diff --git a/Documentation/access-control.txt b/Documentation/access-control.txt
index cc03334..72e309a 100644
--- a/Documentation/access-control.txt
+++ b/Documentation/access-control.txt
@@ -859,8 +859,8 @@
 [[category_edit_hashtags]]
 === Edit Hashtags
 
-This category permits users to add or remove hashtags on a change that
-is uploaded for review.
+This category permits users to add or remove
+link:intro-user.html#hashtags[hashtags] on a change that is uploaded for review.
 
 The change owner, branch owners, project owners, and site administrators
 can always edit or remove hashtags (even without having the `Edit Hashtags`
diff --git a/Documentation/cmd-stream-events.txt b/Documentation/cmd-stream-events.txt
index ba346e7..a75f610 100644
--- a/Documentation/cmd-stream-events.txt
+++ b/Documentation/cmd-stream-events.txt
@@ -151,7 +151,8 @@
 
 === Hashtags Changed
 
-Sent when the hashtags have been added to or removed from a change.
+Sent when the link:intro-user.html#hashtags[hashtags] have been added to or
+removed from a change.
 
 type:: "hashtags-changed"
 
diff --git a/Documentation/config-gerrit.txt b/Documentation/config-gerrit.txt
index 1b6d116..f15363d 100644
--- a/Documentation/config-gerrit.txt
+++ b/Documentation/config-gerrit.txt
@@ -2702,6 +2702,10 @@
 limit will truncate the list (but will still set `_more_changes` on
 result lists). Set to 0 for no limit.
 +
+When `index.type` is set to `ELASTICSEARCH`, this value should not exceed
+the `index.max_result_window` value configured on the Elasticsearch
+server.
++
 Defaults to no limit.
 
 [[index.maxPages]]index.maxPages::
diff --git a/Documentation/config-gitweb.txt b/Documentation/config-gitweb.txt
index fcfd0e1..d49acfee 100644
--- a/Documentation/config-gitweb.txt
+++ b/Documentation/config-gitweb.txt
@@ -17,8 +17,9 @@
 Linux distributions.
 
 ----
-  git config --file $site_path/etc/gerrit.config gitweb.cgi /usr/lib/cgi-bin/gitweb.cgi
-  git config --file $site_path/etc/gerrit.config --unset gitweb.url
+  git config -f $site_path/etc/gerrit.config gitweb.type gitweb
+  git config -f $site_path/etc/gerrit.config gitweb.cgi /usr/lib/cgi-bin/gitweb.cgi
+  git config -f $site_path/etc/gerrit.config --unset gitweb.url
 ----
 
 Alternatively, if Gerrit is served behind reverse proxy, it can
@@ -28,8 +29,9 @@
 To enable this feature, set both: `gitweb.cgi` and `gitweb.url`.
 
 ----
-  git config --file $site_path/etc/gerrit.config gitweb.cgi /usr/lib/cgi-bin/gitweb.cgi
-  git config --file $site_path/etc/gerrit.config gitweb.url /pretty/path/to/gitweb
+  git config -f $site_path/etc/gerrit.config gitweb.type gitweb
+  git config -f $site_path/etc/gerrit.config gitweb.cgi /usr/lib/cgi-bin/gitweb.cgi
+  git config -f $site_path/etc/gerrit.config gitweb.url /pretty/path/to/gitweb
 ----
 
 After updating `'$site_path'/etc/gerrit.config`, the Gerrit server must
@@ -77,13 +79,13 @@
 On Ubuntu:
 
 ----
-  $ sudo apt-get install gitweb
+  sudo apt-get install gitweb
 ----
 
 With Yum:
 
 ----
-  $ yum install gitweb
+  yum install gitweb
 ----
 
 ===== Configure Gitweb
@@ -125,14 +127,14 @@
 Link gitweb to `/var/www/gitweb`, check `/etc/gitweb.conf` if unsure of paths:
 
 ----
-  $ sudo ln -s /usr/share/gitweb /var/www/gitweb
+  sudo ln -s /usr/share/gitweb /var/www/gitweb
 ----
 
 Add the gitweb directory to the Apache configuration by creating a "gitweb"
 file inside the Apache conf.d directory:
 
 ----
-  $ touch /etc/apache/conf.d/gitweb
+  touch /etc/apache/conf.d/gitweb
 ----
 
 Add the following to /etc/apache/conf.d/gitweb:
@@ -152,7 +154,7 @@
 ===== Restart the Apache Web Server
 
 ----
-  $ sudo /etc/init.d/apache2 restart
+  sudo /etc/init.d/apache2 restart
 ----
 
 Now you should be able to view your repository projects online:
@@ -184,7 +186,7 @@
 following to check:
 
 ----
-$ perl -mCGI -mEncode -mFcntl -mFile::Find -mFile::Basename -e ""
+  perl -mCGI -mEncode -mFcntl -mFile::Find -mFile::Basename -e ""
 ----
 
 You may encounter the following exception:
@@ -220,21 +222,22 @@
 namespace is available.
 
 ----
-$ git config -f $site_path/etc/gerrit.config --unset gitweb.cgi
-$ git config -f $site_path/etc/gerrit.config gitweb.url https://gitweb.corporation.com
+  git config -f $site_path/etc/gerrit.config gitweb.type gitweb
+  git config -f $site_path/etc/gerrit.config --unset gitweb.cgi
+  git config -f $site_path/etc/gerrit.config gitweb.url https://gitweb.corporation.com
 ----
 
-If you're not following the traditional \{projectName\}.git project naming conventions,
+If you're not following the traditional `\{projectName\}.git` project naming conventions,
 you will want to customize Gerrit to read them. Add the following:
 
 ----
-$ git config -f $site_path/etc/gerrit.config gitweb.type custom
-$ git config -f $site_path/etc/gerrit.config gitweb.project ?p=\${project}\;a=summary
-$ git config -f $site_path/etc/gerrit.config gitweb.revision ?p=\${project}\;a=commit\;h=\${commit}
-$ git config -f $site_path/etc/gerrit.config gitweb.branch ?p=\${project}\;a=shortlog\;h=\${branch}
-$ git config -f $site_path/etc/gerrit.config gitweb.roottree ?p=\${project}\;a=tree\;hb=\${commit}
-$ git config -f $site_path/etc/gerrit.config gitweb.file ?p=\${project}\;hb=\${commit}\;f=\${file}
-$ git config -f $site_path/etc/gerrit.config gitweb.filehistory ?p=\${project}\;a=history\;hb=\${branch}\;f=\${file}
+  git config -f $site_path/etc/gerrit.config gitweb.type custom
+  git config -f $site_path/etc/gerrit.config gitweb.project ?p=\${project}\;a=summary
+  git config -f $site_path/etc/gerrit.config gitweb.revision ?p=\${project}\;a=commit\;h=\${commit}
+  git config -f $site_path/etc/gerrit.config gitweb.branch ?p=\${project}\;a=shortlog\;h=\${branch}
+  git config -f $site_path/etc/gerrit.config gitweb.roottree ?p=\${project}\;a=tree\;hb=\${commit}
+  git config -f $site_path/etc/gerrit.config gitweb.file ?p=\${project}\;hb=\${commit}\;f=\${file}
+  git config -f $site_path/etc/gerrit.config gitweb.filehistory ?p=\${project}\;a=history\;hb=\${branch}\;f=\${file}
 ----
 
 After updating `'$site_path'/etc/gerrit.config`, the Gerrit server must
diff --git a/Documentation/intro-user.txt b/Documentation/intro-user.txt
index 29e02f0..3805c80 100644
--- a/Documentation/intro-user.txt
+++ b/Documentation/intro-user.txt
@@ -472,6 +472,10 @@
 the use of the command line flag `--push-option`, aliased to `-o`,
 followed by `topic=...`.
 
+Gerrit may be link:config-gerrit.html#change.submitWholeTopic[configured] to
+submit all changes in a topic together with a single click, even when topics
+span multiple projects.
+
 .Set Topic on Push
 ----
   $ git push origin HEAD:refs/for/master%topic=multi-master
@@ -480,6 +484,30 @@
   $ git push origin HEAD:refs/heads/master -o topic=multi-master
 ----
 
+[[hashtags]]
+== Using Hashtags
+
+Hashtags are freeform strings associated with a change, like on social media
+platforms. In Gerrit, you explicitly associate hashtags with changes using a
+dedicated area of the UI; they are not parsed from commit messages or comments.
+
+Similar to topics, hashtags can be used to group related changes together, and
+to search using the link:user-search.html#hashtag[`hashtag:`] operator. Unlike
+topics, a change can have multiple hashtags, and they are only used for
+informational grouping; changes with the same hashtags are not necessarily
+submitted together.
+
+The hashtag feature is only available when running under
+link:note-db.html[NoteDb].
+
+.Set Hashtag on Push
+----
+  $ git push origin HEAD:refs/for/master%t=stable-bugfix
+
+  // this is the same as:
+  $ git push origin HEAD:refs/heads/master -o t=stable-bugfix
+----
+
 [[wip]]
 == Work-in-Progress Changes
 
diff --git a/Documentation/user-search.txt b/Documentation/user-search.txt
index ba42304..0f030a6 100644
--- a/Documentation/user-search.txt
+++ b/Documentation/user-search.txt
@@ -191,6 +191,11 @@
 often combined with 'branch:' and 'project:' operators to select
 all related changes in a series.
 
