Fix Current state appears in timeline in private history with many messages after (#2830)

The problem was that we weren't getting enough recent events, as most of
them were removed by the history visibility filter. Now we're getting
all events between the given input range and re-slice the returned
values after applying history visibility.

syncapi/streams/stream_pdu.go

@@ -227,14 +227,10 @@ func (p *PDUStreamProvider) addRoomDeltaToResponse(
 	stateFilter *gomatrixserverlib.StateFilter,
 	req *types.SyncRequest,
 ) (types.StreamPosition, error) {
-	if delta.MembershipPos > 0 && delta.Membership == gomatrixserverlib.Leave {
+
-		// make sure we don't leak recent events after the leave event.
+	originalLimit := eventFilter.Limit
-		// TODO: History visibility makes this somewhat complex to handle correctly. For example:
+	if r.Backwards {
-		// TODO: This doesn't work for join -> leave in a single /sync request (see events prior to join).
+		eventFilter.Limit = int(r.From - r.To)
-		// TODO: This will fail on join -> leave -> sensitive msg -> join -> leave
-		//       in a single /sync request
-		// This is all "okay" assuming history_visibility == "shared" which it is by default.
-		r.To = delta.MembershipPos
 	}
 	recentStreamEvents, limited, err := snapshot.RecentEvents(
 		ctx, delta.RoomID, r,
@@ -303,6 +299,12 @@ func (p *PDUStreamProvider) addRoomDeltaToResponse(
 		logrus.WithError(err).Error("unable to apply history visibility filter")
 	}
 
+	if r.Backwards && len(events) > originalLimit {
+		// We're going backwards and the events are ordered chronologically, so take the last `limit` events
+		events = events[len(events)-originalLimit:]
+		limited = true
+	}
+
 	if len(delta.StateEvents) > 0 {
 		updateLatestPosition(delta.StateEvents[len(delta.StateEvents)-1].EventID())
 	}

sytest-whitelist

@@ -757,3 +757,4 @@ Can get rooms/{roomId}/messages for a departed room (SPEC-216)
 Local device key changes appear in /keys/changes
 Can get rooms/{roomId}/members at a given point
 Can filter rooms/{roomId}/members
+Current state appears in timeline in private history with many messages after