Federation consumer `adds_state_event_ids` tweak (#2441)

* Don't ask roomserver for events we already have in federation API

* Check number of events returned is as expected

* Preallocate array

* Improve shape a bit

1 files changed, 11 insertions, 14 deletions

diff --git a/federationapi/consumers/roomserver.go b/federationapi/consumers/roomserver.go
index ff2c8e5d..80317ee6 100644
--- a/federationapi/consumers/roomserver.go
+++ b/federationapi/consumers/roomserver.go
@@ -146,28 +146,25 @@ func (s *OutputRoomEventConsumer) processInboundPeek(orp api.OutputNewInboundPee
 // processMessage updates the list of currently joined hosts in the room
 // and then sends the event to the hosts that were joined before the event.
 func (s *OutputRoomEventConsumer) processMessage(ore api.OutputNewRoomEvent) error {
-	eventsRes := &api.QueryEventsByIDResponse{}
-	if len(ore.AddsStateEventIDs) > 0 {
+	addsStateEvents, missingEventIDs := ore.NeededStateEventIDs()
+
+	// Ask the roomserver and add in the rest of the results into the set.
+	// Finally, work out if there are any more events missing.
+	if len(missingEventIDs) > 0 {
 		eventsReq := &api.QueryEventsByIDRequest{
-			EventIDs: ore.AddsStateEventIDs,
+			EventIDs: missingEventIDs,
 		}
+		eventsRes := &api.QueryEventsByIDResponse{}
 		if err := s.rsAPI.QueryEventsByID(s.ctx, eventsReq, eventsRes); err != nil {
 			return fmt.Errorf("s.rsAPI.QueryEventsByID: %w", err)
 		}
-
-		found := false
-		for _, event := range eventsRes.Events {
-			if event.EventID() == ore.Event.EventID() {
-				found = true
-				break
-			}
-		}
-		if !found {
-			eventsRes.Events = append(eventsRes.Events, ore.Event)
+		if len(eventsRes.Events) != len(missingEventIDs) {
+			return fmt.Errorf("missing state events")
 		}
+		addsStateEvents = append(addsStateEvents, eventsRes.Events...)
 	}
 
-	addsJoinedHosts, err := joinedHostsFromEvents(gomatrixserverlib.UnwrapEventHeaders(eventsRes.Events))
+	addsJoinedHosts, err := joinedHostsFromEvents(gomatrixserverlib.UnwrapEventHeaders(addsStateEvents))
 	if err != nil {
 		return err
 	}