+[[hashtag]]
+hashtag:'HASHTAG'::
++
+Changes whose link:intro-user.html#hashtags[hashtag] matches 'HASHTAG' exactly.
+
 [[ref]]
 ref:'REF'::
 +
diff --git a/Documentation/user-upload.txt b/Documentation/user-upload.txt
index 9be39a6..c12a38c 100644
--- a/Documentation/user-upload.txt
+++ b/Documentation/user-upload.txt
@@ -243,12 +243,11 @@
 [[topic]]
 ==== Topic
 
-To include a short tag associated with all of the changes in the
-same group, such as the local topic branch name, append it after
-the destination branch name or add it with the command line flag
-`--push-option`, aliased to `-o`. In this example the short topic
-tag 'driver/i42' will be saved on each change this push creates or
-updates:
+To include a short link:intro-user.html#topics[topic] associated with all
+of the changes in the same group, such as the local topic branch name,
+append it after the destination branch name or add it with the command line
+flag `--push-option`, aliased to `-o`. In this example the short topic name
+'driver/i42' will be saved on each change this push creates or updates:
 
 ----
   git push ssh://john.doe@git.example.com:29418/kernel/common HEAD:refs/for/experimental%topic=driver/i42
@@ -257,6 +256,20 @@
   git push ssh://john.doe@git.example.com:29418/kernel/common HEAD:refs/for/experimental -o topic=driver/i42
 ----
 
+[[hashtag]]
+==== Hashtag
+
+To include a link:intro-user.html#hashtags[hashtag] associated with all of the
+changes in the same group, use the `hashtag` or `t` option:
+
+----
+  // these are all equivalent
+  git push ssh://john.doe@git.example.com:29418/kernel/common HEAD:refs/for/experimental%hashtag=stable-fix
+  git push ssh://john.doe@git.example.com:29418/kernel/common HEAD:refs/for/experimental%t=stable-fix
+  git push ssh://john.doe@git.example.com:29418/kernel/common HEAD:refs/for/experimental -o hashtag=stable-fix
+  git push ssh://john.doe@git.example.com:29418/kernel/common HEAD:refs/for/experimental -o t=stable-fix
+----
+
 [[private]]
 ==== Private Changes
 
diff --git a/java/com/google/gerrit/pgm/init/InitIndex.java b/java/com/google/gerrit/pgm/init/InitIndex.java
index 6ad0a6b..740f4ce 100644
--- a/java/com/google/gerrit/pgm/init/InitIndex.java
+++ b/java/com/google/gerrit/pgm/init/InitIndex.java
@@ -40,6 +40,7 @@
   private final SitePaths site;
   private final InitFlags initFlags;
   private final Section gerrit;
+  private final Section.Factory sections;
 
   @Inject
   InitIndex(ConsoleUI ui, Section.Factory sections, SitePaths site, InitFlags initFlags) {
@@ -48,6 +49,7 @@
     this.gerrit = sections.get("gerrit", null);
     this.site = site;
     this.initFlags = initFlags;
+    this.sections = sections;
   }
 
   @Override
@@ -59,10 +61,15 @@
     }
 
     if (type == IndexType.ELASTICSEARCH) {
-      index.select("Transport protocol", "protocol", "http", Sets.newHashSet("http", "https"));
-      index.string("Hostname", "hostname", "localhost");
-      index.string("Port", "port", "9200");
-      index.string("Index Name", "name", "gerrit");
+      Section elasticsearch = sections.get("elasticsearch", null);
+      elasticsearch.string("Index Prefix", "prefix", "gerrit");
+      String name = ui.readString("default", "Server Name");
+
+      Section defaultServer = sections.get("elasticsearch", name);
+      defaultServer.select(
+          "Transport protocol", "protocol", "http", Sets.newHashSet("http", "https"));
+      defaultServer.string("Hostname", "hostname", "localhost");
+      defaultServer.string("Port", "port", "9200");
     }
 
     if ((site.isNew || isEmptySite()) && type == IndexType.LUCENE) {
diff --git a/java/com/google/gerrit/server/CommentsUtil.java b/java/com/google/gerrit/server/CommentsUtil.java
index 1c71c70..56b1724 100644
--- a/java/com/google/gerrit/server/CommentsUtil.java
+++ b/java/com/google/gerrit/server/CommentsUtil.java
@@ -491,25 +491,21 @@
   }
 
   public static void setCommentRevId(Comment c, PatchListCache cache, Change change, PatchSet ps)
-      throws OrmException {
+      throws PatchListNotAvailableException {
     checkArgument(
         c.key.patchSetId == ps.getId().get(),
         "cannot set RevId for patch set %s on comment %s",
         ps.getId(),
         c);
     if (c.revId == null) {
-      try {
-        if (Side.fromShort(c.side) == Side.PARENT) {
-          if (c.side < 0) {
-            c.revId = ObjectId.toString(cache.getOldId(change, ps, -c.side));
-          } else {
-            c.revId = ObjectId.toString(cache.getOldId(change, ps, null));
-          }
+      if (Side.fromShort(c.side) == Side.PARENT) {
+        if (c.side < 0) {
+          c.revId = ObjectId.toString(cache.getOldId(change, ps, -c.side));
         } else {
-          c.revId = ps.getRevision().get();
+          c.revId = ObjectId.toString(cache.getOldId(change, ps, null));
         }
-      } catch (PatchListNotAvailableException e) {
-        throw new OrmException(e);
+      } else {
+        c.revId = ps.getRevision().get();
       }
     }
   }
@@ -576,7 +572,11 @@
       // Draft may have been created by a different real user; copy the current real user. (Only
       // applies to X-Gerrit-RunAs, since modifying drafts via on_behalf_of is not allowed.)
       ctx.getUser().updateRealAccountId(d::setRealAuthor);
-      setCommentRevId(d, patchListCache, notes.getChange(), ps);
+      try {
+        setCommentRevId(d, patchListCache, notes.getChange(), ps);
+      } catch (PatchListNotAvailableException e) {
+        throw new OrmException(e);
+      }
     }
     putComments(ctx.getDb(), ctx.getUpdate(psId), PUBLISHED, drafts);
   }
diff --git a/java/com/google/gerrit/server/change/ChangeKindCacheImpl.java b/java/com/google/gerrit/server/change/ChangeKindCacheImpl.java
index fc9ae4b..7b44d8f 100644
--- a/java/com/google/gerrit/server/change/ChangeKindCacheImpl.java
+++ b/java/com/google/gerrit/server/change/ChangeKindCacheImpl.java
@@ -241,8 +241,15 @@
           return ChangeKind.NO_CHANGE;
         }
 
-        if ((prior.getParentCount() != 1 || next.getParentCount() != 1)
-            && (!onlyFirstParentChanged(prior, next) || prior.getParentCount() == 0)) {
+        if (prior.getParentCount() == 0 || next.getParentCount() == 0) {
+          // At this point we have considered all the kinds that could be applicable to root
+          // commits; the remainder of the checks in this method all assume that both commits have
+          // at least one parent.
+          return ChangeKind.REWORK;
+        }
+
+        if ((prior.getParentCount() > 1 || next.getParentCount() > 1)
+            && !onlyFirstParentChanged(prior, next)) {
           // Trivial rebases done by machine only work well on 1 parent.
           return ChangeKind.REWORK;
         }
diff --git a/java/com/google/gerrit/server/index/change/AllChangesIndexer.java b/java/com/google/gerrit/server/index/change/AllChangesIndexer.java
index 599596f..54bf0dc 100644
--- a/java/com/google/gerrit/server/index/change/AllChangesIndexer.java
+++ b/java/com/google/gerrit/server/index/change/AllChangesIndexer.java
@@ -111,20 +111,24 @@
     SortedSet<ProjectHolder> projects = new TreeSet<>();
     int changeCount = 0;
     Stopwatch sw = Stopwatch.createStarted();
+    int projectsFailed = 0;
     for (Project.NameKey name : projectCache.all()) {
       try (Repository repo = repoManager.openRepository(name)) {
         long size = estimateSize(repo);
         changeCount += size;
         projects.add(new ProjectHolder(name, size));
       } catch (IOException e) {
-        log.error("Error collecting projects", e);
-        return new Result(sw, false, 0, 0);
+        log.error("Error collecting project {}", name, e);
+        projectsFailed++;
+        if (projectsFailed > projects.size() / 2) {
+          log.error("Over 50% of the projects could not be collected: aborted");
+          return new Result(sw, false, 0, 0);
+        }
       }
       pm.update(1);
     }
     pm.endTask();
     setTotalWork(changeCount);
-
     return indexAll(index, projects);
   }
 
diff --git a/java/com/google/gerrit/server/mail/receive/MailProcessor.java b/java/com/google/gerrit/server/mail/receive/MailProcessor.java
index 9917261..949cd82 100644
--- a/java/com/google/gerrit/server/mail/receive/MailProcessor.java
+++ b/java/com/google/gerrit/server/mail/receive/MailProcessor.java
@@ -46,6 +46,7 @@
 import com.google.gerrit.server.mail.send.InboundEmailRejectionSender;
 import com.google.gerrit.server.notedb.ChangeNotes;
 import com.google.gerrit.server.patch.PatchListCache;
+import com.google.gerrit.server.patch.PatchListNotAvailableException;
 import com.google.gerrit.server.query.change.ChangeData;
 import com.google.gerrit.server.query.change.InternalChangeQuery;
 import com.google.gerrit.server.update.BatchUpdate;
@@ -274,7 +275,7 @@
 
