Speed up SS room sorting (#17468)

We do this by bulk fetching the latest stream ordering.

---------

Co-authored-by: Andrew Morgan <1342360+anoadragon453@users.noreply.github.com>

1 files changed, 21 insertions, 22 deletions

diff --git a/synapse/handlers/sliding_sync.py b/synapse/handlers/sliding_sync.py
index 886d7c7159..554ab59bf3 100644
--- a/synapse/handlers/sliding_sync.py
+++ b/synapse/handlers/sliding_sync.py
@@ -1230,34 +1230,33 @@ class SlidingSyncHandler:
         # Assemble a map of room ID to the `stream_ordering` of the last activity that the
         # user should see in the room (<= `to_token`)
         last_activity_in_room_map: Dict[str, int] = {}
-        for room_id, room_for_user in sync_room_map.items():
-            # If they are fully-joined to the room, let's find the latest activity
-            # at/before the `to_token`.
-            if room_for_user.membership == Membership.JOIN:
-                last_event_result = (
-                    await self.store.get_last_event_pos_in_room_before_stream_ordering(
-                        room_id, to_token.room_key
-                    )
-                )
-
-                # If the room has no events at/before the `to_token`, this is probably a
-                # mistake in the code that generates the `sync_room_map` since that should
-                # only give us rooms that the user had membership in during the token range.
-                assert last_event_result is not None
 
-                _, event_pos = last_event_result
-
-                last_activity_in_room_map[room_id] = event_pos.stream
-            else:
-                # Otherwise, if the user has left/been invited/knocked/been banned from
-                # a room, they shouldn't see anything past that point.
+        for room_id, room_for_user in sync_room_map.items():
+            if room_for_user.membership != Membership.JOIN:
+                # If the user has left/been invited/knocked/been banned from a
+                # room, they shouldn't see anything past that point.
                 #
-                # FIXME: It's possible that people should see beyond this point in
-                # invited/knocked cases if for example the room has
+                # FIXME: It's possible that people should see beyond this point
+                # in invited/knocked cases if for example the room has
                 # `invite`/`world_readable` history visibility, see
                 # https://github.com/matrix-org/matrix-spec-proposals/pull/3575#discussion_r1653045932
                 last_activity_in_room_map[room_id] = room_for_user.event_pos.stream
 
+        # For fully-joined rooms, we find the latest activity at/before the
+        # `to_token`.
+        joined_room_positions = (
+            await self.store.bulk_get_last_event_pos_in_room_before_stream_ordering(
+                [
+                    room_id
+                    for room_id, room_for_user in sync_room_map.items()
+                    if room_for_user.membership == Membership.JOIN
+                ],
+                to_token.room_key,
+            )
+        )
+
+        last_activity_in_room_map.update(joined_room_positions)
+
         return sorted(
             sync_room_map.values(),
             # Sort by the last activity (stream_ordering) in the room