.SKIP_DIFFSTAT);
 
     PushOneCommit.Result change = createChange();
@@ -4438,14 +4499,16 @@
   }
 
   @Test
-  @GerritConfig(name = "index.change.indexMergeable", value = "true")
+  @GerritConfig(
+      name = "change.mergeabilityComputationBehavior",
+      value = "API_REF_UPDATED_AND_CHANGE_REINDEX")
   public void changeQueryReturnsMergeableWhenGerritIndexMergeable() throws Exception {
     String changeId = createChange().getChangeId();
     assertThat(gApi.changes().query(changeId).get().get(0).mergeable).isTrue();
   }
 
   @Test
-  @GerritConfig(name = "index.change.indexMergeable", value = "false")
+  @GerritConfig(name = "change.mergeabilityComputationBehavior", value = "NEVER")
   public void changeQueryDoesNotReturnMergeableWhenGerritDoesNotIndexMergeable() throws Exception {
     String changeId = createChange().getChangeId();
     assertThat(gApi.changes().query(changeId).get().get(0).mergeable).isNull();
diff --git a/javatests/com/google/gerrit/acceptance/api/change/PostReviewIT.java b/javatests/com/google/gerrit/acceptance/api/change/PostReviewIT.java
index 524a05e..707076c 100644
--- a/javatests/com/google/gerrit/acceptance/api/change/PostReviewIT.java
+++ b/javatests/com/google/gerrit/acceptance/api/change/PostReviewIT.java
@@ -16,7 +16,10 @@
 
 import static com.google.common.truth.Truth.assertThat;
 import static com.google.gerrit.testing.GerritJUnit.assertThrows;
+import static java.util.stream.Collectors.toList;
 import static org.mockito.ArgumentMatchers.any;
+import static org.mockito.ArgumentMatchers.argThat;
+import static org.mockito.ArgumentMatchers.eq;
 import static org.mockito.Mockito.clearInvocations;
 import static org.mockito.Mockito.mock;
 import static org.mockito.Mockito.when;
@@ -25,19 +28,23 @@
 import com.google.common.collect.ImmutableList;
 import com.google.common.collect.ImmutableMap;
 import com.google.common.collect.Iterables;
+import com.google.common.truth.Correspondence;
 import com.google.gerrit.acceptance.AbstractDaemonTest;
 import com.google.gerrit.acceptance.PushOneCommit;
+import com.google.gerrit.acceptance.config.GerritConfig;
 import com.google.gerrit.extensions.annotations.Exports;
 import com.google.gerrit.extensions.api.changes.DraftInput;
 import com.google.gerrit.extensions.api.changes.ReviewInput;
 import com.google.gerrit.extensions.api.changes.ReviewInput.CommentInput;
 import com.google.gerrit.extensions.api.changes.ReviewInput.DraftHandling;
+import com.google.gerrit.extensions.api.changes.ReviewInput.RobotCommentInput;
 import com.google.gerrit.extensions.client.Side;
 import com.google.gerrit.extensions.common.ChangeMessageInfo;
+import com.google.gerrit.extensions.common.RobotCommentInfo;
 import com.google.gerrit.extensions.config.FactoryModule;
 import com.google.gerrit.extensions.restapi.BadRequestException;
+import com.google.gerrit.extensions.restapi.RestApiException;
 import com.google.gerrit.extensions.validators.CommentForValidation;
-import com.google.gerrit.extensions.validators.CommentForValidation.CommentType;
 import com.google.gerrit.extensions.validators.CommentValidationContext;
 import com.google.gerrit.extensions.validators.CommentValidator;
 import com.google.gerrit.server.restapi.change.PostReview;
@@ -46,9 +53,12 @@
 import com.google.inject.Inject;
 import com.google.inject.Module;
 import java.sql.Timestamp;
+import java.util.Collection;
+import java.util.List;
 import org.junit.Before;
 import org.junit.Test;
 import org.mockito.ArgumentCaptor;
+import org.mockito.ArgumentMatcher;
 import org.mockito.Captor;
 
 /** Tests for comment validation in {@link PostReview}. */
@@ -57,8 +67,43 @@
   @Inject private TestCommentHelper testCommentHelper;
 
   private static final String COMMENT_TEXT = "The comment text";
+  private static final CommentForValidation FILE_COMMENT_FOR_VALIDATION =
+      CommentForValidation.create(
+          CommentForValidation.CommentSource.HUMAN,
+          CommentForValidation.CommentType.FILE_COMMENT,
+          COMMENT_TEXT,
+          COMMENT_TEXT.length());
+  private static final CommentForValidation INLINE_COMMENT_FOR_VALIDATION =
+      CommentForValidation.create(
+          CommentForValidation.CommentSource.HUMAN,
+          CommentForValidation.CommentType.INLINE_COMMENT,
+          COMMENT_TEXT,
+          COMMENT_TEXT.length());
+  private static final CommentForValidation CHANGE_MESSAGE_FOR_VALIDATION =
+      CommentForValidation.create(
+          CommentForValidation.CommentSource.HUMAN,
+          CommentForValidation.CommentType.CHANGE_MESSAGE,
+          COMMENT_TEXT,
+          COMMENT_TEXT.length());
 
-  @Captor private ArgumentCaptor<ImmutableList<CommentForValidation>> capture;
+  @Captor private ArgumentCaptor<ImmutableList<CommentForValidation>> captor;
+
+  private static class SingleCommentMatcher
+      implements ArgumentMatcher<ImmutableList<CommentForValidation>> {
+    final CommentForValidation left;
+
+    SingleCommentMatcher(CommentForValidation left) {
+      this.left = left;
+    }
+
+    @Override
+    public boolean matches(ImmutableList<CommentForValidation> rightList) {
+      CommentForValidation right = Iterables.getOnlyElement(rightList);
+      return left.getSource() == right.getSource()
+          && left.getType() == right.getType()
+          && left.getText().equals(right.getText());
+    }
+  }
 
   @Override
   public Module createModule() {
@@ -84,13 +129,7 @@
   public void validateCommentsInInput_commentOK() throws Exception {
     PushOneCommit.Result r = createChange();
     when(mockCommentValidator.validateComments(
-            CommentValidationContext.builder()
-                .changeId(r.getChange().getId().get())
-                .project(r.getChange().project().get())
-                .build(),
-            ImmutableList.of(
-                CommentForValidation.create(
-                    CommentForValidation.CommentType.FILE_COMMENT, COMMENT_TEXT))))
+            eq(contextFor(r)), argThat(new SingleCommentMatcher(FILE_COMMENT_FOR_VALIDATION))))
         .thenReturn(ImmutableList.of());
 
     ReviewInput input = new ReviewInput();
@@ -107,15 +146,9 @@
   @Test
   public void validateCommentsInInput_commentRejected() throws Exception {
     PushOneCommit.Result r = createChange();
-    CommentForValidation commentForValidation =
-        CommentForValidation.create(CommentType.FILE_COMMENT, COMMENT_TEXT);
     when(mockCommentValidator.validateComments(
-            CommentValidationContext.builder()
-                .changeId(r.getChange().getId().get())
-                .project(r.getChange().project().get())
-                .build(),
-            ImmutableList.of(CommentForValidation.create(CommentType.FILE_COMMENT, COMMENT_TEXT))))
-        .thenReturn(ImmutableList.of(commentForValidation.failValidation("Oh no!")));
+            eq(contextFor(r)), argThat(new SingleCommentMatcher(FILE_COMMENT_FOR_VALIDATION))))
+        .thenReturn(ImmutableList.of(FILE_COMMENT_FOR_VALIDATION.failValidation("Oh no!")));
 
     ReviewInput input = new ReviewInput();
     CommentInput comment = newComment(r.getChange().currentFilePaths().get(0));
@@ -161,19 +194,13 @@
   public void validateDrafts_draftOK() throws Exception {
     PushOneCommit.Result r = createChange();
     when(mockCommentValidator.validateComments(
-            CommentValidationContext.builder()
-                .changeId(r.getChange().getId().get())
-                .project(r.getChange().project().get())
-                .build(),
-            ImmutableList.of(
-                CommentForValidation.create(
-                    CommentForValidation.CommentType.INLINE_COMMENT, COMMENT_TEXT))))
+            eq(contextFor(r)), argThat(new SingleCommentMatcher((INLINE_COMMENT_FOR_VALIDATION)))))
         .thenReturn(ImmutableList.of());
 
     DraftInput draft =
         testCommentHelper.newDraft(
             r.getChange().currentFilePaths().get(0), Side.REVISION, 1, COMMENT_TEXT);
-    gApi.changes().id(r.getChangeId()).revision(r.getCommit().getName()).createDraft(draft).get();
+    gApi.changes().id(r.getChangeId()).revision(r.getCommit().getName()).createDraft(draft);
     assertThat(testCommentHelper.getPublishedComments(r.getChangeId())).isEmpty();
 
     ReviewInput input = new ReviewInput();
@@ -186,17 +213,9 @@
   @Test
   public void validateDrafts_draftRejected() throws Exception {
     PushOneCommit.Result r = createChange();
-    CommentForValidation commentForValidation =
-        CommentForValidation.create(CommentType.INLINE_COMMENT, COMMENT_TEXT);
     when(mockCommentValidator.validateComments(
-            CommentValidationContext.builder()
-                .changeId(r.getChange().getId().get())
-                .project(r.getChange().project().get())
-                .build(),
-            ImmutableList.of(
-                CommentForValidation.create(
-                    CommentForValidation.CommentType.INLINE_COMMENT, COMMENT_TEXT))))
-        .thenReturn(ImmutableList.of(commentForValidation.failValidation("Oh no!")));
+            eq(contextFor(r)), argThat(new SingleCommentMatcher(INLINE_COMMENT_FOR_VALIDATION))))
+        .thenReturn(ImmutableList.of(INLINE_COMMENT_FOR_VALIDATION.failValidation("Oh no!")));
 
     DraftInput draft =
         testCommentHelper.newDraft(
@@ -233,7 +252,7 @@
     testCommentHelper.addDraft(r.getChangeId(), r.getCommit().getName(), draftFile);
     assertThat(testCommentHelper.getPublishedComments(r.getChangeId())).isEmpty();
 
-    when(mockCommentValidator.validateComments(any(), capture.capture()))
+    when(mockCommentValidator.validateComments(any(), captor.capture()))
         .thenReturn(ImmutableList.of());
 
     ReviewInput input = new ReviewInput();
@@ -241,25 +260,33 @@
     gApi.changes().id(r.getChangeId()).current().review(input);
     assertThat(testCommentHelper.getPublishedComments(r.getChangeId())).hasSize(2);
 
-    assertThat(capture.getAllValues()).hasSize(1);
-    assertThat(capture.getValue())
+    assertThat(captor.getAllValues()).hasSize(1);
+    assertThat(captor.getValue())
+        .comparingElementsUsing(
+            Correspondence.<CommentForValidation, CommentForValidation>from(
+                (a, b) ->
+                    a.getSource() == b.getSource()
+                        && a.getType() == b.getType()
+                        && a.getText().equals(b.getText()),
+                "matches (ignoring size approximation)"))
         .containsExactly(
             CommentForValidation.create(
-                CommentForValidation.CommentType.INLINE_COMMENT, draftInline.message),
+                CommentForValidation.CommentSource.HUMAN,
+                CommentForValidation.CommentType.INLINE_COMMENT,
+                draftInline.message,
+                draftInline.message.length()),
             CommentForValidation.create(
-                CommentForValidation.CommentType.FILE_COMMENT, draftFile.message));
+                CommentForValidation.CommentSource.HUMAN,
+                CommentForValidation.CommentType.FILE_COMMENT,
+                draftFile.message,
+                draftFile.message.length()));
   }
 
   @Test
   public void validateCommentsInChangeMessage_messageOK() throws Exception {
     PushOneCommit.Result r = createChange();
     when(mockCommentValidator.validateComments(
-            CommentValidationContext.builder()
-                .changeId(r.getChange().getId().get())
-                .project(r.getChange().project().get())
-                .build(),
-            ImmutableList.of(
-                CommentForValidation.create(CommentType.CHANGE_MESSAGE, COMMENT_TEXT))))
+            contextFor(r), ImmutableList.of(CHANGE_MESSAGE_FOR_VALIDATION)))
         .thenReturn(ImmutableList.of());
 
     ReviewInput input = new ReviewInput().message(COMMENT_TEXT);
@@ -274,16 +301,9 @@
   @Test
   public void validateCommentsInChangeMessage_messageRejected() throws Exception {
     PushOneCommit.Result r = createChange();
-    CommentForValidation commentForValidation =
-        CommentForValidation.create(CommentType.CHANGE_MESSAGE, COMMENT_TEXT);
     when(mockCommentValidator.validateComments(
-            CommentValidationContext.builder()
-                .changeId(r.getChange().getId().get())
-                .project(r.getChange().project().get())
-                .build(),
-            ImmutableList.of(
-                CommentForValidation.create(CommentType.CHANGE_MESSAGE, COMMENT_TEXT))))
-        .thenReturn(ImmutableList.of(commentForValidation.failValidation("Oh no!")));
+            contextFor(r), ImmutableList.of(CHANGE_MESSAGE_FOR_VALIDATION)))
+        .thenReturn(ImmutableList.of(CHANGE_MESSAGE_FOR_VALIDATION.failValidation("Oh no!")));
 
     ReviewInput input = new ReviewInput().message(COMMENT_TEXT);
     assertThat(gApi.changes().id(r.getChangeId()).get().messages)
@@ -308,7 +328,56 @@
     assertThat(message.message).doesNotContain(COMMENT_TEXT);
   }
 
+  @Test
+  @GerritConfig(name = "change.maxComments", value = "4")
+  public void restrictNumberOfComments() throws Exception {
+    when(mockCommentValidator.validateComments(any(), any())).thenReturn(ImmutableList.of());
+
+    PushOneCommit.Result r = createChange();
+    String filePath = r.getChange().currentFilePaths().get(0);
+    CommentInput commentInput = new CommentInput();
+    commentInput.line = 1;
+    commentInput.message = "foo";
+    commentInput.path = filePath;
+    RobotCommentInput robotCommentInput =
+        TestCommentHelper.createRobotCommentInputWithMandatoryFields(filePath);
+    ReviewInput reviewInput = new ReviewInput();
+    reviewInput.comments = ImmutableMap.of(filePath, ImmutableList.of(commentInput));
+    reviewInput.robotComments = ImmutableMap.of(filePath, ImmutableList.of(robotCommentInput));
+    gApi.changes().id(r.getChangeId()).current().review(reviewInput);
+
+    // reviewInput still has both a user and a robot comment (and deduplication is false). We also
+    // create a draft so that in total there would be 5 comments. The limit is set to 4, so this
+    // verifies that all three channels are considered.
+    DraftInput draftInline = testCommentHelper.newDraft(filePath, Side.REVISION, 1, "a draft");
+    testCommentHelper.addDraft(r.getChangeId(), r.getPatchSetId().getId(), draftInline);
+    reviewInput.drafts = DraftHandling.PUBLISH;
+    reviewInput.omitDuplicateComments = false;
+
+    BadRequestException exception =
+        assertThrows(
+            BadRequestException.class,
+            () -> gApi.changes().id(r.getChangeId()).current().review(reviewInput));
+    assertThat(exception)
+        .hasMessageThat()
+        .contains("Exceeding maximum number of comments: 2 (existing) + 3 (new) > 4");
+
+    assertThat(testCommentHelper.getPublishedComments(r.getChangeId())).hasSize(1);
+    assertThat(getRobotComments(r.getChangeId())).hasSize(1);
+  }
+
+  private List<RobotCommentInfo> getRobotComments(String changeId) throws RestApiException {
+    return gApi.changes().id(changeId).robotComments().values().stream()
+        .flatMap(Collection::stream)
+        .collect(toList());
+  }
+
   private static CommentInput newComment(String path) {
     return TestCommentHelper.populate(new CommentInput(), path, PostReviewIT.COMMENT_TEXT);
   }
+
+  private static CommentValidationContext contextFor(PushOneCommit.Result result) {
+    return CommentValidationContext.create(
+        result.getChange().getId().get(), result.getChange().project().get());
+  }
 }
diff --git a/javatests/com/google/gerrit/acceptance/api/revision/RevisionIT.java b/javatests/com/google/gerrit/acceptance/api/revision/RevisionIT.java
index e6b2190..36b7265 100644
--- a/javatests/com/google/gerrit/acceptance/api/revision/RevisionIT.java
+++ b/javatests/com/google/gerrit/acceptance/api/revision/RevisionIT.java
@@ -1102,7 +1102,9 @@
   }
 
   @Test
-  @GerritConfig(name = "index.change.indexMergeable", value = "true")
+  @GerritConfig(
+      name = "change.mergeabilityComputationBehavior",
+      value = "API_REF_UPDATED_AND_CHANGE_REINDEX")
   public void mergeable() throws Exception {
     ObjectId initial = repo().exactRef(HEAD).getLeaf().getObjectId();
 
diff --git a/javatests/com/google/gerrit/acceptance/api/revision/RobotCommentsIT.java b/javatests/com/google/gerrit/acceptance/api/revision/RobotCommentsIT.java
index 9cabe4b..56d164b 100644
--- a/javatests/com/google/gerrit/acceptance/api/revision/RobotCommentsIT.java
+++ b/javatests/com/google/gerrit/acceptance/api/revision/RobotCommentsIT.java
@@ -178,9 +178,8 @@
 
   @Test
   public void hugeRobotCommentIsRejected() {
-    int defaultSizeLimit = 1024 * 1024;
-    int sizeOfRest = 451;
-    fixReplacementInfo.replacement = getStringFor(defaultSizeLimit - sizeOfRest + 1);
+    int defaultSizeLimit = 1 << 20;
+    fixReplacementInfo.replacement = getStringFor(defaultSizeLimit + 1);
 