     @Override
     public boolean updateChange(ChangeContext ctx)
-        throws OrmException, UnprocessableEntityException {
+        throws OrmException, UnprocessableEntityException, PatchListNotAvailableException {
       patchSet = psUtil.get(ctx.getDb(), ctx.getNotes(), psId);
       notes = ctx.getNotes();
       if (patchSet == null) {
@@ -371,7 +372,7 @@
 
     private Comment persistentCommentFromMailComment(
         ChangeContext ctx, MailComment mailComment, PatchSet patchSetForComment)
-        throws OrmException, UnprocessableEntityException {
+        throws OrmException, UnprocessableEntityException, PatchListNotAvailableException {
       String fileName;
       // The patch set that this comment is based on is different if this
       // comment was sent in reply to a comment on a previous patch set.
diff --git a/java/com/google/gerrit/server/notedb/PrimaryStorageMigrator.java b/java/com/google/gerrit/server/notedb/PrimaryStorageMigrator.java
index e33ece9..69cc2eb 100644
--- a/java/com/google/gerrit/server/notedb/PrimaryStorageMigrator.java
+++ b/java/com/google/gerrit/server/notedb/PrimaryStorageMigrator.java
@@ -80,6 +80,18 @@
 public class PrimaryStorageMigrator {
   private static final Logger log = LoggerFactory.getLogger(PrimaryStorageMigrator.class);
 
+  /**
+   * Exception thrown during migration if the change has no {@code noteDbState} field at the
+   * beginning of the migration.
+   */
+  public static class NoNoteDbStateException extends RuntimeException {
+    private static final long serialVersionUID = 1L;
+
+    private NoNoteDbStateException(Change.Id id) {
+      super("change " + id + " has no note_db_state; rebuild it first");
+    }
+  }
+
   private final AllUsersName allUsers;
   private final ChangeNotes.Factory changeNotesFactory;
   private final ChangeRebuilder rebuilder;
@@ -280,9 +292,11 @@
                     NoteDbChangeState state = NoteDbChangeState.parse(change);
                     if (state == null) {
                       // Could rebuild the change here, but that's more complexity, and this
-                      // really shouldn't happen.
-                      throw new OrmRuntimeException(
-                          "change " + id + " has no note_db_state; rebuild it first");
+                      // normally shouldn't happen.
+                      //
+                      // Known cases where this happens are described in and handled by
+                      // NoteDbMigrator#canSkipPrimaryStorageMigration.
+                      throw new NoNoteDbStateException(id);
                     }
                     // If the change is already read-only, then the lease is held by another
                     // (likely failed) migrator thread. Fail early, as we can't take over
diff --git a/java/com/google/gerrit/server/notedb/rebuild/ChangeRebuilderImpl.java b/java/com/google/gerrit/server/notedb/rebuild/ChangeRebuilderImpl.java
index 9ab06e8..92a878c 100644
--- a/java/com/google/gerrit/server/notedb/rebuild/ChangeRebuilderImpl.java
+++ b/java/com/google/gerrit/server/notedb/rebuild/ChangeRebuilderImpl.java
@@ -522,8 +522,7 @@
   }
 
   private void flushEventsToDraftUpdate(
-      NoteDbUpdateManager manager, EventList<DraftCommentEvent> events, Change change)
-      throws OrmException {
+      NoteDbUpdateManager manager, EventList<DraftCommentEvent> events, Change change) {
     if (events.isEmpty()) {
       return;
     }
diff --git a/java/com/google/gerrit/server/notedb/rebuild/CommentEvent.java b/java/com/google/gerrit/server/notedb/rebuild/CommentEvent.java
index c8a649e..611f32e 100644
--- a/java/com/google/gerrit/server/notedb/rebuild/CommentEvent.java
+++ b/java/com/google/gerrit/server/notedb/rebuild/CommentEvent.java
@@ -24,9 +24,13 @@
 import com.google.gerrit.server.CommentsUtil;
 import com.google.gerrit.server.notedb.ChangeUpdate;
 import com.google.gerrit.server.patch.PatchListCache;
-import com.google.gwtorm.server.OrmException;
+import com.google.gerrit.server.patch.PatchListNotAvailableException;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
 
 class CommentEvent extends Event {
+  private static final Logger log = LoggerFactory.getLogger(CommentEvent.class);
+
   public final Comment c;
   private final Change change;
   private final PatchSet ps;
@@ -57,10 +61,19 @@
   }
 
   @Override
-  void apply(ChangeUpdate update) throws OrmException {
+  void apply(ChangeUpdate update) {
     checkUpdate(update);
     if (c.revId == null) {
-      setCommentRevId(c, cache, change, ps);
+      try {
+        setCommentRevId(c, cache, change, ps);
+      } catch (PatchListNotAvailableException e) {
+        log.warn(
+            "Unable to determine parent commit of patch set {} ({}); omitting inline comment",
+            ps.getId(),
+            ps.getRevision(),
+            c);
+        return;
+      }
     }
     update.putComment(PatchLineComment.Status.PUBLISHED, c);
   }
diff --git a/java/com/google/gerrit/server/notedb/rebuild/DraftCommentEvent.java b/java/com/google/gerrit/server/notedb/rebuild/DraftCommentEvent.java
index 914930c..3bc3a58 100644
--- a/java/com/google/gerrit/server/notedb/rebuild/DraftCommentEvent.java
+++ b/java/com/google/gerrit/server/notedb/rebuild/DraftCommentEvent.java
@@ -24,9 +24,13 @@
 import com.google.gerrit.server.notedb.ChangeDraftUpdate;
 import com.google.gerrit.server.notedb.ChangeUpdate;
 import com.google.gerrit.server.patch.PatchListCache;
-import com.google.gwtorm.server.OrmException;
+import com.google.gerrit.server.patch.PatchListNotAvailableException;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
 
 class DraftCommentEvent extends Event {
+  private static final Logger log = LoggerFactory.getLogger(DraftCommentEvent.class);
+
   public final Comment c;
   private final Change change;
   private final PatchSet ps;
@@ -56,9 +60,18 @@
     throw new UnsupportedOperationException();
   }
 
-  void applyDraft(ChangeDraftUpdate draftUpdate) throws OrmException {
+  void applyDraft(ChangeDraftUpdate draftUpdate) {
     if (c.revId == null) {
-      setCommentRevId(c, cache, change, ps);
+      try {
+        setCommentRevId(c, cache, change, ps);
+      } catch (PatchListNotAvailableException e) {
+        log.warn(
+            "Unable to determine parent commit of patch set {} ({}); omitting draft inline comment",
+            ps.getId(),
+            ps.getRevision(),
+            c);
+        return;
+      }
     }
     draftUpdate.putComment(c);
   }
diff --git a/java/com/google/gerrit/server/notedb/rebuild/NoteDbMigrator.java b/java/com/google/gerrit/server/notedb/rebuild/NoteDbMigrator.java
index 8e8f232..216e95a 100644
--- a/java/com/google/gerrit/server/notedb/rebuild/NoteDbMigrator.java
+++ b/java/com/google/gerrit/server/notedb/rebuild/NoteDbMigrator.java
@@ -33,6 +33,7 @@
 import com.google.common.base.Stopwatch;
 import com.google.common.collect.ImmutableList;
 import com.google.common.collect.ImmutableListMultimap;
+import com.google.common.collect.Iterables;
 import com.google.common.collect.MultimapBuilder;
 import com.google.common.collect.Ordering;
 import com.google.common.collect.SetMultimap;
@@ -66,6 +67,7 @@
 import com.google.gerrit.server.notedb.NoteDbUpdateManager;
 import com.google.gerrit.server.notedb.NotesMigrationState;
 import com.google.gerrit.server.notedb.PrimaryStorageMigrator;
+import com.google.gerrit.server.notedb.PrimaryStorageMigrator.NoNoteDbStateException;
 import com.google.gerrit.server.notedb.RepoSequence;
 import com.google.gerrit.server.notedb.rebuild.ChangeRebuilder.NoPatchSetsException;
 import com.google.gerrit.server.project.NoSuchChangeException;
@@ -605,7 +607,19 @@
                       executor.submit(
                           () -> {
                             try (ManualRequestContext ctx = contextHelper.open()) {
-                              primaryStorageMigrator.migrateToNoteDbPrimary(id);
+                              try {
+                                primaryStorageMigrator.migrateToNoteDbPrimary(id);
+                              } catch (NoNoteDbStateException e) {
+                                if (canSkipPrimaryStorageMigration(
+                                    ctx.getReviewDbProvider().get(), id)) {
+                                  log.warn(
+                                      "Change {} previously failed to rebuild;"
+                                          + " skipping primary storage migration",
+                                      e);
+                                } else {
+                                  throw e;
+                                }
+                              }
                               return true;
                             } catch (Exception e) {
                               log.error("Error migrating primary storage for " + id, e);
@@ -628,6 +642,38 @@
     return disableReviewDb(prev);
   }
 
