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

chrome/browser/sync/engine/model_changing_syncer_command.h

 // 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.
 
 #ifndef CHROME_BROWSER_SYNC_ENGINE_MODEL_CHANGING_SYNCER_COMMAND_H_
 #define CHROME_BROWSER_SYNC_ENGINE_MODEL_CHANGING_SYNCER_COMMAND_H_
 #pragma once
 
 #include "base/compiler_specific.h"
+#include "chrome/browser/sync/engine/model_safe_worker.h"
 #include "chrome/browser/sync/engine/syncer_command.h"
 #include "chrome/browser/sync/util/unrecoverable_error_info.h"
 
 namespace browser_sync {
 namespace sessions {
 class SyncSession;
 }
 
 // An abstract SyncerCommand which dispatches its Execute step to the
 // model-safe worker thread.  Classes derived from ModelChangingSyncerCommand
@@ skipping 14 matching lines @@
   virtual void ExecuteImpl(sessions::SyncSession* session) OVERRIDE;
 
   // wrapper so implementations don't worry about storing work_session
   UnrecoverableErrorInfo StartChangingModel() {
     // TODO(lipalani): |ModelChangingExecuteImpl| should return an
     // UnrecoverableErrorInfo struct.
     ModelChangingExecuteImpl(work_session_);
     return UnrecoverableErrorInfo();
   }
 
+  std::set<ModelSafeGroup> GetGroupsToChangeForTest(
+      const sessions::SyncSession& session) const {
+    return GetGroupsToChange(session);
+  }
+
+ protected:
+  // This should return the set of groups in |session| that need to be
+  // changed.  The returned set should be a subset of
+  // session.GetEnabledGroups().  Subclasses can guarantee this either
+  // by calling session.GetEnabledGroups() or
+  // session.GetEnabledGroupsWithConflicts() and filtering that, or
+  // using GetGroupForModelType() (which handles top-level/unspecified
+  // nodes) to project from model types to groups.
+  virtual std::set<ModelSafeGroup> GetGroupsToChange(
+      const sessions::SyncSession& session) const = 0;
+
   // Sometimes, a command has work to do that needs to touch global state
   // belonging to multiple ModelSafeGroups, but in a way that is known to be
   // safe.  This will be called once, prior to ModelChangingExecuteImpl,
   // *without* a ModelSafeGroup restriction in place on the SyncSession.
   // Returns true on success, false on failure.
   // TODO(tim): Remove this (bug 36594).
   virtual bool ModelNeutralExecuteImpl(sessions::SyncSession* session);
 
   // Abstract method to be implemented by subclasses to handle logic that
   // operates on the model.  This is invoked with a SyncSession ModelSafeGroup
   // restriction in place so that bits of state belonging to data types
   // running on an unsafe thread are siloed away.
   virtual void ModelChangingExecuteImpl(sessions::SyncSession* session) = 0;
 
  private:
   // ExecuteImpl is expected to be run by SyncerCommand to set work_session.
   // StartChangingModel is called to start this command running.
   // Implementations will implement ModelChangingExecuteImpl and not
   // worry about storing the session or setting it. They are given work_session.
   sessions::SyncSession* work_session_;
 
   DISALLOW_COPY_AND_ASSIGN(ModelChangingSyncerCommand);
 };
 
 }  // namespace browser_sync
 
 #endif  // CHROME_BROWSER_SYNC_ENGINE_MODEL_CHANGING_SYNCER_COMMAND_H_