Allow to listen to garbage collector events

A new event type is introduced that allows to listen to garbage
collector events. This enables to trigger actions that depend on
changes in the file size footprint of a project such as quota
enforcement or usage data monitoring.

Change-Id: I1eb913a618df5fe972f3f516af504c13871e5fa5

Documentation/dev-plugins.txt

@@ -404,6 +404,10 @@ Update of HEAD on a project
 +
 Publication of usage data
 
+* `com.google.gerrit.extensions.events.GarbageCollectorListener`:
++
+Garbage collection ran on a project
+
 [[stream-events]]
 == Sending Events to the Events Stream
 

gerrit-extension-api/src/main/java/com/google/gerrit/extensions/events/GarbageCollectorListener.java

@@ -0,0 +1,43 @@
+// Copyright (C) 2014 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.events;
+
+import com.google.gerrit.extensions.annotations.ExtensionPoint;
+
+import java.util.Properties;
+
+/**
+ * Notified whenever the garbage collector has run successfully on a project.
+ */
+@ExtensionPoint
+public interface GarbageCollectorListener {
+  public interface Event {
+    /**
+     * The name of the Gerrit project that has been garbage collected
+     * @return
+     */
+    String getProjectName();
+
+    /**
+     * Properties describing the result of the garbage collection performed by
+     * JGit
+     *
+     * @see org.eclipse.jgit.api.GarbageCollectCommand#call()
+     */
+    Properties getStatistics();
+  }
+
+  void onGarbageCollected(Event event);
+}

gerrit-server/src/main/java/com/google/gerrit/server/config/GerritGlobalModule.java

@@ -27,6 +27,7 @@ import com.google.gerrit.extensions.events.HeadUpdatedListener;
 import com.google.gerrit.extensions.events.LifecycleListener;
 import com.google.gerrit.extensions.events.NewProjectCreatedListener;
 import com.google.gerrit.extensions.events.ProjectDeletedListener;
+import com.google.gerrit.extensions.events.GarbageCollectorListener;
 import com.google.gerrit.extensions.events.UsageDataPublishedListener;
 import com.google.gerrit.extensions.registration.DynamicItem;
 import com.google.gerrit.extensions.registration.DynamicMap;
@@ -253,6 +254,7 @@ public class GerritGlobalModule extends FactoryModule {
     DynamicSet.setOf(binder(), PreUploadHook.class);
     DynamicSet.setOf(binder(), NewProjectCreatedListener.class);
     DynamicSet.setOf(binder(), ProjectDeletedListener.class);
+    DynamicSet.setOf(binder(), GarbageCollectorListener.class);
     DynamicSet.setOf(binder(), HeadUpdatedListener.class);
     DynamicSet.setOf(binder(), UsageDataPublishedListener.class);
     DynamicSet.bind(binder(), GitReferenceUpdatedListener.class).to(ReindexAfterUpdate.class);

gerrit-server/src/main/java/com/google/gerrit/server/git/GarbageCollection.java

@@ -16,6 +16,9 @@ package com.google.gerrit.server.git;
 
 import com.google.common.collect.Sets;
 import com.google.gerrit.common.data.GarbageCollectionResult;
+import com.google.gerrit.extensions.events.GarbageCollectorListener;
+import com.google.gerrit.extensions.events.GarbageCollectorListener.Event;
+import com.google.gerrit.extensions.registration.DynamicSet;
 import com.google.gerrit.reviewdb.client.Project;
 import com.google.gerrit.server.config.GcConfig;
 import com.google.inject.Inject;
@@ -48,6 +51,7 @@ public class GarbageCollection {
   private final GitRepositoryManager repoManager;
   private final GarbageCollectionQueue gcQueue;
   private final GcConfig gcConfig;
+  private final DynamicSet<GarbageCollectorListener> listeners;
 
   public interface Factory {
     GarbageCollection create();
@@ -55,10 +59,12 @@ public class GarbageCollection {
 
   @Inject
   GarbageCollection(GitRepositoryManager repoManager,
-      GarbageCollectionQueue gcQueue, GcConfig config) {
+      GarbageCollectionQueue gcQueue, GcConfig config,
+      DynamicSet<GarbageCollectorListener> listeners) {
     this.repoManager = repoManager;
     this.gcQueue = gcQueue;
     this.gcConfig = config;
+    this.listeners = listeners;
   }
 
   public GarbageCollectionResult run(List<Project.NameKey> projectNames) {
@@ -93,6 +99,7 @@ public class GarbageCollection {
         Properties statistics = gc.call();
         logGcInfo(p, "after: ", statistics);
         print(writer, "done.\n\n");
+        fire(p, statistics);
       } catch (RepositoryNotFoundException e) {
         logGcError(writer, p, e);
         result.addError(new GarbageCollectionResult.Error(
@@ -112,6 +119,27 @@ public class GarbageCollection {
     return result;
   }
 
+  private void fire(final Project.NameKey p, final Properties statistics) {
+    Event event = new GarbageCollectorListener.Event() {
+      @Override
+      public String getProjectName() {
+        return p.get();
+      }
+
+      @Override
+      public Properties getStatistics() {
+        return statistics;
+      }
+    };
+    for (GarbageCollectorListener l : listeners) {
+      try {
+        l.onGarbageCollected(event);
+      } catch (RuntimeException e) {
+        log.warn("Failure in GarbageCollectorListener", e);
+      }
+    }
+  }
+
   private static void logGcInfo(Project.NameKey projectName, String msg) {
     logGcInfo(projectName, msg, null);
   }