Stub out broken function only used for cache

synapse/storage/signatures.py

@@ -25,7 +25,9 @@ from synapse.util.caches.descriptors import cached, cachedList
 class SignatureWorkerStore(SQLBaseStore):
     @cached()
     def get_event_reference_hash(self, event_id):
-        return self._get_event_reference_hashes_txn(event_id)
+        # This is a dummy function to allow get_event_reference_hashes
+        # to use its cache
+        raise NotImplementedError()
 
     @cachedList(cached_method_name="get_event_reference_hash",
                 list_name="event_ids", num_args=1)