Also don't answer /make_join

synapse/handlers/federation.py

@@ -748,6 +748,19 @@ async def on_make_join_request(
         # (and return a 404 otherwise)
         room_version = await self.store.get_room_version(room_id)
 
+        if await self.store.is_partial_state_room(room_id):
+            # If our server is still only partially joined, we can't give a complete
+            # response to make_join, so return a 404 as we would if we weren't in the
+            # room at all.
+            logger.info(
+                "Rejecting make_join to %s because it's a partial state room", room_id
+            )
+            raise SynapseError(
+                404,
+                "Unable to handle make_join right now; this server is not fully joined.",
+                errcode=Codes.NOT_FOUND,
+            )
+
         # now check that we are *still* in the room
         is_in_room = await self._event_auth_handler.check_host_in_room(
             room_id, self.server_name