Reinstate linearizer for federation_server.on_context_state_request

synapse/federation/federation_server.py

@@ -211,11 +211,17 @@ class FederationServer(FederationBase):
         if not in_room:
             raise AuthError(403, "Host not in room.")
 
-        resp = yield self._state_resp_cache.wrap(
-            (room_id, event_id),
-            self._on_context_state_request_compute,
-            room_id, event_id,
-        )
+        # we grab the linearizer to protect ourselves from servers which hammer
+        # us. In theory we might already have the response to this query
+        # in the cache so we could return it without waiting for the linearizer
+        # - but that's non-trivial to get right, and anyway somewhat defeats
+        # the point of the linearizer.
+        with (yield self._server_linearizer.queue((origin, room_id))):
+            resp = yield self._state_resp_cache.wrap(
+                (room_id, event_id),
+                self._on_context_state_request_compute,
+                room_id, event_id,
+            )
 
         defer.returnValue((200, resp))