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

chrome/browser/sync/engine/apply_updates_command.cc

 // Copyright (c) 2011 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "chrome/browser/sync/engine/apply_updates_command.h"
 
 #include "base/location.h"
 #include "chrome/browser/sync/engine/update_applicator.h"
 #include "chrome/browser/sync/sessions/sync_session.h"
 #include "chrome/browser/sync/syncable/directory_manager.h"
 #include "chrome/browser/sync/syncable/syncable.h"
 
 namespace browser_sync {
 
 using sessions::SyncSession;
 
 ApplyUpdatesCommand::ApplyUpdatesCommand() {}
 ApplyUpdatesCommand::~ApplyUpdatesCommand() {}
 
+std::set<ModelSafeGroup> ApplyUpdatesCommand::GetGroupsToChange(
+    const sessions::SyncSession& session) const {
+  std::set<ModelSafeGroup> groups_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);
+  syncable::Directory::UnappliedUpdateMetaHandles handles;
+  dir->GetUnappliedUpdateMetaHandles(&trans, &handles);
+  for (syncable::Directory::UnappliedUpdateMetaHandles::const_iterator it =
+           handles.begin(); it != handles.end(); ++it) {
+    const syncable::Entry entry(&trans, syncable::GET_BY_HANDLE, *it);
+    groups_with_unapplied_updates.insert(
+        GetGroupForModelType(entry.GetServerModelType(),
+                             session.routing_info()));
+  }
+
+  return groups_with_unapplied_updates;
+}
+
 void ApplyUpdatesCommand::ModelChangingExecuteImpl(SyncSession* session) {
   syncable::ScopedDirLookup dir(session->context()->directory_manager(),
                                 session->context()->account_name());
   if (!dir.good()) {
     LOG(ERROR) << "Scoped dir lookup failed!";
     return;
   }
   syncable::WriteTransaction trans(FROM_HERE, syncable::SYNCER, dir);
   syncable::Directory::UnappliedUpdateMetaHandles handles;
   dir->GetUnappliedUpdateMetaHandles(&trans, &handles);
@@ skipping 24 matching lines @@
       syncable::ModelType model_type = syncable::ModelTypeFromInt(i);
       if (status.updates_request_types()[i]) {
         // This gets persisted to the directory's backing store.
         dir->set_initial_sync_ended_for_type(model_type, true);
       }
     }
   }
 }
 
 }  // namespace browser_sync