Fix missing sync events during historical batch imports (#12319)

Discovered after much in-depth investigation in #12281.

Closes: #12281
Closes: #3305

Signed off by: Nick Mills-Barrett nick@beeper.com

1 files changed, 7 insertions, 8 deletions

diff --git a/synapse/handlers/sync.py b/synapse/handlers/sync.py

index 6c8b17c420..5125126a80 100644
--- a/synapse/handlers/sync.py
+++ b/synapse/handlers/sync.py
@@ -661,16 +661,15 @@ class SyncHandler:
             stream_position: point at which to get state
             state_filter: The state filter used to fetch state from the database.
         """
-        # FIXME this claims to get the state at a stream position, but
-        # get_recent_events_for_room operates by topo ordering. This therefore
-        # does not reliably give you the state at the given stream position.
-        # (https://github.com/matrix-org/synapse/issues/3305)
-        last_events, _ = await self.store.get_recent_events_for_room(
-            room_id, end_token=stream_position.room_key, limit=1
+        # FIXME: This gets the state at the latest event before the stream ordering,
+        # which might not be the same as the "current state" of the room at the time
+        # of the stream token if there were multiple forward extremities at the time.
+        last_event = await self.store.get_last_event_in_room_before_stream_ordering(
+            room_id,
+            end_token=stream_position.room_key,
         )
 
-        if last_events:
-            last_event = last_events[-1]
+        if last_event:
             state = await self.get_state_after_event(
                 last_event, state_filter=state_filter or StateFilter.all()
             )