Update sync API to support attachments.

sync/api/attachments/attachment_service_base.h

Index: sync/api/attachments/attachment_service_base.h
diff --git a/sync/api/attachments/attachment_service_base.h b/sync/api/attachments/attachment_service_base.h
new file mode 100644
index 0000000000000000000000000000000000000000..5d89446e59aa0289878aaa4b6c05f85c354ee399
--- /dev/null
+++ b/sync/api/attachments/attachment_service_base.h
@@ -0,0 +1,30 @@
+// 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_BASE_H_
+#define SYNC_API_ATTACHMENTS_ATTACHMENT_SERVICE_BASE_H_
+
+#include "base/memory/weak_ptr.h"
+#include "sync/api/attachments/attachment_service.h"
+#include "sync/base/sync_export.h"
+
+namespace syncer {
+
+// AttachmentServiceBase is an AttachmentService that provides WeakPtrs.  It is
+// designed to be used in conjunction with AttachmentServiceProxy.
+class SYNC_EXPORT AttachmentServiceBase : public AttachmentService {
+ public:
+  AttachmentServiceBase();
+  virtual ~AttachmentServiceBase();
+
+  // Implementation is intentionally omitted.  A derived class should provide
+  // its own implementation and ensure that its WeakPtrFactory is destroyed
+  // immediately after the derived class's destructor is invoked.  See
+  // SupportsWeakPtr class comment for rationale.
+  virtual base::WeakPtr<AttachmentServiceBase> AsWeakPtr() = 0;

[tim (not reviewing), 2014/03/27 17:01:44]

The more I look at it, the less I think of this as a responsibility of
AttachmentService and more as a responsibility of GenericChangeProcessor, which
owns the AS. GCP is the only thing that vends pointers to an AS -- it can ensure
that it only vends weak ptrs via a weak_ptr_factory member declared after the AS
scoped_ptr, and you're done.  No AttachmentServiceBase, no worrying about AS
implementations keeping the factory as the last member, no referencing
SupportsWeakPtr... WDYT?

[maniscalco, 2014/03/27 17:36:24]

I like this approach.  It's nice to eliminate AttachmentServiceBase (one fewer
class to understand).  Done.

+};
+
+}  // namespace syncer
+
+#endif  // SYNC_API_ATTACHMENTS_ATTACHMENT_SERVICE_BASE_H_