session/player.go: Ensure the player is not currently changing slots …

…to prevent desync.

server/session/handler_inventory_transaction.go

@@ -29,16 +29,22 @@ func (h *InventoryTransactionHandler) Handle(p packet.Packet, s *Session) error
 		h.resendInventories(s)
 		return nil
 	case *protocol.UseItemOnEntityTransactionData:
+		s.changingSlot.Store(true)
+		defer s.changingSlot.Store(false)
 		if err := s.c.SetHeldSlot(int(data.HotBarSlot)); err != nil {
 			return err
 		}
 		return h.handleUseItemOnEntityTransaction(data, s)
 	case *protocol.UseItemTransactionData:
+		s.changingSlot.Store(true)
+		defer s.changingSlot.Store(false)
 		if err := s.c.SetHeldSlot(int(data.HotBarSlot)); err != nil {
 			return err
 		}
 		return h.handleUseItemTransaction(data, s)
 	case *protocol.ReleaseItemTransactionData:
+		s.changingSlot.Store(true)
+		defer s.changingSlot.Store(false)
 		if err := s.c.SetHeldSlot(int(data.HotBarSlot)); err != nil {
 			return err
 		}

server/session/handler_mob_equipment.go

@@ -29,6 +29,8 @@ func (*MobEquipmentHandler) Handle(p packet.Packet, s *Session) error {
 			// out of sync.
 			s.log.Debugf("failed processing packet from %v (%v): *packet.MobEquipment: client-side item must be identical to server-side item, but got differences: client: %v vs server: %v", s.conn.RemoteAddr(), s.c.Name(), clientSide, actual)
 		}
+		s.changingSlot.Store(true)
+		defer s.changingSlot.Store(false)
 		return s.c.SetHeldSlot(int(pk.InventorySlot))
 	default:
 		return fmt.Errorf("only main inventory should be involved in slot chnage, got window ID %v", pk.WindowID)

server/session/player.go

@@ -569,6 +569,9 @@ func (s *Session) SetHeldSlot(slot int) error {
 	if slot < 0 || slot > 8 {
 		return fmt.Errorf("slot exceeds hotbar range 0-8: slot is %v", slot)
 	}
+	if s.changingSlot.Load() {
+		return nil
+	}
 
 	s.heldSlot.Store(uint32(slot))
 

server/session/session.go

@@ -71,6 +71,7 @@ type Session struct {
 	openedWindow                   atomic.Value[*inventory.Inventory]
 	openedPos                      atomic.Value[cube.Pos]
 	swingingArm                    atomic.Bool
+	changingSlot                   atomic.Bool
 
 	recipes map[uint32]recipe.Recipe