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

In this change:
- Move towards following class hierarchy: http://www.plantuml.com:80/plantuml/png/ZPBTJiCm38NlynIvv4TzWsaI525nCI5jkw-on1EZQHmvwQG9U7TC4-ZeKiITvNm-EiSExbv1Hxc6VOszYs24mDHQeG6xFNcGRqBAknYLVkd0nKr40l7nmqrU1deDeRUHYrfPkrEw3LmJx848YCkWqODfkECZBIOAZuHin9abWxUo9b0Hdjt38RGJyEhwZ4YZI9kJq_mmRp0pAPKnd7pGgG8t6usTHyVe7_FPtY3mbOth9ghGDjGxTnwlaEq-ySjv-KmCwZflxvVyE5bSIa4Mw7ZGyDHvAyHurPkgkhZgz9PuoNp75tDhAUZcJ68cwkATHyfXnFWn4_PFDsKuNLuKLrFodGS-0G00
- Remove AttachmenService::GetStore. Now attachment store is owned by
  model type, there is no need to get it from AttachmentService
- Introduce AttachmentReferrer. There is no functionality behind it yet
  and interface is not complete for it. These will come in the next change.

BUG=457735
R=maniscalco@chromium.org
TEST=Not exposed in chrome. Only unit tests available (sync_unit_tests)

Committed: https://crrev.com/6adcada8a799057c31c0f17550c9e2747a8df847
Cr-Commit-Position: refs/heads/master@{#319733}

Review URL: https://codereview.chromium.org/986743004

Cr-Commit-Position: refs/heads/master@{#320015}

8 files changed, 215 insertions, 110 deletions

diff --git a/sync/api/attachments/attachment_store.cc b/sync/api/attachments/attachment_store.cc
index 91b3f95..7cf774e 100644
--- a/sync/api/attachments/attachment_store.cc
+++ b/sync/api/attachments/attachment_store.cc
@@ -10,16 +10,53 @@
 #include "base/message_loop/message_loop.h"
 #include "base/sequenced_task_runner.h"
 #include "base/thread_task_runner_handle.h"
-#include "sync/internal_api/public/attachments/attachment_store_handle.h"
+#include "sync/internal_api/public/attachments/attachment_store_frontend.h"
 #include "sync/internal_api/public/attachments/in_memory_attachment_store.h"
 #include "sync/internal_api/public/attachments/on_disk_attachment_store.h"
 
 namespace syncer {
 
-AttachmentStore::AttachmentStore() {}
-AttachmentStore::~AttachmentStore() {}
+AttachmentStore::AttachmentStore(
+    const scoped_refptr<AttachmentStoreFrontend>& frontend,
+    AttachmentReferrer referrer)
+    : frontend_(frontend), referrer_(referrer) {
+}
+
+AttachmentStore::~AttachmentStore() {
+}
+
+void AttachmentStore::Read(const AttachmentIdList& ids,
+                           const ReadCallback& callback) {
+  frontend_->Read(ids, callback);
+}
 
-scoped_refptr<AttachmentStore> AttachmentStore::CreateInMemoryStore() {
+void AttachmentStore::Write(const AttachmentList& attachments,
+                            const WriteCallback& callback) {
+  frontend_->Write(referrer_, attachments, callback);
+}
+
+void AttachmentStore::Drop(const AttachmentIdList& ids,
+                           const DropCallback& callback) {
+  frontend_->Drop(referrer_, ids, callback);
+}
+
+void AttachmentStore::ReadMetadata(const AttachmentIdList& ids,
+                                   const ReadMetadataCallback& callback) {
+  frontend_->ReadMetadata(ids, callback);
+}
+
+void AttachmentStore::ReadAllMetadata(const ReadMetadataCallback& callback) {
+  frontend_->ReadAllMetadata(referrer_, callback);
+}
+
+scoped_ptr<AttachmentStore> AttachmentStore::CreateAttachmentStoreForSync()
+    const {
+  scoped_ptr<AttachmentStore> attachment_store(
+      new AttachmentStore(frontend_, SYNC));
+  return attachment_store.Pass();
+}
+
+scoped_ptr<AttachmentStore> AttachmentStore::CreateInMemoryStore() {
   // Both frontend and backend of attachment store will live on current thread.
   scoped_refptr<base::SingleThreadTaskRunner> runner;
   if (base::ThreadTaskRunnerHandle::IsSet()) {
@@ -32,34 +69,38 @@ scoped_refptr<AttachmentStore> AttachmentStore::CreateInMemoryStore() {
   }
   scoped_ptr<AttachmentStoreBackend> backend(
       new InMemoryAttachmentStore(runner));
-  return scoped_refptr<AttachmentStore>(
-      new AttachmentStoreHandle(backend.Pass(), runner));
+  scoped_refptr<AttachmentStoreFrontend> frontend(
+      new AttachmentStoreFrontend(backend.Pass(), runner));
+  scoped_ptr<AttachmentStore> attachment_store(
+      new AttachmentStore(frontend, MODEL_TYPE));
+  return attachment_store.Pass();
 }
 
