`/sync` performance optimizations (#2927)

Since #2849 there is no limit for the current state we fetch to
calculate history visibility. In large rooms this can cause us to fetch
thousands of membership events we don't really care about.
This now only gets the state event types and senders in our timeline,
which should significantly reduce the amount of events we fetch from the
database.

Also removes `MaxTopologicalPosition`, as it is an unnecessary DB call,
given we use the result in `topological_position < $1` calls.

1 files changed, 24 insertions, 11 deletions

diff --git a/syncapi/streams/stream_pdu.go b/syncapi/streams/stream_pdu.go
index 4664276c..44013e37 100644
--- a/syncapi/streams/stream_pdu.go
+++ b/syncapi/streams/stream_pdu.go
@@ -384,19 +384,32 @@ func applyHistoryVisibilityFilter(
 	roomID, userID string,
 	recentEvents []*gomatrixserverlib.HeaderedEvent,
 ) ([]*gomatrixserverlib.HeaderedEvent, error) {
-	// We need to make sure we always include the latest states events, if they are in the timeline.
-	// We grep at least limit * 2 events, to ensure we really get the needed events.
-	filter := gomatrixserverlib.DefaultStateFilter()
-	stateEvents, err := snapshot.CurrentState(ctx, roomID, &filter, nil)
-	if err != nil {
-		// Not a fatal error, we can continue without the stateEvents,
-		// they are only needed if there are state events in the timeline.
-		logrus.WithError(err).Warnf("Failed to get current room state for history visibility")
+	// We need to make sure we always include the latest state events, if they are in the timeline.
+	alwaysIncludeIDs := make(map[string]struct{})
+	var stateTypes []string
+	var senders []string
+	for _, ev := range recentEvents {
+		if ev.StateKey() != nil {
+			stateTypes = append(stateTypes, ev.Type())
+			senders = append(senders, ev.Sender())
+		}
 	}
-	alwaysIncludeIDs := make(map[string]struct{}, len(stateEvents))
-	for _, ev := range stateEvents {
-		alwaysIncludeIDs[ev.EventID()] = struct{}{}
+
+	// Only get the state again if there are state events in the timeline
+	if len(stateTypes) > 0 {
+		filter := gomatrixserverlib.DefaultStateFilter()
+		filter.Types = &stateTypes
+		filter.Senders = &senders
+		stateEvents, err := snapshot.CurrentState(ctx, roomID, &filter, nil)
+		if err != nil {
+			return nil, fmt.Errorf("failed to get current room state for history visibility calculation: %w", err)
+		}
+
+		for _, ev := range stateEvents {
+			alwaysIncludeIDs[ev.EventID()] = struct{}{}
+		}
 	}
+
 	startTime := time.Now()
 	events, err := internal.ApplyHistoryVisibilityFilter(ctx, snapshot, rsAPI, recentEvents, alwaysIncludeIDs, userID, "sync")
 	if err != nil {