     BadRequestException thrown =
         assertThrows(
@@ -191,9 +190,9 @@
 
   @Test
   public void reasonablyLargeRobotCommentIsAccepted() throws Exception {
-    int defaultSizeLimit = 1024 * 1024;
-    int sizeOfRest = 451;
-    fixReplacementInfo.replacement = getStringFor(defaultSizeLimit - sizeOfRest);
+    int defaultSizeLimit = 1 << 20;
+    // Allow for a few hundred bytes in other fields.
+    fixReplacementInfo.replacement = getStringFor(defaultSizeLimit - 666);
 
     testCommentHelper.addRobotComment(changeId, withFixRobotCommentInput);
 
@@ -204,7 +203,7 @@
   @Test
   @GerritConfig(name = "change.robotCommentSizeLimit", value = "10k")
   public void maximumAllowedSizeOfRobotCommentCanBeAdjusted() {
-    int sizeLimit = 10 * 1024;
+    int sizeLimit = 10 << 20;
     fixReplacementInfo.replacement = getStringFor(sizeLimit);
 
     BadRequestException thrown =
@@ -217,8 +216,8 @@
   @Test
   @GerritConfig(name = "change.robotCommentSizeLimit", value = "0")
   public void zeroForMaximumAllowedSizeOfRobotCommentRemovesRestriction() throws Exception {
-    int defaultSizeLimit = 1024 * 1024;
-    fixReplacementInfo.replacement = getStringFor(defaultSizeLimit);
+    int defaultSizeLimit = 1 << 20;
+    fixReplacementInfo.replacement = getStringFor(2 * defaultSizeLimit);
 
     testCommentHelper.addRobotComment(changeId, withFixRobotCommentInput);
 
@@ -230,8 +229,8 @@
   @GerritConfig(name = "change.robotCommentSizeLimit", value = "-1")
   public void negativeValueForMaximumAllowedSizeOfRobotCommentRemovesRestriction()
       throws Exception {
-    int defaultSizeLimit = 1024 * 1024;
-    fixReplacementInfo.replacement = getStringFor(defaultSizeLimit);
+    int defaultSizeLimit = 1 << 20;
+    fixReplacementInfo.replacement = getStringFor(2 * defaultSizeLimit);
 
     testCommentHelper.addRobotComment(changeId, withFixRobotCommentInput);
 
diff --git a/javatests/com/google/gerrit/acceptance/rest/change/AbstractSubmit.java b/javatests/com/google/gerrit/acceptance/rest/change/AbstractSubmit.java
index 0c0a8ed..cdffa3c 100644
--- a/javatests/com/google/gerrit/acceptance/rest/change/AbstractSubmit.java
+++ b/javatests/com/google/gerrit/acceptance/rest/change/AbstractSubmit.java
@@ -1205,7 +1205,9 @@
   }
 
   @Test
-  @GerritConfig(name = "index.change.indexMergeable", value = "true")
+  @GerritConfig(
+      name = "change.mergeabilityComputationBehavior",
+      value = "API_REF_UPDATED_AND_CHANGE_REINDEX")
   public void submitSchedulesOpenChangesOfSameBranchForReindexing() throws Throwable {
     // Create a merged change.
     PushOneCommit push =
diff --git a/javatests/com/google/gerrit/acceptance/rest/change/ActionsIT.java b/javatests/com/google/gerrit/acceptance/rest/change/ActionsIT.java
index 911a04d..28b8cae 100644
--- a/javatests/com/google/gerrit/acceptance/rest/change/ActionsIT.java
+++ b/javatests/com/google/gerrit/acceptance/rest/change/ActionsIT.java
@@ -78,22 +78,6 @@
     gApi.changes().id(changeId).current().submit();
     Map<String, ActionInfo> actions = getChangeActions(changeId);
     assertThat(actions).containsKey("revert");
-    assertThat(actions).doesNotContainKey("revert_submission");
-  }
-
-  @Test
-  public void changeActionTwoMergedChangesHaveReverts() throws Exception {
-    String changeId1 = createChangeWithTopic().getChangeId();
-    String changeId2 = createChangeWithTopic().getChangeId();
-    gApi.changes().id(changeId1).current().review(ReviewInput.approve());
-    gApi.changes().id(changeId2).current().review(ReviewInput.approve());
-    gApi.changes().id(changeId2).current().submit();
-    Map<String, ActionInfo> actions1 = getChangeActions(changeId1);
-    assertThat(actions1).containsKey("revert");
-    assertThat(actions1).containsKey("revert_submission");
-    Map<String, ActionInfo> actions2 = getChangeActions(changeId2);
-    assertThat(actions2).containsKey("revert");
-    assertThat(actions2).containsKey("revert_submission");
   }
 
   @Test
diff --git a/javatests/com/google/gerrit/acceptance/rest/change/IndexChangeIT.java b/javatests/com/google/gerrit/acceptance/rest/change/IndexChangeIT.java
index ea3a6a0..61dc4d4 100644
--- a/javatests/com/google/gerrit/acceptance/rest/change/IndexChangeIT.java
+++ b/javatests/com/google/gerrit/acceptance/rest/change/IndexChangeIT.java
@@ -59,7 +59,9 @@
   }
 
   @Test
-  @GerritConfig(name = "index.change.indexMergeable", value = "true")
+  @GerritConfig(
+      name = "change.mergeabilityComputationBehavior",
+      value = "API_REF_UPDATED_AND_CHANGE_REINDEX")
   public void indexChangeAfterOwnerLosesVisibility() throws Exception {
     // Create a test group with 2 users as members
     TestAccount user2 = accountCreator.user2();
diff --git a/javatests/com/google/gerrit/acceptance/rest/config/ServerInfoIT.java b/javatests/com/google/gerrit/acceptance/rest/config/ServerInfoIT.java
index 9573eb0..14a04cd 100644
--- a/javatests/com/google/gerrit/acceptance/rest/config/ServerInfoIT.java
+++ b/javatests/com/google/gerrit/acceptance/rest/config/ServerInfoIT.java
@@ -169,7 +169,8 @@
     assertThat(i.change.updateDelay).isEqualTo(300);
     assertThat(i.change.disablePrivateChanges).isNull();
     assertThat(i.change.submitWholeTopic).isNull();
-    assertThat(i.change.excludeMergeableInChangeInfo).isNull();
+    assertThat(i.change.mergeabilityComputationBehavior)
+        .isEqualTo("API_REF_UPDATED_AND_CHANGE_REINDEX");
 
     // download
     assertThat(i.download.archives).containsExactly("tar", "tbz2", "tgz", "txz");
@@ -180,9 +181,6 @@
     assertThat(i.gerrit.allUsers).isEqualTo(AllUsersNameProvider.DEFAULT);
     assertThat(i.gerrit.reportBugUrl).isNull();
 
-    // index
-    assertThat(i.index.change.indexMergeable).isNull(); // false in all tests
-
     // plugin
     assertThat(i.plugin.jsResourcePaths).isEmpty();
 
@@ -204,16 +202,9 @@
   }
 
   @Test
-  @GerritConfig(name = "change.api.excludeMergeableInChangeInfo", value = "true")
-  public void serverConfigWithExcludeMergeableInChangeInfo() throws Exception {
+  @GerritConfig(name = "change.mergeabilityComputationBehavior", value = "NEVER")
+  public void mergeabilityComputationBehavior_neverCompute() throws Exception {
     ServerInfo i = gApi.config().server().getInfo();
-    assertThat(i.change.excludeMergeableInChangeInfo).isTrue();
-  }
-
-  @Test
-  @GerritConfig(name = "index.change.indexMergeable", value = "true")
-  public void indexMergeableIsTrueWhenTrueInConfig() throws Exception {
-    ServerInfo i = gApi.config().server().getInfo();
-    assertThat(i.index.change.indexMergeable).isTrue();
+    assertThat(i.change.mergeabilityComputationBehavior).isEqualTo("NEVER");
   }
 }
diff --git a/javatests/com/google/gerrit/acceptance/server/event/EventPayloadIT.java b/javatests/com/google/gerrit/acceptance/server/event/EventPayloadIT.java
index d973934..3bc863f 100644
--- a/javatests/com/google/gerrit/acceptance/server/event/EventPayloadIT.java
+++ b/javatests/com/google/gerrit/acceptance/server/event/EventPayloadIT.java
@@ -32,32 +32,26 @@
   @Test
   public void defaultOptions() throws Exception {
     RevisionCreatedListener listener =
-        new RevisionCreatedListener() {
-          @Override
-          public void onRevisionCreated(Event event) {
-            assertThat(event.getChange().submittable).isNotNull();
-            assertThat(event.getRevision().files).isNotEmpty();
-          }
+        event -> {
+          assertThat(event.getChange().submittable).isNotNull();
+          assertThat(event.getRevision().files).isNotEmpty();
         };
-    try (Registration registration = extensionRegistry.newRegistration().add(listener)) {
+    try (Registration ignored = extensionRegistry.newRegistration().add(listener)) {
       createChange();
     }
   }
 
   @Test
-  @GerritConfig(name = "event.payload.listChangeOptions", value = "SKIP_MERGEABLE")
+  @GerritConfig(name = "event.payload.listChangeOptions", value = "SKIP_DIFFSTAT")
   public void configuredOptions() throws Exception {
     RevisionCreatedListener listener =
-        new RevisionCreatedListener() {
-          @Override
-          public void onRevisionCreated(Event event) {
-            assertThat(event.getChange().submittable).isNull();
-            assertThat(event.getChange().mergeable).isNull();
-            assertThat(event.getRevision().files).isNull();
-            assertThat(event.getChange().subject).isNotEmpty();
-          }
+        event -> {
+          assertThat(event.getChange().submittable).isNull();
+          assertThat(event.getChange().insertions).isNull();
+          assertThat(event.getRevision().files).isNull();
+          assertThat(event.getChange().subject).isNotEmpty();
         };
-    try (Registration registration = extensionRegistry.newRegistration().add(listener)) {
+    try (Registration ignored = extensionRegistry.newRegistration().add(listener)) {
       createChange();
     }
   }
diff --git a/javatests/com/google/gerrit/acceptance/server/git/receive/ReceiveCommitsCommentValidationIT.java b/javatests/com/google/gerrit/acceptance/server/git/receive/ReceiveCommitsCommentValidationIT.java
index ccfe783..eb1f907 100644
--- a/javatests/com/google/gerrit/acceptance/server/git/receive/ReceiveCommitsCommentValidationIT.java
+++ b/javatests/com/google/gerrit/acceptance/server/git/receive/ReceiveCommitsCommentValidationIT.java
@@ -31,7 +31,6 @@
 import com.google.gerrit.extensions.client.Side;
 import com.google.gerrit.extensions.config.FactoryModule;
 import com.google.gerrit.extensions.validators.CommentForValidation;
-import com.google.gerrit.extensions.validators.CommentForValidation.CommentType;
 import com.google.gerrit.extensions.validators.CommentValidationContext;
 import com.google.gerrit.extensions.validators.CommentValidator;
 import com.google.gerrit.testing.TestCommentHelper;
@@ -49,9 +48,15 @@
   @Inject private CommentValidator mockCommentValidator;
   @Inject private TestCommentHelper testCommentHelper;
 
-  private static final int MAX_COMMENT_LENGTH = 666;
+  private static final int COMMENT_SIZE_LIMIT = 666;
 
   private static final String COMMENT_TEXT = "The comment text";
+  private static final CommentForValidation COMMENT_FOR_VALIDATION =
+      CommentForValidation.create(
+          CommentForValidation.CommentSource.HUMAN,
+          CommentForValidation.CommentType.FILE_COMMENT,
+          COMMENT_TEXT,
+          COMMENT_TEXT.length());
 
   @Captor private ArgumentCaptor<ImmutableList<CommentForValidation>> capture;
   @Captor private ArgumentCaptor<CommentValidationContext> captureCtx;
@@ -82,13 +87,9 @@
     String changeId = result.getChangeId();
     String revId = result.getCommit().getName();
     when(mockCommentValidator.validateComments(
-            CommentValidationContext.builder()
-                .changeId(result.getChange().getId().get())
-                .project(result.getChange().project().get())
-                .build(),
-            ImmutableList.of(
-                CommentForValidation.create(
-                    CommentForValidation.CommentType.FILE_COMMENT, COMMENT_TEXT))))
+            CommentValidationContext.create(
+                result.getChange().getId().get(), result.getChange().project().get()),
+            ImmutableList.of(COMMENT_FOR_VALIDATION)))
         .thenReturn(ImmutableList.of());
     DraftInput comment = testCommentHelper.newDraft(COMMENT_TEXT);
     testCommentHelper.addDraft(changeId, revId, comment);
@@ -101,20 +102,14 @@
 
   @Test
   public void validateComments_commentRejected() throws Exception {
-    CommentForValidation commentForValidation =
-        CommentForValidation.create(CommentType.FILE_COMMENT, COMMENT_TEXT);
     PushOneCommit.Result result = createChange();
     String changeId = result.getChangeId();
     String revId = result.getCommit().getName();
     when(mockCommentValidator.validateComments(
-            CommentValidationContext.builder()
-                .changeId(result.getChange().getId().get())
-                .project(result.getChange().project().get())
-                .build(),
-            ImmutableList.of(
-                CommentForValidation.create(
-                    CommentForValidation.CommentType.FILE_COMMENT, COMMENT_TEXT))))
-        .thenReturn(ImmutableList.of(commentForValidation.failValidation("Oh no!")));
+            CommentValidationContext.create(
+                result.getChange().getId().get(), result.getChange().project().get()),
+            ImmutableList.of(COMMENT_FOR_VALIDATION)))
+        .thenReturn(ImmutableList.of(COMMENT_FOR_VALIDATION.failValidation("Oh no!")));
     DraftInput comment = testCommentHelper.newDraft(COMMENT_TEXT);
     testCommentHelper.addDraft(changeId, revId, comment);
     assertThat(testCommentHelper.getPublishedComments(result.getChangeId())).isEmpty();
@@ -149,18 +144,24 @@
     assertThat(capture.getAllValues().get(0))
         .containsExactly(
             CommentForValidation.create(
-                CommentForValidation.CommentType.INLINE_COMMENT, draftInline.message),
+                CommentForValidation.CommentSource.HUMAN,
+                CommentForValidation.CommentType.INLINE_COMMENT,
+                draftInline.message,
+                draftInline.message.length()),
             CommentForValidation.create(
-                CommentForValidation.CommentType.FILE_COMMENT, draftFile.message));
+                CommentForValidation.CommentSource.HUMAN,
+                CommentForValidation.CommentType.FILE_COMMENT,
+                draftFile.message,
+                draftFile.message.length()));
   }
 
   @Test
-  @GerritConfig(name = "change.maxCommentLength", value = "" + MAX_COMMENT_LENGTH)
+  @GerritConfig(name = "change.commentSizeLimit", value = "" + COMMENT_SIZE_LIMIT)
   public void validateComments_enforceLimits_commentTooLarge() throws Exception {
     when(mockCommentValidator.validateComments(any(), any())).thenReturn(ImmutableList.of());
     PushOneCommit.Result result = createChange();
     String changeId = result.getChangeId();
-    int commentLength = MAX_COMMENT_LENGTH + 1;
+    int commentLength = COMMENT_SIZE_LIMIT + 1;
     DraftInput comment =
         testCommentHelper.newDraft(new String(new char[commentLength]).replace("\0", "x"));
     testCommentHelper.addDraft(changeId, result.getCommit().getName(), comment);
@@ -169,7 +170,33 @@
     Result amendResult = amendChange(changeId, "refs/for/master%publish-comments", admin, testRepo);
     amendResult.assertOkStatus();
     amendResult.assertMessage(
-        String.format("Comment too large (%d > %d)", commentLength, MAX_COMMENT_LENGTH));
+        String.format("Comment size exceeds limit (%d > %d)", commentLength, COMMENT_SIZE_LIMIT));
     assertThat(testCommentHelper.getPublishedComments(result.getChangeId())).isEmpty();
   }
+
+  @Test
+  @GerritConfig(name = "change.maxComments", value = "3")
+  public void countComments_limitNumberOfComments() throws Exception {
+    when(mockCommentValidator.validateComments(any(), any())).thenReturn(ImmutableList.of());
+    PushOneCommit.Result result = createChange();
+    String changeId = result.getChangeId();
+    String revId = result.getCommit().getName();
+    String filePath = result.getChange().currentFilePaths().get(0);
+    DraftInput draftInline = testCommentHelper.newDraft(filePath, Side.REVISION, 1, COMMENT_TEXT);
+    testCommentHelper.addDraft(changeId, revId, draftInline);
+    amendChange(changeId, "refs/for/master%publish-comments", admin, testRepo);
+    assertThat(testCommentHelper.getPublishedComments(result.getChangeId())).hasSize(1);
+
+    for (int i = 1; i < 3; ++i) {
+      testCommentHelper.addRobotComment(
+          changeId,
+          TestCommentHelper.createRobotCommentInput(result.getChange().currentFilePaths().get(0)));
+    }
+
+    draftInline = testCommentHelper.newDraft(filePath, Side.REVISION, 1, COMMENT_TEXT);
+    testCommentHelper.addDraft(changeId, revId, draftInline);
+    Result amendResult = amendChange(changeId, "refs/for/master%publish-comments", admin, testRepo);
+    assertThat(testCommentHelper.getPublishedComments(result.getChangeId())).hasSize(1);
+    amendResult.assertMessage("exceeding maximum number of comments");
+  }
 }
diff --git a/javatests/com/google/gerrit/acceptance/server/git/receive/ReceiveCommitsLimitsIT.java b/javatests/com/google/gerrit/acceptance/server/git/receive/ReceiveCommitsLimitsIT.java
new file mode 100644
index 0000000..c32827a
--- /dev/null
+++ b/javatests/com/google/gerrit/acceptance/server/git/receive/ReceiveCommitsLimitsIT.java
@@ -0,0 +1,67 @@
+// Copyright (C) 2020 The Android Open Source Project
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package com.google.gerrit.acceptance.server.git.receive;
+
+import static com.google.common.truth.Truth.assertThat;
+
+import com.google.common.cache.Cache;
+import com.google.common.collect.ImmutableMap;
+import com.google.gerrit.acceptance.AbstractDaemonTest;
+import com.google.gerrit.acceptance.PushOneCommit;
+import com.google.gerrit.acceptance.config.GerritConfig;
+import com.google.gerrit.server.patch.DiffSummary;
+import com.google.gerrit.server.patch.DiffSummaryKey;
+import com.google.inject.Inject;
+import com.google.inject.name.Named;
+import org.junit.Test;
+
+/** Tests for applying limits to e.g. number of files per change. */
+public class ReceiveCommitsLimitsIT extends AbstractDaemonTest {
+
+  @Inject
+  private @Named("diff_summary") Cache<DiffSummaryKey, DiffSummary> diffSummaryCache;
+
+  @Test
+  @GerritConfig(name = "change.maxFiles", value = "1")
+  public void limitFileCount() throws Exception {
+    PushOneCommit.Result result =
+        pushFactory
+            .create(
+                admin.newIdent(),
+                testRepo,
+                "foo",
+                ImmutableMap.of("foo", "foo-1.0", "bar", "bar-1.0"))
+            .to("refs/for/master");
+    result.assertErrorStatus("Exceeding maximum number of files per change (2 > 1)");
+  }
+
+  @Test
+  public void cacheKeyMatches() throws Exception {
+    int cacheSizeBefore = diffSummaryCache.asMap().size();
+    PushOneCommit.Result result =
+        pushFactory
+            .create(
+                admin.newIdent(),
+                testRepo,
+                "foo",
+                ImmutableMap.of("foo", "foo-1.0", "bar", "bar-1.0"))
+            .to("refs/for/master");
+    result.assertOkStatus();
+
+    // Assert that we only performed the diff computation once. This would e.g. catch
+    // bugs/deviations in the computation of the cache key.
+    assertThat(diffSummaryCache.asMap()).hasSize(cacheSizeBefore + 1);
+  }
+}
diff --git a/javatests/com/google/gerrit/acceptance/server/mail/AbstractMailIT.java b/javatests/com/google/gerrit/acceptance/server/mail/AbstractMailIT.java
index 768c269..3b0e27b 100644
--- a/javatests/com/google/gerrit/acceptance/server/mail/AbstractMailIT.java
+++ b/javatests/com/google/gerrit/acceptance/server/mail/AbstractMailIT.java
@@ -33,7 +33,9 @@
 public class AbstractMailIT extends AbstractDaemonTest {
   @Inject private RequestScopeOperations requestScopeOperations;
 