-scoped_refptr<AttachmentStore> AttachmentStore::CreateOnDiskStore(
+scoped_ptr<AttachmentStore> AttachmentStore::CreateOnDiskStore(
     const base::FilePath& path,
     const scoped_refptr<base::SequencedTaskRunner>& backend_task_runner,
     const InitCallback& callback) {
   scoped_ptr<OnDiskAttachmentStore> backend(
       new OnDiskAttachmentStore(base::ThreadTaskRunnerHandle::Get(), path));
 
-  scoped_refptr<AttachmentStore> attachment_store =
-      new AttachmentStoreHandle(backend.Pass(), backend_task_runner);
-  attachment_store->Init(callback);
-
-  return attachment_store;
-}
-
-AttachmentStoreBackend::AttachmentStoreBackend(
-    const scoped_refptr<base::SequencedTaskRunner>& callback_task_runner)
-    : callback_task_runner_(callback_task_runner) {
-}
+  scoped_refptr<AttachmentStoreFrontend> frontend =
+      new AttachmentStoreFrontend(backend.Pass(), backend_task_runner);
+  scoped_ptr<AttachmentStore> attachment_store(
+      new AttachmentStore(frontend, MODEL_TYPE));
+  frontend->Init(callback);
 
-AttachmentStoreBackend::~AttachmentStoreBackend() {
+  return attachment_store.Pass();
 }
 
-void AttachmentStoreBackend::PostCallback(const base::Closure& callback) {
-  callback_task_runner_->PostTask(FROM_HERE, callback);
+scoped_ptr<AttachmentStore> AttachmentStore::CreateMockStoreForTest(
+    scoped_ptr<AttachmentStoreBackend> backend) {
+  scoped_refptr<base::SingleThreadTaskRunner> runner =
+      base::ThreadTaskRunnerHandle::Get();
+  scoped_refptr<AttachmentStoreFrontend> attachment_store_frontend(
+      new AttachmentStoreFrontend(backend.Pass(), runner));
+  scoped_ptr<AttachmentStore> attachment_store(
+      new AttachmentStore(attachment_store_frontend, MODEL_TYPE));
+  return attachment_store.Pass();
 }
 
 }  // namespace syncer
diff --git a/sync/api/attachments/attachment_store.h b/sync/api/attachments/attachment_store.h
index b183de1..e8d8f5c 100644
--- a/sync/api/attachments/attachment_store.h
+++ b/sync/api/attachments/attachment_store.h
@@ -15,14 +15,13 @@
 
 namespace base {
 class FilePath;
-class RefCountedMemory;
 class SequencedTaskRunner;
 }  // namespace base
 
 namespace syncer {
 
-class Attachment;
-class AttachmentId;
+class AttachmentStoreFrontend;
+class AttachmentStoreBackend;
 
 // AttachmentStore is a place to locally store and access Attachments.
 //
@@ -32,8 +31,7 @@ class AttachmentId;
 // implementations.
 // Destroying this object does not necessarily cancel outstanding async
 // operations. If you need cancel like semantics, use WeakPtr in the callbacks.
-class SYNC_EXPORT AttachmentStore
-    : public base::RefCountedThreadSafe<AttachmentStore> {
+class SYNC_EXPORT AttachmentStore {
  public:
   // TODO(maniscalco): Consider udpating Read and Write methods to support
   // resumable transfers (bug 353292).
@@ -49,6 +47,14 @@ class SYNC_EXPORT AttachmentStore
   static const int RESULT_SIZE =
       10;  // Size of the Result enum; used for histograms.
 
