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

chrome/browser/sync/engine/model_changing_syncer_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/model_changing_syncer_command.h"
 
 #include "base/basictypes.h"
 #include "base/callback_old.h"
-#include "chrome/browser/sync/engine/model_safe_worker.h"
 #include "chrome/browser/sync/sessions/status_controller.h"
 #include "chrome/browser/sync/sessions/sync_session.h"
 #include "chrome/browser/sync/util/unrecoverable_error_info.h"
 
 namespace browser_sync {
 
 void ModelChangingSyncerCommand::ExecuteImpl(sessions::SyncSession* session) {
   work_session_ = session;
   if (!ModelNeutralExecuteImpl(work_session_)) {
     return;
   }
 
-  // Project the list of active types (i.e., types in the routing
-  // info) to a list of groups.
-  //
-  // TODO(akalin): Make this overrideable by subclasses (who might be
-  // working on a subset of |active_groups|).  (See
-  // http://crbug.com/97832.)
-  std::set<ModelSafeGroup> active_groups;
-  const ModelSafeRoutingInfo& routing_info = session->routing_info();
-  for (ModelSafeRoutingInfo::const_iterator it = routing_info.begin();
-       it != routing_info.end(); ++it) {
-    active_groups.insert(it->second);
-  }
-  // Always work on GROUP_PASSIVE, since that's the group that
-  // top-level folders map to.
-  active_groups.insert(GROUP_PASSIVE);
-
+  const std::set<ModelSafeGroup>& groups_to_change =
+      GetGroupsToChange(*work_session_);
   for (size_t i = 0; i < session->workers().size(); ++i) {
-    ModelSafeWorker* worker = session->workers()[i];
+    ModelSafeWorker* worker = work_session_->workers()[i];
     ModelSafeGroup group = worker->GetModelSafeGroup();
     // Skip workers whose group isn't active.
-    if (active_groups.find(group) == active_groups.end()) {
+    if (groups_to_change.count(group) == 0u) {
       VLOG(2) << "Skipping worker for group "
               << ModelSafeGroupToString(group);
       continue;
     }
 
     sessions::StatusController* status =
         work_session_->mutable_status_controller();
     sessions::ScopedModelSafeGroupRestriction r(status, group);
     WorkCallback c = base::Bind(
         &ModelChangingSyncerCommand::StartChangingModel,
         // We wait until the callback is executed. So it is safe to use
         // unretained.
         base::Unretained(this));
 
     // TODO(lipalani): Check the return value for an unrecoverable error.
     ignore_result(worker->DoWorkAndWaitUntilDone(c));
 
   }
 }
 
 bool ModelChangingSyncerCommand::ModelNeutralExecuteImpl(
     sessions::SyncSession* session) {
   return true;
 }
 
 }  // namespace browser_sync