[Sync] Signal UIModelWorker to abort on sync shutdown.

components/sync/driver/glue/ui_model_worker.h

 // Copyright (c) 2012 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 COMPONENTS_SYNC_DRIVER_GLUE_UI_MODEL_WORKER_H_
 #define COMPONENTS_SYNC_DRIVER_GLUE_UI_MODEL_WORKER_H_
 
 #include "base/macros.h"
 #include "base/memory/ref_counted.h"
 #include "base/single_thread_task_runner.h"
+#include "base/synchronization/waitable_event.h"
 #include "components/sync/engine/model_safe_worker.h"
 
 namespace syncer {
 
 // A ModelSafeWorker for UI models (e.g. bookmarks) that
 // accepts work requests from the syncapi that need to be fulfilled
 // from the MessageLoop home to the native model.
 class UIModelWorker : public ModelSafeWorker {
  public:
-  explicit UIModelWorker(scoped_refptr<base::SingleThreadTaskRunner> ui_thread);
+  explicit UIModelWorker(
+      const scoped_refptr<base::SingleThreadTaskRunner>& ui_thread);
 
-  // ModelSafeWorker implementation. Called on syncapi SyncerThread.
+  // ModelSafeWorker implementation.
+  void RequestStop() override;
   ModelSafeGroup GetModelSafeGroup() override;
   bool IsOnModelThread() override;
 
  protected:
   SyncerError DoWorkAndWaitUntilDoneImpl(const WorkCallback& work) override;
 
  private:
   ~UIModelWorker() override;
 
   // A reference to the UI thread's task runner.
   const scoped_refptr<base::SingleThreadTaskRunner> ui_thread_;
 
+  // Signal set when work on native thread is done, aborted due to the thread
+  // being torn down, or aborted do to sync stopping.

[Nicolas Zea, 2016/11/10 22:27:07]

typo: do -> due

[Nicolas Zea, 2016/11/10 22:27:07]

nit: i think it's worth explaining why we need this (namely that the UI thread
can join on the sync thread, and this prevents the sync thread from waiting on
work on the UI thread)

[maxbogue, 2016/11/12 03:19:52]

Done.

[maxbogue, 2016/11/12 03:19:52]

Done.

+  base::WaitableEvent work_done_or_abandoned_;
+
   DISALLOW_COPY_AND_ASSIGN(UIModelWorker);
 };
 
 }  // namespace syncer
 
 #endif  // COMPONENTS_SYNC_DRIVER_GLUE_UI_MODEL_WORKER_H_