+  /**
+   * Checks whether a change is so corrupt that it can be completely skipped by the primary storage
+   * migration step.
+   *
+   * <p>To get to the point where this method is called from {@link #setNoteDbPrimary}, it means we
+   * attempted to rebuild it, and encountered an error that was then caught in {@link
+   * #rebuildProject} and skipped. As a result, there is no {@code noteDbState} field in the change
+   * by the time we get to {@link #setNoteDbPrimary}, so {@code migrateToNoteDbPrimary} throws an
+   * exception.
+   *
+   * <p>We have to do this hacky double-checking because we don't have a way for the rebuilding
+   * phase to communicate to the primary storage migration phase that the change is skippable. It
+   * would be possible to store this info in some field in this class, but there is no guarantee
+   * that the rebuild and primary storage migration phases are run in the same JVM invocation.
+   *
+   * <p>In an ideal world, we could do this through the {@link
+   * com.google.gerrit.server.notedb.NoteDbChangeState.PrimaryStorage} enum, having a separate value
+   * for errors. However, that would be an invasive change touching many non-migration-related parts
+   * of the NoteDb migration code, which is too risky to attempt in the stable branch where this bug
+   * had to be fixed.
+   *
+   * <p>As of this writing, the only case where this happens is when a change has no patch sets.
+   */
+  private static boolean canSkipPrimaryStorageMigration(ReviewDb db, Change.Id id) {
+    try {
+      return Iterables.isEmpty(db.patchSets().byChange(id));
+    } catch (Exception e) {
+      log.error("Error checking if change " + id + " is corrupt, assuming yes", e);
+      return true;
+    }
+  }
+
   private NotesMigrationState disableReviewDb(NotesMigrationState prev) throws IOException {
     return saveState(prev, NOTE_DB, c -> setAutoMigrate(c, false));
   }
diff --git a/java/com/google/gerrit/server/project/ProjectCacheImpl.java b/java/com/google/gerrit/server/project/ProjectCacheImpl.java
index 68270e2..e3d752d 100644
--- a/java/com/google/gerrit/server/project/ProjectCacheImpl.java
+++ b/java/com/google/gerrit/server/project/ProjectCacheImpl.java
@@ -155,6 +155,7 @@
         Throwables.throwIfInstanceOf(e.getCause(), IOException.class);
         throw new IOException(e);
       }
+      log.warn("Cannot find project {}", projectName.get(), e);
       return null;
     }
   }
diff --git a/java/com/google/gerrit/server/restapi/change/CreateDraftComment.java b/java/com/google/gerrit/server/restapi/change/CreateDraftComment.java
index afcc8f7..be689ca 100644
--- a/java/com/google/gerrit/server/restapi/change/CreateDraftComment.java
+++ b/java/com/google/gerrit/server/restapi/change/CreateDraftComment.java
@@ -34,6 +34,7 @@
 import com.google.gerrit.server.PatchSetUtil;
 import com.google.gerrit.server.change.RevisionResource;
 import com.google.gerrit.server.patch.PatchListCache;
+import com.google.gerrit.server.patch.PatchListNotAvailableException;
 import com.google.gerrit.server.update.BatchUpdate;
 import com.google.gerrit.server.update.BatchUpdateOp;
 import com.google.gerrit.server.update.ChangeContext;
@@ -108,7 +109,8 @@
 
     @Override
     public boolean updateChange(ChangeContext ctx)
-        throws ResourceNotFoundException, OrmException, UnprocessableEntityException {
+        throws ResourceNotFoundException, OrmException, UnprocessableEntityException,
+            PatchListNotAvailableException {
       PatchSet ps = psUtil.get(ctx.getDb(), ctx.getNotes(), psId);
       if (ps == null) {
         throw new ResourceNotFoundException("patch set not found: " + psId);
diff --git a/java/com/google/gerrit/server/restapi/change/DeleteDraftComment.java b/java/com/google/gerrit/server/restapi/change/DeleteDraftComment.java
index ee57c20..e81f9f1 100644
--- a/java/com/google/gerrit/server/restapi/change/DeleteDraftComment.java
+++ b/java/com/google/gerrit/server/restapi/change/DeleteDraftComment.java
@@ -29,6 +29,7 @@
 import com.google.gerrit.server.PatchSetUtil;
 import com.google.gerrit.server.change.DraftCommentResource;
 import com.google.gerrit.server.patch.PatchListCache;
+import com.google.gerrit.server.patch.PatchListNotAvailableException;
 import com.google.gerrit.server.update.BatchUpdate;
 import com.google.gerrit.server.update.BatchUpdateOp;
 import com.google.gerrit.server.update.ChangeContext;
@@ -87,7 +88,8 @@
     }
 
     @Override
-    public boolean updateChange(ChangeContext ctx) throws ResourceNotFoundException, OrmException {
+    public boolean updateChange(ChangeContext ctx)
+        throws ResourceNotFoundException, OrmException, PatchListNotAvailableException {
       Optional<Comment> maybeComment =
           commentsUtil.getDraft(ctx.getDb(), ctx.getNotes(), ctx.getIdentifiedUser(), key);
       if (!maybeComment.isPresent()) {
diff --git a/java/com/google/gerrit/server/restapi/change/PostReview.java b/java/com/google/gerrit/server/restapi/change/PostReview.java
index 8e78ec0..5ea72c0 100644
--- a/java/com/google/gerrit/server/restapi/change/PostReview.java
+++ b/java/com/google/gerrit/server/restapi/change/PostReview.java
@@ -839,7 +839,8 @@
 
     @Override
     public boolean updateChange(ChangeContext ctx)
-        throws OrmException, ResourceConflictException, UnprocessableEntityException, IOException {
+        throws OrmException, ResourceConflictException, UnprocessableEntityException, IOException,
+            PatchListNotAvailableException {
       user = ctx.getIdentifiedUser();
       notes = ctx.getNotes();
       ps = psUtil.get(ctx.getDb(), ctx.getNotes(), psId);
@@ -881,7 +882,7 @@
     }
 
     private boolean insertComments(ChangeContext ctx)
-        throws OrmException, UnprocessableEntityException {
+        throws OrmException, UnprocessableEntityException, PatchListNotAvailableException {
       Map<String, List<CommentInput>> map = in.comments;
       if (map == null) {
         map = Collections.emptyMap();
@@ -941,7 +942,8 @@
       return !toPublish.isEmpty();
     }
 
-    private boolean insertRobotComments(ChangeContext ctx) throws OrmException {
+    private boolean insertRobotComments(ChangeContext ctx)
+        throws OrmException, PatchListNotAvailableException {
       if (in.robotComments == null) {
         return false;
       }
@@ -952,7 +954,8 @@
       return !newRobotComments.isEmpty();
     }
 
-    private List<RobotComment> getNewRobotComments(ChangeContext ctx) throws OrmException {
+    private List<RobotComment> getNewRobotComments(ChangeContext ctx)
+        throws OrmException, PatchListNotAvailableException {
       List<RobotComment> toAdd = new ArrayList<>(in.robotComments.size());
 
       Set<CommentSetEntry> existingIds =
@@ -972,7 +975,8 @@
     }
 
     private RobotComment createRobotCommentFromInput(
-        ChangeContext ctx, String path, RobotCommentInput robotCommentInput) throws OrmException {
+        ChangeContext ctx, String path, RobotCommentInput robotCommentInput)
+        throws PatchListNotAvailableException {
       RobotComment robotComment =
           commentsUtil.newRobotComment(
               ctx,
diff --git a/java/com/google/gerrit/server/restapi/change/PutDraftComment.java b/java/com/google/gerrit/server/restapi/change/PutDraftComment.java
index 3017d89..e6ede34 100644
--- a/java/com/google/gerrit/server/restapi/change/PutDraftComment.java
+++ b/java/com/google/gerrit/server/restapi/change/PutDraftComment.java
@@ -33,6 +33,7 @@
 import com.google.gerrit.server.change.DraftCommentResource;
 import com.google.gerrit.server.notedb.ChangeUpdate;
 import com.google.gerrit.server.patch.PatchListCache;
+import com.google.gerrit.server.patch.PatchListNotAvailableException;
 import com.google.gerrit.server.update.BatchUpdate;
 import com.google.gerrit.server.update.BatchUpdateOp;
 import com.google.gerrit.server.update.ChangeContext;
@@ -113,7 +114,8 @@
     }
 
