From 99b146825a1a8257d05440ae3e331c68b8e1575a Mon Sep 17 00:00:00 2001
From: Jason Little <j.little@famedly.com>
Date: Wed, 30 Apr 2025 09:29:42 -0500
Subject: fix: Always recheck `/messages` pagination data if a backfill might
 have been needed (#28)

---
 synapse/handlers/pagination.py | 36 ++++++++++++++++++++----------------
 1 file changed, 20 insertions(+), 16 deletions(-)

(limited to 'synapse/handlers/pagination.py')

diff --git a/synapse/handlers/pagination.py b/synapse/handlers/pagination.py
index 4070b74b7a..81cda38549 100644
--- a/synapse/handlers/pagination.py
+++ b/synapse/handlers/pagination.py
@@ -577,27 +577,31 @@ class PaginationHandler:
                 or missing_too_many_events
                 or not_enough_events_to_fill_response
             ):
-                did_backfill = await self.hs.get_federation_handler().maybe_backfill(
+                # Historical Note: There used to be a check here for if backfill was
+                # successful or not
+                await self.hs.get_federation_handler().maybe_backfill(
                     room_id,
                     curr_topo,
                     limit=pagin_config.limit,
                 )
 
-                # If we did backfill something, refetch the events from the database to
-                # catch anything new that might have been added since we last fetched.
-                if did_backfill:
-                    (
-                        events,
-                        next_key,
-                        _,
-                    ) = await self.store.paginate_room_events_by_topological_ordering(
-                        room_id=room_id,
-                        from_key=from_token.room_key,
-                        to_key=to_room_key,
-                        direction=pagin_config.direction,
-                        limit=pagin_config.limit,
-                        event_filter=event_filter,
-                    )
+                # Regardless if we backfilled or not, another worker or even a
+                # simultaneous request may have backfilled for us while we were held
+                # behind the linearizer. This should not have too much additional
+                # database load as it will only be triggered if a backfill *might* have
+                # been needed
+                (
+                    events,
+                    next_key,
+                    _,
+                ) = await self.store.paginate_room_events_by_topological_ordering(
+                    room_id=room_id,
+                    from_key=from_token.room_key,
+                    to_key=to_room_key,
+                    direction=pagin_config.direction,
+                    limit=pagin_config.limit,
+                    event_filter=event_filter,
+                )
             else:
                 # Otherwise, we can backfill in the background for eventual
                 # consistency's sake but we don't need to block the client waiting
-- 
cgit 1.5.1