Limit presence in /sync responses (#2394)

* Use filter and limit presence count

* More limiting

* More limiting

* Fix unit test

* Also limit presence by last_active_ts

* Update query, use "from" as the initial lastPos

* Get 1000 presence events, they are filtered later

Co-authored-by: Neil Alexander <neilalexander@users.noreply.github.com>

syncapi/streams/stream_presence.go

@@ -53,7 +53,8 @@ func (p *PresenceStreamProvider) IncrementalSync(
 	req *types.SyncRequest,
 	from, to types.StreamPosition,
 ) types.StreamPosition {
-	presences, err := p.DB.PresenceAfter(ctx, from)
+	// We pull out a larger number than the filter asks for, since we're filtering out events later
+	presences, err := p.DB.PresenceAfter(ctx, from, gomatrixserverlib.EventFilter{Limit: 1000})
 	if err != nil {
 		req.Log.WithError(err).Error("p.DB.PresenceAfter failed")
 		return from
@@ -72,6 +73,7 @@ func (p *PresenceStreamProvider) IncrementalSync(
 			req.Log.WithError(err).Error("unable to refresh notifier lists")
 			return from
 		}
+	NewlyJoinedLoop:
 		for _, roomID := range newlyJoined {
 			roomUsers := p.notifier.JoinedUsers(roomID)
 			for i := range roomUsers {
@@ -86,11 +88,14 @@ func (p *PresenceStreamProvider) IncrementalSync(
 					req.Log.WithError(err).Error("unable to query presence for user")
 					return from
 				}
+				if len(presences) > req.Filter.Presence.Limit {
+					break NewlyJoinedLoop
+				}
 			}
 		}
 	}
 
-	lastPos := to
+	lastPos := from
 	for _, presence := range presences {
 		if presence == nil {
 			continue
@@ -135,6 +140,9 @@ func (p *PresenceStreamProvider) IncrementalSync(
 		if presence.StreamPos > lastPos {
 			lastPos = presence.StreamPos
 		}
+		if len(req.Response.Presence.Events) == req.Filter.Presence.Limit {
+			break
+		}
 		p.cache.Store(cacheKey, presence)
 	}