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

sync/internal_api/public/attachments/attachment_store_handle.h

Index: sync/internal_api/public/attachments/attachment_store_handle.h
diff --git a/sync/internal_api/public/attachments/attachment_store_handle.h b/sync/internal_api/public/attachments/attachment_store_handle.h
new file mode 100644
index 0000000000000000000000000000000000000000..bbf79104c3b99eabe25a7210221596963ab8371b
--- /dev/null
+++ b/sync/internal_api/public/attachments/attachment_store_handle.h
@@ -0,0 +1,68 @@
+// Copyright 2014 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_ATTACHMENTS_ATTACHMENT_STORE_HANDLE_H_
+#define SYNC_INTERNAL_API_PUBLIC_ATTACHMENTS_ATTACHMENT_STORE_HANDLE_H_
+
+#include <map>
+
+#include "base/memory/ref_counted.h"
+#include "base/memory/scoped_ptr.h"
+#include "base/stl_util.h"
+#include "base/threading/non_thread_safe.h"
+#include "sync/api/attachments/attachment_store.h"
+#include "sync/base/sync_export.h"
+
+namespace base {
+class SequencedTaskRunner;
+}  // namespace base
+
+namespace sync_pb {
+class AttachmentId;
+}  // namespace sync_pb
+
+namespace syncer {
+
+class Attachment;
+
+// AttachmentStoreHandle is helper to post AttachmentStore calls to backend on
+// different thread. Backend is expected to know on which thread to post
+// callbacks with results.
+// AttachmentStoreHandle takes ownership of backend. Backend is deleted on
+// backend thread.
+// AttachmentStoreHandle is not thread safe, it should only be accessed from
+// model thread.
+class SYNC_EXPORT AttachmentStoreHandle : public AttachmentStore,
+                                          public base::NonThreadSafe {
+ public:
+  AttachmentStoreHandle(
+      scoped_ptr<AttachmentStoreBackend> backend,
+      const scoped_refptr<base::SequencedTaskRunner>& backend_task_runner);
+
+  // AttachmentStore implementation.
+  void Init(const InitCallback& callback) override;
+  void Read(const AttachmentIdList& id, const ReadCallback& callback) override;
+  void Write(const AttachmentList& attachments,
+             const WriteCallback& callback) override;
+  void Drop(const AttachmentIdList& id, const DropCallback& callback) override;
+  void ReadMetadata(const AttachmentIdList& ids,
+                    const ReadMetadataCallback& callback) override;
+  void ReadAllMetadata(const ReadMetadataCallback& callback) override;
+
+ private:
+  ~AttachmentStoreHandle() override;
+
+  // AttachmentStoreHandle controls backend's lifetime. It is safe for
+  // AttachmentStoreHandle to bind backend through base::Unretained for posts.
+  // Backend is deleted on backend_task_runner, after that backend_ pointer is
+  // invalid.
+  scoped_ptr<AttachmentStoreBackend> backend_;
+  scoped_refptr<base::SequencedTaskRunner> backend_task_runner_;
+
+  DISALLOW_COPY_AND_ASSIGN(AttachmentStoreHandle);
+};
+
+}  // namespace syncer
+
+#endif  // SYNC_INTERNAL_API_PUBLIC_ATTACHMENTS_ATTACHMENT_STORE_HANDLE_H_