RebuildNotedb: batch changes by project

If we batch changes by project before rebuilding them and writing them
to the notedb, we can use a single BatchRefUpdate for all of the
changes in a project, which reduces overhead for writing every change
to the notedb.

Additionally, within the code the rebuilds each change, I synchronized
on the BatchRefUpdate object because it is not thread safe. Since all
changes in a project will be using the same one, we can just
synchronize the function calls that modify that BatchRefUpdate.

Change-Id: I4af196fa720180b0846e9a6e7cc6d9083a75f695

gerrit-server/src/main/java/com/google/gerrit/server/notedb/ChangeRebuilder.java

@@ -72,12 +72,12 @@ public class ChangeRebuilder {
     this.updateFactory = updateFactory;
   }
 
-  public ListenableFuture<?> rebuildAsync(
-      final Change change, ListeningExecutorService executor) {
+  public ListenableFuture<?> rebuildAsync(final Change change,
+      ListeningExecutorService executor, final BatchRefUpdate bru) {
     return executor.submit(new Callable<Void>() {
-      @Override
+        @Override
       public Void call() throws Exception {
-        rebuild(change, null);
+        rebuild(change, bru);
         return null;
       }
     });
@@ -109,7 +109,7 @@ public class ChangeRebuilder {
             controlFactory.controlFor(change, user), e.when);
         update.setPatchSetId(e.psId);
         if (batch == null) {
-          batch = update.openUpdate();
+          batch = update.openUpdateInBatch(bru);
         }
       }
       e.apply(update);
@@ -118,7 +118,15 @@ public class ChangeRebuilder {
       if (update != null) {
         writeToBatch(batch, update);
       }
-      batch.commit();
+
+      // Since the BatchMetaDataUpdates generated by all ChangeRebuilders on a
+      // given project are backed by the same BatchRefUpdate, we need to
+      // synchronize on the BatchRefUpdate. Therefore, since commit on a
+      // BatchMetaDataUpdate is the only method that modifies a BatchRefUpdate,
+      // we can just synchronize this call.
+      synchronized (bru) {
+        batch.commit();
+      }
     }
   }