+  // Each attachment can have references from sync or model type. Tracking these
+  // references is needed for lifetime management of attachment, it can only be
+  // deleted from the store when it doesn't have references.
+  enum AttachmentReferrer {
+    MODEL_TYPE,
+    SYNC,
+  };
+
   typedef base::Callback<void(const Result&)> InitCallback;
   typedef base::Callback<void(const Result&,
                               scoped_ptr<AttachmentMap>,
@@ -59,15 +65,7 @@ class SYNC_EXPORT AttachmentStore
                               scoped_ptr<AttachmentMetadataList>)>
       ReadMetadataCallback;
 
-  AttachmentStore();
-
-  // Asynchronously initializes attachment store.
-  //
-  // This method should not be called by consumer of this interface. It is
-  // called by factory methods in AttachmentStore class. When initialization is
-  // complete |callback| is invoked with result, in case of failure result is
-  // UNSPECIFIED_ERROR.
-  virtual void Init(const InitCallback& callback) = 0;
+  ~AttachmentStore();
 
   // Asynchronously reads the attachments identified by |ids|.
   //
@@ -81,8 +79,7 @@ class SYNC_EXPORT AttachmentStore
   //
   // Reads on individual attachments are treated atomically; |callback| will not
   // read only part of an attachment.
-  virtual void Read(const AttachmentIdList& ids,
-                    const ReadCallback& callback) = 0;
+  void Read(const AttachmentIdList& ids, const ReadCallback& callback);
 
   // Asynchronously writes |attachments| to the store.
   //
@@ -93,8 +90,7 @@ class SYNC_EXPORT AttachmentStore
   // not be written |callback|'s Result will be UNSPECIFIED_ERROR. When this
   // happens, some or none of the attachments may have been written
   // successfully.
-  virtual void Write(const AttachmentList& attachments,
-                     const WriteCallback& callback) = 0;
+  void Write(const AttachmentList& attachments, const WriteCallback& callback);
 
   // Asynchronously drops |attchments| from this store.
   //
@@ -105,8 +101,7 @@ class SYNC_EXPORT AttachmentStore
   // could not be dropped, |callback|'s Result will be UNSPECIFIED_ERROR. When
   // this happens, some or none of the attachments may have been dropped
   // successfully.
-  virtual void Drop(const AttachmentIdList& ids,
-                    const DropCallback& callback) = 0;
+  void Drop(const AttachmentIdList& ids, const DropCallback& callback);
 
   // Asynchronously reads metadata for the attachments identified by |ids|.
   //
@@ -114,72 +109,52 @@ class SYNC_EXPORT AttachmentStore
   // read metadata for all attachments specified in ids. If any of the
   // metadata entries do not exist or could not be read, |callback|'s Result
   // will be UNSPECIFIED_ERROR.
-  virtual void ReadMetadata(const AttachmentIdList& ids,
-                            const ReadMetadataCallback& callback) = 0;
+  void ReadMetadata(const AttachmentIdList& ids,
+                    const ReadMetadataCallback& callback);
 