-  protected MailMessage.Builder messageBuilderWithDefaultFields() {
+  static final String FILE_NAME = "gerrit-server/test.txt";
+
+  MailMessage.Builder messageBuilderWithDefaultFields() {
     MailMessage.Builder b = MailMessage.builder();
     b.id("some id");
     b.from(user.getEmailAddress());
@@ -43,16 +45,15 @@
     return b;
   }
 
-  protected String createChangeWithReview() throws Exception {
+  String createChangeWithReview() throws Exception {
     return createChangeWithReview(admin);
   }
 
-  protected String createChangeWithReview(TestAccount reviewer) throws Exception {
+  String createChangeWithReview(TestAccount reviewer) throws Exception {
     // Create change
-    String file = "gerrit-server/test.txt";
     String contents = "contents \nlorem \nipsum \nlorem";
     PushOneCommit push =
-        pushFactory.create(admin.newIdent(), testRepo, "first subject", file, contents);
+        pushFactory.create(admin.newIdent(), testRepo, "first subject", FILE_NAME, contents);
     PushOneCommit.Result r = push.to("refs/for/master");
     String changeId = r.getChangeId();
 
@@ -61,8 +62,8 @@
     ReviewInput input = new ReviewInput();
     input.message = "I have two comments";
     input.comments = new HashMap<>();
-    CommentInput c1 = newComment(file, Side.REVISION, 0, "comment on file");
-    CommentInput c2 = newComment(file, Side.REVISION, 2, "inline comment");
+    CommentInput c1 = newComment(FILE_NAME, Side.REVISION, 0, "comment on file");
+    CommentInput c2 = newComment(FILE_NAME, Side.REVISION, 2, "inline comment");
     input.comments.put(c1.path, ImmutableList.of(c1, c2));
     revision(r).review(input);
     return changeId;
@@ -94,7 +95,7 @@
    * @param fc1 Comment in reply to a comment of file 1.
    * @return A string with all inline comments and the original quoted email.
    */
-  protected static String newPlaintextBody(
+  static String newPlaintextBody(
       String changeURL, String changeMessage, String c1, String f1, String fc1) {
     return (changeMessage == null ? "" : changeMessage + "\n")
         + "> Foo Bar has posted comments on this change. (  \n"
@@ -137,7 +138,7 @@
         + "> \n";
   }
 
-  protected static String textFooterForChange(int changeNumber, String timestamp) {
+  static String textFooterForChange(int changeNumber, String timestamp) {
     return "Gerrit-Change-Number: "
         + changeNumber
         + "\n"
diff --git a/javatests/com/google/gerrit/acceptance/server/mail/MailProcessorIT.java b/javatests/com/google/gerrit/acceptance/server/mail/MailProcessorIT.java
index 2409f52..502194a 100644
--- a/javatests/com/google/gerrit/acceptance/server/mail/MailProcessorIT.java
+++ b/javatests/com/google/gerrit/acceptance/server/mail/MailProcessorIT.java
@@ -21,9 +21,14 @@
 import static org.mockito.Mockito.when;
 
 import com.google.common.collect.ImmutableList;
+import com.google.common.collect.ImmutableMap;
 import com.google.common.collect.Iterables;
+import com.google.gerrit.acceptance.config.GerritConfig;
 import com.google.gerrit.acceptance.testsuite.account.AccountOperations;
 import com.google.gerrit.extensions.annotations.Exports;
+import com.google.gerrit.extensions.api.changes.ReviewInput;
+import com.google.gerrit.extensions.api.changes.ReviewInput.CommentInput;
+import com.google.gerrit.extensions.api.changes.ReviewInput.RobotCommentInput;
 import com.google.gerrit.extensions.common.ChangeInfo;
 import com.google.gerrit.extensions.common.ChangeMessageInfo;
 import com.google.gerrit.extensions.common.CommentInfo;
@@ -341,17 +346,53 @@
     assertThat(message.body()).contains("rejected one or more comments");
   }
 
+  @Test
+  @GerritConfig(name = "change.maxComments", value = "4")
+  public void limitNumberOfComments() throws Exception {
+    String changeId = createChangeWithReview();
+    CommentInput commentInput = new CommentInput();
+    commentInput.line = 1;
+    commentInput.message = "foo";
+    commentInput.path = FILE_NAME;
+    RobotCommentInput robotCommentInput =
+        TestCommentHelper.createRobotCommentInputWithMandatoryFields(FILE_NAME);
+    ReviewInput reviewInput = new ReviewInput();
+    reviewInput.comments = ImmutableMap.of(FILE_NAME, ImmutableList.of(commentInput));
+    reviewInput.robotComments = ImmutableMap.of(FILE_NAME, ImmutableList.of(robotCommentInput));
+    gApi.changes().id(changeId).current().review(reviewInput);
+
+    ChangeInfo changeInfo = gApi.changes().id(changeId).get();
+    List<CommentInfo> comments = gApi.changes().id(changeId).current().commentsAsList();
+    String ts =
+        MailProcessingUtil.rfcDateformatter.format(
+            ZonedDateTime.ofInstant(comments.get(0).updated.toInstant(), ZoneId.of("UTC")));
+
+    MailMessage.Builder b = messageBuilderWithDefaultFields();
+    String txt = newPlaintextBody(getChangeUrl(changeInfo) + "/1", null, null, COMMENT_TEXT, null);
+    b.textContent(txt + textFooterForChange(changeInfo._number, ts));
+
+    Collection<CommentInfo> commentsBefore = testCommentHelper.getPublishedComments(changeId);
+    mailProcessor.process(b.build());
+    assertThat(testCommentHelper.getPublishedComments(changeId)).isEqualTo(commentsBefore);
+
+    assertNotifyTo(user);
+    Message message = sender.nextMessage();
+    assertThat(message.body()).contains("rejected one or more comments");
+  }
+
   private String getChangeUrl(ChangeInfo changeInfo) {
     return canonicalWebUrl.get() + "c/" + changeInfo.project + "/+/" + changeInfo._number;
   }
 
   private void setupFailValidation(
       CommentForValidation.CommentType type, String failProject, int failChange) {
-    CommentForValidation commentForValidation = CommentForValidation.create(type, COMMENT_TEXT);
+    CommentForValidation commentForValidation =
+        CommentForValidation.create(
+            CommentForValidation.CommentSource.HUMAN, type, COMMENT_TEXT, COMMENT_TEXT.length());
 
     when(mockCommentValidator.validateComments(
-            CommentValidationContext.builder().changeId(failChange).project(failProject).build(),
-            ImmutableList.of(CommentForValidation.create(type, COMMENT_TEXT))))
+            CommentValidationContext.create(failChange, failProject),
+            ImmutableList.of(commentForValidation)))
         .thenReturn(ImmutableList.of(commentForValidation.failValidation("Oh no!")));
   }
 }
diff --git a/javatests/com/google/gerrit/pgm/BUILD b/javatests/com/google/gerrit/pgm/BUILD
index 0cae937..b68aacc 100644
--- a/javatests/com/google/gerrit/pgm/BUILD
+++ b/javatests/com/google/gerrit/pgm/BUILD
@@ -5,7 +5,7 @@
     name = "pgm_tests",
     srcs = glob(["**/*.java"]),
     deps = [
-        "//java/com/google/gerrit/pgm/http",
+        "//java/com/google/gerrit/pgm/http/jetty",
         "//java/com/google/gerrit/pgm/init/api",
         "//java/com/google/gerrit/server",
         "//java/com/google/gerrit/server/securestore/testing",
diff --git a/javatests/com/google/gerrit/server/git/DeleteZombieCommentsRefsTest.java b/javatests/com/google/gerrit/server/git/DeleteZombieCommentsRefsTest.java
new file mode 100644
index 0000000..bfcb9e4
--- /dev/null
+++ b/javatests/com/google/gerrit/server/git/DeleteZombieCommentsRefsTest.java
@@ -0,0 +1,247 @@
+// Copyright (C) 2020 The Android Open Source Project
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package com.google.gerrit.server.git;
+
+import static com.google.common.collect.ImmutableList.toImmutableList;
+import static com.google.common.truth.Truth.assertThat;
+import static java.nio.charset.StandardCharsets.UTF_8;
+
+import com.google.gerrit.entities.Account;
+import com.google.gerrit.entities.Change;
+import com.google.gerrit.entities.Project;
+import com.google.gerrit.entities.RefNames;
+import com.google.gerrit.git.RefUpdateUtil;
+import com.google.gerrit.server.config.AllUsersName;
+import com.google.gerrit.server.notedb.DeleteZombieCommentsRefs;
+import com.google.gerrit.server.util.time.TimeUtil;
+import com.google.gerrit.testing.InMemoryRepositoryManager;
+import java.io.IOException;
+import java.util.ArrayList;
+import java.util.List;
+import org.eclipse.jgit.lib.BatchRefUpdate;
+import org.eclipse.jgit.lib.CommitBuilder;
+import org.eclipse.jgit.lib.Constants;
+import org.eclipse.jgit.lib.ObjectId;
+import org.eclipse.jgit.lib.ObjectInserter;
+import org.eclipse.jgit.lib.PersonIdent;
+import org.eclipse.jgit.lib.Ref;
+import org.eclipse.jgit.lib.RefUpdate;
+import org.eclipse.jgit.lib.Repository;
+import org.eclipse.jgit.lib.TreeFormatter;
+import org.eclipse.jgit.revwalk.RevBlob;
+import org.eclipse.jgit.revwalk.RevWalk;
+import org.eclipse.jgit.transport.ReceiveCommand;
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.junit.runners.JUnit4;
+
+@RunWith(JUnit4.class)
+public class DeleteZombieCommentsRefsTest {
+  private InMemoryRepositoryManager repoManager = new InMemoryRepositoryManager();
+  private Project.NameKey allUsersProject = Project.nameKey("All-Users");
+
+  @Test
+  public void cleanZombieDraftsSmall() throws Exception {
+    try (Repository usersRepo = repoManager.createRepository(allUsersProject)) {
+      Ref ref1 = createRefWithNonEmptyTreeCommit(usersRepo, 1, 1000001);
+      Ref ref2 = createRefWithEmptyTreeCommit(usersRepo, 1, 1000002);
+
+      DeleteZombieCommentsRefs clean =
+          new DeleteZombieCommentsRefs(new AllUsersName("All-Users"), repoManager, null);
+      clean.execute();
+
+      /* Check that ref1 still exists, and ref2 is deleted */
+      assertThat(usersRepo.exactRef(ref1.getName())).isNotNull();
+      assertThat(usersRepo.exactRef(ref2.getName())).isNull();
+    }
+  }
+
+  @Test
+  public void cleanZombieDraftsWithPercentage() throws Exception {
+    try (Repository usersRepo = repoManager.createRepository(allUsersProject)) {
+      Ref ref1 = createRefWithNonEmptyTreeCommit(usersRepo, 1005, 1000001);
+      Ref ref2 = createRefWithEmptyTreeCommit(usersRepo, 1006, 1000002);
+      Ref ref3 = createRefWithEmptyTreeCommit(usersRepo, 1060, 1000002);
+
+      assertThat(usersRepo.getRefDatabase().getRefs()).hasSize(3);
+
+      int cleanupPercentage = 50;
+      DeleteZombieCommentsRefs clean =
+          new DeleteZombieCommentsRefs(
+              new AllUsersName("All-Users"), repoManager, cleanupPercentage);
+      clean.execute();
+
+      /* ref1 not deleted, ref2 deleted, ref3 not deleted because of the clean percentage */
+      assertThat(usersRepo.getRefDatabase().getRefs()).hasSize(2);
+      assertThat(usersRepo.exactRef(ref1.getName())).isNotNull();
+      assertThat(usersRepo.exactRef(ref2.getName())).isNull();
+      assertThat(usersRepo.exactRef(ref3.getName())).isNotNull();
+
+      /* Re-execute the cleanup and make sure nothing's changed */
+      clean.execute();
+      assertThat(usersRepo.getRefDatabase().getRefs()).hasSize(2);
+      assertThat(usersRepo.exactRef(ref1.getName())).isNotNull();
+      assertThat(usersRepo.exactRef(ref2.getName())).isNull();
+      assertThat(usersRepo.exactRef(ref3.getName())).isNotNull();
+
+      /* Increase the cleanup percentage */
+      cleanupPercentage = 70;
+      clean =
+          new DeleteZombieCommentsRefs(
+              new AllUsersName("All-Users"), repoManager, cleanupPercentage);
+
+      clean.execute();
+
+      /* Now ref3 is deleted */
+      assertThat(usersRepo.getRefDatabase().getRefs()).hasSize(1);
+      assertThat(usersRepo.exactRef(ref1.getName())).isNotNull();
+      assertThat(usersRepo.exactRef(ref2.getName())).isNull();
+      assertThat(usersRepo.exactRef(ref3.getName())).isNull();
+    }
+  }
+
+  @Test
+  public void cleanZombieDraftsLarge() throws Exception {
+    try (Repository usersRepo = repoManager.createRepository(allUsersProject)) {
+      int goodRefsCnt = 5000;
+      int zombieRefsCnt = 5000;
+      int userIdGoodRefs = 1000001;
+      int userIdBadRefs = 1000002;
+
+      Ref nonEmptyBaseRef = createRefWithNonEmptyTreeCommit(usersRepo, 1, userIdGoodRefs);
+      Ref emptyBaseRef = createRefWithEmptyTreeCommit(usersRepo, 1, userIdBadRefs);
+
+      List<String> goodRefs =
+          createNRefsOnCommit(
+              usersRepo, nonEmptyBaseRef.getObjectId(), goodRefsCnt, userIdGoodRefs);
+      List<String> badRefs =
+          createNRefsOnCommit(usersRepo, emptyBaseRef.getObjectId(), zombieRefsCnt, userIdBadRefs);
+
+      goodRefs.add(0, nonEmptyBaseRef.getName());
+      badRefs.add(0, emptyBaseRef.getName());
+
+      assertThat(usersRepo.getRefDatabase().getRefs().size())
+          .isEqualTo(goodRefs.size() + badRefs.size());
+
+      DeleteZombieCommentsRefs clean =
+          new DeleteZombieCommentsRefs(new AllUsersName("All-Users"), repoManager, null);
+      clean.execute();
+
+      assertThat(
+              usersRepo.getRefDatabase().getRefs().stream()
+                  .map(Ref::getName)
+                  .collect(toImmutableList()))
+          .containsExactlyElementsIn(goodRefs);
+
+      assertThat(
+              usersRepo.getRefDatabase().getRefs().stream()
+                  .map(Ref::getName)
+                  .collect(toImmutableList()))
+          .containsNoneIn(badRefs);
+    }
+  }
+
+  private static List<String> createNRefsOnCommit(
+      Repository usersRepo, ObjectId commitId, int n, int uuid) throws IOException {
+    List<String> refNames = new ArrayList<>();
+    BatchRefUpdate bru = usersRepo.getRefDatabase().newBatchUpdate();
+    bru.setAtomic(true);
+    for (int i = 2; i <= n + 1; i++) {
+      String refName = getRefName(i, uuid);
+      bru.addCommand(
+          new ReceiveCommand(ObjectId.zeroId(), commitId, refName, ReceiveCommand.Type.CREATE));
+      refNames.add(refName);
+    }
+    RefUpdateUtil.executeChecked(bru, usersRepo);
+    return refNames;
+  }
+
+  private static String getRefName(int changeId, int userId) {
+    Change.Id cId = Change.id(changeId);
+    Account.Id aId = Account.id(userId);
+    return RefNames.refsDraftComments(cId, aId);
+  }
+
+  private static Ref createRefWithNonEmptyTreeCommit(Repository usersRepo, int changeId, int userId)
+      throws IOException {
+    RevWalk rw = new RevWalk(usersRepo);
+    ObjectId fileObj = createBlob(usersRepo, String.format("file %d content", changeId));
+    ObjectId treeObj =
+        createTree(usersRepo, rw.lookupBlob(fileObj), String.format("file%d.txt", changeId));
+    ObjectId commitObj = createCommit(usersRepo, treeObj, null);
+    Ref refObj = createRef(usersRepo, commitObj, getRefName(changeId, userId));
+    return refObj;
+  }
+
+  private static Ref createRefWithEmptyTreeCommit(Repository usersRepo, int changeId, int userId)
+      throws IOException {
+    ObjectId treeEmpty = createTree(usersRepo, null, "");
+    ObjectId commitObj = createCommit(usersRepo, treeEmpty, null);
+    Ref refObj = createRef(usersRepo, commitObj, getRefName(changeId, userId));
+    return refObj;
+  }
+
+  private static Ref createRef(Repository repo, ObjectId commitId, String refName)
+      throws IOException {
+    RefUpdate update = repo.updateRef(refName);
+    update.setNewObjectId(commitId);
+    update.setForceUpdate(true);
+    update.update();
+    return repo.exactRef(refName);
+  }
+
+  private static ObjectId createCommit(Repository repo, ObjectId treeId, ObjectId parentCommit)
+      throws IOException {
+    try (ObjectInserter oi = repo.newObjectInserter()) {
+      PersonIdent committer =
+          new PersonIdent(new PersonIdent("Foo Bar", "foo.bar@baz.com"), TimeUtil.nowTs());
+      CommitBuilder cb = new CommitBuilder();
+      cb.setTreeId(treeId);
+      cb.setCommitter(committer);
+      cb.setAuthor(committer);
+      cb.setMessage("Test commit");
+      if (parentCommit != null) {
+        cb.setParentIds(parentCommit);
+      }
+      ObjectId commitId = oi.insert(cb);
+      oi.flush();
+      oi.close();
+      return commitId;
+    }
+  }
+
+  private static ObjectId createTree(Repository repo, RevBlob blob, String blobName)
+      throws IOException {
+    try (ObjectInserter oi = repo.newObjectInserter()) {
+      TreeFormatter formatter = new TreeFormatter();
+      if (blob != null) {
+        formatter.append(blobName, blob);
+      }
+      ObjectId treeId = oi.insert(formatter);
+      oi.flush();
+      oi.close();
+      return treeId;
+    }
+  }
+
+  private static ObjectId createBlob(Repository repo, String content) throws IOException {
+    try (ObjectInserter oi = repo.newObjectInserter()) {
+      ObjectId blobId = oi.insert(Constants.OBJ_BLOB, content.getBytes(UTF_8));
+      oi.flush();
+      oi.close();
+      return blobId;
+    }
+  }
+}
diff --git a/javatests/com/google/gerrit/server/query/change/AbstractQueryChangesTest.java b/javatests/com/google/gerrit/server/query/change/AbstractQueryChangesTest.java
index 9c12052..38e4ca4 100644
--- a/javatests/com/google/gerrit/server/query/change/AbstractQueryChangesTest.java
+++ b/javatests/com/google/gerrit/server/query/change/AbstractQueryChangesTest.java
@@ -2025,7 +2025,9 @@
   }
 
