[USS] Add SyncMetadataStore interface

components/sync/model/metadata_change_list.h

 // 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 COMPONENTS_SYNC_MODEL_METADATA_CHANGE_LIST_H_
 #define COMPONENTS_SYNC_MODEL_METADATA_CHANGE_LIST_H_
 
 #include <string>
 
+#include "base/optional.h"
+#include "components/sync/model/model_error.h"
+
 namespace sync_pb {
 class EntityMetadata;
 class ModelTypeState;
 }  // namespace sync_pb
 
 namespace syncer {
 
 // Interface used by the processor and service to communicate about metadata.
 // The purpose of the interface is to record changes to data type global and
 // per entity metadata for the purpose of propagating changes to the datatype
@@ skipping 16 matching lines @@
 
   // Requests metadata entry to be updated in the storage.
   // Please note that the update might contain a deleted entry if
   // metadata.is_deleted() is true (as opposed to clearing the entry from the
   // storage completely by calling the Clear method).
   virtual void UpdateMetadata(const std::string& storage_key,
                               const sync_pb::EntityMetadata& metadata) = 0;
 
   // Requests metadata entry to be cleared from the storage.
   virtual void ClearMetadata(const std::string& storage_key) = 0;
+
+  // Returns the value of ModelError and unsets it.
+  virtual base::Optional<syncer::ModelError> TakeError() = 0;

[skym, 2017/04/06 23:01:58]

What was the decision yesterday about returning errors from MCLs? Is it just
that we'll talk about it next week again?

[pavely, 2017/04/06 23:36:02]

The decision was that we don't change the pattern in this CL, instead consider
this as a separate change. If we decide to return errors from individual
functions I think we'll need to refactor SMTP's functions to handle these
errors.

[Gang Wu, 2017/04/21 05:28:03]

Done.

 };
 
 }  // namespace syncer
 
 #endif  // COMPONENTS_SYNC_MODEL_METADATA_CHANGE_LIST_H_