Use membership check rather than referring to state directly

roomserver/internal/perform/perform_join.go

@@ -268,21 +268,19 @@ func (r *Joiner) performJoinRoomByID(
 	case nil:
 		// The room join is local. Send the new join event into the
 		// roomserver. First of all check that the user isn't already
-		// a member of the room.
-		alreadyJoined := false
-		for _, se := range buildRes.StateEvents {
-			if !se.StateKeyEquals(userID) {
-				continue
-			}
-			if membership, merr := se.Membership(); merr == nil {
-				alreadyJoined = (membership == gomatrixserverlib.Join)
-				break
-			}
+		// a member of the room. This is best-effort (as in we won't
+		// fail if we can't find the existing membership) because there
+		// is really no harm in just sending another membership event.
+		membershipReq := &api.QueryMembershipForUserRequest{
+			RoomID: req.RoomIDOrAlias,
+			UserID: userID,
 		}
+		membershipRes := &api.QueryMembershipForUserResponse{}
+		_ = r.Queryer.QueryMembershipForUser(ctx, membershipReq, membershipRes)
 
 		// If we haven't already joined the room then send an event
 		// into the room changing our membership status.
-		if !alreadyJoined {
+		if !membershipRes.RoomExists || !membershipRes.IsInRoom {
 			inputReq := rsAPI.InputRoomEventsRequest{
 				InputRoomEvents: []rsAPI.InputRoomEvent{
 					{