Don't do unnecessary work for rooms that don't exist

roomserver/internal/input/input_events.go

@@ -128,9 +128,17 @@ func (r *Inputer) processRoomEvent(
 		}
 	}
 
+	// Don't waste time processing the event if the room doesn't exist.
+	// A room entry locally will only be created in response to a create
+	// event.
+	isCreateEvent := event.Type() == gomatrixserverlib.MRoomCreate && event.StateKeyEquals("")
+	if !updater.RoomExists() && !isCreateEvent {
+		return rollbackTransaction, fmt.Errorf("room does not exist")
+	}
+
 	var missingAuth, missingPrev bool
 	serverRes := &fedapi.QueryJoinedHostServerNamesInRoomResponse{}
-	if event.Type() != gomatrixserverlib.MRoomCreate || !event.StateKeyEquals("") {
+	if !isCreateEvent {
 		missingAuthIDs, missingPrevIDs, err := updater.MissingAuthPrevEvents(ctx, event)
 		if err != nil {
 			return rollbackTransaction, fmt.Errorf("updater.MissingAuthPrevEvents: %w", err)

roomserver/storage/shared/room_updater.go

@@ -16,6 +16,7 @@ type RoomUpdater struct {
 	latestEvents            []types.StateAtEventAndReference
 	lastEventIDSent         string
 	currentStateSnapshotNID types.StateSnapshotNID
+	roomExists              bool
 }
 
 func rollback(txn *sql.Tx) {
@@ -33,7 +34,7 @@ func NewRoomUpdater(ctx context.Context, d *Database, txn *sql.Tx, roomInfo *typ
 	// succeed, processing a create event which creates the room, or it won't.
 	if roomInfo == nil {
 		return &RoomUpdater{
-			transaction{ctx, txn}, d, nil, nil, "", 0,
+			transaction{ctx, txn}, d, nil, nil, "", 0, false,
 		}, nil
 	}
 
@@ -57,10 +58,15 @@ func NewRoomUpdater(ctx context.Context, d *Database, txn *sql.Tx, roomInfo *typ
 		}
 	}
 	return &RoomUpdater{
-		transaction{ctx, txn}, d, roomInfo, stateAndRefs, lastEventIDSent, currentStateSnapshotNID,
+		transaction{ctx, txn}, d, roomInfo, stateAndRefs, lastEventIDSent, currentStateSnapshotNID, true,
 	}, nil
 }
 
+// RoomExists returns true if the room exists and false otherwise.
+func (u *RoomUpdater) RoomExists() bool {
+	return u.roomExists
+}
+
 // Implements sqlutil.Transaction
 func (u *RoomUpdater) Commit() error {
 	if u.txn == nil { // SQLite mode probably