Fix getting an event for a room the server forgot it was in

synapse/handlers/federation.py

@@ -125,8 +125,20 @@ class FederationHandler(BaseHandler):
         )
         if not is_in_room and not event.internal_metadata.is_outlier():
             logger.debug("Got event for room we're not in.")
-            current_state = state
 
+            try:
+                event_stream_id, max_stream_id = yield self._persist_auth_tree(
+                    auth_chain, state, event
+                )
+            except AuthError as e:
+                raise FederationError(
+                    "ERROR",
+                    e.code,
+                    e.msg,
+                    affected=event.event_id,
+                )
+
+        else:
             event_ids = set()
             if state:
                 event_ids |= {e.event_id for e in state}