   // Asynchronously reads metadata for all attachments in the store.
   //
   // |callback| will be invoked when finished. If any of the metadata entries
   // could not be read, |callback|'s Result will be UNSPECIFIED_ERROR.
-  virtual void ReadAllMetadata(const ReadMetadataCallback& callback) = 0;
-
-  // Creates an AttachmentStoreHandle backed by in-memory implementation of
-  // attachment store. For now frontend lives on the same thread as backend.
-  static scoped_refptr<AttachmentStore> CreateInMemoryStore();
-
-  // Creates an AttachmentStoreHandle backed by on-disk implementation of
-  // attachment store. Opens corresponding leveldb database located at |path|.
-  // All backend operations are scheduled to |backend_task_runner|. Opening
-  // attachment store is asynchronous, once it finishes |callback| will be
-  // called on the thread that called CreateOnDiskStore. Calling Read/Write/Drop
-  // before initialization completed is allowed.  Later if initialization fails
-  // these operations will fail with STORE_INITIALIZATION_FAILED error.
-  static scoped_refptr<AttachmentStore> CreateOnDiskStore(
+  void ReadAllMetadata(const ReadMetadataCallback& callback);
+
+  // Given current AttachmentStore (this) creates separate AttachmentStore that
+  // will be used by sync components (AttachmentService). Resulting
+  // AttachmentStore is backed by the same frontend/backend.
+  scoped_ptr<AttachmentStore> CreateAttachmentStoreForSync() const;
+
+  // Creates an AttachmentStore backed by in-memory implementation of attachment
+  // store. For now frontend lives on the same thread as backend.
+  static scoped_ptr<AttachmentStore> CreateInMemoryStore();
+
+  // Creates an AttachmentStore backed by on-disk implementation of attachment
+  // store. Opens corresponding leveldb database located at |path|. All backend
+  // operations are scheduled to |backend_task_runner|. Opening attachment store
+  // is asynchronous, once it finishes |callback| will be called on the thread
+  // that called CreateOnDiskStore. Calling Read/Write/Drop before
+  // initialization completed is allowed.  Later if initialization fails these
+  // operations will fail with STORE_INITIALIZATION_FAILED error.
+  static scoped_ptr<AttachmentStore> CreateOnDiskStore(
       const base::FilePath& path,
       const scoped_refptr<base::SequencedTaskRunner>& backend_task_runner,
       const InitCallback& callback);
 
- protected:
-  friend class base::RefCountedThreadSafe<AttachmentStore>;
-  virtual ~AttachmentStore();
-};
-
-// Interface for AttachmentStore backends.
-//
-// AttachmentStoreBackend provides interface for different backends (on-disk,
-// in-memory). Factory methods in AttachmentStore create corresponding backend
-// and pass reference to AttachmentStoreHandle.
-// All functions in AttachmentStoreBackend mirror corresponding functions in
-// AttachmentStore.
-// All callbacks and result codes are used directly from AttachmentStore.
-// AttachmentStoreHandle only passes callbacks and results, there is no need to
-// declare separate set.
-class SYNC_EXPORT AttachmentStoreBackend {
- public:
-  explicit AttachmentStoreBackend(
-      const scoped_refptr<base::SequencedTaskRunner>& callback_task_runner);
-  virtual ~AttachmentStoreBackend();
-  virtual void Init(const AttachmentStore::InitCallback& callback) = 0;
-  virtual void Read(const AttachmentIdList& ids,
-                    const AttachmentStore::ReadCallback& callback) = 0;
-  virtual void Write(const AttachmentList& attachments,
-                     const AttachmentStore::WriteCallback& callback) = 0;
-  virtual void Drop(const AttachmentIdList& ids,
-                    const AttachmentStore::DropCallback& callback) = 0;
-  virtual void ReadMetadata(
-      const AttachmentIdList& ids,
-      const AttachmentStore::ReadMetadataCallback& callback) = 0;
-  virtual void ReadAllMetadata(
-      const AttachmentStore::ReadMetadataCallback& callback) = 0;
-
- protected:
-  // Helper function to post callback on callback_task_runner.
-  void PostCallback(const base::Closure& callback);
+  // Creates set of AttachmentStore/AttachmentStoreFrontend instances for tests
+  // that provide their own implementation of AttachmentstoreBackend for
+  // mocking.
+  static scoped_ptr<AttachmentStore> CreateMockStoreForTest(
+      scoped_ptr<AttachmentStoreBackend> backend);
 
  private:
-  scoped_refptr<base::SequencedTaskRunner> callback_task_runner_;
+  AttachmentStore(const scoped_refptr<AttachmentStoreFrontend>& frontend,
+                  AttachmentReferrer referrer);
+
+  scoped_refptr<AttachmentStoreFrontend> frontend_;
+  // Modification operations with attachment store will be performed on behalf
+  // of |referrer_|.
+  const AttachmentReferrer referrer_;
 
-  DISALLOW_COPY_AND_ASSIGN(AttachmentStoreBackend);
+  DISALLOW_COPY_AND_ASSIGN(AttachmentStore);
 };
 