     @Override
-    public boolean updateChange(ChangeContext ctx) throws ResourceNotFoundException, OrmException {
+    public boolean updateChange(ChangeContext ctx)
+        throws ResourceNotFoundException, OrmException, PatchListNotAvailableException {
       Optional<Comment> maybeComment =
           commentsUtil.getDraft(ctx.getDb(), ctx.getNotes(), ctx.getIdentifiedUser(), key);
       if (!maybeComment.isPresent()) {
diff --git a/java/com/google/gerrit/server/schema/Schema_160.java b/java/com/google/gerrit/server/schema/Schema_160.java
index b65870f..c5a4422 100644
--- a/java/com/google/gerrit/server/schema/Schema_160.java
+++ b/java/com/google/gerrit/server/schema/Schema_160.java
@@ -18,6 +18,7 @@
 import static com.google.gerrit.server.git.UserConfigSections.MY;
 
 import com.google.common.annotations.VisibleForTesting;
+import com.google.common.collect.ImmutableList;
 import com.google.gerrit.reviewdb.client.Account;
 import com.google.gerrit.reviewdb.client.RefNames;
 import com.google.gerrit.reviewdb.server.ReviewDb;
@@ -40,17 +41,34 @@
 import org.eclipse.jgit.lib.TextProgressMonitor;
 
 /**
- * Remove "My Drafts" menu items for all users.
+ * Remove "My Drafts" menu items for all users and server-wide default preferences.
  *
  * <p>Since draft changes no longer exist, these menu items are obsolete.
  *
- * <p>Only matches menu items (with any name) where the URL exactly matches the <a
+ * <p>Only matches menu items (with any name) where the URL exactly matches one of the following,
+ * with or without leading {@code #}:
+ *
+ * <ul>
+ *   <li>/q/is:draft
+ *   <li>/q/owner:self+is:draft
+ * </ul>
+ *
+ * In particular, this includes the <a
  * href="https://gerrit.googlesource.com/gerrit/+/v2.14.4/gerrit-server/src/main/java/com/google/gerrit/server/account/GeneralPreferencesLoader.java#144">default
- * version from 2.14 and earlier</a>. Other menus containing {@code is:draft} in other positions are
- * not affected; this is still a valid predicate that matches no changes.
+ * from version 2.14 and earlier</a>.
+ *
+ * <p>Other menus containing {@code is:draft} in other positions are not affected; this is still a
+ * valid predicate that matches no changes.
  */
 public class Schema_160 extends SchemaVersion {
-  @VisibleForTesting static final String DEFAULT_DRAFT_ITEM = "#/q/owner:self+is:draft";
+  @VisibleForTesting static final ImmutableList<String> DEFAULT_DRAFT_ITEMS;
+
+  static {
+    String ownerSelfIsDraft = "/q/owner:self+is:draft";
+    String isDraft = "/q/is:draft";
+    DEFAULT_DRAFT_ITEMS =
+        ImmutableList.of(ownerSelfIsDraft, '#' + ownerSelfIsDraft, isDraft, '#' + isDraft);
+  }
 
   private final GitRepositoryManager repoManager;
   private final AllUsersName allUsersName;
@@ -75,10 +93,9 @@
         ProgressMonitor pm = new TextProgressMonitor();
         pm.beginTask("Removing \"My Drafts\" menu items", ProgressMonitor.UNKNOWN);
         for (Account.Id id : (Iterable<Account.Id>) Accounts.readUserRefs(repo)::iterator) {
-          if (removeMyDrafts(repo, id)) {
-            pm.update(1);
-          }
+          removeMyDrafts(repo, RefNames.refsUsers(id), pm);
         }
+        removeMyDrafts(repo, RefNames.REFS_USERS_DEFAULT, pm);
         pm.endTask();
       }
     } catch (IOException | ConfigInvalidException e) {
@@ -86,24 +103,26 @@
     }
   }
 
-  private boolean removeMyDrafts(Repository repo, Account.Id id)
+  private void removeMyDrafts(Repository repo, String ref, ProgressMonitor pm)
       throws IOException, ConfigInvalidException {
     MetaDataUpdate md = new MetaDataUpdate(GitReferenceUpdated.DISABLED, allUsersName, repo);
     PersonIdent ident = serverIdent.get();
     md.getCommitBuilder().setAuthor(ident);
     md.getCommitBuilder().setCommitter(ident);
-    Prefs prefs = new Prefs(id);
+    Prefs prefs = new Prefs(ref);
     prefs.load(repo);
     prefs.removeMyDrafts();
     prefs.commit(md);
-    return prefs.dirty();
+    if (prefs.dirty()) {
+      pm.update(1);
+    }
   }
 
   private static class Prefs extends VersionedAccountPreferences {
     private boolean dirty;
 
-    Prefs(Account.Id id) {
-      super(RefNames.refsUsers(id));
+    Prefs(String ref) {
+      super(ref);
     }
 
     @Override
@@ -118,7 +137,8 @@
     void removeMyDrafts() {
       Config cfg = getConfig();
       for (String item : cfg.getSubsections(MY)) {
-        if (DEFAULT_DRAFT_ITEM.equals(cfg.getString(MY, item, KEY_URL))) {
+        String value = cfg.getString(MY, item, KEY_URL);
+        if (DEFAULT_DRAFT_ITEMS.contains(value)) {
           cfg.unsetSection(MY, item);
           dirty = true;
         }
diff --git a/java/com/google/gerrit/server/schema/VersionedAccountPreferences.java b/java/com/google/gerrit/server/schema/VersionedAccountPreferences.java
index d57b14d..0213f3b 100644
--- a/java/com/google/gerrit/server/schema/VersionedAccountPreferences.java
+++ b/java/com/google/gerrit/server/schema/VersionedAccountPreferences.java
@@ -25,12 +25,16 @@
 
 /** Preferences for user accounts during schema migrations. */
 class VersionedAccountPreferences extends VersionedMetaData {
-  private static final String PREFERENCES = "preferences.config";
+  static final String PREFERENCES = "preferences.config";
 
   static VersionedAccountPreferences forUser(Account.Id id) {
     return new VersionedAccountPreferences(RefNames.refsUsers(id));
   }
 
+  static VersionedAccountPreferences forDefault() {
+    return new VersionedAccountPreferences(RefNames.REFS_USERS_DEFAULT);
+  }
+
   private final String ref;
   private Config cfg;
 
diff --git a/java/com/google/gerrit/testing/NoteDbChecker.java b/java/com/google/gerrit/testing/NoteDbChecker.java
index 77d581b..b5dd9e9 100644
--- a/java/com/google/gerrit/testing/NoteDbChecker.java
+++ b/java/com/google/gerrit/testing/NoteDbChecker.java
@@ -19,6 +19,8 @@
 import static java.util.stream.Collectors.toList;
 
 import com.google.common.base.Joiner;
+import com.google.common.collect.ImmutableListMultimap;
+import com.google.common.collect.ListMultimap;
 import com.google.gerrit.reviewdb.client.Change;
 import com.google.gerrit.reviewdb.client.Project;
 import com.google.gerrit.reviewdb.client.RefNames;
@@ -78,14 +80,17 @@
   }
 
   public void rebuildAndCheckAllChanges() throws Exception {
-    rebuildAndCheckChanges(getUnwrappedDb().changes().all().toList().stream().map(Change::getId));
+    rebuildAndCheckChanges(
+        getUnwrappedDb().changes().all().toList().stream().map(Change::getId),
+        ImmutableListMultimap.of());
   }
 
   public void rebuildAndCheckChanges(Change.Id... changeIds) throws Exception {
-    rebuildAndCheckChanges(Arrays.stream(changeIds));
+    rebuildAndCheckChanges(Arrays.stream(changeIds), ImmutableListMultimap.of());
   }
 
-  private void rebuildAndCheckChanges(Stream<Change.Id> changeIds) throws Exception {
+  private void rebuildAndCheckChanges(
+      Stream<Change.Id> changeIds, ListMultimap<Change.Id, String> expectedDiffs) throws Exception {
     ReviewDb db = getUnwrappedDb();
 
     List<ChangeBundle> allExpected = readExpected(changeIds);
@@ -105,7 +110,7 @@
         }
       }
 
-      checkActual(allExpected, msgs);
+      checkActual(allExpected, expectedDiffs, msgs);
     } finally {
       notesMigration.setReadChanges(oldRead);
       notesMigration.setWriteChanges(oldWrite);
@@ -113,7 +118,14 @@
   }
 
   public void checkChanges(Change.Id... changeIds) throws Exception {
-    checkActual(readExpected(Arrays.stream(changeIds)), new ArrayList<>());
+    checkActual(
+        readExpected(Arrays.stream(changeIds)), ImmutableListMultimap.of(), new ArrayList<>());
+  }
+
+  public void rebuildAndCheckChange(Change.Id changeId, String... expectedDiff) throws Exception {
+    ImmutableListMultimap.Builder<Change.Id, String> b = ImmutableListMultimap.builder();
+    b.putAll(changeId, Arrays.asList(expectedDiff));
+    rebuildAndCheckChanges(Stream.of(changeId), b.build());
   }
 
   public void assertNoChangeRef(Project.NameKey project, Change.Id changeId) throws Exception {
@@ -160,7 +172,11 @@
     }
   }
 
