fix: prevent race condition on order subtotal calculation

internal/processor/order_processor.go

@@ -338,7 +338,7 @@ func (p *OrderProcessorImpl) AddToOrder(ctx context.Context, orderID uuid.UUID,
 			ProductID:        itemReq.ProductID,
 			ProductVariantID: itemReq.ProductVariantID,
 			Quantity:         itemReq.Quantity,
-			UnitPrice:        unitPrice, // Use price from database
+			UnitPrice:        unitPrice,
 			TotalPrice:       itemTotalPrice,
 			UnitCost:         unitCost,
 			TotalCost:        itemTotalCost,
@@ -594,6 +594,10 @@ func (p *OrderProcessorImpl) VoidOrder(ctx context.Context, req *models.VoidOrde
 				return fmt.Errorf("order item does not belong to this order")
 			}
 
+			if orderItem.Status == entities.OrderItemStatusCancelled {
+				return fmt.Errorf("order item %s is already cancelled", orderItemID)
+			}
+
 			if itemVoid.Quantity > orderItem.Quantity {
 				return fmt.Errorf("void quantity cannot exceed original quantity for item %d", itemVoid.OrderItemID)
 			}
@@ -614,9 +618,15 @@ func (p *OrderProcessorImpl) VoidOrder(ctx context.Context, req *models.VoidOrde
 			return fmt.Errorf("outlet not found: %w", err)
 		}
 
+		// Reload order to get latest state
+		order, err = p.orderRepo.GetByID(ctx, req.OrderID)
+		if err != nil {
+			return fmt.Errorf("failed to reload order: %w", err)
+		}
+
 		order.Subtotal -= totalVoidedAmount
 		order.TotalCost -= totalVoidedCost
-		order.TaxAmount = order.Subtotal * outlet.TaxRate // Recalculate tax using outlet's tax rate
+		order.TaxAmount = order.Subtotal * outlet.TaxRate
 		order.TotalAmount = order.Subtotal + order.TaxAmount - order.DiscountAmount
 
 		if err := p.orderRepo.Update(ctx, order); err != nil {

internal/repository/tx_manager.go

@@ -2,6 +2,7 @@ package repository
 
 import (
 	"context"
+	"database/sql"
 
 	"gorm.io/gorm"
 )
@@ -37,3 +38,15 @@ func (m *TxManager) WithTransaction(ctx context.Context, fn func(ctx context.Con
 		return fn(ctxTx)
 	})
 }
+
+// WithTransactionOptions runs fn inside a DB transaction with custom TxOptions (e.g. isolation level).
+func (m *TxManager) WithTransactionOptions(ctx context.Context, opts *sql.TxOptions, fn func(ctx context.Context) error) error {
+	if m == nil || m.db == nil {
+		return fn(ctx)
+	}
+
+	return m.db.WithContext(ctx).Transaction(func(tx *gorm.DB) error {
+		ctxTx := context.WithValue(ctx, txKey, tx)
+		return fn(ctxTx)
+	}, opts)
+}

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)
@@ -305,9 +308,17 @@ 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 {
+	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 {
 		fmt.Printf("Warning: failed to handle table release for voided order %s: %v\n", req.OrderID, err)
@@ -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")
 		}