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diff --git a/sync/api/attachments/attachment_service_proxy.h b/sync/api/attachments/attachment_service_proxy.h
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+++ b/sync/api/attachments/attachment_service_proxy.h
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+// 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_API_ATTACHMENTS_ATTACHMENT_SERVICE_PROXY_H_
+#define SYNC_API_ATTACHMENTS_ATTACHMENT_SERVICE_PROXY_H_
+
+#include "base/basictypes.h"
+#include "base/callback.h"
+#include "base/memory/scoped_ptr.h"
+#include "base/memory/weak_ptr.h"
+#include "base/sequenced_task_runner.h"
+#include "base/task_runner.h"
+#include "sync/api/attachments/attachment.h"
+#include "sync/api/attachments/attachment_service.h"
+#include "sync/base/sync_export.h"
+
+namespace syncer {
+
+class SyncData;
+
+// AttachmentServiceProxy wraps an AttachmentService allowing multiple threads
+// to share the wrapped AttachmentService.
+//
+// Callbacks passed to methods on this class will be invoked in the same thread
+// from which the method was called.
+//
+// This class does not own its wrapped AttachmentService object.  This class
+// holds a WeakPtr to the wrapped object.  Once the the wrapped object is
+// destroyed, method calls on this object will be no-ops.
+//
+//  Users of this class should take care to destroy the wrapped object on the
+//  correct thread (wrapped_task_runner).
+//
+// This class is thread-safe.
+class SYNC_EXPORT AttachmentServiceProxy : public AttachmentService {
+ public:
+  // Default copy and assignment are welcome.
+
+  // Construct an invalid AttachmentServiceProxy.
+  AttachmentServiceProxy();
+
+  // Construct an AttachmentServiceProxy that forwards calls to |wrapped| on the
+  // |wrapped_task_runner| thread.
+  AttachmentServiceProxy(
+      const scoped_refptr<base::SequencedTaskRunner>& wrapped_task_runner,
+      base::WeakPtr<syncer::AttachmentService> wrapped);
+
+  virtual ~AttachmentServiceProxy();
+
+  // AttachmentService implementation.
+  virtual void GetOrDownloadAttachments(const AttachmentIdList& attachment_ids,
+                                        const GetOrDownloadCallback& callback)
+      OVERRIDE;
+  virtual void DropAttachments(const AttachmentIdList& attachment_ids,
+                               const DropCallback& callback) OVERRIDE;
+  virtual void OnSyncDataAdd(const SyncData& sync_data) OVERRIDE;
+  virtual void OnSyncDataDelete(const SyncData& sync_data) OVERRIDE;
+  virtual void OnSyncDataUpdate(const AttachmentIdList& old_attachment_ids,
+                                const SyncData& updated_sync_data) OVERRIDE;
+
+ private:
+  scoped_refptr<base::SequencedTaskRunner> wrapped_task_runner_;
+  // wrapped_ must only be dereferenced on the wrapped_task_runner_ thread.
+  base::WeakPtr<AttachmentService> wrapped_;
+};
+
+}  // namespace syncer
+
+#endif  // SYNC_API_ATTACHMENTS_ATTACHMENT_SERVICE_PROXY_H_