Noop get_new_messages_for_device if token hasn't changed

synapse/handlers/sync.py

@@ -571,6 +571,7 @@ class SyncHandler(object):
                 user_id, device_id, since_stream_id
             )
 
+        if since_stream_id and since_stream_id == int(now_token.to_device_key):
             logger.debug("Getting messages up to %d", now_token.to_device_key)
             messages, stream_id = yield self.store.get_new_messages_for_device(
                 user_id, device_id, now_token.to_device_key
@@ -580,6 +581,8 @@ class SyncHandler(object):
                 "to_device_key", stream_id
             )
             sync_result_builder.to_device = messages
+        else:
+            sync_result_builder.to_device = []
 
     @defer.inlineCallbacks
     def _generate_sync_entry_for_account_data(self, sync_result_builder):