Fix it for knocks too while we're here

Author: richvdh

changelog.d/10223.bugfix

@@ -1 +1 @@
-Fix a long-standing bug which meant that invite rejections were not sent out over federation in a timely manner.
+Fix a long-standing bug which meant that invite rejections and knocks were not sent out over federation in a timely manner.

synapse/handlers/federation.py

@@ -2091,6 +2091,13 @@ async def on_send_knock_request(
 
         event.internal_metadata.outlier = False
 
+        # Send this event on behalf of the other server.
+        #
+        # The remote server isn't a full participant in the room at this point, so
+        # may not have an up-to-date list of the other homeservers participating in
+        # the room, so we send it on their behalf.
+        event.internal_metadata.send_on_behalf_of = origin
+
         context = await self.state_handler.compute_event_context(event)
 
         event_allowed = await self.third_party_event_rules.check_event_allowed(