   @Test
-  @GerritConfig(name = "index.change.indexMergeable", value = "true")
+  @GerritConfig(
+      name = "change.mergeabilityComputationBehavior",
+      value = "API_REF_UPDATED_AND_CHANGE_REINDEX")
   public void mergeable() throws Exception {
     TestRepository<Repo> repo = createProject("repo");
     RevCommit commit1 = repo.parseBody(repo.commit().add("file1", "contents1").create());
@@ -2043,7 +2045,7 @@
     // If a change gets submitted, the remaining open changes get reindexed asynchronously to update
     // their mergeability information. If the further assertions in this test are done before the
     // asynchronous reindex completed they fail because the mergeability information in the index
-    // was not updated yet. To avoid this flakiness indexMergeable is switched off for the
+    // was not updated yet. To avoid this flakiness indexing mergeable is switched off for the
     // tests and we index change2 synchronously here.
     gApi.changes().id(change2.getChangeId()).index();
 
@@ -3083,7 +3085,7 @@
   }
 
   @Test
-  @GerritConfig(name = "index.change.indexMergeable", value = "false")
+  @GerritConfig(name = "change.mergeabilityComputationBehavior", value = "NEVER")
   public void mergeableFailsWhenNotIndexed() throws Exception {
     TestRepository<Repo> repo = createProject("repo");
     RevCommit commit1 = repo.parseBody(repo.commit().add("file1", "contents1").create());
diff --git a/lib/BUILD b/lib/BUILD
index 199f4fb..f0c0aad 100644
--- a/lib/BUILD
+++ b/lib/BUILD
@@ -114,6 +114,7 @@
     name = "caffeine",
     data = ["//lib:LICENSE-Apache2.0"],
     visibility = [
+        "//java/com/google/gerrit/acceptance:__pkg__",
         "//java/com/google/gerrit/server/cache/mem:__pkg__",
     ],
     exports = ["@caffeine//jar"],
@@ -128,6 +129,7 @@
     name = "caffeine-guava",
     data = ["//lib:LICENSE-Apache2.0"],
     visibility = [
+        "//java/com/google/gerrit/acceptance:__pkg__",
         "//java/com/google/gerrit/server/cache/mem:__pkg__",
     ],
     exports = [":caffeine-guava-renamed"],
diff --git a/lib/jackson/BUILD b/lib/jackson/BUILD
index 3eed77a..d5253a0 100644
--- a/lib/jackson/BUILD
+++ b/lib/jackson/BUILD
@@ -4,6 +4,7 @@
     name = "jackson-core",
     data = ["//lib:LICENSE-Apache2.0"],
     visibility = [
+        "//java/com/google/gerrit/acceptance:__pkg__",
         "//java/com/google/gerrit/elasticsearch:__pkg__",
         "//plugins:__pkg__",
     ],
diff --git a/plugins/BUILD b/plugins/BUILD
index ccb97e8..8d9682f 100644
--- a/plugins/BUILD
+++ b/plugins/BUILD
@@ -93,7 +93,18 @@
 ]
 
 java_binary(
+    name = "bouncycastle-deploy-env",
+    main_class = "Dummy",
+    runtime_deps = [
+        "//lib/bouncycastle:bcpg",
+        "//lib/bouncycastle:bcpkix",
+        "//lib/bouncycastle:bcprov",
+    ],
+)
+
+java_binary(
     name = "plugin-api",
+    deploy_env = ["bouncycastle-deploy-env"],
     main_class = "Dummy",
     visibility = ["//visibility:public"],
     runtime_deps = [":plugin-lib"],
diff --git a/polygerrit-ui/app/elements/change/gr-change-actions/gr-change-actions.html b/polygerrit-ui/app/elements/change/gr-change-actions/gr-change-actions.html
index 283bd74..f4bd6a6 100644
--- a/polygerrit-ui/app/elements/change/gr-change-actions/gr-change-actions.html
+++ b/polygerrit-ui/app/elements/change/gr-change-actions/gr-change-actions.html
@@ -205,9 +205,6 @@
       <gr-confirm-revert-dialog id="confirmRevertDialog"
           class="confirmDialog"
           on-confirm="_handleRevertDialogConfirm"
-          commit-message="[[commitMessage]]"
-          change="[[change]]"
-          changes="[[_revertChanges]]"
           on-cancel="_handleConfirmDialogCancel"
           hidden></gr-confirm-revert-dialog>
       <gr-confirm-revert-submission-dialog id="confirmRevertSubmissionDialog"
diff --git a/polygerrit-ui/app/elements/change/gr-change-actions/gr-change-actions.js b/polygerrit-ui/app/elements/change/gr-change-actions/gr-change-actions.js
index c0a603e..299c12a 100644
--- a/polygerrit-ui/app/elements/change/gr-change-actions/gr-change-actions.js
+++ b/polygerrit-ui/app/elements/change/gr-change-actions/gr-change-actions.js
@@ -923,9 +923,13 @@
 
     showRevertDialog() {
       const query = 'submissionid:' + this.change.submission_id;
+      /* A chromium plugin expects that the modifyRevertMsg hook will only
+      be called after the revert button is pressed, hence we populate the
+      revert dialog after revert button is pressed. */
       this.$.restAPI.getChanges('', query)
           .then(changes => {
-            this._revertChanges = changes;
+            this.$.confirmRevertDialog.populate(this.change,
+                this.commitMessage, changes);
             this._showActionDialog(this.$.confirmRevertDialog);
           });
     }
diff --git a/polygerrit-ui/app/elements/change/gr-change-actions/gr-change-actions_test.html b/polygerrit-ui/app/elements/change/gr-change-actions/gr-change-actions_test.html
index 6495adb..088bebd 100644
--- a/polygerrit-ui/app/elements/change/gr-change-actions/gr-change-actions_test.html
+++ b/polygerrit-ui/app/elements/change/gr-change-actions/gr-change-actions_test.html
@@ -820,6 +820,11 @@
         element.change = {
           current_revision: 'abc1234',
         };
+        sandbox.stub(element.$.restAPI, 'getChanges')
+            .returns(Promise.resolve([
+              {change_id: '12345678901234', topic: 'T', subject: 'random'},
+              {change_id: '23456', topic: 'T', subject: 'a'.repeat(100)},
+            ]));
         sandbox.stub(element.$.confirmRevertDialog,
             '_populateRevertSubmissionMessage', () => 'original msg');
         flush(() => {
@@ -970,7 +975,7 @@
           });
         });
 
-        test('confirm revert dialog shows one radio button', done => {
+        test('confirm revert dialog shows no radio button', done => {
           const revertButton = element.shadowRoot
               .querySelector('gr-button[data-action-key="revert"]');
           MockInteractions.tap(revertButton);
@@ -978,7 +983,7 @@
             const confirmRevertDialog = element.$.confirmRevertDialog;
             const radioInputs = confirmRevertDialog.shadowRoot
                 .querySelectorAll('input[name="revertOptions"]');
-            assert.equal(radioInputs.length, 1);
+            assert.equal(radioInputs.length, 0);
             const msg = 'Revert "random commit message"\n\n'
               + 'This reverts commit 2000.\n\nReason '
               + 'for revert: <INSERT REASONING HERE>\n';
diff --git a/polygerrit-ui/app/elements/change/gr-change-view/gr-change-view.html b/polygerrit-ui/app/elements/change/gr-change-view/gr-change-view.html
index 02b017b..623e8d1 100644
--- a/polygerrit-ui/app/elements/change/gr-change-view/gr-change-view.html
+++ b/polygerrit-ui/app/elements/change/gr-change-view/gr-change-view.html
@@ -613,7 +613,8 @@
             <span>Comment Threads</span></gr-tooltip-content>
         </paper-tab>
       </paper-tabs>
-      <template is="dom-if" if="[[_showMessagesView]]">
+      <template is="dom-if" if="[[_isSelectedView(_currentView,
+        _commentTabs.CHANGE_LOG)]]">
         <gr-messages-list
             class="hideOnMobileOverlay"
             change-num="[[_changeNum]]"
@@ -626,7 +627,8 @@
             on-message-anchor-tap="_handleMessageAnchorTap"
             on-reply="_handleMessageReply"></gr-messages-list>
       </template>
-      <template is="dom-if" if="[[!_showMessagesView]]">
+      <template is="dom-if" if="[[_isSelectedView(_currentView,
+        _commentTabs.COMMENT_THREADS)]]">
         <gr-thread-list
             threads="[[_commentThreads]]"
             change="[[_change]]"
diff --git a/polygerrit-ui/app/elements/change/gr-change-view/gr-change-view.js b/polygerrit-ui/app/elements/change/gr-change-view/gr-change-view.js
index 7536c2c..393cbc3 100644
--- a/polygerrit-ui/app/elements/change/gr-change-view/gr-change-view.js
+++ b/polygerrit-ui/app/elements/change/gr-change-view/gr-change-view.js
@@ -59,6 +59,11 @@
     UNIFIED: 'UNIFIED_DIFF',
   };
 
+  const CommentTabs = {
+    CHANGE_LOG: 0,
+    COMMENT_THREADS: 1,
+  };
+
   const CHANGE_DATA_TIMING_LABEL = 'ChangeDataLoaded';
   const CHANGE_RELOAD_TIMING_LABEL = 'ChangeReloaded';
   const SEND_REPLY_TIMING_LABEL = 'SendReply';
@@ -193,6 +198,10 @@
           type: String,
           value: '',
         },
+        _commentTabs: {
+          type: Object,
+          value: CommentTabs,
+        },
         _lineHeight: Number,
         _changeIdCommitMessageError: {
           type: String,
@@ -280,9 +289,9 @@
           type: Boolean,
           value: undefined,
         },
-        _showMessagesView: {
-          type: Boolean,
-          value: true,
+        _currentView: {
+          type: Number,
+          value: CommentTabs.CHANGE_LOG,
         },
         _showFileTabContent: {
           type: Boolean,
@@ -460,7 +469,11 @@
     }
 
     _handleCommentTabChange() {
-      this._showMessagesView = this.$.commentTabs.selected === 0;
+      this._currentView = this.$.commentTabs.selected;
+    }
+
+    _isSelectedView(currentView, view) {
+      return currentView === view;
     }
 
     _handleFileTabChange(e) {
@@ -475,8 +488,7 @@
 
         const tabName = this._selectedFilesTabPluginEndpoint || 'files';
         const source = e && e.type ? e.type : '';
-        this.$.reporting.reportInteraction('tab-changed',
-            `tabname: ${tabName}, source: ${source}`);
+        this.$.reporting.reportInteraction('tab-changed', {tabName, source});
       }
     }
 
@@ -488,7 +500,7 @@
       }
       this.shadowRoot.querySelector('#primaryTabs').selected = idx + 1;
       this.shadowRoot.querySelector('#primaryTabs').scrollIntoView();
-      this.$.reporting.reportInteraction('show-tab', e.detail.tab);
+      this.$.reporting.reportInteraction('show-tab', {tabName: e.detail.tab});
     }
 
     _handleEditCommitMessage(e) {
@@ -773,8 +785,7 @@
       // get messed up if changed here, because it requires the tabs to be on
       // the streen, and they are hidden shortly after this. The tab switching
       // animation will happen in post render tasks.
-      this._showMessagesView = true;
-
+      this._currentView = CommentTabs.CHANGE_LOG;
       if (value.view !== Gerrit.Nav.View.CHANGE) {
         this._initialLoadComplete = false;
         return;
@@ -809,6 +820,7 @@
         }
         this._reloadPatchNumDependentResources().then(() => {
           this._sendShowChangeEvent();
+          this._setPrimaryTab();
         });
         return;
       }
diff --git a/polygerrit-ui/app/elements/change/gr-change-view/gr-change-view_test.html b/polygerrit-ui/app/elements/change/gr-change-view/gr-change-view_test.html
index 1ef79cf..46f6292 100644
--- a/polygerrit-ui/app/elements/change/gr-change-view/gr-change-view_test.html
+++ b/polygerrit-ui/app/elements/change/gr-change-view/gr-change-view_test.html
@@ -63,6 +63,11 @@
     let navigateToChangeStub;
     const TEST_SCROLL_TOP_PX = 100;
 
+    const CommentTabs = {
+      CHANGE_LOG: 0,
+      COMMENT_THREADS: 1,
+    };
+
     setup(() => {
       sandbox = sinon.sandbox.create();
       stub('gr-endpoint-decorator', {
@@ -375,7 +380,7 @@
 
     test('thread list modified', () => {
       sandbox.spy(element, '_handleReloadDiffComments');
-      element._showMessagesView = false;
+      element._currentView = CommentTabs.COMMENT_THREADS;
       flushAsynchronousOperations();
 
       return element._reloadComments().then(() => {
@@ -404,57 +409,66 @@
       });
     });
 
-    test('thread list and change log tabs', done => {
-      element._changeNum = '1';
-      element._patchRange = {
-        basePatchNum: 'PARENT',
-        patchNum: 1,
-      };
-      element._change = {
-        change_id: 'Iad9dc96274af6946f3632be53b106ef80f7ba6ca',
-        revisions: {
-          rev2: {_number: 2, commit: {parents: []}},
-          rev1: {_number: 1, commit: {parents: []}},
-          rev13: {_number: 13, commit: {parents: []}},
-          rev3: {_number: 3, commit: {parents: []}},
-        },
-        current_revision: 'rev3',
-        status: 'NEW',
-        labels: {
-          test: {
-            all: [],
-            default_value: 0,
-            values: [],
-            approved: {},
+    suite('thread list and change log tabs', () => {
+      setup(() => {
+        element._changeNum = '1';
+        element._patchRange = {
+          basePatchNum: 'PARENT',
+          patchNum: 1,
+        };
+        element._change = {
+          change_id: 'Iad9dc96274af6946f3632be53b106ef80f7ba6ca',
+          revisions: {
+            rev2: {_number: 2, commit: {parents: []}},
+            rev1: {_number: 1, commit: {parents: []}},
+            rev13: {_number: 13, commit: {parents: []}},
+            rev3: {_number: 3, commit: {parents: []}},
           },
-        },
-      };
-      sandbox.stub(element.$.relatedChanges, 'reload');
-      sandbox.stub(element, '_reload').returns(Promise.resolve());
-      sandbox.spy(element, '_paramsChanged');
-      element.params = {view: 'change', changeNum: '1'};
+          current_revision: 'rev3',
+          status: 'NEW',
+          labels: {
+            test: {
+              all: [],
+              default_value: 0,
+              values: [],
+              approved: {},
+            },
+          },
+        };
+        sandbox.stub(element.$.relatedChanges, 'reload');
+        sandbox.stub(element, '_reload').returns(Promise.resolve());
+        sandbox.spy(element, '_paramsChanged');
+        element.params = {view: 'change', changeNum: '1'};
+      });
 
