Sync: Avoid deadlock in SyncBackendRegistrar / ModelSafeWorker on sync backend shutdown.

sync/internal_api/public/engine/model_safe_worker.h

 // Copyright 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 SYNC_INTERNAL_API_PUBLIC_ENGINE_MODEL_SAFE_WORKER_H_
 #define SYNC_INTERNAL_API_PUBLIC_ENGINE_MODEL_SAFE_WORKER_H_
 
 #include <map>
 #include <string>
 #include <vector>
@@ skipping 115 matching lines @@
   // is to be destroyed so no more work can be done.
   base::WaitableEvent work_done_or_stopped_;
 
   // Notified when working thread of the worker is to be destroyed.
   WorkerLoopDestructionObserver* observer_;
 
   // Remember working loop for posting task to unregister destruction
   // observation from sync thread when shutting down sync.
   base::Lock working_loop_lock_;
   base::MessageLoop* working_loop_;
-  base::WaitableEvent working_loop_set_wait_;
+
+  // Callback passed with UnregisterForLoopDestruction. Normally this
+  // remains unset/unused and is stored only if |working_loop_| isn't
+  // initialized by the time UnregisterForLoopDestruction is called.
+  // It is safe to copy and thread safe.
+  // See comments in model_safe_worker.cc for more details.
+  base::Callback<void(ModelSafeGroup)> unregister_done_callback_;
 };
 
 // A map that details which ModelSafeGroup each ModelType
 // belongs to.  Routing info can change in response to the user enabling /
 // disabling sync for certain types, as well as model association completions.
 typedef std::map<ModelType, ModelSafeGroup> ModelSafeRoutingInfo;
 
 // Caller takes ownership of return value.
 SYNC_EXPORT_PRIVATE base::DictionaryValue* ModelSafeRoutingInfoToValue(
     const ModelSafeRoutingInfo& routing_info);
 
 SYNC_EXPORT std::string ModelSafeRoutingInfoToString(
     const ModelSafeRoutingInfo& routing_info);
 
 SYNC_EXPORT ModelTypeSet GetRoutingInfoTypes(
     const ModelSafeRoutingInfo& routing_info);
 
 SYNC_EXPORT ModelSafeGroup GetGroupForModelType(
     const ModelType type,
     const ModelSafeRoutingInfo& routes);
 
 }  // namespace syncer
 
 #endif  // SYNC_INTERNAL_API_PUBLIC_ENGINE_MODEL_SAFE_WORKER_H_