 }  // namespace syncer
diff --git a/sync/api/attachments/attachment_store_backend.cc b/sync/api/attachments/attachment_store_backend.cc
new file mode 100644
index 0000000..44363cc
--- /dev/null
+++ b/sync/api/attachments/attachment_store_backend.cc
@@ -0,0 +1,24 @@
+// 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.
+
+#include "sync/api/attachments/attachment_store_backend.h"
+
+#include "base/location.h"
+#include "base/sequenced_task_runner.h"
+
+namespace syncer {
+
+AttachmentStoreBackend::AttachmentStoreBackend(
+    const scoped_refptr<base::SequencedTaskRunner>& callback_task_runner)
+    : callback_task_runner_(callback_task_runner) {
+}
+
+AttachmentStoreBackend::~AttachmentStoreBackend() {
+}
+
+void AttachmentStoreBackend::PostCallback(const base::Closure& callback) {
+  callback_task_runner_->PostTask(FROM_HERE, callback);
+}
+
+}  // namespace syncer
diff --git a/sync/api/attachments/attachment_store_backend.h b/sync/api/attachments/attachment_store_backend.h
new file mode 100644
index 0000000..74859fe
--- /dev/null
+++ b/sync/api/attachments/attachment_store_backend.h
@@ -0,0 +1,64 @@
+// 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 SYNC_API_ATTACHMENTS_ATTACHMENT_STORE_BACKEND_H_
+#define SYNC_API_ATTACHMENTS_ATTACHMENT_STORE_BACKEND_H_
+
+#include "base/callback.h"
+#include "base/memory/ref_counted.h"
+#include "sync/api/attachments/attachment.h"
+#include "sync/api/attachments/attachment_id.h"
+#include "sync/api/attachments/attachment_store.h"
+#include "sync/base/sync_export.h"
+
+namespace base {
+class SequencedTaskRunner;
+}  // namespace base
+
+namespace syncer {
+
+// Interface for AttachmentStore backends.
+//
+// AttachmentStoreBackend provides interface for different backends (on-disk,
+// in-memory). Factory methods in AttachmentStore create corresponding backend
+// and pass reference to AttachmentStore.
+// All functions in AttachmentStoreBackend mirror corresponding functions in
+// AttachmentStore.
+// All callbacks and result codes are used directly from AttachmentStore.
+// AttachmentStoreFrontend only passes callbacks and results without modifying
+// them, there is no need to declare separate set.
+class SYNC_EXPORT AttachmentStoreBackend {
+ public:
+  explicit AttachmentStoreBackend(
+      const scoped_refptr<base::SequencedTaskRunner>& callback_task_runner);
+  virtual ~AttachmentStoreBackend();
+  virtual void Init(const AttachmentStore::InitCallback& callback) = 0;
+  virtual void Read(const AttachmentIdList& ids,
+                    const AttachmentStore::ReadCallback& callback) = 0;
+  virtual void Write(AttachmentStore::AttachmentReferrer referrer,
+                     const AttachmentList& attachments,
+                     const AttachmentStore::WriteCallback& callback) = 0;
+  virtual void Drop(AttachmentStore::AttachmentReferrer referrer,
+                    const AttachmentIdList& ids,
+                    const AttachmentStore::DropCallback& callback) = 0;
+  virtual void ReadMetadata(
+      const AttachmentIdList& ids,
+      const AttachmentStore::ReadMetadataCallback& callback) = 0;
+  virtual void ReadAllMetadata(
+      AttachmentStore::AttachmentReferrer referrer,
+      const AttachmentStore::ReadMetadataCallback& callback) = 0;
+
+ protected:
+  // Helper function to post callback on callback_task_runner.
+  void PostCallback(const base::Closure& callback);
+
+ private:
+  scoped_refptr<base::SequencedTaskRunner> callback_task_runner_;
+
+  DISALLOW_COPY_AND_ASSIGN(AttachmentStoreBackend);
+};
+
+}  // namespace syncer
+
+#endif  // SYNC_API_ATTACHMENTS_ATTACHMENT_STORE_BACKEND_H_
diff --git a/sync/api/fake_syncable_service.cc b/sync/api/fake_syncable_service.cc
index ec7b90c..d2177e9 100644
--- a/sync/api/fake_syncable_service.cc
+++ b/sync/api/fake_syncable_service.cc
@@ -26,8 +26,8 @@ void FakeSyncableService::set_process_sync_changes_error(
 }
 
 void FakeSyncableService::set_attachment_store(
-    const scoped_refptr<AttachmentStore>& attachment_store) {
-  attachment_store_ = attachment_store;
+    scoped_ptr<AttachmentStore> attachment_store) {
+  attachment_store_ = attachment_store.Pass();
 }
 
 const AttachmentService* FakeSyncableService::attachment_service() const {
@@ -70,8 +70,9 @@ SyncError FakeSyncableService::ProcessSyncChanges(
   return process_sync_changes_error_;
 }
 
