[Sync] DeviceInfo implementation of MergeSyncData.

sync/api/model_type_service.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 SYNC_API_MODEL_TYPE_SERVICE_H_
 #define SYNC_API_MODEL_TYPE_SERVICE_H_
 
 #include <string>
 #include <vector>
 
@@ skipping 20 matching lines @@
   typedef std::vector<std::string> ClientTagList;
 
   ModelTypeService();
 
   virtual ~ModelTypeService();
 
   // Creates an object used to communicate changes in the sync metadata to the
   // model type store.
   virtual scoped_ptr<MetadataChangeList> CreateMetadataChangeList() = 0;
 
-  // Perform the initial merge of data from the sync server. Should only need
-  // to be called when sync is first turned on, not on every restart.
+  // Perform the initial merge between local and sync data. This should only be
+  // called when a data type is first enabled to start syncing, and there is no
+  // sync metadata. Best effort should be made to match local and sync data. The
+  // keys in the |entity_data_map| will have been created via GetClientTag(...),
+  // and if a local and sync data should match/merge but disagree on tags, the
+  // service should use the sync data's tag. Any local pieces of data that are
+  // not present in sync should immediately be Put(...) to the processor before
+  // returning. The same MetadataChangeList that was passed into this function
+  // can be passed to Put(...) calls. Delete(...) can also be called but should
+  // not be needed for most model types. Durable storage writes, if not able to
+  // combine all change atomically, should save the metadata after the data
+  // changes, so that this merge will be re-driven by sync if is not completely
+  // saved during the current run.
   virtual syncer::SyncError MergeSyncData(
       scoped_ptr<MetadataChangeList> metadata_change_list,
       EntityDataMap entity_data_map) = 0;
 
   // Apply changes from the sync server locally.
   // Please note that |entity_changes| might have fewer entries than
   // |metadata_change_list| in case when some of the data changes are filtered
   // out, or even be empty in case when a commit confirmation is processed and
   // only the metadata needs to persisted.
   virtual syncer::SyncError ApplySyncChanges(
@@ skipping 23 matching lines @@
   void clear_change_processor();
 
  private:
   // Recieves ownership in set_change_processor(...).
   scoped_ptr<ModelTypeChangeProcessor> change_processor_;
 };
 
 }  // namespace syncer_v2
 
 #endif  // SYNC_API_MODEL_TYPE_SERVICE_H_