Change get_pos_of_last_change to return upper bound

synapse/storage/event_federation.py

@@ -347,9 +347,8 @@ class EventFederationStore(SQLBaseStore):
     def get_forward_extremeties_for_room(self, room_id, stream_ordering):
         # We want to make the cache more effective, so we clamp to the last
         # change before the given ordering.
-        last_change = self._events_stream_cache.get_pos_of_last_change(room_id)
-        if last_change:
-            stream_ordering = min(last_change, stream_ordering)
+        last_change = self._events_stream_cache.get_max_pos_of_last_change(room_id)
+        stream_ordering = min(last_change, stream_ordering)
 
         return self._get_forward_extremeties_for_room(room_id, stream_ordering)
 

synapse/util/caches/stream_change_cache.py

@@ -122,7 +122,8 @@ class StreamChangeCache(object):
                 self._earliest_known_stream_pos = max(k, self._earliest_known_stream_pos)
                 self._entity_to_key.pop(r, None)
 
-    def get_pos_of_last_change(self, entity):
-        """Returns the stream pos of the last change for an entitiy, if known.
+    def get_max_pos_of_last_change(self, entity):
+        """Returns an upper bound of the stream id of the last change to an
+        entity.
         """
-        return self._entity_to_key.get(entity, None)
+        return self._entity_to_key.get(entity, self._earliest_known_stream_pos)