-  private void checkActual(List<ChangeBundle> allExpected, List<String> msgs) throws Exception {
+  private void checkActual(
+      List<ChangeBundle> allExpected,
+      ListMultimap<Change.Id, String> expectedDiffs,
+      List<String> msgs)
+      throws Exception {
     ReviewDb db = getUnwrappedDb();
     boolean oldRead = notesMigration.readChanges();
     boolean oldWrite = notesMigration.rawWriteChangesSetting();
@@ -181,9 +197,14 @@
           continue;
         }
         List<String> diff = expected.differencesFrom(actual);
-        if (!diff.isEmpty()) {
+        List<String> expectedDiff = expectedDiffs.get(c.getId());
+        if (!diff.equals(expectedDiff)) {
           msgs.add("Differences between ReviewDb and NoteDb for " + c + ":");
           msgs.addAll(diff);
+          if (!expectedDiff.isEmpty()) {
+            msgs.add("Expected differences:");
+            msgs.addAll(expectedDiff);
+          }
           msgs.add("");
         } else {
           System.err.println("NoteDb conversion of change " + c.getId() + " successful");
diff --git a/javatests/com/google/gerrit/acceptance/server/notedb/BUILD b/javatests/com/google/gerrit/acceptance/server/notedb/BUILD
index 20c256f..4965402 100644
--- a/javatests/com/google/gerrit/acceptance/server/notedb/BUILD
+++ b/javatests/com/google/gerrit/acceptance/server/notedb/BUILD
@@ -7,5 +7,8 @@
         "notedb",
         "server",
     ],
+    # TODO(dborowitz): Fix leaks in local disk tests so we can reduce heap size.
+    # http://crbug.com/gerrit/8567
+    vm_args = ["-Xmx1024m"],
     deps = ["//java/com/google/gerrit/server/schema"],
 )
diff --git a/javatests/com/google/gerrit/acceptance/server/notedb/ChangeRebuilderIT.java b/javatests/com/google/gerrit/acceptance/server/notedb/ChangeRebuilderIT.java
index c0dcc9c..d18ef34 100644
--- a/javatests/com/google/gerrit/acceptance/server/notedb/ChangeRebuilderIT.java
+++ b/javatests/com/google/gerrit/acceptance/server/notedb/ChangeRebuilderIT.java
@@ -15,6 +15,7 @@
 package com.google.gerrit.acceptance.server.notedb;
 
 import static com.google.common.truth.Truth.assertThat;
+import static com.google.common.truth.Truth.assert_;
 import static com.google.common.truth.TruthJUnit.assume;
 import static com.google.gerrit.reviewdb.client.RefNames.changeMetaRef;
 import static com.google.gerrit.reviewdb.client.RefNames.refsDraftComments;
@@ -23,6 +24,7 @@
 import static java.util.concurrent.TimeUnit.DAYS;
 import static java.util.concurrent.TimeUnit.MILLISECONDS;
 import static java.util.concurrent.TimeUnit.SECONDS;
+import static java.util.stream.Collectors.joining;
 import static java.util.stream.Collectors.toList;
 import static org.eclipse.jgit.lib.Constants.OBJ_BLOB;
 
@@ -53,6 +55,7 @@
 import com.google.gerrit.reviewdb.client.PatchSetApproval;
 import com.google.gerrit.reviewdb.client.Project;
 import com.google.gerrit.reviewdb.client.RefNames;
+import com.google.gerrit.reviewdb.client.RevId;
 import com.google.gerrit.reviewdb.server.ReviewDb;
 import com.google.gerrit.reviewdb.server.ReviewDbUtil;
 import com.google.gerrit.server.ChangeUtil;
@@ -69,6 +72,8 @@
 import com.google.gerrit.server.notedb.NoteDbUpdateManager;
 import com.google.gerrit.server.notedb.TestChangeRebuilderWrapper;
 import com.google.gerrit.server.notedb.rebuild.ChangeRebuilder.NoPatchSetsException;
+import com.google.gerrit.server.patch.PatchListCache;
+import com.google.gerrit.server.patch.PatchListNotAvailableException;
 import com.google.gerrit.server.patch.PatchSetInfoFactory;
 import com.google.gerrit.server.patch.PatchSetInfoNotAvailableException;
 import com.google.gerrit.server.project.testing.Util;
@@ -95,6 +100,7 @@
 import java.util.Map;
 import java.util.Optional;
 import java.util.concurrent.TimeUnit;
+import java.util.stream.Stream;
 import org.apache.http.Header;
 import org.apache.http.message.BasicHeader;
 import org.eclipse.jgit.junit.TestRepository;
@@ -145,6 +151,8 @@
 
   @Inject private PatchSetInfoFactory patchSetInfoFactory;
 
+  @Inject private PatchListCache patchListCache;
+
   @Before
   public void setUp() throws Exception {
     assume().that(NoteDbMode.get()).isEqualTo(NoteDbMode.OFF);
@@ -1017,8 +1025,43 @@
       db.rollback();
     }
 
-    exception.expect(NoPatchSetsException.class);
-    checker.rebuildAndCheckChanges(id);
+    try {
+      checker.rebuildAndCheckChanges(id);
+      assert_().fail("expected NoPatchSetsException");
+    } catch (NoPatchSetsException e) {
+      // Expected.
+    }
+
+    Change c = db.changes().get(id);
+    assertThat(c.getNoteDbState()).isNull();
+    checker.assertNoChangeRef(project, id);
+  }
+
+  @Test
+  public void rebuildChangeWithNoEntitiesOtherThanChange() throws Exception {
+    PushOneCommit.Result r = createChange();
+    Change.Id id = r.getPatchSetId().getParentKey();
+    db.changes().beginTransaction(id);
+    try {
+      db.changeMessages().delete(db.changeMessages().byChange(id));
+      db.patchSets().delete(db.patchSets().byChange(id));
+      db.patchSetApprovals().delete(db.patchSetApprovals().byChange(id));
+      db.patchComments().delete(db.patchComments().byChange(id));
+      db.commit();
+    } finally {
+      db.rollback();
+    }
+
+    try {
+      checker.rebuildAndCheckChanges(id);
+      assert_().fail("expected NoPatchSetsException");
+    } catch (NoPatchSetsException e) {
+      // Expected.
+    }
+
+    Change c = db.changes().get(id);
+    assertThat(c.getNoteDbState()).isNull();
+    checker.assertNoChangeRef(project, id);
   }
 
   @Test
@@ -1339,6 +1382,67 @@
     assertChangeUpToDate(true, id);
   }
 
+  @Test
+  public void missingPatchSetCommitOkForCommentsNotOnParentSide() throws Exception {
+    PushOneCommit.Result r = createChange();
+    Change.Id id = r.getChange().getId();
+
+    putDraft(user, id, 1, "draft comment", null, Side.REVISION);
+    putComment(user, id, 1, "published comment", null, Side.REVISION);
+
+    ReviewDb db = getUnwrappedDb();
+    PatchSet ps = db.patchSets().get(new PatchSet.Id(id, 1));
+    ps.setRevision(new RevId("deadbeefdeadbeefdeadbeefdeadbeefdeadbeef"));
+    db.patchSets().update(Collections.singleton(ps));
+
+    try {
+      patchListCache.getOldId(db.changes().get(id), ps, null);
+      assert_().fail("Expected PatchListNotAvailableException");
+    } catch (PatchListNotAvailableException e) {
+      // Expected.
+    }
+
+    checker.rebuildAndCheckChanges(id);
+  }
+
+  @Test
+  public void missingPatchSetCommitOmitsCommentsOnParentSide() throws Exception {
+    PushOneCommit.Result r = createChange();
+    Change.Id id = r.getChange().getId();
+
+    CommentInfo draftInfo = putDraft(user, id, 1, "draft comment", null, Side.PARENT);
+    putComment(user, id, 1, "published comment", null, Side.PARENT);
+    CommentInfo commentInfo =
+        gApi.changes()
+            .id(id.get())
+            .comments()
+            .values()
+            .stream()
+            .flatMap(List::stream)
+            .findFirst()
+            .get();
+
+    ReviewDb db = getUnwrappedDb();
+    PatchSet ps = db.patchSets().get(new PatchSet.Id(id, 1));
+    ps.setRevision(new RevId("deadbeefdeadbeefdeadbeefdeadbeefdeadbeef"));
+    db.patchSets().update(Collections.singleton(ps));
+
+    try {
+      patchListCache.getOldId(db.changes().get(id), ps, null);
+      assert_().fail("Expected PatchListNotAvailableException");
+    } catch (PatchListNotAvailableException e) {
+      // Expected.
+    }
+
+    checker.rebuildAndCheckChange(
+        id,
+        Stream.of(draftInfo.id, commentInfo.id)
+            .sorted()
+            .map(c -> id + ",1," + PushOneCommit.FILE_NAME + "," + c)
+            .collect(
+                joining(", ", "PatchLineComment.Key sets differ: [", "] only in A; [] only in B")));
+  }
+
   private void assertChangesReadOnly(RestApiException e) throws Exception {
     Throwable cause = e.getCause();
     assertThat(cause).isInstanceOf(UpdateException.class);
@@ -1388,16 +1492,24 @@
     }
   }
 
