Allow to filter out refs from the fetch-replication deltas

Other plugins, e.g. the global-refdb, may want to influence the behaviour
of the pull-replication plugin and prevent refs from being replicated.

Introduce the refs filtering interface defined already in
the replication plugin with I99a7cb4fb2 but applied to the refs
deltas to fetch.

Bug: Issue 15637
Change-Id: I19d58638181fa4bab204aba30b2cda73241f7ed0

src/main/java/com/googlesource/gerrit/plugins/replication/pull/FetchOne.java

diff --git a/src/main/java/com/googlesource/gerrit/plugins/replication/pull/FetchOne.java b/src/main/java/com/googlesource/gerrit/plugins/replication/pull/FetchOne.java
index 24da012..c028d84 100644
--- a/src/main/java/com/googlesource/gerrit/plugins/replication/pull/FetchOne.java
+++ b/src/main/java/com/googlesource/gerrit/plugins/replication/pull/FetchOne.java
@@ -22,6 +22,7 @@
 import com.google.common.collect.ListMultimap;
 import com.google.common.collect.Sets;
 import com.google.gerrit.entities.Project;
+import com.google.gerrit.extensions.registration.DynamicItem;
 import com.google.gerrit.metrics.Timer1;
 import com.google.gerrit.server.git.GitRepositoryManager;
 import com.google.gerrit.server.git.PerThreadRequestScope;
@@ -100,6 +101,7 @@
   private final AtomicBoolean canceledWhileRunning;
   private final FetchFactory fetchFactory;
   private final Optional<PullReplicationApiRequestMetrics> apiRequestMetrics;
+  private DynamicItem<ReplicationFetchFilter> replicationFetchFilter;
 
   @Inject
   FetchOne(
@@ -133,6 +135,12 @@
     this.apiRequestMetrics = apiRequestMetrics;
   }
 
+  @Inject(optional = true)
+  public void setReplicationFetchFilter(
+      DynamicItem<ReplicationFetchFilter> replicationFetchFilter) {
+    this.replicationFetchFilter = replicationFetchFilter;
+  }
+
   @Override
   public void cancel() {
     repLog.info("[{}] Replication task from {} was canceled", taskIdHex, getURI());
@@ -438,13 +446,20 @@
       return configRefSpecs;
     }
 
-    return delta.stream()
+    return runRefsFilter(delta).stream()
         .map(ref -> refToFetchRefSpec(ref, configRefSpecs))
         .filter(Optional::isPresent)
         .map(Optional::get)
         .collect(Collectors.toList());
   }
 
+  private Set<String> runRefsFilter(Set<String> refs) {
+    return Optional.ofNullable(replicationFetchFilter)
+        .flatMap(filter -> Optional.ofNullable(filter.get()))
+        .map(f -> f.filter(this.projectName.get(), refs))
+        .orElse(refs);
+  }
+
   private Optional<RefSpec> refToFetchRefSpec(String ref, List<RefSpec> configRefSpecs) {
     for (RefSpec refSpec : configRefSpecs) {
       if (refSpec.matchSource(ref)) {

src/main/java/com/googlesource/gerrit/plugins/replication/pull/ReplicationExtensionPointModule.java

diff --git a/src/main/java/com/googlesource/gerrit/plugins/replication/pull/ReplicationExtensionPointModule.java b/src/main/java/com/googlesource/gerrit/plugins/replication/pull/ReplicationExtensionPointModule.java
new file mode 100644
index 0000000..7b35668
--- /dev/null
+++ b/src/main/java/com/googlesource/gerrit/plugins/replication/pull/ReplicationExtensionPointModule.java
@@ -0,0 +1,32 @@
+// Copyright (C) 2023 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.googlesource.gerrit.plugins.replication.pull;
+
+import com.google.gerrit.extensions.registration.DynamicItem;
+import com.google.inject.AbstractModule;
+
+/**
+ * Gerrit libModule for applying a fetch-filter for pull replications.
+ *
+ * <p>It should be used only when an actual filter is defined, otherwise the default plugin
+ * behaviour will be fetching refs without any filtering.
+ */
+public class ReplicationExtensionPointModule extends AbstractModule {
+
+  @Override
+  protected void configure() {
+    DynamicItem.itemOf(binder(), ReplicationFetchFilter.class);
+  }
+}

src/main/java/com/googlesource/gerrit/plugins/replication/pull/ReplicationFetchFilter.java

diff --git a/src/main/java/com/googlesource/gerrit/plugins/replication/pull/ReplicationFetchFilter.java b/src/main/java/com/googlesource/gerrit/plugins/replication/pull/ReplicationFetchFilter.java
new file mode 100644
index 0000000..dfe6fbd
--- /dev/null
+++ b/src/main/java/com/googlesource/gerrit/plugins/replication/pull/ReplicationFetchFilter.java
@@ -0,0 +1,29 @@
+// Copyright (C) 2023 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.googlesource.gerrit.plugins.replication.pull;
+
+import com.google.gerrit.extensions.annotations.ExtensionPoint;
+import java.util.Set;
+
+/**
+ * Filter that is invoked before a set of remote refs are fetched from a remote instance.
+ *
+ * <p>It can be used to filter out unwanted fetches.
+ */
+@ExtensionPoint
+public interface ReplicationFetchFilter {
+
+  public Set<String> filter(String projectName, Set<String> fetchRefs);
+}

src/main/resources/Documentation/extension-point.md

diff --git a/src/main/resources/Documentation/extension-point.md b/src/main/resources/Documentation/extension-point.md
new file mode 100644
index 0000000..33127c5
--- /dev/null
+++ b/src/main/resources/Documentation/extension-point.md
@@ -0,0 +1,47 @@
+@PLUGIN@ extension points
+==============
+
+The @PLUGIN@ plugin exposes an extension point to allow influencing its
+behaviour from another plugin or a script.
+Extension points are available only when the plugin extension points module
+is loaded as [libModule](/config-gerrit.html#gerrit.installModule) and
+implemented by another plugin which depends on this as `provided`
+dependency.
+
+### Install extension libModule
+
+The @PLUGIN@ plugin's extension points are defined in the
+`c.g.g.p.r.p.ReplicationExtensionPointModule` that needs to be configured
+as libModule.
+
+Create a symbolic link from `$GERRIT_SITE/plugins/@PLUGIN@.jar` into
+`$GERRIT_SITE/lib` and then add the @PLUGIN@ extension module to the
+`gerrit.config`.
+
+Example:
+
+```
+[gerrit]
+  installModule = com.googlesource.gerrit.plugins.replication.pull.ReplicationExtensionPointModule
+```
+
+> **NOTE**: Use and configuration of the @PLUGIN@ plugin as library module
+requires a Gerrit server restart and does not support hot plugin install or
+upgrade.
+
+
+### Extension points
+
+* `com.googlesource.gerrit.plugins.replication.pull.ReplicationFetchFilter`
+
+  Filter out the refs fetched from a remote instance.
+  Only one filter at a time is supported. Filter implementation needs to
+  bind a `DynamicItem`.
+
+  Default: no filtering
+
+  Example:
+
+  ```
+  DynamicItem.bind(binder(), ReplicationFetchFilter.class).to(ReplicationFetchFilterImpl.class);
+  ```