-      // When the change is hard reloaded, paramsChanged will not set the tab.
-      // It will be set in postLoadTasks, which requires the flush() to detect.
-      assert.isTrue(element._paramsChanged.called);
-      assert.isUndefined(element.$.commentTabs.selected);
+      test('tab switch works correctly', done => {
+        assert.isTrue(element._paramsChanged.called);
+        assert.equal(element.$.commentTabs.selected, CommentTabs.CHANGE_LOG);
 
-      // Wait for tab to get selected
-      flush(() => {
-        assert.equal(element.$.commentTabs.selected, 0);
-        assert.isTrue(element._showMessagesView);
-        // Switch to comment thread tab
-        MockInteractions.tap(element.$$('paper-tab.commentThreads'));
-        assert.equal(element.$.commentTabs.selected, 1);
-        assert.isFalse(element._showMessagesView);
+        // Wait for tab to get selected
+        flush(() => {
+          assert.equal(element.$.commentTabs.selected, CommentTabs.CHANGE_LOG);
+          assert.equal(element._currentView, CommentTabs.CHANGE_LOG);
+          // Switch to comment thread tab
+          MockInteractions.tap(element.$$('paper-tab.commentThreads'));
+          assert.equal(element.$.commentTabs.selected, 1);
+          assert.equal(element._currentView, CommentTabs.COMMENT_THREADS);
 
-        // When the change is partially reloaded (ex: Shift+R), the content
-        // is swapped out before the tab, so messages list will display even
-        // though the tab for comment threads is still temporarily selected.
-        element._paramsChanged(element.params);
-        assert.equal(element.$.commentTabs.selected, 1);
-        assert.isTrue(element._showMessagesView);
-        done();
+          // When the change is partially reloaded (ex: Shift+R), the content
+          // is swapped out before the tab, so messages list will display even
+          // though the tab for comment threads is still temporarily selected.
+          element._paramsChanged(element.params);
+          assert.equal(element.$.commentTabs.selected,
+              CommentTabs.COMMENT_THREADS);
+          assert.equal(element._currentView, CommentTabs.CHANGE_LOG);
+          flush(() => {
+            // Correct tab is selected after the patchset is changed
+            assert.equal(element.$.commentTabs.selected,
+                CommentTabs.CHANGE_LOG);
+            assert.equal(element._currentView, CommentTabs.CHANGE_LOG);
+            done();
+          });
+        });
       });
     });
 
diff --git a/polygerrit-ui/app/elements/change/gr-comment-list/gr-comment-list.html b/polygerrit-ui/app/elements/change/gr-comment-list/gr-comment-list.html
index a7e65a3..52c0c89 100644
--- a/polygerrit-ui/app/elements/change/gr-comment-list/gr-comment-list.html
+++ b/polygerrit-ui/app/elements/change/gr-comment-list/gr-comment-list.html
@@ -36,18 +36,15 @@
         word-wrap: break-word;
       }
       .file {
-        border-top: 1px solid var(--border-color);
-        font-weight: var(--font-weight-bold);
-        margin: 10px 0 3px;
-        padding: 10px 0 5px;
+        padding: var(--spacing-s) 0;
       }
       .container {
         display: flex;
-        margin: var(--spacing-m) 0;
+        padding: var(--spacing-s) 0;
       }
       .lineNum {
-        margin-right: var(--spacing-m);
-        min-width: 10em;
+        margin-right: var(--spacing-s);
+        min-width: 135px;
         text-align: right;
       }
       .message {
@@ -57,16 +54,16 @@
       @media screen and (max-width: 50em) {
         .container {
           flex-direction: column;
-          margin: 0 0 var(--spacing-m) var(--spacing-m);
         }
         .lineNum {
+          margin-right: 0;
           min-width: initial;
           text-align: left;
         }
       }
     </style>
     <template is="dom-repeat" items="[[_computeFilesFromComments(comments)]]" as="file">
-      <div class="file">[[computeDisplayPath(file)]]:</div>
+      <div class="file"><a class="fileLink" href="[[_computeDiffURL(file, changeNum, comments)]]">[[computeDisplayPath(file)]]</a></div>
       <template is="dom-repeat"
                 items="[[_computeCommentsForFile(comments, file)]]" as="comment">
         <div class="container">
@@ -74,7 +71,7 @@
              href$="[[_computeDiffLineURL(file, changeNum, comment.patch_set, comment)]]">
              <span hidden$="[[!comment.line]]">
                <span>[[_computePatchDisplayName(comment)]]</span>
-               Line <span>[[comment.line]]</span>:
+               Line <span>[[comment.line]]</span>
              </span>
              <span hidden$="[[comment.line]]">
                File comment:
diff --git a/polygerrit-ui/app/elements/change/gr-comment-list/gr-comment-list.js b/polygerrit-ui/app/elements/change/gr-comment-list/gr-comment-list.js
index 58bdbbc..16c55cd 100644
--- a/polygerrit-ui/app/elements/change/gr-comment-list/gr-comment-list.js
+++ b/polygerrit-ui/app/elements/change/gr-comment-list/gr-comment-list.js
@@ -52,18 +52,29 @@
       return comment.side === 'PARENT';
     }
 
-    _computeDiffLineURL(file, changeNum, patchNum, comment) {
+    _computeDiffURL(filePath, changeNum, allComments) {
+      if ([filePath, changeNum, allComments].some(arg => arg === undefined)) {
+        return;
+      }
+      const fileComments = this._computeCommentsForFile(allComments, filePath);
+      // This can happen for files that don't exist anymore in the current ps.
+      if (fileComments.length === 0) return;
+      return Gerrit.Nav.getUrlForDiffById(changeNum, this.projectName,
+          filePath, fileComments[0].patch_set);
+    }
+
+    _computeDiffLineURL(filePath, changeNum, patchNum, comment) {
       const basePatchNum = comment.hasOwnProperty('parent') ?
         -comment.parent : null;
-      return Gerrit.Nav.getUrlForDiffById(this.changeNum, this.projectName,
-          file, patchNum, basePatchNum, comment.line,
+      return Gerrit.Nav.getUrlForDiffById(changeNum, this.projectName,
+          filePath, patchNum, basePatchNum, comment.line,
           this._isOnParent(comment));
     }
 
-    _computeCommentsForFile(comments, file) {
+    _computeCommentsForFile(comments, filePath) {
       // Changes are not picked up by the dom-repeat due to the array instance
       // identity not changing even when it has elements added/removed from it.
-      return (comments[file] || []).slice();
+      return (comments[filePath] || []).slice();
     }
 
     _computePatchDisplayName(comment) {
diff --git a/polygerrit-ui/app/elements/change/gr-confirm-revert-dialog/gr-confirm-revert-dialog.html b/polygerrit-ui/app/elements/change/gr-confirm-revert-dialog/gr-confirm-revert-dialog.html
index 0ad3ed3..144cf20 100644
--- a/polygerrit-ui/app/elements/change/gr-confirm-revert-dialog/gr-confirm-revert-dialog.html
+++ b/polygerrit-ui/app/elements/change/gr-confirm-revert-dialog/gr-confirm-revert-dialog.html
@@ -67,56 +67,40 @@
         <div class="error" hidden$="[[!_showErrorMessage]]">
           <span> A reason is required </span>
         </div>
-        <div class="revertSubmissionLayout">
-          <input
-            name="revertOptions"
-            type="radio"
-            id="revertSingleChange"
-            on-change="_handleRevertSingleChangeClicked"
-            checked="[[_computeIfSingleRevert(_revertType)]]">
-          <label for="revertSingleChange" class="label revertSingleChange">
-            Revert single change
-          </label>
-        </div>
         <template is="dom-if" if="[[_showRevertSubmission]]">
           <div class="revertSubmissionLayout">
             <input
               name="revertOptions"
               type="radio"
+              id="revertSingleChange"
+              on-change="_handleRevertSingleChangeClicked"
+              checked="[[_computeIfSingleRevert(_revertType)]]">
+            <label for="revertSingleChange" class="label revertSingleChange">
+              Revert single change
+            </label>
+          </div>
+          <div class="revertSubmissionLayout">
+            <input
+              name="revertOptions"
+              type="radio"
               id="revertSubmission"
               on-change="_handleRevertSubmissionClicked"
               checked="[[_computeIfRevertSubmission(_revertType)]]">
             <label for="revertSubmission" class="label revertSubmission">
-              Revert entire submission ([[changes.length]] Changes)
+              Revert entire submission ([[_changesCount]] Changes)
             </label>
         </template>
         <gr-endpoint-decorator name="confirm-revert-change">
-          <!-- Duplicating the text-area as a plugin in the case of a single
-          revert will override the entire textarea which should not happen
-          for multiple revert -->
-          <template is="dom-if" if="[[_computeIfSingleRevert(_revertType)]]">
-            <label for="messageInput">
-                Revert Commit Message
-              </label>
-            <iron-autogrow-textarea
-              id="messageInput"
-              class="message"
-              autocomplete="on"
-              max-rows="15"
-              bind-value="{{_message}}"></iron-autogrow-textarea>
-          </template>
-        </gr-endpoint-decorator>
-        <template is="dom-if" if="[[_computeIfRevertSubmission(_revertType)]]">
           <label for="messageInput">
             Revert Commit Message
           </label>
           <iron-autogrow-textarea
-              id="messageInput"
-              class="message"
-              autocomplete="on"
-              max-rows="15"
-              bind-value="{{_message}}"></iron-autogrow-textarea>
-        </template>
+            id="messageInput"
+            class="message"
+            autocomplete="on"
+            max-rows="15"
+            bind-value="{{_message}}"></iron-autogrow-textarea>
+        </gr-endpoint-decorator>
       </div>
     </gr-dialog>
     <gr-js-api-interface id="jsAPI"></gr-js-api-interface>
diff --git a/polygerrit-ui/app/elements/change/gr-confirm-revert-dialog/gr-confirm-revert-dialog.js b/polygerrit-ui/app/elements/change/gr-confirm-revert-dialog/gr-confirm-revert-dialog.js
index b07738a..05660bf 100644
--- a/polygerrit-ui/app/elements/change/gr-confirm-revert-dialog/gr-confirm-revert-dialog.js
+++ b/polygerrit-ui/app/elements/change/gr-confirm-revert-dialog/gr-confirm-revert-dialog.js
@@ -62,22 +62,14 @@
           type: Boolean,
           value: false,
         },
-        /* List of changes with the same topic
-        value is empty if only a single change is being reverted */
-        changes: {
-          type: Array,
-          value() { return []; },
-        },
-        change: Object,
-        /* commit message is _latestCommitMessage coming from gr-change-view
-        read only and is not meant to be modified */
-        commitMessage: String,
+        _changesCount: Number,
         _showErrorMessage: {
           type: Boolean,
           value: false,
         },
         /* store the default revert messages per revert type so that we can
-        check if user has edited the revert message or not */
+        check if user has edited the revert message or not
+        Set when populate() is called */
         _originalRevertMessages: {
           type: Array,
           value() { return []; },
@@ -90,12 +82,6 @@
       };
     }
 
-    static get observers() {
-      return [
-        'onInputUpdate(change, commitMessage, changes)',
-      ];
-    }
-
     _computeIfSingleRevert(revertType) {
       return revertType === REVERT_TYPES.REVERT_SINGLE_CHANGE;
     }
@@ -109,12 +95,12 @@
           message, commitMessage);
     }
 
-    onInputUpdate(change, commitMessage, changes) {
-      if (!change || !changes) return;
+    populate(change, commitMessage, changes) {
+      this._changesCount = changes.length;
       // The option to revert a single change is always available
       this._populateRevertSingleChangeMessage(
           change, commitMessage, change.current_revision);
-      this._populateRevertSubmissionMessage(change, changes);
+      this._populateRevertSubmissionMessage(change, changes, commitMessage);
     }
 
     _populateRevertSingleChangeMessage(change, commitMessage, commitHash) {
@@ -144,12 +130,12 @@
       return subject.substring(0, CHANGE_SUBJECT_LIMIT) + '...';
     }
 
-    _modifyRevertSubmissionMsg(change, msg) {
+    _modifyRevertSubmissionMsg(change, msg, commitMessage) {
       return this.$.jsAPI.modifyRevertSubmissionMsg(change, msg,
-          this.commitMessage);
+          commitMessage);
     }
 
-    _populateRevertSubmissionMessage(change, changes) {
+    _populateRevertSubmissionMessage(change, changes, commitMessage) {
       // Follow the same convention of the revert
       const commitHash = change.current_revision;
       if (!commitHash) {
@@ -166,7 +152,8 @@
         this._message += change.change_id.substring(0, 10) + ':'
           + this._getTrimmedChangeSubject(change.subject) + '\n';
       });
-      this._message = this._modifyRevertSubmissionMsg(change, this._message);
+      this._message = this._modifyRevertSubmissionMsg(change, this._message,
+          commitMessage);
       this._revertType = REVERT_TYPES.REVERT_SUBMISSION;
       this._revertMessages[this._revertType] = this._message;
       this._originalRevertMessages[this._revertType] = this._message;
diff --git a/polygerrit-ui/app/elements/change/gr-message/gr-message.html b/polygerrit-ui/app/elements/change/gr-message/gr-message.html
index fbd5d68d..e77bf57 100644
--- a/polygerrit-ui/app/elements/change/gr-message/gr-message.html
+++ b/polygerrit-ui/app/elements/change/gr-message/gr-message.html
@@ -45,44 +45,19 @@
       :host(.expanded) {
         cursor: auto;
       }
-      :host > div {
-        padding: 0 var(--spacing-l);
-      }
-      gr-avatar {
-        position: absolute;
-        left: var(--spacing-l);
-      }
       .collapsed .contentContainer {
-        align-items: baseline;
+        align-items: center;
         color: var(--deemphasized-text-color);
         display: flex;
         white-space: nowrap;
       }
       .contentContainer {
-        margin-left: calc(var(--spacing-l) + 2.5em);
-        padding: var(--spacing-m) 0;
+        padding: var(--spacing-m) var(--spacing-l);
       }
-      .showAvatar.collapsed .contentContainer {
-        margin-left: calc(var(--spacing-l) + 1.75em);
-      }
-      .hideAvatar.collapsed .contentContainer,
-      .hideAvatar.expanded .contentContainer {
-        margin-left: 0;
-      }
-      .showAvatar.collapsed .contentContainer,
-      .hideAvatar.collapsed .contentContainer,
-      .hideAvatar.expanded .contentContainer {
-        padding: var(--spacing-m) 0;
-      }
-      .collapsed gr-avatar {
-        top: var(--spacing-m);
-        height: var(--line-height-normal);
-        width: var(--line-height-normal);
-      }
-      .expanded gr-avatar {
-        top: var(--spacing-l);
-        height: var(--line-height-h1);
-        width: var(--line-height-h1);
+      .collapsed .contentContainer {
+        /* For expanded state we inherit the alternating background color
+           that is set in gr-messages-list. */
+        background-color: var(--background-color-primary);
       }
       .name {
         font-weight: var(--font-weight-bold);
@@ -129,14 +104,29 @@
         color: var(--primary-text-color);
         margin-right: var(--spacing-s);
       }
+      .authorLabel {
+        min-width: 160px;
+        display: inline-block;
+      }
       .expanded .author {
         cursor: pointer;
-        margin-bottom: var(--spacing-s);
+        margin-bottom: var(--spacing-m);
+      }
+      .expanded .content {
+        padding-left: 40px;
       }
       .dateContainer {
         position: absolute;
+        /* right and top values should match .contentContainer padding */
         right: var(--spacing-l);
-        top: 10px;
+        top: var(--spacing-m);
+      }
+      .dateContainer .patchset {
+        margin-right: var(--spacing-m);
+        color: var(--deemphasized-text-color);
+      }
+      .dateContainer .patchset:before {
+        content: 'Patchset ';
       }
       span.date {
         color: var(--deemphasized-text-color);
@@ -148,16 +138,23 @@
         cursor: pointer;
         vertical-align: top;
       }
-      .replyContainer {
-        padding: var(--spacing-m) 0 0 0;
-      }
       .score {
-        border: 1px solid rgba(0,0,0,.12);
         border-radius: var(--border-radius);
         color: var(--primary-text-color);
         display: inline-block;
-        margin: -1px 0;
-        padding: 0 var(--spacing-xxs);
+        padding: 0 var(--spacing-s);
+        text-align: center;
+      }
+      .score,
+      .commentsSummary {
+        margin-right: var(--spacing-s);
+        min-width: 115px;
+      }
+      .expanded .commentsSummary {
+        display: none;
+      }
+      .commentsIcon {
+        vertical-align: top;
       }
       .score.negative {
         background-color: var(--vote-color-disliked);
@@ -176,35 +173,53 @@
           font-weight: var(--font-weight-bold);
         };
       }
+      @media screen and (max-width: 50em) {
+        .expanded .content {
+          padding-left: 0;
+        }
+        .score,
+        .commentsSummary,
+        .authorLabel {
+          min-width: 0px;
+        }
+        .dateContainer .patchset:before {
+          content: 'PS ';
+        }
+      }
     </style>
-    <div class$="[[_computeClass(_expanded, showAvatar, message)]]">
-      <gr-avatar account="[[author]]" image-size="100"></gr-avatar>
+    <div class$="[[_computeClass(_expanded)]]">
       <div class="contentContainer">
         <div class="author" on-click="_handleAuthorClick">
           <span hidden$="[[!showOnBehalfOf]]">
             <span class="name">[[message.real_author.name]]</span>
             on behalf of
           </span>
-          <gr-account-label
-              account="[[author]]"
-              hide-avatar></gr-account-label>
+          <gr-account-label account="[[author]]" class="authorLabel"></gr-account-label>
           <template is="dom-repeat" items="[[_getScores(message)]]" as="score">
             <span class$="score [[_computeScoreClass(score, labelExtremes)]]">
               [[score.label]] [[score.value]]
             </span>
           </template>
         </div>
+        <template is="dom-if" if="[[_commentCountText]]">
+          <div class="commentsSummary">
+            <iron-icon icon="gr-icons:comment" class="commentsIcon"></iron-icon>
+            <span class="numberOfComments">[[_commentCountText]]</span>
+          </div>
+        </template>
         <template is="dom-if" if="[[message.message]]">
           <div class="content">
-            <div class="message hideOnOpen">[[message.message]]</div>
+            <div class="message hideOnOpen">[[_messageContentCollapsed]]</div>
             <gr-formatted-text
                 no-trailing-margin
                 class="message hideOnCollapsed"
-                content="[[message.message]]"
+                content="[[_messageContentExpanded]]"
                 config="[[_projectConfig.commentlinks]]"></gr-formatted-text>
-            <div class="replyContainer" hidden$="[[!showReplyButton]]" hidden>
-              <gr-button link small on-click="_handleReplyTap">Reply</gr-button>
-            </div>
+            <template is="dom-if" if="[[!_isMessageContentEmpty(message.message)]]">
+              <div class="replyContainer" hidden$="[[!showReplyButton]]" hidden>
+                <gr-button link small on-click="_handleReplyTap">Reply</gr-button>
+              </div>
+            </template>
             <gr-comment-list
                 comments="[[comments]]"
                 change-num="[[changeNum]]"
@@ -228,6 +243,9 @@
           </div>
         </template>
         <span class="dateContainer">
+          <template is="dom-if" if="[[message._revision_number]]">
+            <span class="patchset">[[message._revision_number]]</span>
+          </template>
           <template is="dom-if" if="[[!message.id]]">
             <span class="date">
               <gr-date-formatter
@@ -248,7 +266,7 @@
               id="expandToggle"
               on-click="_toggleExpanded"
               title="Toggle expanded state"
-              icon="[[_computeExpandToggleIcon(_expanded)]]">
+              icon="[[_computeExpandToggleIcon(_expanded)]]"></iron-icon>
         </span>
       </div>
     </div>
diff --git a/polygerrit-ui/app/elements/change/gr-message/gr-message.js b/polygerrit-ui/app/elements/change/gr-message/gr-message.js
index 59f12f5..95109bb 100644
--- a/polygerrit-ui/app/elements/change/gr-message/gr-message.js
+++ b/polygerrit-ui/app/elements/change/gr-message/gr-message.js
@@ -69,10 +69,6 @@
           type: Boolean,
           computed: '_computeIsAutomated(message)',
         },
-        showAvatar: {
-          type: Boolean,
-          computed: '_computeShowAvatar(author, config)',
-        },
         showOnBehalfOf: {
           type: Boolean,
           computed: '_computeShowOnBehalfOf(message)',
@@ -101,6 +97,18 @@
           type: Object,
           computed: '_computeExpanded(message.expanded)',
         },
+        _messageContentExpanded: {
+          type: String,
+          computed: '_computeMessageContentExpanded(message.message)',
+        },
+        _messageContentCollapsed: {
+          type: String,
+          computed: '_computeMessageContentCollapsed(message.message)',
+        },
+        _commentCountText: {
+          type: Number,
+          computed: '_computeCommentCountText(comments)',
+        },
         _loggedIn: {
           type: Boolean,
           value: false,
@@ -140,12 +148,51 @@
       }
     }
 
