Fix deadlock

roomserver/storage/shared/storage.go

@@ -468,27 +468,38 @@ func (d *Database) StoreEvent(
 		if !isRejected { // ignore rejected redaction events
 			redactionEvent, redactedEventID, err = d.handleRedactions(ctx, txn, eventNID, event)
 		}
-
+		return nil
-		var roomInfo *types.RoomInfo
-		var updater *LatestEventsUpdater
-		roomInfo, err = d.RoomInfo(ctx, event.RoomID())
-		if err != nil {
-			return fmt.Errorf("d.RoomInfo: %w", err)
-		}
-		updater, err = d.GetLatestEventsForUpdate(ctx, *roomInfo)
-		if err != nil {
-			return fmt.Errorf("NewLatestEventsUpdater: %w", err)
-		}
-		if err = updater.StorePreviousEvents(eventNID, event.PrevEvents()); err != nil {
-			return fmt.Errorf("updater.StorePreviousEvents: %w", err)
-		}
-		succeeded := false
-		return sqlutil.EndTransaction(updater, &succeeded)
 	})
 	if err != nil {
 		return 0, types.StateAtEvent{}, nil, "", fmt.Errorf("d.Writer.Do: %w", err)
 	}
 
+	// We should attempt to update the previous events table with any
+	// references that this new event makes. We do this using a latest
+	// events updater because it somewhat works as a mutex, ensuring
+	// that there's a row-level lock on the latest room events (well,
+	// on Postgres at least).
+	var roomInfo *types.RoomInfo
+	var updater *LatestEventsUpdater
+	if prevEvents := event.PrevEvents(); len(prevEvents) > 0 {
+		roomInfo, err = d.RoomInfo(ctx, event.RoomID())
+		if err != nil {
+			return 0, types.StateAtEvent{}, nil, "", fmt.Errorf("d.RoomInfo: %w", err)
+		}
+		if roomInfo == nil && len(prevEvents) > 0 {
+			return 0, types.StateAtEvent{}, nil, "", fmt.Errorf("expected room %q to exist", event.RoomID())
+		}
+		updater, err = d.GetLatestEventsForUpdate(ctx, *roomInfo)
+		if err != nil {
+			return 0, types.StateAtEvent{}, nil, "", fmt.Errorf("NewLatestEventsUpdater: %w", err)
+		}
+		if err = updater.StorePreviousEvents(eventNID, prevEvents); err != nil {
+			return 0, types.StateAtEvent{}, nil, "", fmt.Errorf("updater.StorePreviousEvents: %w", err)
+		}
+		succeeded := false
+		err = sqlutil.EndTransaction(updater, &succeeded)
+	}
+
 	return roomNID, types.StateAtEvent{
 		BeforeStateSnapshotNID: stateNID,
 		StateEntry: types.StateEntry{
@@ -498,7 +509,7 @@ func (d *Database) StoreEvent(
 			},
 			EventNID: eventNID,
 		},
-	}, redactionEvent, redactedEventID, nil
+	}, redactionEvent, redactedEventID, err
 }
 
 func (d *Database) PublishRoom(ctx context.Context, roomID string, publish bool) error {