Add AttachmentDownloader interface, change signature of AttachmentStore::Read

sync/api/attachments/attachment_store.h

 // 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_STORE_H_
 #define SYNC_API_ATTACHMENTS_ATTACHMENT_STORE_H_
 
 #include "base/callback.h"
 #include "base/memory/ref_counted.h"
 #include "base/memory/scoped_ptr.h"
@@ skipping 20 matching lines @@
   virtual ~AttachmentStore();
 
   // TODO(maniscalco): Consider udpating Read and Write methods to support
   // resumable transfers (bug 353292).
 
   enum Result {
     SUCCESS,            // No error, all completed successfully.
     UNSPECIFIED_ERROR,  // An unspecified error occurred for one or more items.
   };
 
-  typedef base::Callback<void(const Result&, scoped_ptr<AttachmentMap>)>
-      ReadCallback;
+  typedef base::Callback<void(const Result&,
+                              scoped_ptr<AttachmentMap>,
+                              scoped_ptr<AttachmentIdList>)> ReadCallback;
   typedef base::Callback<void(const Result&)> WriteCallback;
   typedef base::Callback<void(const Result&)> DropCallback;
 
   // Asynchronously reads the attachments identified by |ids|.
   //
-  // |callback| will be invoked when finished. If any of the attachments do not
-  // exist or could not be read, |callback|'s Result will be UNSPECIFIED_ERROR
-  // and |callback| may receive a partial result. That is, AttachmentMap may be
-  // empty or may contain the attachments that were read successfully.
+  // |callback| will be invoked when finished. AttachmentStore will attempt to
+  // read all attachments specified in ids. If any of the attachments do not
+  // exist or could not be read, |callback|'s Result will be UNSPECIFIED_ERROR.
+  // Callback's AttachmentMap will contain all attachments that were
+  // successfully read, AttachmentIdList will contain attachment ids of
+  // attachments that are unavailable in attachment store, these need to be
+  // downloaded from server.
   //
   // Reads on individual attachments are treated atomically; |callback| will not
   // read only part of an attachment.
   virtual void Read(const AttachmentIdList& ids,
                     const ReadCallback& callback) = 0;
 
   // Asynchronously writes |attachments| to the store.
   //
   // Will not overwrite stored attachments. Attempting to overwrite an
   // attachment that already exists is not an error.
@@ skipping 14 matching lines @@
   // could not be dropped, |callback|'s Result will be UNSPECIFIED_ERROR. When
   // this happens, some or none of the attachments may have been dropped
   // successfully.
   virtual void Drop(const AttachmentIdList& ids,
                     const DropCallback& callback) = 0;
 };
 
 }  // namespace syncer
 
 #endif  // SYNC_API_ATTACHMENTS_ATTACHMENT_STORE_H_