-    _computeAuthor(message) {
-      return message.author || message.updated_by;
+    _computeCommentCountText(comments) {
+      if (!comments) return undefined;
+      let count = 0;
+      for (const file in comments) {
+        if (comments.hasOwnProperty(file)) {
+          const commentArray = comments[file] || [];
+          count += commentArray.length;
+        }
+      }
+      if (count === 0) {
+        return undefined;
+      } else if (count === 1) {
+        return '1 comment';
+      } else {
+        return `${count} comments`;
+      }
     }
 
-    _computeShowAvatar(author, config) {
-      return !!(author && config && config.plugin && config.plugin.has_avatars);
+    _computeMessageContentExpanded(content) {
+      return this._computeMessageContent(content, true);
+    }
+
+    _computeMessageContentCollapsed(content) {
+      return this._computeMessageContent(content, false);
+    }
+
+    _computeMessageContent(content, isExpanded) {
+      if (!content) return '';
+      const lines = content.split('\n');
+      const filteredLines = lines.filter(line => {
+        if (!isExpanded && line.startsWith('>')) return false;
+        if (line.startsWith('Patch Set ')) return false;
+        if (line.startsWith('(') && line.endsWith(' comment)')) return false;
+        if (line.startsWith('(') && line.endsWith(' comments)')) return false;
+        return true;
+      });
+      return filteredLines.join('\n').trim();
+    }
+
+    _isMessageContentEmpty(content) {
+      return this._computeMessageContent(content).trim().length === 0;
+    }
+
+    _computeAuthor(message) {
+      return message.author || message.updated_by;
     }
 
     _computeShowOnBehalfOf(message) {
@@ -236,10 +283,9 @@
       return classes.join(' ');
     }
 
-    _computeClass(expanded, showAvatar, message) {
+    _computeClass(expanded) {
       const classes = [];
       classes.push(expanded ? 'expanded' : 'collapsed');
-      classes.push(showAvatar ? 'showAvatar' : 'hideAvatar');
       return classes.join(' ');
     }
 
diff --git a/polygerrit-ui/app/elements/change/gr-messages-list/gr-messages-list.html b/polygerrit-ui/app/elements/change/gr-messages-list/gr-messages-list.html
index d71beb4..8d76510 100644
--- a/polygerrit-ui/app/elements/change/gr-messages-list/gr-messages-list.html
+++ b/polygerrit-ui/app/elements/change/gr-messages-list/gr-messages-list.html
@@ -64,6 +64,12 @@
         align-items: center;
         display: flex;
       }
+      gr-message:nth-child(2n) {
+        background-color: var(--background-color-secondary);
+      }
+      gr-message:nth-child(2n+1) {
+        background-color: var(--background-color-tertiary);
+      }
     </style>
     <div class="header">
         <span
diff --git a/polygerrit-ui/app/elements/change/gr-messages-list/gr-messages-list.js b/polygerrit-ui/app/elements/change/gr-messages-list/gr-messages-list.js
index 2c47dc1..cd759d3 100644
--- a/polygerrit-ui/app/elements/change/gr-messages-list/gr-messages-list.js
+++ b/polygerrit-ui/app/elements/change/gr-messages-list/gr-messages-list.js
@@ -359,6 +359,16 @@
     _processedMessagesChanged(messages) {
       if (messages) {
         this._visibleMessages = messages.slice(-MAX_INITIAL_SHOWN_MESSAGES);
+
+        if (messages.length === 0) return;
+        const tags = messages.map(message => message.tag || message.type ||
+            (message.comments ? 'comments' : 'none'));
+        const tagsCounted = tags.reduce((acc, val) => {
+          acc[val] = (acc[val] || 0) + 1;
+          return acc;
+        }, {all: messages.length});
+        this.$.reporting.reportInteraction('messages-count',
+            JSON.stringify(tagsCounted));
       }
     }
 
diff --git a/polygerrit-ui/app/elements/change/gr-reply-dialog/gr-reply-dialog.html b/polygerrit-ui/app/elements/change/gr-reply-dialog/gr-reply-dialog.html
index c177283..d54669f 100644
--- a/polygerrit-ui/app/elements/change/gr-reply-dialog/gr-reply-dialog.html
+++ b/polygerrit-ui/app/elements/change/gr-reply-dialog/gr-reply-dialog.html
@@ -86,19 +86,17 @@
       }
       .peopleList {
         display: flex;
-        padding-top: var(--spacing-xxs);
       }
       .peopleListLabel {
         color: var(--deemphasized-text-color);
         margin-top: var(--spacing-xs);
-        min-width: 7em;
+        min-width: 6em;
         padding-right: var(--spacing-m);
       }
       gr-account-list {
         display: flex;
         flex-wrap: wrap;
         flex: 1;
-        min-height: 1.8em;
       }
       #reviewerConfirmationOverlay {
         padding: var(--spacing-l);
diff --git a/polygerrit-ui/app/elements/change/gr-reviewer-list/gr-reviewer-list.html b/polygerrit-ui/app/elements/change/gr-reviewer-list/gr-reviewer-list.html
index a5875ab1..132ce11 100644
--- a/polygerrit-ui/app/elements/change/gr-reviewer-list/gr-reviewer-list.html
+++ b/polygerrit-ui/app/elements/change/gr-reviewer-list/gr-reviewer-list.html
@@ -32,7 +32,15 @@
         opacity: .8;
         pointer-events: none;
       }
-      .container > :not(:first-child) {
+      .container {
+        display: block;
+        /* This is a bit of a hack. We tried to use margin-top with
+           :not(:first-child) before, but :first-child does not understand
+           whether a child is visible or not. So adding a margin for every
+           child and then a negative one at the top does the trick. */
+        margin-top: calc(0px - var(--spacing-s));
+      }
+      .container > * {
         margin-top: var(--spacing-s);
       }
       gr-button {
diff --git a/polygerrit-ui/app/elements/core/gr-error-manager/gr-error-manager_test.html b/polygerrit-ui/app/elements/core/gr-error-manager/gr-error-manager_test.html
index 76d12e9..4f9b103 100644
--- a/polygerrit-ui/app/elements/core/gr-error-manager/gr-error-manager_test.html
+++ b/polygerrit-ui/app/elements/core/gr-error-manager/gr-error-manager_test.html
@@ -196,7 +196,7 @@
             let toast = toastSpy.lastCall.returnValue;
             assert.isOk(toast);
             assert.include(
-                Polymer.dom(toast.root).textContent, 'Credentails expired.');
+                Polymer.dom(toast.root).textContent, 'Credentials expired.');
             assert.include(
                 Polymer.dom(toast.root).textContent, 'Refresh credentials');
 
@@ -267,7 +267,7 @@
             // toast
             const toast = toastSpy.lastCall.returnValue;
             assert.include(
-                Polymer.dom(toast.root).textContent, 'Credentails expired.');
+                Polymer.dom(toast.root).textContent, 'Credentials expired.');
             assert.include(
                 Polymer.dom(toast.root).textContent, 'Refresh credentials');
             done();
@@ -313,7 +313,7 @@
           flush(() => {
             let toast = toastSpy.lastCall.returnValue;
             assert.include(
-                Polymer.dom(toast.root).textContent, 'Credentails expired.');
+                Polymer.dom(toast.root).textContent, 'Credentials expired.');
             assert.include(
                 Polymer.dom(toast.root).textContent, 'Refresh credentials');
 
@@ -325,7 +325,7 @@
               toast = toastSpy.lastCall.returnValue;
               assert.isOk(toast);
               assert.include(
-                  Polymer.dom(toast.root).textContent, 'Credentails expired.');
+                  Polymer.dom(toast.root).textContent, 'Credentials expired.');
               done();
             });
           });
@@ -463,4 +463,4 @@
       });
     });
   });
-</script>
\ No newline at end of file
+</script>
diff --git a/polygerrit-ui/app/elements/core/gr-reporting/gr-reporting.js b/polygerrit-ui/app/elements/core/gr-reporting/gr-reporting.js
index 889f1f3..2ed0206 100644
--- a/polygerrit-ui/app/elements/core/gr-reporting/gr-reporting.js
+++ b/polygerrit-ui/app/elements/core/gr-reporting/gr-reporting.js
@@ -96,11 +96,9 @@
   const DRAFT_ACTION_TIMER = 'TimeBetweenDraftActions';
   const DRAFT_ACTION_TIMER_MAX = 2 * 60 * 1000; // 2 minutes.
 
-  const pending = [];
+  let pending = [];
 
   // Variables that hold context info in global scope
-  const loadedPlugins = [];
-  const detectedExtensions = [];
   let reportRepoName = undefined;
 
   const onError = function(oldOnError, msg, url, line, column, error) {
@@ -152,7 +150,7 @@
           if (task.duration > 200) {
             GrReporting.prototype.reporter(TIMING.TYPE,
                 TIMING.CATEGORY_UI_LATENCY, `Task ${task.name}`,
-                Math.round(task.duration), false);
+                Math.round(task.duration), {}, false);
           }
         }
       });
@@ -160,6 +158,12 @@
     }
   }
 
+  document.addEventListener('visibilitychange', () => {
+    const eventName = `Visibility changed to ${document.visibilityState}`;
+    GrReporting.prototype.reporter(INTERACTION_TYPE, undefined, eventName,
+        undefined, {}, true);
+  });
+
   // The Polymer pass of JSCompiler requires this to be reassignable
   // eslint-disable-next-line prefer-const
   let GrReporting = Polymer({
@@ -197,94 +201,75 @@
         !this._baselines.hasOwnProperty(TIMER.METRICS_PLUGIN_LOADED);
     },
 
-    reporter(...args) {
-      const report = (this._isMetricsPluginLoaded() && !pending.length) ?
-        this.defaultReporter : this.cachingReporter;
-      const contextInfo = {
-        loadedPlugins,
-        detectedExtensions,
-        repoName: reportRepoName,
-        isInBackgroundTab: document.visibilityState === 'hidden',
-        startTimeMs: this.now(),
-      };
-      args.splice(4, 0, contextInfo);
-      report.apply(this, args);
-    },
-
     /**
-     * The default reporter reports events immediately.
+     * Reporter reports events. Events will be queued if metrics plugin is not
+     * yet installed.
      *
      * @param {string} type
      * @param {string} category
      * @param {string} eventName
      * @param {string|number} eventValue
-     * @param {Object} contextInfo
+     * @param {Object} eventDetails
      * @param {boolean|undefined} opt_noLog If true, the event will not be
      *     logged to the JS console.
      */
-    defaultReporter(type, category, eventName, eventValue, contextInfo,
-        opt_noLog) {
-      const detail = {
+    reporter(type, category, eventName, eventValue, eventDetails, opt_noLog) {
+      const eventInfo = this._createEventInfo(type, category,
+          eventName, eventValue, eventDetails);
+      if (type === ERROR.TYPE && category === ERROR.CATEGORY) {
+        console.error(eventValue && eventValue.error || eventName);
+      }
+
+      // We report events immediately when metrics plugin is loaded
+      if (this._isMetricsPluginLoaded() && !pending.length) {
+        this._reportEvent(eventInfo, opt_noLog);
+      } else {
+        // We cache until metrics plugin is loaded
+        pending.push([eventInfo, opt_noLog]);
+        if (this._isMetricsPluginLoaded()) {
+          pending.forEach(([eventInfo, opt_noLog]) => {
+            this._reportEvent(eventInfo, opt_noLog);
+          });
+          pending = [];
+        }
+      }
+    },
+
+    _reportEvent(eventInfo, opt_noLog) {
+      const {type, value, name} = eventInfo;
+      document.dispatchEvent(new CustomEvent(type, {detail: eventInfo}));
+      if (opt_noLog) { return; }
+      if (type !== ERROR.TYPE) {
+        if (value !== undefined) {
+          console.log(`Reporting: ${name}: ${value}`);
+        } else {
+          console.log(`Reporting: ${name}`);
+        }
+      }
+    },
+
+    _createEventInfo(type, category, name, value, eventDetails) {
+      const eventInfo = {
         type,
         category,
-        name: eventName,
-        value: eventValue,
+        name,
+        value,
+        eventStart: this.now(),
       };
-      if (category === TIMING.CATEGORY_UI_LATENCY && contextInfo) {
-        detail.loadedPlugins = contextInfo.loadedPlugins;
-        detail.detectedExtensions = contextInfo.detectedExtensions;
-      }
-      if (contextInfo && contextInfo.repoName) {
-        detail.repoName = contextInfo.repoName;
-      }
-      if (contextInfo && contextInfo.isInBackgroundTab !== undefined) {
-        detail.inBackgroundTab = contextInfo.isInBackgroundTab;
-      }
-      if (contextInfo && contextInfo.startTimeMs) {
-        detail.eventStart = contextInfo.startTimeMs;
-      }
-      document.dispatchEvent(new CustomEvent(type, {detail}));
-      if (opt_noLog) { return; }
-      if (type === ERROR.TYPE && category === ERROR.CATEGORY) {
-        console.error(eventValue && eventValue.error || eventName);
-      } else {
-        if (eventValue !== undefined) {
-          console.log(`Reporting: ${eventName}: ${eventValue}`);
-        } else {
-          console.log(`Reporting: ${eventName}`);
-        }
-      }
-    },
 
-    /**
-     * The caching reporter will queue reports until plugins have loaded, and
-     * log events immediately if they're reported after plugins have loaded.
-     *
-     * @param {string} type
-     * @param {string} category
-     * @param {string} eventName
-     * @param {string|number} eventValue
-     * @param {Object} contextInfo
-     * @param {boolean|undefined} opt_noLog If true, the event will not be
-     *     logged to the JS console.
-     */
-    cachingReporter(type, category, eventName, eventValue, contextInfo,
-        opt_noLog) {
-      if (type === ERROR.TYPE && category === ERROR.CATEGORY) {
-        console.error(eventValue && eventValue.error || eventName);
+      if (typeof(eventDetails) === 'object' &&
+        Object.entries(eventDetails).length !== 0) {
+        eventInfo.eventDetails = JSON.stringify(eventDetails);
       }
-      if (this._isMetricsPluginLoaded()) {
-        if (pending.length) {
-          for (const args of pending.splice(0)) {
-            this.defaultReporter(...args);
-          }
-        }
-        this.defaultReporter(type, category, eventName, eventValue, contextInfo,
-            opt_noLog);
-      } else {
-        pending.push([type, category, eventName, eventValue, contextInfo,
-          opt_noLog]);
+      if (reportRepoName) {
+        eventInfo.repoName = reportRepoName;
       }
+      const isInBackgroundTab = document.visibilityState === 'hidden';
+      if (isInBackgroundTab !== undefined) {
+        eventInfo.inBackgroundTab = isInBackgroundTab;
+      }
+
+      return eventInfo;
     },
 