-  private void putDraft(TestAccount account, Change.Id id, int line, String msg, Boolean unresolved)
+  private CommentInfo putDraft(
+      TestAccount account, Change.Id id, int line, String msg, Boolean unresolved)
+      throws Exception {
+    return putDraft(account, id, line, msg, unresolved, Side.REVISION);
+  }
+
+  private CommentInfo putDraft(
+      TestAccount account, Change.Id id, int line, String msg, Boolean unresolved, Side side)
       throws Exception {
     DraftInput in = new DraftInput();
+    in.side = side;
     in.line = line;
     in.message = msg;
     in.path = PushOneCommit.FILE_NAME;
     in.unresolved = unresolved;
     AcceptanceTestRequestScope.Context old = setApiUser(account);
     try {
-      gApi.changes().id(id.get()).current().createDraft(in);
+      return gApi.changes().id(id.get()).current().createDraft(in).get();
     } finally {
       atrScope.set(old);
     }
@@ -1405,7 +1517,14 @@
 
   private void putComment(TestAccount account, Change.Id id, int line, String msg, String inReplyTo)
       throws Exception {
+    putComment(account, id, line, msg, inReplyTo, Side.REVISION);
+  }
+
+  private void putComment(
+      TestAccount account, Change.Id id, int line, String msg, String inReplyTo, Side side)
+      throws Exception {
     CommentInput in = new CommentInput();
+    in.side = side;
     in.line = line;
     in.message = msg;
     in.inReplyTo = inReplyTo;
diff --git a/javatests/com/google/gerrit/acceptance/server/notedb/OnlineNoteDbMigrationIT.java b/javatests/com/google/gerrit/acceptance/server/notedb/OnlineNoteDbMigrationIT.java
index d4990af..27dc951 100644
--- a/javatests/com/google/gerrit/acceptance/server/notedb/OnlineNoteDbMigrationIT.java
+++ b/javatests/com/google/gerrit/acceptance/server/notedb/OnlineNoteDbMigrationIT.java
@@ -18,6 +18,7 @@
 import static com.google.common.truth.Truth.assertThat;
 import static com.google.common.truth.Truth8.assertThat;
 import static com.google.common.truth.TruthJUnit.assume;
+import static com.google.gerrit.server.notedb.NoteDbChangeState.NOTE_DB_PRIMARY_STATE;
 import static com.google.gerrit.server.notedb.NotesMigrationState.NOTE_DB;
 import static com.google.gerrit.server.notedb.NotesMigrationState.READ_WRITE_NO_SEQUENCE;
 import static com.google.gerrit.server.notedb.NotesMigrationState.READ_WRITE_WITH_SEQUENCE_NOTE_DB_PRIMARY;
@@ -43,11 +44,15 @@
 import com.google.gerrit.extensions.registration.DynamicSet;
 import com.google.gerrit.extensions.registration.RegistrationHandle;
 import com.google.gerrit.reviewdb.client.Change;
+import com.google.gerrit.reviewdb.client.PatchSet;
 import com.google.gerrit.reviewdb.client.Project;
 import com.google.gerrit.reviewdb.client.RefNames;
 import com.google.gerrit.reviewdb.server.ReviewDb;
+import com.google.gerrit.server.CommentsUtil;
 import com.google.gerrit.server.Sequences;
 import com.google.gerrit.server.config.SitePaths;
+import com.google.gerrit.server.notedb.ChangeBundle;
+import com.google.gerrit.server.notedb.ChangeBundleReader;
 import com.google.gerrit.server.notedb.NoteDbChangeState;
 import com.google.gerrit.server.notedb.NoteDbChangeState.PrimaryStorage;
 import com.google.gerrit.server.notedb.NoteDbChangeState.RefState;
@@ -76,6 +81,7 @@
 import org.eclipse.jgit.lib.ObjectLoader;
 import org.eclipse.jgit.lib.ObjectReader;
 import org.eclipse.jgit.lib.Ref;
+import org.eclipse.jgit.lib.RefUpdate;
 import org.eclipse.jgit.lib.Repository;
 import org.eclipse.jgit.storage.file.FileBasedConfig;
 import org.eclipse.jgit.util.FS;
@@ -101,10 +107,12 @@
   // migration state may result in various kinds of wrappers showing up unexpectedly.
   @Inject @ReviewDbFactory private SchemaFactory<ReviewDb> schemaFactory;
 
-  @Inject private SitePaths sitePaths;
+  @Inject private ChangeBundleReader changeBundleReader;
+  @Inject private CommentsUtil commentsUtil;
+  @Inject private DynamicSet<NotesMigrationStateListener> listeners;
   @Inject private Provider<NoteDbMigrator.Builder> migratorBuilderProvider;
   @Inject private Sequences sequences;
-  @Inject private DynamicSet<NotesMigrationStateListener> listeners;
+  @Inject private SitePaths sitePaths;
 
   private FileBasedConfig noteDbConfig;
   private List<RegistrationHandle> addedListeners;
@@ -433,6 +441,67 @@
   }
 
   @Test
+  public void fullMigrationOneChangeWithNoPatchSets() throws Exception {
+    PushOneCommit.Result r1 = createChange();
+    PushOneCommit.Result r2 = createChange();
+    Change.Id id1 = r1.getChange().getId();
+    Change.Id id2 = r2.getChange().getId();
+
+    db.changes().beginTransaction(id2);
+    try {
+      db.patchSets().delete(db.patchSets().byChange(id2));
+      db.commit();
+    } finally {
+      db.rollback();
+    }
+
+    migrate(b -> b);
+    assertNotesMigrationState(NOTE_DB, false, false);
+
+    try (ReviewDb db = schemaFactory.open();
+        Repository repo = repoManager.openRepository(project)) {
+      assertThat(repo.exactRef(RefNames.changeMetaRef(id1))).isNotNull();
+      assertThat(db.changes().get(id1).getNoteDbState()).isEqualTo(NOTE_DB_PRIMARY_STATE);
+
+      // A change with no patch sets is so corrupt that it is completely skipped by the migration
+      // process.
+      assertThat(repo.exactRef(RefNames.changeMetaRef(id2))).isNull();
+      assertThat(db.changes().get(id2).getNoteDbState()).isNull();
+    }
+  }
+
+  @Test
+  public void fullMigrationMissingPatchSetRefs() throws Exception {
+    PushOneCommit.Result r = createChange();
+    Change.Id id = r.getChange().getId();
+
+    try (Repository repo = repoManager.openRepository(project)) {
+      RefUpdate u = repo.updateRef(new PatchSet.Id(id, 1).toRefName());
+      u.setForceUpdate(true);
+      assertThat(u.delete()).isEqualTo(RefUpdate.Result.FORCED);
+    }
+
+    ChangeBundle reviewDbBundle;
+    try (ReviewDb db = schemaFactory.open()) {
+      reviewDbBundle = changeBundleReader.fromReviewDb(db, id);
+    }
+
+    migrate(b -> b);
+    assertNotesMigrationState(NOTE_DB, false, false);
+
+    try (ReviewDb db = schemaFactory.open();
+        Repository repo = repoManager.openRepository(project)) {
+      // Change migrated successfully even though it was missing patch set refs.
+      assertThat(repo.exactRef(RefNames.changeMetaRef(id))).isNotNull();
+      assertThat(db.changes().get(id).getNoteDbState()).isEqualTo(NOTE_DB_PRIMARY_STATE);
+
+      ChangeBundle noteDbBundle =
+          ChangeBundle.fromNotes(commentsUtil, notesFactory.createChecked(db, project, id));
+      assertThat(noteDbBundle.differencesFrom(reviewDbBundle)).isEmpty();
+    }
+  }
+
+  @Test
   public void autoMigrationConfig() throws Exception {
     createChange();
 
diff --git a/javatests/com/google/gerrit/server/schema/Schema_159_to_160_Test.java b/javatests/com/google/gerrit/server/schema/Schema_159_to_160_Test.java
index 8dc4244..a01d611 100644
--- a/javatests/com/google/gerrit/server/schema/Schema_159_to_160_Test.java
+++ b/javatests/com/google/gerrit/server/schema/Schema_159_to_160_Test.java
@@ -14,14 +14,19 @@
 
 package com.google.gerrit.server.schema;
 
