Only send catch up events if they're the latest in the room

changelog.d/9621.misc

@@ -0,0 +1 @@
+Only send catch up events if they're the latest in the room, reducing the number of events a server receives after recovering from downtime.

synapse/storage/databases/main/transactions.py

@@ -428,7 +428,8 @@ def _get_catch_up_room_event_ids_txn(
     ) -> List[str]:
         q = """
                 SELECT event_id FROM destination_rooms
-                 JOIN events USING (stream_ordering)
+                 INNER JOIN events USING (stream_ordering)
+                 INNER JOIN event_forward_extremities USING (event_id)
                 WHERE destination = ?
                   AND stream_ordering > ?
                 ORDER BY stream_ordering