     /**
@@ -310,14 +295,14 @@
       }
     },
 
-    _reportPerformanceTiming(eventName) {
+    _reportPerformanceTiming(eventName, eventDetails) {
       const eventTiming = this.performanceTiming[eventName];
       if (eventTiming > 0) {
         const elapsedTime = eventTiming -
             this.performanceTiming.navigationStart;
         // NavResTime - Navigation and resource timings.
         this.reporter(TIMING.TYPE, TIMING.CATEGORY_UI_LATENCY,
-            `NavResTime - ${eventName}`, elapsedTime, true);
+            `NavResTime - ${eventName}`, elapsedTime, eventDetails, true);
       }
     },
 
@@ -399,22 +384,19 @@
 
     reportExtension(name) {
       this.reporter(EXTENSION.TYPE, EXTENSION.DETECTED, name);
-      if (!detectedExtensions.includes(name)) {
-        detectedExtensions.push(name);
-      }
     },
 
     pluginLoaded(name) {
       if (name.startsWith('metrics-')) {
         this.timeEnd(TIMER.METRICS_PLUGIN_LOADED);
       }
-      loadedPlugins.push(name);
     },
 
     pluginsLoaded(pluginsList) {
       this.timeEnd(TIMER.PLUGINS_LOADED);
       this.reporter(
-          PLUGINS.TYPE, PLUGINS.INSTALLED, (pluginsList || []).join(','));
+          PLUGINS.TYPE, PLUGINS.INSTALLED, PLUGINS.INSTALLED, undefined,
+          {pluginsList: pluginsList || []}, true);
     },
 
     /**
@@ -470,9 +452,11 @@
      *
      * @param {string} name Timing name.
      * @param {number} time The time to report as an integer of milliseconds.
+     * @param {Object} eventDetails non sensitive details
      */
-    _reportTiming(name, time) {
-      this.reporter(TIMING.TYPE, TIMING.CATEGORY_UI_LATENCY, name, time);
+    _reportTiming(name, time, eventDetails) {
+      this.reporter(TIMING.TYPE, TIMING.CATEGORY_UI_LATENCY, name, time,
+          eventDetails);
     },
 
     /**
@@ -532,11 +516,12 @@
      */
     reportRpcTiming(anonymizedUrl, elapsed) {
       this.reporter(TIMING.TYPE, TIMING.CATEGORY_RPC, 'RPC-' + anonymizedUrl,
-          elapsed, true);
+          elapsed, {}, true);
     },
 
-    reportInteraction(eventName, opt_msg) {
-      this.reporter(INTERACTION_TYPE, this.category, eventName, opt_msg);
+    reportInteraction(eventName, details) {
+      this.reporter(INTERACTION_TYPE, this.category, eventName, undefined,
+          details, true);
     },
 
     /**
diff --git a/polygerrit-ui/app/elements/core/gr-reporting/gr-reporting_test.html b/polygerrit-ui/app/elements/core/gr-reporting/gr-reporting_test.html
index c7aa896..11fe132 100644
--- a/polygerrit-ui/app/elements/core/gr-reporting/gr-reporting_test.html
+++ b/polygerrit-ui/app/elements/core/gr-reporting/gr-reporting_test.html
@@ -67,7 +67,7 @@
       sandbox.stub(element, 'now').returns(42);
       element.appStarted();
       assert.isTrue(
-          element.reporter.calledWithExactly(
+          element.reporter.calledWithMatch(
               'timing-report', 'UI Latency', 'App Started', 42
           ));
     });
@@ -75,7 +75,7 @@
     test('WebComponentsReady', () => {
       sandbox.stub(element, 'now').returns(42);
       element.timeEnd('WebComponentsReady');
-      assert.isTrue(element.reporter.calledWithExactly(
+      assert.isTrue(element.reporter.calledWithMatch(
           'timing-report', 'UI Latency', 'WebComponentsReady', 42
       ));
     });
@@ -86,7 +86,7 @@
           element.reporter.calledWithExactly(
               'timing-report', 'UI Latency', 'NavResTime - loadEventEnd',
               fakePerformance.loadEventEnd - fakePerformance.navigationStart,
-              true)
+              undefined, true)
       );
     });
 
@@ -166,10 +166,10 @@
       element.timeEnd('bar');
       nowStub.returns(3);
       element.timeEnd('foo');
-      assert.isTrue(element.reporter.calledWithExactly(
+      assert.isTrue(element.reporter.calledWithMatch(
           'timing-report', 'UI Latency', 'foo', 3
       ));
-      assert.isTrue(element.reporter.calledWithExactly(
+      assert.isTrue(element.reporter.calledWithMatch(
           'timing-report', 'UI Latency', 'bar', 1
       ));
     });
@@ -179,7 +179,7 @@
       const timer = element.getTimer('foo-bar');
       nowStub.returns(150);
       timer.end();
-      assert.isTrue(element.reporter.calledWithExactly(
+      assert.isTrue(element.reporter.calledWithMatch(
           'timing-report', 'UI Latency', 'foo-bar', 50));
     });
 
@@ -236,9 +236,9 @@
       nowStub.returns(1100);
       element.timeEndWithAverage('foo', 'bar', 10);
       assert.isTrue(element.reporter.calledTwice);
-      assert.isTrue(element.reporter.calledWithExactly(
+      assert.isTrue(element.reporter.calledWithMatch(
           'timing-report', 'UI Latency', 'foo', 100));
-      assert.isTrue(element.reporter.calledWithExactly(
+      assert.isTrue(element.reporter.calledWithMatch(
           'timing-report', 'UI Latency', 'bar', 10));
     });
 
@@ -249,17 +249,36 @@
       ));
     });
 
+    test('reportInteraction', () => {
+      element.reporter.restore();
+      sandbox.spy(element, '_reportEvent');
+      element.pluginsLoaded(); // so we don't cache
+      element.reportInteraction('button-click', {name: 'sendReply'});
+      assert.isTrue(element._reportEvent.getCall(2).calledWithMatch(
+          {
+            type: 'interaction',
+            name: 'button-click',
+            eventDetails: JSON.stringify({name: 'sendReply'}),
+          }
+      ));
+    });
+
     test('report start time', () => {
       element.reporter.restore();
       sandbox.stub(element, 'now').returns(42);
-      sandbox.spy(element, 'defaultReporter');
+      sandbox.spy(element, '_reportEvent');
       const dispatchStub = sandbox.spy(document, 'dispatchEvent');
       element.pluginsLoaded();
       element.time('timeAction');
       element.timeEnd('timeAction');
-      assert.isTrue(element.defaultReporter.getCall(2).calledWithMatch(
-          'timing-report', 'UI Latency', 'timeAction', 0,
-          {startTimeMs: 42}
+      assert.isTrue(element._reportEvent.getCall(2).calledWithMatch(
+          {
+            type: 'timing-report',
+            category: 'UI Latency',
+            name: 'timeAction',
+            value: 0,
+            eventStart: 42,
+          }
       ));
       assert.equal(dispatchStub.getCall(2).args[0].detail.eventStart, 42);
     });
@@ -267,50 +286,46 @@
     suite('plugins', () => {
       setup(() => {
         element.reporter.restore();
-        sandbox.stub(element, 'defaultReporter');
+        sandbox.stub(element, '_reportEvent');
       });
 
       test('pluginsLoaded reports time', () => {
         sandbox.stub(element, 'now').returns(42);
         element.pluginsLoaded();
-        assert.isTrue(element.defaultReporter.calledWith(
-            'timing-report', 'UI Latency', 'PluginsLoaded', 42
+        assert.isTrue(element._reportEvent.calledWithMatch(
+            {
+              type: 'timing-report',
+              category: 'UI Latency',
+              name: 'PluginsLoaded',
+              value: 42,
+            }
         ));
       });
 
       test('pluginsLoaded reports plugins', () => {
         element.pluginsLoaded(['foo', 'bar']);
-        assert.isTrue(element.defaultReporter.calledWith(
-            'lifecycle', 'Plugins installed', 'foo,bar'
+        assert.isTrue(element._reportEvent.calledWithMatch(
+            {
+              type: 'lifecycle',
+              category: 'Plugins installed',
+              eventDetails: JSON.stringify({pluginsList: ['foo', 'bar']}),
+            }
         ));
       });
 
       test('caches reports if plugins are not loaded', () => {
         element.timeEnd('foo');
-        assert.isFalse(element.defaultReporter.called);
+        assert.isFalse(element._reportEvent.called);
       });
 
       test('reports if plugins are loaded', () => {
         element.pluginsLoaded();
-        assert.isTrue(element.defaultReporter.called);
-      });
-
-      test('reports plugins in timing events', () => {
-        element.pluginsLoaded = [];
-        sandbox.stub(element, 'now').returns(42);
-        element.pluginLoaded('metrics-xyz1');
-        // element.pluginLoaded('foo');
-        element.time('timeAction');
-        element.timeEnd('timeAction');
-        assert.isTrue(element.defaultReporter.getCall(1).calledWithMatch(
-            'timing-report', 'UI Latency', 'timeAction', 0,
-            {loadedPlugins: ['metrics-xyz1']}
-        ));
+        assert.isTrue(element._reportEvent.called);
       });
 
       test('reports if metrics plugin xyz is loaded', () => {
         element.pluginLoaded('metrics-xyz');
-        assert.isTrue(element.defaultReporter.called);
+        assert.isTrue(element._reportEvent.called);
       });
 
       test('reports cached events preserving order', () => {
@@ -319,17 +334,18 @@
         element.timeEnd('foo');
         element.pluginsLoaded();
         element.timeEnd('bar');
-        assert.isTrue(element.defaultReporter.getCall(0).calledWith(
-            'timing-report', 'UI Latency', 'foo'
+        assert.isTrue(element._reportEvent.getCall(0).calledWithMatch(
+            {type: 'timing-report', category: 'UI Latency', name: 'foo'}
         ));
-        assert.isTrue(element.defaultReporter.getCall(1).calledWith(
-            'timing-report', 'UI Latency', 'PluginsLoaded'
+        assert.isTrue(element._reportEvent.getCall(1).calledWithMatch(
+            {type: 'timing-report', category: 'UI Latency',
+              name: 'PluginsLoaded'}
         ));
-        assert.isTrue(element.defaultReporter.getCall(2).calledWith(
-            'lifecycle', 'Plugins installed'
+        assert.isTrue(element._reportEvent.getCall(2).calledWithMatch(
+            {type: 'lifecycle', category: 'Plugins installed'}
         ));
-        assert.isTrue(element.defaultReporter.getCall(3).calledWith(
-            'timing-report', 'UI Latency', 'bar'
+        assert.isTrue(element._reportEvent.getCall(3).calledWithMatch(
+            {type: 'timing-report', category: 'UI Latency', name: 'bar'}
         ));
       });
     });
diff --git a/polygerrit-ui/app/elements/diff/gr-diff-cursor/gr-diff-cursor.js b/polygerrit-ui/app/elements/diff/gr-diff-cursor/gr-diff-cursor.js
index 05a7525..726ac10 100644
--- a/polygerrit-ui/app/elements/diff/gr-diff-cursor/gr-diff-cursor.js
+++ b/polygerrit-ui/app/elements/diff/gr-diff-cursor/gr-diff-cursor.js
@@ -237,6 +237,11 @@
       this.moveToNextChunk(true);
     }
 
+    moveToLastChunk() {
+      this.$.cursorManager.moveToEnd();
+      this.moveToPreviousChunk();
+    }
+
     reInitCursor() {
       this._updateStops();
       if (this.initialLineNumber) {
diff --git a/polygerrit-ui/app/elements/diff/gr-diff-cursor/gr-diff-cursor_test.html b/polygerrit-ui/app/elements/diff/gr-diff-cursor/gr-diff-cursor_test.html
index dff9e79..24ca3d1 100644
--- a/polygerrit-ui/app/elements/diff/gr-diff-cursor/gr-diff-cursor_test.html
+++ b/polygerrit-ui/app/elements/diff/gr-diff-cursor/gr-diff-cursor_test.html
@@ -108,6 +108,18 @@
       assert.equal(cursorElement.diffRow, firstDeltaRow);
     });
 
+    test('moveToLastChunk', () => {
+      const chunks = Array.from(Polymer.dom(diffElement.root).querySelectorAll(
+          '.section.delta'));
+      assert.isAbove(chunks.length, 1);
+      assert.equal(chunks.indexOf(cursorElement.diffRow.parentElement), 0);
+
+      cursorElement.moveToLastChunk();
+
+      assert.equal(chunks.indexOf(cursorElement.diffRow.parentElement),
+          chunks.length - 1);
+    });
+
     test('cursor scroll behavior', () => {
       cursorElement._handleDiffRenderStart();
       assert.equal(cursorElement._scrollBehavior, 'keep-visible');
diff --git a/polygerrit-ui/app/elements/diff/gr-diff-host/gr-diff-host.js b/polygerrit-ui/app/elements/diff/gr-diff-host/gr-diff-host.js
index cb23308..a15ca37 100644
--- a/polygerrit-ui/app/elements/diff/gr-diff-host/gr-diff-host.js
+++ b/polygerrit-ui/app/elements/diff/gr-diff-host/gr-diff-host.js
@@ -590,7 +590,7 @@
         this.$.reporting.reportInteraction(EVENT_ZERO_REBASE);
       } else {
         this.$.reporting.reportInteraction(EVENT_NONZERO_REBASE,
-            percentRebaseDelta);
+            {percentRebaseDelta});
       }
     }
 
@@ -1023,13 +1023,16 @@
 
     _handleNormalizeRange(event) {
       this.$.reporting.reportInteraction('normalize-range',
-          `Modified invalid comment range on l. ${event.detail.lineNum}` +
-          ` of the ${event.detail.side} side`);
+          {
+            side: event.detail.side,
+            lineNum: event.detail.lineNum,
+          });
     }
 
     _handleDiffContextExpanded(event) {
       this.$.reporting.reportInteraction(
-          'diff-context-expanded', event.detail.numLines);
+          'diff-context-expanded', {numLines: event.detail.numLines}
+      );
     }
   }
 
diff --git a/polygerrit-ui/app/elements/diff/gr-diff-host/gr-diff-host_test.html b/polygerrit-ui/app/elements/diff/gr-diff-host/gr-diff-host_test.html
index 6d05876..d28f2a3 100644
--- a/polygerrit-ui/app/elements/diff/gr-diff-host/gr-diff-host_test.html
+++ b/polygerrit-ui/app/elements/diff/gr-diff-host/gr-diff-host_test.html
@@ -1069,8 +1069,10 @@
         };
         element._reportDiff(diff);
         assert.isTrue(reportStub.calledOnce);
-        assert.equal(reportStub.lastCall.args[0], 'rebase-percent-nonzero');
-        assert.strictEqual(reportStub.lastCall.args[1], 50);
+        assert.isTrue(reportStub.calledWith(
+            'rebase-percent-nonzero',
+            {percentRebaseDelta: 50}
+        ));
       });
 
       test('diff w/ all rebase delta', () => {
@@ -1081,8 +1083,10 @@
         }]};
         element._reportDiff(diff);
         assert.isTrue(reportStub.calledOnce);
-        assert.equal(reportStub.lastCall.args[0], 'rebase-percent-nonzero');
-        assert.strictEqual(reportStub.lastCall.args[1], 100);
+        assert.isTrue(reportStub.calledWith(
+            'rebase-percent-nonzero',
+            {percentRebaseDelta: 100}
+        ));
       });
 
       test('diff against parent event', () => {
diff --git a/polygerrit-ui/app/elements/shared/gr-account-label/gr-account-label.html b/polygerrit-ui/app/elements/shared/gr-account-label/gr-account-label.html
index 7ed7962..4bf1f1b 100644
--- a/polygerrit-ui/app/elements/shared/gr-account-label/gr-account-label.html
+++ b/polygerrit-ui/app/elements/shared/gr-account-label/gr-account-label.html
@@ -33,10 +33,9 @@
         content: var(--account-label-suffix);
       }
       gr-avatar {
-        height: 1.3em;
-        width: 1.3em;
-        margin-right: var(--spacing-xs);
-        vertical-align: -.25em;
+        height: var(--line-height-normal);
+        width: var(--line-height-normal);
+        vertical-align: top;
       }
       .text {
         @apply --gr-account-label-text-style;
diff --git a/polygerrit-ui/app/elements/shared/gr-account-list/gr-account-list.html b/polygerrit-ui/app/elements/shared/gr-account-list/gr-account-list.html
index 6103d6f..37591d8 100644
--- a/polygerrit-ui/app/elements/shared/gr-account-list/gr-account-list.html
+++ b/polygerrit-ui/app/elements/shared/gr-account-list/gr-account-list.html
@@ -32,6 +32,7 @@
         display: flex;
         flex: 1;
         min-width: 10em;
+        margin: var(--spacing-xs) var(--spacing-xs) var(--spacing-xs) 0;
       }
       .group {
         --account-label-suffix: ' (group)';
diff --git a/polygerrit-ui/app/elements/shared/gr-button/gr-button.js b/polygerrit-ui/app/elements/shared/gr-button/gr-button.js
index 681717f..a078be7 100644
--- a/polygerrit-ui/app/elements/shared/gr-button/gr-button.js
+++ b/polygerrit-ui/app/elements/shared/gr-button/gr-button.js
@@ -102,7 +102,7 @@
         }
       }
       this.$.reporting.reportInteraction('button-click',
-          path.trim().toLowerCase());
+          {path: path.trim().toLowerCase()});
     }
 
