Fix how we check for self redaction

synapse/handlers/message.py

@@ -729,10 +729,24 @@ class EventCreationHandler(object):
         assert not self.config.worker_app
 
         if ratelimit:
-            is_admin_redaction = (
-                event.type == EventTypes.Redaction
-                and event.sender != requester.user.to_string()
-            )
+            # We check if this is a room admin redacting an event so that we
+            # can apply different ratelimiting. We do this by simply checking
+            # its not a self-redaction (to avoid having to look up whether the
+            # user is actually admin or not).
+            is_admin_redaction = False
+            if event.type == EventTypes.Redaction:
+                original_event = yield self.store.get_event(
+                    event.redacts,
+                    check_redacted=False,
+                    get_prev_content=False,
+                    allow_rejected=False,
+                    allow_none=True,
+                )
+
+                is_admin_redaction = (
+                    original_event and event.sender != original_event.sender
+                )
+
             yield self.base_handler.ratelimit(
                 requester, is_admin_redaction=is_admin_redaction
             )