Fix AttributeError

synapse/events/__init__.py

@@ -55,6 +55,9 @@ class _EventInternalMetadata(object):
     def get_dict(self):
         return dict(self.__dict__)
 
+    def is_outlier(self):
+        return hasattr(self, "outlier") and self.outlier
+
 
 def _event_dict_property(key):
         def getter(self):

synapse/handlers/federation.py

@@ -687,7 +687,7 @@ class FederationHandler(BaseHandler):
             event.event_id, event.signatures,
         )
 
-        is_new_state = not event.internal_metadata.outlier
+        is_new_state = not event.internal_metadata.is_outlier()
 
         known_ids = set(
             [s.event_id for s in context.auth_events.values()]