Speed up sorting of sliding sync rooms in initial request (#17734)

We do this by using the event stream cache.

---------

Co-authored-by: Devon Hudson <devon.dmytro@gmail.com>

changelog.d/17734.misc

@@ -0,0 +1 @@
+Minor speed up of initial sliding sync requests.

synapse/handlers/sliding_sync/room_lists.py

@@ -420,6 +420,10 @@ class SlidingSyncRoomLists:
                                     Dict[str, RoomsForUserType], filtered_sync_room_map
                                 ),
                                 to_token,
+                                # We only need to sort the rooms up to the end
+                                # of the largest range. Both sides of range are
+                                # inclusive so we `+ 1`.
+                                limit=max(range[1] + 1 for range in list_config.ranges),
                             )
 
                         for range in list_config.ranges:
@@ -462,7 +466,7 @@ class SlidingSyncRoomLists:
                             )
 
                     lists[list_key] = SlidingSyncResult.SlidingWindowList(
-                        count=len(sorted_room_info),
+                        count=len(filtered_sync_room_map),
                         ops=ops,
                     )
 
@@ -1980,15 +1984,21 @@ class SlidingSyncRoomLists:
         self,
         sync_room_map: Dict[str, RoomsForUserType],
         to_token: StreamToken,
+        limit: Optional[int] = None,
     ) -> List[RoomsForUserType]:
         """
         Sort by `stream_ordering` of the last event that the user should see in the
         room. `stream_ordering` is unique so we get a stable sort.
 
+        If `limit` is specified then sort may return fewer entries, but will
+        always return at least the top N rooms. This is useful as we don't always
+        need to sort the full list, but are just interested in the top N.
+
         Args:
             sync_room_map: Dictionary of room IDs to sort along with membership
                 information in the room at the time of `to_token`.
             to_token: We sort based on the events in the room at this token (<= `to_token`)
+            limit: The number of rooms that we need to return from the top of the list.
 
         Returns:
             A sorted list of room IDs by `stream_ordering` along with membership information.
@@ -1998,8 +2008,23 @@ class SlidingSyncRoomLists:
         # user should see in the room (<= `to_token`)
         last_activity_in_room_map: Dict[str, int] = {}
 
+        # Same as above, except for positions that we know are in the event
+        # stream cache.
+        cached_positions: Dict[str, int] = {}
+
+        earliest_cache_position = (
+            self.store._events_stream_cache.get_earliest_known_position()
+        )
+
         for room_id, room_for_user in sync_room_map.items():
-            if room_for_user.membership != Membership.JOIN:
+            if room_for_user.membership == Membership.JOIN:
+                # For joined rooms check the stream change cache.
+                cached_position = (
+                    self.store._events_stream_cache.get_max_pos_of_last_change(room_id)
+                )
+                if cached_position is not None:
+                    cached_positions[room_id] = cached_position
+            else:
                 # If the user has left/been invited/knocked/been banned from a
                 # room, they shouldn't see anything past that point.
                 #
@@ -2009,6 +2034,48 @@ class SlidingSyncRoomLists:
                 # 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
 
+                # If the stream position is in range of the stream change cache
+                # we can include it.
+                if room_for_user.event_pos.stream > earliest_cache_position:
+                    cached_positions[room_id] = room_for_user.event_pos.stream
+
+        # If we are only asked for the top N rooms, and we have enough from
+        # looking in the stream change cache, then we can return early. This
+        # is because the cache must include all entries above
+        # `.get_earliest_known_position()`.
+        if limit is not None and len(cached_positions) >= limit:
+            # ... but first we need to handle the case where the cached max
+            # position is greater than the to_token, in which case we do
+            # actually query the DB. This should happen rarely, so can do it in
+            # a loop.
+            for room_id, position in list(cached_positions.items()):
+                if position > to_token.room_key.stream:
+                    result = await self.store.get_last_event_pos_in_room_before_stream_ordering(
+                        room_id, to_token.room_key
+                    )
+                    if (
+                        result is not None
+                        and result[1].stream > earliest_cache_position
+                    ):
+                        # We have a stream position in the cached range.
+                        cached_positions[room_id] = result[1].stream
+                    else:
+                        # No position in the range, so we remove the entry.
+                        cached_positions.pop(room_id)
+
+        if limit is not None and len(cached_positions) >= limit:
+            return sorted(
+                (
+                    room
+                    for room in sync_room_map.values()
+                    if room.room_id in cached_positions
+                ),
+                # Sort by the last activity (stream_ordering) in the room
+                key=lambda room_info: cached_positions[room_info.room_id],
+                # We want descending order
+                reverse=True,
+            )
+
         # For fully-joined rooms, we find the latest activity at/before the
         # `to_token`.
         joined_room_positions = (