lock refunds against duplicates

backend/src/main/java/com/petshop/backend/repository/RefundRepository.java

@@ -1,14 +1,23 @@
 package com.petshop.backend.repository;
 
 import com.petshop.backend.entity.Refund;
+import jakarta.persistence.LockModeType;
 import org.springframework.data.jpa.repository.JpaRepository;
+import org.springframework.data.jpa.repository.Lock;
+import org.springframework.data.jpa.repository.Query;
+import org.springframework.data.repository.query.Param;
 import org.springframework.stereotype.Repository;
 
 import java.util.List;
+import java.util.Optional;
 
 @Repository
 public interface RefundRepository extends JpaRepository<Refund, Long> {
     List<Refund> findByCustomerIdOrderByCreatedAtDesc(Long customerId);
     List<Refund> findAllByOrderByCreatedAtDesc();
     List<Refund> findBySaleId(Long saleId);
+
+    @Lock(LockModeType.PESSIMISTIC_WRITE)
+    @Query("SELECT r FROM Refund r WHERE r.id = :id")
+    Optional<Refund> findByIdForUpdate(@Param("id") Long id);
 }

backend/src/main/java/com/petshop/backend/repository/SaleRepository.java

@@ -1,10 +1,12 @@
 package com.petshop.backend.repository;
 
 import com.petshop.backend.entity.Sale;
+import jakarta.persistence.LockModeType;
 import org.springframework.data.domain.Page;
 import org.springframework.data.domain.Pageable;
 import java.time.LocalDateTime;
 import org.springframework.data.jpa.repository.JpaRepository;
+import org.springframework.data.jpa.repository.Lock;
 import org.springframework.data.jpa.repository.Query;
 import org.springframework.data.repository.query.Param;
 import org.springframework.stereotype.Repository;
@@ -42,5 +44,9 @@ public interface SaleRepository extends JpaRepository<Sale, Long> {
 
     List<Sale> findByOriginalSaleSaleId(Long originalSaleId);
 
+    @Lock(LockModeType.PESSIMISTIC_WRITE)
+    @Query("SELECT s FROM Sale s WHERE s.saleId = :id")
+    Optional<Sale> findByIdForUpdate(@Param("id") Long id);
+
     Optional<Sale> findByCartCartId(Long cartId);
 }

backend/src/main/java/com/petshop/backend/service/RefundService.java

@@ -112,7 +112,7 @@ public class RefundService {
 
     @Transactional
     public RefundResponse updateRefundStatus(Long id, String status) {
-        Refund refund = refundRepository.findById(id)
+        Refund refund = refundRepository.findByIdForUpdate(id)
                 .orElseThrow(() -> new ResourceNotFoundException("Refund not found"));
 
         Refund.RefundStatus newStatus;

backend/src/main/java/com/petshop/backend/service/SaleService.java

@@ -107,7 +107,7 @@ public class SaleService {
         }
 
         if (sale.getIsRefund() && request.getOriginalSaleId() != null) {
-            Sale originalSale = saleRepository.findById(request.getOriginalSaleId())
+            Sale originalSale = saleRepository.findByIdForUpdate(request.getOriginalSaleId())
                     .orElseThrow(() -> new ResourceNotFoundException("Original sale not found with id: " + request.getOriginalSaleId()));
             sale.setOriginalSale(originalSale);
         }