[Sync] Stop accessing BrowserContextKeyedServiceFactory on non-UI thread.

components/sync/driver/sync_client.h

 // Copyright 2015 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_SYNC_CLIENT_H_
 #define COMPONENTS_SYNC_DRIVER_SYNC_CLIENT_H_
 
 #include "base/callback_forward.h"
 #include "base/macros.h"
 #include "base/memory/ref_counted.h"
@@ skipping 41 matching lines @@
 class SyncableService;
 
 // Interface for clients of the Sync API to plumb through necessary dependent
 // components. This interface is purely for abstracting dependencies, and
 // should not contain any non-trivial functional logic.
 //
 // Note: on some platforms, getters might return nullptr. Callers are expected
 // to handle these scenarios gracefully.
 class SyncClient {
  public:
+  using ServiceProvider = base::Callback<base::WeakPtr<SyncableService>()>;
+
   SyncClient();
   virtual ~SyncClient();
 
   // Initializes the sync client with the specified sync service.
   virtual void Initialize() = 0;
 
   // Returns SequencedWorkerPool to be used by ProfileSyncService for blocking
   // operations.
   virtual base::SequencedWorkerPool* GetBlockingPool() = 0;
 
@@ skipping 21 matching lines @@
   // Returns a callback that will be invoked when password sync state has
   // potentially been changed.
   virtual base::Closure GetPasswordStateChangedCallback() = 0;
 
   virtual autofill::PersonalDataManager* GetPersonalDataManager() = 0;
   virtual BookmarkUndoService* GetBookmarkUndoServiceIfExists() = 0;
   virtual invalidation::InvalidationService* GetInvalidationService() = 0;
   virtual scoped_refptr<ExtensionsActivity> GetExtensionsActivity() = 0;
   virtual sync_sessions::SyncSessionsClient* GetSyncSessionsClient() = 0;
 
-  // Returns a weak pointer to the syncable service specified by |type|.
-  // Weak pointer may be unset if service is already destroyed.
-  // Note: Should only be dereferenced from the model type thread.
-  virtual base::WeakPtr<SyncableService> GetSyncableServiceForType(
-      ModelType type) = 0;
+  // Returns a callback to retrieve a syncable service specified by |type|.
+  // Both the provider and the resulting weak pointer should only be accessed

[pavely, 2017/03/27 19:38:25]

nit: "should only be accessed" => "will only be accessed" ?
You are describing to implementers guarantees about how this API will be used by
sync engine.

[skym, 2017/03/27 21:45:26]

Done.

+  // on the model thread.
+  virtual ServiceProvider GetSyncableServiceForType(ModelType type) = 0;
 
   // Returns a weak pointer to the ModelTypeSyncBridge specified by |type|. Weak
   // pointer may be unset if service is already destroyed.
   // Note: Should only be dereferenced from the model type thread.
   virtual base::WeakPtr<ModelTypeSyncBridge> GetSyncBridgeForModelType(
       ModelType type) = 0;
 
   // Creates and returns a new ModelSafeWorker for the group, or null if one
   // cannot be created.
   // TODO(maxbogue): Move this inside SyncApiComponentFactory.
   virtual scoped_refptr<ModelSafeWorker> CreateModelWorkerForGroup(
       ModelSafeGroup group) = 0;
 
   // Returns the current SyncApiComponentFactory instance.
   virtual SyncApiComponentFactory* GetSyncApiComponentFactory() = 0;
 
  private:
   DISALLOW_COPY_AND_ASSIGN(SyncClient);
 };
 
 }  // namespace syncer
 
 #endif  // COMPONENTS_SYNC_DRIVER_SYNC_CLIENT_H_