-scoped_refptr<AttachmentStore> FakeSyncableService::GetAttachmentStore() {
-  return attachment_store_;
+scoped_ptr<AttachmentStore> FakeSyncableService::GetAttachmentStoreForSync() {
+  return attachment_store_ ? attachment_store_->CreateAttachmentStoreForSync()
+                           : scoped_ptr<AttachmentStore>();
 }
 
 void FakeSyncableService::SetAttachmentService(
diff --git a/sync/api/fake_syncable_service.h b/sync/api/fake_syncable_service.h
index 7403ef3..9b75647 100644
--- a/sync/api/fake_syncable_service.h
+++ b/sync/api/fake_syncable_service.h
@@ -23,8 +23,7 @@ class FakeSyncableService : public SyncableService {
   void set_process_sync_changes_error(const SyncError& error);
 
   // Setter for AttachmentStore.
-  void set_attachment_store(
-      const scoped_refptr<AttachmentStore>& attachment_store);
+  void set_attachment_store(scoped_ptr<AttachmentStore> attachment_store);
 
   // AttachmentService should be set when this syncable service is connected,
   // just before MergeDataAndStartSyncing. NULL is returned by default.
@@ -44,7 +43,7 @@ class FakeSyncableService : public SyncableService {
   SyncDataList GetAllSyncData(ModelType type) const override;
   SyncError ProcessSyncChanges(const tracked_objects::Location& from_here,
                                const SyncChangeList& change_list) override;
-  scoped_refptr<AttachmentStore> GetAttachmentStore() override;
+  scoped_ptr<AttachmentStore> GetAttachmentStoreForSync() override;
   void SetAttachmentService(
       scoped_ptr<AttachmentService> attachment_service) override;
 
@@ -54,7 +53,7 @@ class FakeSyncableService : public SyncableService {
   SyncError process_sync_changes_error_;
   bool syncing_;
   ModelType type_;
-  scoped_refptr<AttachmentStore> attachment_store_;
+  scoped_ptr<AttachmentStore> attachment_store_;
   scoped_ptr<AttachmentService> attachment_service_;
 };
 
diff --git a/sync/api/syncable_service.cc b/sync/api/syncable_service.cc
index 7b0a973..f0cfe67 100644
--- a/sync/api/syncable_service.cc
+++ b/sync/api/syncable_service.cc
@@ -8,8 +8,8 @@ namespace syncer {
 
 SyncableService::~SyncableService() {}
 
-scoped_refptr<AttachmentStore> SyncableService::GetAttachmentStore() {
-  return scoped_refptr<AttachmentStore>();
+scoped_ptr<AttachmentStore> SyncableService::GetAttachmentStoreForSync() {
+  return scoped_ptr<AttachmentStore>();
 }
 
 void SyncableService::SetAttachmentService(
diff --git a/sync/api/syncable_service.h b/sync/api/syncable_service.h
index f83a017..0d5aae4 100644
--- a/sync/api/syncable_service.h
+++ b/sync/api/syncable_service.h
@@ -66,15 +66,16 @@ class SYNC_EXPORT SyncableService
   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.
+  // Returns AttachmentStore for use 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
-  // to return pointer to it.
-  virtual scoped_refptr<AttachmentStore> GetAttachmentStore();
+  // attachments should create attachment store, implement GetAttachmentStore
+  // to return result of AttachmentStore::CreateAttachmentStoreForSync() from
+  // attachment store object.
+  virtual scoped_ptr<AttachmentStore> GetAttachmentStoreForSync();
 
   // Called by sync to provide AttachmentService to be used to download
   // attachments.