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

sync/internal_api/public/attachments/attachment_store_frontend.h

Index: sync/internal_api/public/attachments/attachment_store_frontend.h
diff --git a/sync/internal_api/public/attachments/attachment_store_frontend.h b/sync/internal_api/public/attachments/attachment_store_frontend.h
new file mode 100644
index 0000000000000000000000000000000000000000..ec4a33f24f661e381029968b56f333212e32238a
--- /dev/null
+++ b/sync/internal_api/public/attachments/attachment_store_frontend.h
@@ -0,0 +1,69 @@
+// 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_INTERNAL_API_PUBLIC_ATTACHMENTS_ATTACHMENT_STORE_FRONTEND_H_
+#define SYNC_INTERNAL_API_PUBLIC_ATTACHMENTS_ATTACHMENT_STORE_FRONTEND_H_
+
+#include "base/memory/ref_counted.h"
+#include "base/memory/scoped_ptr.h"
+#include "base/threading/non_thread_safe.h"
+#include "sync/api/attachments/attachment_store.h"
+
+namespace base {
+class SequencedTaskRunner;
+}  // namespace base
+
+namespace syncer {
+
+class AttachmentStoreBackend;
+
+// AttachmentStoreFrontend is helper to post AttachmentStore calls to backend on
+// different thread. Backend is expected to know on which thread to post
+// callbacks with results.
+// AttachmentStoreFrontend takes ownership of backend. Backend is deleted on
+// backend thread.
+// AttachmentStoreFrontend is not thread safe, it should only be accessed from
+// model thread.
+// Method signatures of AttachmentStoreFrontend match exactly methods of
+// AttachmentStoreBackend.
+class SYNC_EXPORT AttachmentStoreFrontend
+    : public base::RefCounted<AttachmentStoreFrontend>,
+      public base::NonThreadSafe {
+ public:
+  AttachmentStoreFrontend(
+      scoped_ptr<AttachmentStoreBackend> backend,
+      const scoped_refptr<base::SequencedTaskRunner>& backend_task_runner);
+
+  void Init(const AttachmentStore::InitCallback& callback);
+
+  void Read(const AttachmentIdList& ids,
+            const AttachmentStore::ReadCallback& callback);
+
+  void Write(AttachmentStore::AttachmentReferrer referrer,
+             const AttachmentList& attachments,
+             const AttachmentStore::WriteCallback& callback);
+
+  void Drop(AttachmentStore::AttachmentReferrer referrer,
+            const AttachmentIdList& ids,
+            const AttachmentStore::DropCallback& callback);
+
+  void ReadMetadata(const AttachmentIdList& ids,
+                    const AttachmentStore::ReadMetadataCallback& callback);
+
+  void ReadAllMetadata(AttachmentStore::AttachmentReferrer referrer,
+                       const AttachmentStore::ReadMetadataCallback& callback);
+
+ private:
+  friend class base::RefCounted<AttachmentStoreFrontend>;
+  virtual ~AttachmentStoreFrontend();
+
+  scoped_ptr<AttachmentStoreBackend> backend_;
+  scoped_refptr<base::SequencedTaskRunner> backend_task_runner_;
+
+  DISALLOW_COPY_AND_ASSIGN(AttachmentStoreFrontend);
+};
+
+}  // namespace syncer
+
+#endif  // SYNC_INTERNAL_API_PUBLIC_ATTACHMENTS_ATTACHMENT_STORE_FRONTEND_H_