Fix mainline ordering in state res v2 (#8971)

This had two effects 1) it'd give the wrong answer and b) would iterate
*all* power levels in the auth chain of each event. The latter of which
can be *very* expensive for certain types of IRC bridge rooms that have
large numbers of power level changes.

1 files changed, 1 insertions, 1 deletions

diff --git a/synapse/state/v2.py b/synapse/state/v2.py

index f85124bf81..e585954bd8 100644
--- a/synapse/state/v2.py
+++ b/synapse/state/v2.py
@@ -658,7 +658,7 @@ async def _get_mainline_depth_for_event(
     # We do an iterative search, replacing `event with the power level in its
     # auth events (if any)
     while tmp_event:
-        depth = mainline_map.get(event.event_id)
+        depth = mainline_map.get(tmp_event.event_id)
         if depth is not None:
             return depth