Sliding Sync: ensure bump stamp ignores backfilled events (#17478)

Backfill events have a negative stream ordering, and so its not useful
to use to compare with other (positive) stream orderings.

Plus, the Rust SDK currently assumes `bump_stamp` is positive.

1 files changed, 8 insertions, 2 deletions

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

index 36665db8e1..f1f6f30b95 100644
--- a/synapse/handlers/sliding_sync.py
+++ b/synapse/handlers/sliding_sync.py
@@ -1758,8 +1758,14 @@ class SlidingSyncHandler:
         bump_stamp = room_membership_for_user_at_to_token.event_pos.stream
         # But if we found a bump event, use that instead
         if last_bump_event_result is not None:
-            _, bump_event_pos = last_bump_event_result
-            bump_stamp = bump_event_pos.stream
+            _, new_bump_event_pos = last_bump_event_result
+
+            # If we've just joined a remote room, then the last bump event may
+            # have been backfilled (and so have a negative stream ordering).
+            # These negative stream orderings can't sensibly be compared, so
+            # instead we use the membership event position.
+            if new_bump_event_pos.stream > 0:
+                bump_stamp = new_bump_event_pos.stream
 
         return SlidingSyncResult.RoomResult(
             name=room_name,