fix: prevent race condition on order subtotal calculation

internal/service/order_service.go

@@ -3,6 +3,7 @@ package service
 import (
 	"apskel-pos-be/internal/appcontext"
 	"context"
+	"database/sql"
 	"fmt"
 	"time"
 
@@ -228,7 +229,9 @@ func (s *OrderServiceImpl) AddToOrder(ctx context.Context, orderID uuid.UUID, re
 	var response *models.AddToOrderResponse
 	var ingredientTransactions []*contract.CreateOrderIngredientTransactionRequest
 
-	err := s.txManager.WithTransaction(ctx, func(txCtx context.Context) error {
+	err := s.txManager.WithTransactionOptions(ctx, &sql.TxOptions{
+		Isolation: sql.LevelSerializable,
+	}, func(txCtx context.Context) error {
 		addResp, err := s.orderProcessor.AddToOrder(txCtx, orderID, req)
 		if err != nil {
 			return fmt.Errorf("failed to add items to order: %w", err)
@@ -305,8 +308,16 @@ func (s *OrderServiceImpl) VoidOrder(ctx context.Context, req *models.VoidOrderR
 		return fmt.Errorf("invalid user ID")
 	}
 
-	if err := s.orderProcessor.VoidOrder(ctx, req, voidedBy); err != nil {
-		return fmt.Errorf("failed to void order: %w", err)
+	err := s.txManager.WithTransactionOptions(ctx, &sql.TxOptions{
+		Isolation: sql.LevelSerializable,
+	}, func(txCtx context.Context) error {
+		if err := s.orderProcessor.VoidOrder(txCtx, req, voidedBy); err != nil {
+			return fmt.Errorf("failed to void order: %w", err)
+		}
+		return nil
+	})
+	if err != nil {
+		return err
 	}
 
 	if err := s.handleTableReleaseOnVoid(ctx, req.OrderID); err != nil {
@@ -561,9 +572,14 @@ func (s *OrderServiceImpl) validateCreatePaymentRequest(req *models.CreatePaymen
 				return fmt.Errorf("payment item amount must be greater than zero for item %d", i+1)
 			}
 
+			fmt.Printf("[DEBUG] CreatePayment order_id=%s item[%d] order_item_id=%s amount=%.10f\n",
+				req.OrderID, i, item.OrderItemID, item.Amount)
 			totalItemAmount += item.Amount
 		}
 
+		fmt.Printf("[DEBUG] CreatePayment order_id=%s total_amount=%.10f sum_items=%.10f diff=%.10f\n",
+			req.OrderID, req.Amount, totalItemAmount, req.Amount-totalItemAmount)
+
 		if totalItemAmount != req.Amount {
 			return fmt.Errorf("sum of payment item amounts must equal total payment amount")
 		}