[Sync] Make syncer commands avoid posting tasks on threads with no work to do

chrome/browser/sync/engine/apply_updates_command.cc

Index: chrome/browser/sync/engine/apply_updates_command.cc
diff --git a/chrome/browser/sync/engine/apply_updates_command.cc b/chrome/browser/sync/engine/apply_updates_command.cc
index 8b5a6b8db8d28e10eca8fe405b2cb0b2b03df8d0..81d747d6f123e5ab0477e5bf52fa4178072db8c6 100644
--- a/chrome/browser/sync/engine/apply_updates_command.cc
+++ b/chrome/browser/sync/engine/apply_updates_command.cc
@@ -17,6 +17,40 @@ using sessions::SyncSession;
 ApplyUpdatesCommand::ApplyUpdatesCommand() {}
 ApplyUpdatesCommand::~ApplyUpdatesCommand() {}
 
+bool ApplyUpdatesCommand::HasCustomGroupsToChange() const {
+  // TODO(akalin): Set to true.
+  return false;
+}
+
+std::set<ModelSafeGroup> ApplyUpdatesCommand::GetGroupsToChange(
+    const sessions::SyncSession& session) const {
+  std::set<ModelSafeGroup> groups_with_unapplied_updates;
+
+  syncable::ModelTypeBitSet server_types_with_unapplied_updates;
+  {
+    syncable::ScopedDirLookup dir(session.context()->directory_manager(),
+                                  session.context()->account_name());
+    if (!dir.good()) {
+      LOG(ERROR) << "Scoped dir lookup failed!";
+      return groups_with_unapplied_updates;
+    }
+
+    syncable::ReadTransaction trans(FROM_HERE, dir);
+    server_types_with_unapplied_updates =
+        dir->GetServerTypesWithUnappliedUpdates(&trans);
+  }
+
+  for (int i = 0; i < syncable::MODEL_TYPE_COUNT; ++i) {
+    const syncable::ModelType type = syncable::ModelTypeFromInt(i);
+    if (server_types_with_unapplied_updates.test(type)) {
+      groups_with_unapplied_updates.insert(
+          GetGroupForModelType(type, session.routing_info()));
+    }
+  }
+
+  return groups_with_unapplied_updates;
+}
+
 void ApplyUpdatesCommand::ModelChangingExecuteImpl(SyncSession* session) {
   syncable::ScopedDirLookup dir(session->context()->directory_manager(),
                                 session->context()->account_name());
@@ -24,9 +58,27 @@ void ApplyUpdatesCommand::ModelChangingExecuteImpl(SyncSession* session) {
     LOG(ERROR) << "Scoped dir lookup failed!";
     return;
   }
+
   syncable::WriteTransaction trans(FROM_HERE, syncable::SYNCER, dir);
+
+  // Compute server types with unapplied updates that fall under our
+  // group restriction.
+  const syncable::ModelTypeBitSet server_types_with_unapplied_updates =
+      dir->GetServerTypesWithUnappliedUpdates(&trans);
+  syncable::ModelTypeBitSet server_type_restriction;
+  for (int i = 0; i < syncable::MODEL_TYPE_COUNT; ++i) {
+    const syncable::ModelType server_type = syncable::ModelTypeFromInt(i);
+    if (server_types_with_unapplied_updates.test(server_type)) {
+      if (GetGroupForModelType(server_type, session->routing_info()) ==
+          session->status_controller().group_restriction()) {
+        server_type_restriction.set(server_type);
+      }
+    }
+  }
+
   syncable::Directory::UnappliedUpdateMetaHandles handles;
-  dir->GetUnappliedUpdateMetaHandles(&trans, &handles);
+  dir->GetUnappliedUpdateMetaHandles(
+      &trans, server_type_restriction, &handles);
 
   UpdateApplicator applicator(
       session->context()->resolver(),
@@ -42,13 +94,6 @@ void ApplyUpdatesCommand::ModelChangingExecuteImpl(SyncSession* session) {
   // some subset of the currently synced datatypes.
   const sessions::StatusController& status(session->status_controller());
   if (status.ServerSaysNothingMoreToDownload()) {
-    syncable::ScopedDirLookup dir(session->context()->directory_manager(),
-                                  session->context()->account_name());
-    if (!dir.good()) {
-      LOG(ERROR) << "Scoped dir lookup failed!";
-      return;
-    }
-
     for (int i = syncable::FIRST_REAL_MODEL_TYPE;
          i < syncable::MODEL_TYPE_COUNT; ++i) {
       syncable::ModelType model_type = syncable::ModelTypeFromInt(i);