     _disabledChanged(disabled) {
diff --git a/polygerrit-ui/app/elements/shared/gr-cursor-manager/gr-cursor-manager.js b/polygerrit-ui/app/elements/shared/gr-cursor-manager/gr-cursor-manager.js
index 374204b..fd70fd9 100644
--- a/polygerrit-ui/app/elements/shared/gr-cursor-manager/gr-cursor-manager.js
+++ b/polygerrit-ui/app/elements/shared/gr-cursor-manager/gr-cursor-manager.js
@@ -163,6 +163,12 @@
       }
     }
 
+    moveToEnd() {
+      if (this.stops.length) {
+        this.setCursor(this.stops[this.stops.length - 1]);
+      }
+    }
+
     setCursorAtIndex(index, opt_noScroll) {
       this.setCursor(this.stops[index], opt_noScroll);
     }
diff --git a/polygerrit-ui/app/elements/shared/gr-dropdown-list/gr-dropdown-list.html b/polygerrit-ui/app/elements/shared/gr-dropdown-list/gr-dropdown-list.html
index ee9e7a0..f7349eb 100644
--- a/polygerrit-ui/app/elements/shared/gr-dropdown-list/gr-dropdown-list.html
+++ b/polygerrit-ui/app/elements/shared/gr-dropdown-list/gr-dropdown-list.html
@@ -59,16 +59,19 @@
         cursor: pointer;
         flex-direction: column;
         font-size: inherit;
+        // This variable was introduced in Dec 2019. We keep both min-height
+        // rules around, because --paper-item-min-height is not yet upstreamed.
+        --paper-item-min-height: 0;
         --paper-item: {
           min-height: 0;
           padding: 10px 16px;
-        }
+        };
         --paper-item-focused-before: {
           background-color: var(--selection-background-color);
-        }
+        };
         --paper-item-focused: {
           background-color: var(--selection-background-color);
-        }
+        };
       }
       paper-item:hover {
         background-color: var(--hover-background-color);
diff --git a/polygerrit-ui/app/elements/shared/gr-icons/gr-icons.html b/polygerrit-ui/app/elements/shared/gr-icons/gr-icons.html
index 5ecae00..75ab77a 100644
--- a/polygerrit-ui/app/elements/shared/gr-icons/gr-icons.html
+++ b/polygerrit-ui/app/elements/shared/gr-icons/gr-icons.html
@@ -54,6 +54,8 @@
       <g id="info"><path d="M12 2C6.48 2 2 6.48 2 12s4.48 10 10 10 10-4.48 10-10S17.52 2 12 2zm1 15h-2v-6h2v6zm0-8h-2V7h2v2z"></path></g>
       <!-- This SVG is a copy from material.io https://material.io/icons/#ic_hourglass_full-->
       <g id="hourglass"><path d="M6 2v6h.01L6 8.01 10 12l-4 4 .01.01H6V22h12v-5.99h-.01L18 16l-4-4 4-3.99-.01-.01H18V2H6z"/><path d="M0 0h24v24H0V0z" fill="none"/></g>
+      <!-- This SVG is a copy from material.io https://material.io/icons/#mode_comment-->
+      <g id="comment"><path d="M21.99 4c0-1.1-.89-2-1.99-2H4c-1.1 0-2 .9-2 2v12c0 1.1.9 2 2 2h14l4 4-.01-18z"/><path d="M0 0h24v24H0z" fill="none"/></g>
       <!-- This SVG is a copy from iron-icons https://github.com/PolymerElements/iron-icons/blob/master/iron-icons.js -->
       <g id="error"><path d="M12 2C6.48 2 2 6.48 2 12s4.48 10 10 10 10-4.48 10-10S17.52 2 12 2zm1 15h-2v-2h2v2zm0-4h-2V7h2v6z"></path></g>
       <!-- This is a custom PolyGerrit SVG -->
diff --git a/polygerrit-ui/app/elements/shared/gr-rest-api-interface/gr-auth.js b/polygerrit-ui/app/elements/shared/gr-rest-api-interface/gr-auth.js
index 2978193..7648b7d 100644
--- a/polygerrit-ui/app/elements/shared/gr-rest-api-interface/gr-auth.js
+++ b/polygerrit-ui/app/elements/shared/gr-rest-api-interface/gr-auth.js
@@ -264,7 +264,7 @@
     ERROR: 3,
   };
 
-  Auth.CREDS_EXPIRED_MSG = 'Credentails expired.';
+  Auth.CREDS_EXPIRED_MSG = 'Credentials expired.';
 
   // TODO(taoalpha): this whole thing should be moved to a service
   window.Auth = Auth;
diff --git a/polygerrit-ui/app/elements/shared/gr-rest-api-interface/gr-rest-api-interface.js b/polygerrit-ui/app/elements/shared/gr-rest-api-interface/gr-rest-api-interface.js
index 472a7ea..4aefe46 100644
--- a/polygerrit-ui/app/elements/shared/gr-rest-api-interface/gr-rest-api-interface.js
+++ b/polygerrit-ui/app/elements/shared/gr-rest-api-interface/gr-rest-api-interface.js
@@ -1001,7 +1001,6 @@
         this.ListChangesOption.MESSAGES,
         this.ListChangesOption.SUBMITTABLE,
         this.ListChangesOption.WEB_LINKS,
-        this.ListChangesOption.SKIP_MERGEABLE,
         this.ListChangesOption.SKIP_DIFFSTAT,
       ];
       return this.getConfig(false).then(config => {
@@ -1024,7 +1023,6 @@
       const optionsHex = this.listChangesOptionsToHex(
           this.ListChangesOption.ALL_COMMITS,
           this.ListChangesOption.ALL_REVISIONS,
-          this.ListChangesOption.SKIP_MERGEABLE,
           this.ListChangesOption.SKIP_DIFFSTAT
       );
       return this._getChangeDetail(changeNum, optionsHex, opt_errFn,
diff --git a/polygerrit-ui/app/elements/shared/gr-rest-api-interface/gr-reviewer-updates-parser.js b/polygerrit-ui/app/elements/shared/gr-rest-api-interface/gr-reviewer-updates-parser.js
index 1c62ca3..d599563 100644
--- a/polygerrit-ui/app/elements/shared/gr-rest-api-interface/gr-reviewer-updates-parser.js
+++ b/polygerrit-ui/app/elements/shared/gr-rest-api-interface/gr-reviewer-updates-parser.js
@@ -147,15 +147,16 @@
    */
   GrReviewerUpdatesParser.prototype._getUpdateMessage = function(prev, state) {
     if (prev === 'REMOVED' || !prev) {
-      return 'added to ' + state + ': ';
+      return 'Added to ' + state.toLowerCase() + ': ';
     } else if (state === 'REMOVED') {
       if (prev) {
-        return 'removed from ' + prev + ': ';
+        return 'Removed from ' + prev.toLowerCase() + ': ';
       } else {
-        return 'removed : ';
+        return 'Removed : ';
       }
     } else {
-      return 'moved from ' + prev + ' to ' + state + ': ';
+      return 'Moved from ' + prev.toLowerCase() + ' to ' + state.toLowerCase() +
+          ': ';
     }
   };
 
diff --git a/polygerrit-ui/app/elements/shared/gr-rest-api-interface/gr-reviewer-updates-parser_test.html b/polygerrit-ui/app/elements/shared/gr-rest-api-interface/gr-reviewer-updates-parser_test.html
index 818bfa9..593fcec 100644
--- a/polygerrit-ui/app/elements/shared/gr-rest-api-interface/gr-reviewer-updates-parser_test.html
+++ b/polygerrit-ui/app/elements/shared/gr-rest-api-interface/gr-reviewer-updates-parser_test.html
@@ -239,15 +239,15 @@
       assert.equal(change.reviewer_updates[1].updates.length, 3);
 
       let items = change.reviewer_updates[0].updates;
-      assert.equal(items[0].message, 'added to CC: ');
+      assert.equal(items[0].message, 'Added to cc: ');
       assert.deepEqual(items[0].reviewers, [reviewer1, reviewer2]);
 
       items = change.reviewer_updates[1].updates;
-      assert.equal(items[0].message, 'moved from CC to REVIEWER: ');
+      assert.equal(items[0].message, 'Moved from cc to reviewer: ');
       assert.deepEqual(items[0].reviewers, [reviewer1]);
-      assert.equal(items[1].message, 'removed from REVIEWER: ');
+      assert.equal(items[1].message, 'Removed from reviewer: ');
       assert.deepEqual(items[1].reviewers, [reviewer1]);
-      assert.equal(items[2].message, 'added to REVIEWER: ');
+      assert.equal(items[2].message, 'Added to reviewer: ');
       assert.deepEqual(items[2].reviewers, [reviewer1, reviewer2]);
     });
 
diff --git a/polygerrit-ui/app/elements/shared/gr-textarea/gr-textarea.js b/polygerrit-ui/app/elements/shared/gr-textarea/gr-textarea.js
index a604bcf..07b664b2 100644
--- a/polygerrit-ui/app/elements/shared/gr-textarea/gr-textarea.js
+++ b/polygerrit-ui/app/elements/shared/gr-textarea/gr-textarea.js
@@ -201,7 +201,7 @@
       this.text = this._getText(text);
       this.$.textarea.selectionStart = colonIndex + 1;
       this.$.textarea.selectionEnd = colonIndex + 1;
-      this.$.reporting.reportInteraction('select-emoji');
+      this.$.reporting.reportInteraction('select-emoji', {type: text});
       this._resetEmojiDropdown();
     }
 
diff --git a/polygerrit-ui/app/styles/themes/app-theme.html b/polygerrit-ui/app/styles/themes/app-theme.html
index 8ad2576..db49822 100644
--- a/polygerrit-ui/app/styles/themes/app-theme.html
+++ b/polygerrit-ui/app/styles/themes/app-theme.html
@@ -68,7 +68,7 @@
   /* comment background colors */
   --comment-background-color: #e8eaed;
   --robot-comment-background-color: #e8f0fe;
-  --unresolved-comment-background-color: #fef7f0;
+  --unresolved-comment-background-color: #fef7e0;
   /* vote background colors */
   --vote-color-approved: #9fcc6b;
   --vote-color-disliked: #f7c4cb;
@@ -77,7 +77,7 @@
   --vote-color-rejected: #f7a1ad;
 
   /* misc colors */
-  --border-color: #dadce0;
+  --border-color: #e8e8e8;
 
   /* fonts */
   --font-family: 'Roboto', -apple-system, BlinkMacSystemFont, 'Segoe UI', Helvetica, Arial, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol';
diff --git a/resources/com/google/gerrit/server/mail/InboundEmailRejection.soy b/resources/com/google/gerrit/server/mail/InboundEmailRejection.soy
index e88c424..16c5c8d 100644
--- a/resources/com/google/gerrit/server/mail/InboundEmailRejection.soy
+++ b/resources/com/google/gerrit/server/mail/InboundEmailRejection.soy
@@ -64,6 +64,7 @@
 {/template}
 
 {template .InboundEmailRejection_COMMENT_REJECTED kind="text"}
-  Gerrit Code Review rejected one or more comments because they did not pass validation.
+  Gerrit Code Review rejected one or more comments because they did not pass validation, or
+  because the maximum number of comments per change would be exceeded.
   {call .InboundEmailRejectionFooter /}
 {/template}
diff --git a/resources/com/google/gerrit/server/mail/InboundEmailRejectionHtml.soy b/resources/com/google/gerrit/server/mail/InboundEmailRejectionHtml.soy
index e17508d..8762e10 100644
--- a/resources/com/google/gerrit/server/mail/InboundEmailRejectionHtml.soy
+++ b/resources/com/google/gerrit/server/mail/InboundEmailRejectionHtml.soy
@@ -81,7 +81,8 @@
 
 {template .InboundEmailRejectionHtml_COMMENT_REJECTED}
   <p>
-    Gerrit Code Review rejected one or more comments because they did not pass validation.
+    Gerrit Code Review rejected one or more comments because they did not pass validation, or
+    because the maximum number of comments per change would be exceeded.
   </p>
   {call .InboundEmailRejectionFooterHtml /}
 {/template}
diff --git a/tools/BUILD b/tools/BUILD
index 32599aa..2d702fe 100644
--- a/tools/BUILD
+++ b/tools/BUILD
@@ -86,6 +86,7 @@
         "-Xep:TypeParameterShadowing:ERROR",
         "-Xep:TypeParameterUnusedInFormals:ERROR",
         "-Xep:URLEqualsHashCode:ERROR",
+        "-Xep:UnusedException:ERROR",
         "-Xep:UnsynchronizedOverridesSynchronized:ERROR",
         "-Xep:WaitNotInLoop:ERROR",
         "-Xep:WildcardImport:ERROR",
diff --git a/tools/bzl/bazelisk_version.bzl b/tools/bzl/bazelisk_version.bzl
deleted file mode 100644
index d8b3d10..0000000
--- a/tools/bzl/bazelisk_version.bzl
+++ /dev/null
@@ -1,16 +0,0 @@
-_template = """
-load("@bazel_skylib//lib:versions.bzl", "versions")
-
-def check_bazel_version():
-  versions.check(minimum_bazel_version = "{version}")
-""".strip()
-
-def _impl(repository_ctx):
-    repository_ctx.symlink(Label("@//:.bazelversion"), ".bazelversion")
-    bazelversion = repository_ctx.read(".bazelversion").strip()
-
-    repository_ctx.file("BUILD", executable = False)
-
-    repository_ctx.file("check.bzl", executable = False, content = _template.format(version = bazelversion))
-
-bazelisk_version = repository_rule(implementation = _impl)
diff --git a/tools/bzl/java.bzl b/tools/bzl/java.bzl
deleted file mode 100644
index 8996b69..0000000
--- a/tools/bzl/java.bzl
+++ /dev/null
@@ -1,28 +0,0 @@
-# Copyright (C) 2016 The Android Open Source Project
-#
-# Licensed under the Apache License, Version 2.0 (the "License");
-# you may not use this file except in compliance with the License.
-# You may obtain a copy of the License at
-#
-# http://www.apache.org/licenses/LICENSE-2.0
-#
-# Unless required by applicable law or agreed to in writing, software
-# distributed under the License is distributed on an "AS IS" BASIS,
-# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-# See the License for the specific language governing permissions and
-# limitations under the License.
-
-# Syntactic sugar for native java_library() rule:
-#   accept exported_deps attributes
-
-load("@rules_java//java:defs.bzl", "java_library")
-
-def java_library2(deps = [], exported_deps = [], exports = [], **kwargs):
-    if exported_deps:
-        deps = deps + exported_deps
-        exports = exports + exported_deps
-    java_library(
-        deps = deps,
-        exports = exports,
-        **kwargs
-    )
diff --git a/tools/bzl/plugin.bzl b/tools/bzl/plugin.bzl
index ed64d1b..64a6e22 100644
--- a/tools/bzl/plugin.bzl
+++ b/tools/bzl/plugin.bzl
@@ -21,6 +21,7 @@
         manifest_entries = [],
         dir_name = None,
         target_suffix = "",
+        deploy_env = [],
         **kwargs):
     java_library(
         name = name + "__plugin",
@@ -43,6 +44,7 @@
         runtime_deps = [
             ":%s__plugin" % name,
         ] + static_jars,
+        deploy_env = deploy_env,
         visibility = ["//visibility:public"],
         **kwargs
     )
diff --git a/tools/coverage.sh b/tools/coverage.sh
index 11e50e6..c92d5cf 100755
--- a/tools/coverage.sh
+++ b/tools/coverage.sh
@@ -33,7 +33,7 @@
 cp -r {java,javatests}/* ${destdir}/java
 
 mkdir -p ${destdir}/plugins
-for plugin in `find plugins/ -type d` -maxdepth 1
+for plugin in `find plugins/ -maxdepth 1 -type d`
 do
   mkdir -p ${destdir}/${plugin}/java
   cp -r plugins/*/{java,javatests}/* ${destdir}/${plugin}/java
diff --git a/tools/nongoogle.bzl b/tools/nongoogle.bzl
index acf01fa..1210504 100644
--- a/tools/nongoogle.bzl
+++ b/tools/nongoogle.bzl
@@ -126,18 +126,18 @@
         sha1 = "dc13ae4faca6df981fc7aeb5a522d9db446d5d50",
     )
 
-    TESTCONTAINERS_VERSION = "1.12.4"
+    TESTCONTAINERS_VERSION = "1.12.5"
 
     maven_jar(
         name = "testcontainers",
         artifact = "org.testcontainers:testcontainers:" + TESTCONTAINERS_VERSION,
-        sha1 = "456b6facac12c4b67130d9056a43c011679e9f0c",
+        sha1 = "ca8a8dfbd3b194fb6c541e3bad858898865ce069",
     )
 
     maven_jar(
         name = "testcontainers-elasticsearch",
         artifact = "org.testcontainers:elasticsearch:" + TESTCONTAINERS_VERSION,
-        sha1 = "9e210c277a35a95a76d03a79e2812575bd07391c",
+        sha1 = "d88f79c83e7057edd23d33621c36d6267b9c4e96",
     )
 
     maven_jar(
diff --git a/tools/remote-bazelrc b/tools/remote-bazelrc
index b26a8e8..826be1d 100644
--- a/tools/remote-bazelrc
+++ b/tools/remote-bazelrc
@@ -70,9 +70,6 @@
 # Enable remote execution so actions are performed on the remote systems.
 build:remote --remote_executor=remotebuildexecution.googleapis.com
 
-# Enable encryption.
-build:remote --tls_enabled=true
-
 # Set a higher timeout value, just in case.
 build:remote --remote_timeout=3600