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

sync/api/attachments/attachment_store_backend.h

Index: sync/api/attachments/attachment_store_backend.h
diff --git a/sync/api/attachments/attachment_store_backend.h b/sync/api/attachments/attachment_store_backend.h
new file mode 100644
index 0000000000000000000000000000000000000000..74859fe72601e816fddde2764617a2fbcbe9aa1a
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+++ 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_