+import static com.google.common.base.MoreObjects.firstNonNull;
 import static com.google.common.collect.ImmutableMap.toImmutableMap;
 import static com.google.common.truth.Truth.assertThat;
+import static com.google.common.truth.Truth8.assertThat;
+import static com.google.gerrit.reviewdb.client.RefNames.REFS_USERS_DEFAULT;
 import static com.google.gerrit.server.git.UserConfigSections.KEY_URL;
 import static com.google.gerrit.server.git.UserConfigSections.MY;
-import static com.google.gerrit.server.schema.Schema_160.DEFAULT_DRAFT_ITEM;
+import static com.google.gerrit.server.schema.Schema_160.DEFAULT_DRAFT_ITEMS;
+import static com.google.gerrit.server.schema.VersionedAccountPreferences.PREFERENCES;
 
 import com.google.common.collect.ImmutableList;
 import com.google.common.collect.ImmutableMap;
+import com.google.common.collect.ImmutableSet;
 import com.google.gerrit.extensions.api.GerritApi;
 import com.google.gerrit.extensions.client.GeneralPreferencesInfo;
 import com.google.gerrit.extensions.client.MenuItem;
@@ -38,8 +43,13 @@
 import com.google.inject.Inject;
 import com.google.inject.Provider;
 import java.util.List;
+import java.util.Optional;
+import java.util.function.Supplier;
+import org.eclipse.jgit.junit.TestRepository;
+import org.eclipse.jgit.lib.BlobBasedConfig;
 import org.eclipse.jgit.lib.Config;
 import org.eclipse.jgit.lib.ObjectId;
+import org.eclipse.jgit.lib.Ref;
 import org.eclipse.jgit.lib.Repository;
 import org.junit.Before;
 import org.junit.Rule;
@@ -67,8 +77,12 @@
   @Test
   public void skipUnmodified() throws Exception {
     ObjectId oldMetaId = metaRef(accountId);
-    assertThat(myMenusFromNoteDb(accountId).values()).doesNotContain(DEFAULT_DRAFT_ITEM);
-    assertThat(myMenusFromApi(accountId).values()).doesNotContain(DEFAULT_DRAFT_ITEM);
+    ImmutableSet<String> fromNoteDb = myMenusFromNoteDb(accountId);
+    ImmutableSet<String> fromApi = myMenusFromApi(accountId);
+    for (String item : DEFAULT_DRAFT_ITEMS) {
+      assertThat(fromNoteDb).doesNotContain(item);
+      assertThat(fromApi).doesNotContain(item);
+    }
 
     schema160.migrateData(db, new TestUpdateUI());
 
@@ -78,22 +92,25 @@
   @Test
   public void deleteItems() throws Exception {
     ObjectId oldMetaId = metaRef(accountId);
-    List<String> defaultNames = ImmutableList.copyOf(myMenusFromApi(accountId).keySet());
+    ImmutableSet<String> defaultNames = myMenusFromApi(accountId);
 
     GeneralPreferencesInfo prefs = gApi.accounts().id(accountId.get()).getPreferences();
-    prefs.my.add(0, new MenuItem("Something else", DEFAULT_DRAFT_ITEM + "+is:mergeable"));
-    prefs.my.add(new MenuItem("Drafts", DEFAULT_DRAFT_ITEM));
-    prefs.my.add(new MenuItem("Totally not drafts", DEFAULT_DRAFT_ITEM));
+    prefs.my.add(0, new MenuItem("Something else", DEFAULT_DRAFT_ITEMS.get(0) + "+is:mergeable"));
+    for (int i = 0; i < DEFAULT_DRAFT_ITEMS.size(); i++) {
+      prefs.my.add(new MenuItem("Draft entry " + i, DEFAULT_DRAFT_ITEMS.get(i)));
+    }
     gApi.accounts().id(accountId.get()).setPreferences(prefs);
 
     List<String> oldNames =
         ImmutableList.<String>builder()
             .add("Something else")
             .addAll(defaultNames)
-            .add("Drafts")
-            .add("Totally not drafts")
+            .add("Draft entry 0")
+            .add("Draft entry 1")
+            .add("Draft entry 2")
+            .add("Draft entry 3")
             .build();
-    assertThat(myMenusFromApi(accountId).keySet()).containsExactlyElementsIn(oldNames).inOrder();
+    assertThat(myMenusFromApi(accountId)).containsExactlyElementsIn(oldNames).inOrder();
 
     schema160.migrateData(db, new TestUpdateUI());
     accountCache.evict(accountId);
@@ -104,14 +121,74 @@
 
     List<String> newNames =
         ImmutableList.<String>builder().add("Something else").addAll(defaultNames).build();
-    assertThat(myMenusFromNoteDb(accountId).keySet()).containsExactlyElementsIn(newNames).inOrder();
-    assertThat(myMenusFromApi(accountId).keySet()).containsExactlyElementsIn(newNames).inOrder();
+    assertThat(myMenusFromNoteDb(accountId)).containsExactlyElementsIn(newNames).inOrder();
+    assertThat(myMenusFromApi(accountId)).containsExactlyElementsIn(newNames).inOrder();
+  }
+
+  @Test
+  public void skipNonExistentRefsUsersDefault() throws Exception {
+    assertThat(readRef(REFS_USERS_DEFAULT)).isEmpty();
+    schema160.migrateData(db, new TestUpdateUI());
+    assertThat(readRef(REFS_USERS_DEFAULT)).isEmpty();
+  }
+
+  @Test
+  public void deleteDefaultItem() throws Exception {
+    assertThat(readRef(REFS_USERS_DEFAULT)).isEmpty();
+    ImmutableSet<String> defaultNames = defaultMenusFromApi();
+
+    // Setting *any* preference causes preferences.config to contain the full set of "my" sections.
+    // This mimics real-world behavior prior to the 2.15 upgrade; see Issue 8439 for details.
+    GeneralPreferencesInfo prefs = gApi.config().server().getDefaultPreferences();
+    prefs.signedOffBy = !firstNonNull(prefs.signedOffBy, false);
+    gApi.config().server().setDefaultPreferences(prefs);
+
+    try (Repository repo = repoManager.openRepository(allUsersName)) {
+      Config cfg = new BlobBasedConfig(null, repo, readRef(REFS_USERS_DEFAULT).get(), PREFERENCES);
+      assertThat(cfg.getSubsections("my")).containsExactlyElementsIn(defaultNames).inOrder();
+
+      // Add more defaults directly in git, the SetPreferences endpoint doesn't respect the "my"
+      // field in the input in 2.15 and earlier.
+      cfg.setString("my", "Drafts", "url", "#/q/owner:self+is:draft");
+      cfg.setString("my", "Something else", "url", "#/q/owner:self+is:draft+is:mergeable");
+      cfg.setString("my", "Totally not drafts", "url", "#/q/owner:self+is:draft");
+      new TestRepository<>(repo)
+          .branch(REFS_USERS_DEFAULT)
+          .commit()
+          .add(PREFERENCES, cfg.toText())
+          .create();
+    }
+
+    List<String> oldNames =
+        ImmutableList.<String>builder()
+            .addAll(defaultNames)
+            .add("Drafts")
+            .add("Something else")
+            .add("Totally not drafts")
+            .build();
+    assertThat(defaultMenusFromApi()).containsExactlyElementsIn(oldNames).inOrder();
+
+    schema160.migrateData(db, new TestUpdateUI());
+
+    assertThat(readRef(REFS_USERS_DEFAULT)).isPresent();
+
+    List<String> newNames =
+        ImmutableList.<String>builder().addAll(defaultNames).add("Something else").build();
+    assertThat(myMenusFromNoteDb(VersionedAccountPreferences::forDefault).keySet())
+        .containsExactlyElementsIn(newNames)
+        .inOrder();
+    assertThat(defaultMenusFromApi()).containsExactlyElementsIn(newNames).inOrder();
+  }
+
+  private ImmutableSet<String> myMenusFromNoteDb(Account.Id id) throws Exception {
+    return myMenusFromNoteDb(() -> VersionedAccountPreferences.forUser(id)).keySet();
   }
 
   // Raw config values, bypassing the defaults set by PreferencesConfig.
-  private ImmutableMap<String, String> myMenusFromNoteDb(Account.Id id) throws Exception {
+  private ImmutableMap<String, String> myMenusFromNoteDb(
+      Supplier<VersionedAccountPreferences> prefsSupplier) throws Exception {
     try (Repository repo = repoManager.openRepository(allUsersName)) {
-      VersionedAccountPreferences prefs = VersionedAccountPreferences.forUser(id);
+      VersionedAccountPreferences prefs = prefsSupplier.get();
       prefs.load(repo);
       Config cfg = prefs.getConfig();
       return cfg.getSubsections(MY)
@@ -120,18 +197,27 @@
     }
   }
 
-  private ImmutableMap<String, String> myMenusFromApi(Account.Id id) throws Exception {
-    return gApi.accounts()
-        .id(id.get())
-        .getPreferences()
-        .my
-        .stream()
-        .collect(toImmutableMap(i -> i.name, i -> i.url));
+  private ImmutableSet<String> myMenusFromApi(Account.Id id) throws Exception {
+    return myMenus(gApi.accounts().id(id.get()).getPreferences()).keySet();
+  }
+
+  private ImmutableSet<String> defaultMenusFromApi() throws Exception {
+    return myMenus(gApi.config().server().getDefaultPreferences()).keySet();
+  }
+
+  private static ImmutableMap<String, String> myMenus(GeneralPreferencesInfo prefs) {
+
+    return prefs.my.stream().collect(toImmutableMap(i -> i.name, i -> i.url));
   }
 
   private ObjectId metaRef(Account.Id id) throws Exception {
+    return readRef(RefNames.refsUsers(id))
+        .orElseThrow(() -> new AssertionError("missing ref for account " + id));
+  }
+
+  private Optional<ObjectId> readRef(String ref) throws Exception {
     try (Repository repo = repoManager.openRepository(allUsersName)) {
-      return repo.exactRef(RefNames.refsUsers(id)).getObjectId();
+      return Optional.ofNullable(repo.exactRef(ref)).map(Ref::getObjectId);
     }
   }
 }