Only send rooms with updates down sliding sync (#17479)

Rather than always including all rooms in range.

Also adds a pre-filter to rooms that checks the stream change cache to
see if anything might have happened.

Based on #17447

---------

Co-authored-by: Eric Eastwood <eric.eastwood@beta.gouv.fr>

1 files changed, 10 insertions, 0 deletions

diff --git a/synapse/storage/databases/main/stream.py b/synapse/storage/databases/main/stream.py

index b034361aec..4207e73c7f 100644
--- a/synapse/storage/databases/main/stream.py
+++ b/synapse/storage/databases/main/stream.py
@@ -2104,3 +2104,13 @@ class StreamWorkerStore(EventsWorkerStore, SQLBaseStore):
             return RoomStreamToken(stream=last_position.stream - 1)
 
         return None
+
+    def get_rooms_that_might_have_updates(
+        self, room_ids: StrCollection, from_token: RoomStreamToken
+    ) -> StrCollection:
+        """Filters given room IDs down to those that might have updates, i.e.
+        removes rooms that definitely do not have updates.
+        """
+        return self._events_stream_cache.get_entities_changed(
+            room_ids, from_token.stream
+        )