[Sync] Refactor AttachmentStore classes. Introduce concept of referrer.

sync/api/syncable_service.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_API_SYNCABLE_SERVICE_H_
 #define SYNC_API_SYNCABLE_SERVICE_H_
 
 #include <vector>
 
 #include "base/callback.h"
@@ skipping 54 matching lines @@
   //          otherwise.
   SyncError ProcessSyncChanges(const tracked_objects::Location& from_here,
                                const SyncChangeList& change_list) override = 0;
 
   // Returns AttachmentStore used by datatype. Attachment store is used by sync
   // when uploading or downloading attachments.
   // GetAttachmentStore is called right before MergeDataAndStartSyncing. If at
   // that time GetAttachmentStore returns NULL then datatype is considered not
   // using attachments and all attempts to upload/download attachments will
   // fail. Default implementation returns NULL. Datatype that uses sync
-  // attachemnts should create attachment store and implement GetAttachmentStore
+  // attachments should create attachment store and implement GetAttachmentStore
   // to return pointer to it.
-  virtual scoped_refptr<AttachmentStore> GetAttachmentStore();
+  virtual AttachmentStore* GetAttachmentStore();

[maniscalco, 2015/03/06 23:15:03]

As we talked about in person, it would be great if SyncableService could create
the AttachmentStoreForSync and return it by scoped_ptr.  That way, we'd never
have to worry about the lifetime of the object returned by GetAttachmentStore()
[Should I free it?  Can I hold on to the pointer and use it later?  How much
later?]

[pavely, 2015/03/09 18:09:29]

Done.

 
   // Called by sync to provide AttachmentService to be used to download
   // attachments.
   // SetAttachmentService is called after GetAttachmentStore and right before
   // MergeDataAndStartSyncing and only if GetAttachmentStore has returned a
   // non-NULL store instance. Default implementation does nothing.
   // Datatype that uses attachments must take ownerhip of the provided
   // AttachmentService instance.
   virtual void SetAttachmentService(
       scoped_ptr<AttachmentService> attachment_service);
 
  protected:
   ~SyncableService() override;
 };
 
 }  // namespace syncer
 
 #endif  // SYNC_API_SYNCABLE_SERVICE_H_