fix: make coupon consumption atomic during submit

pos_next/api/invoices.py

@@ -1302,14 +1302,13 @@ def submit_invoice(invoice=None, data=None):
         if coupon_code:
             # Increment usage counter for POS Coupon
             if frappe.db.table_exists("POS Coupon"):
-                try:
-                    from pos_next.pos_next.doctype.pos_coupon.pos_coupon import increment_coupon_usage
-                    increment_coupon_usage(coupon_code)
-                except Exception as e:
-                    frappe.log_error(
-                        title="Failed to increment coupon usage",
-                        message=f"Coupon: {coupon_code}, Error: {str(e)}"
-                    )
+                from pos_next.pos_next.doctype.pos_coupon.pos_coupon import consume_coupon_usage
+
+                consume_coupon_usage(
+                    coupon_code,
+                    customer=invoice_doc.customer,
+                    company=invoice_doc.company,
+                )
 
         # Auto-set batch numbers for returns
         _auto_set_return_batches(invoice_doc)

pos_next/pos_next/doctype/pos_coupon/pos_coupon.py

@@ -176,18 +176,44 @@ def apply_coupon_discount(coupon, cart_total, net_total=None):
     }
 
 
-def increment_coupon_usage(coupon_code):
-    """Increment the usage counter for a coupon"""
+def consume_coupon_usage(coupon_code, customer=None, company=None):
+    """Atomically validate and increment coupon usage within the current transaction."""
+    coupon_code = coupon_code.upper()
+    locked_coupon = frappe.db.sql(
+        """
+        SELECT name
+        FROM `tabPOS Coupon`
+        WHERE coupon_code = %s
+        FOR UPDATE
+        """,
+        (coupon_code,),
+        as_dict=True,
+    )
+
+    if not locked_coupon:
+        frappe.throw(_("Sorry, this coupon code does not exist"))
+
+    coupon_result = check_coupon_code(coupon_code, customer=customer, company=company)
+    if not coupon_result or not coupon_result.get("valid"):
+        frappe.throw(_(coupon_result.get("msg", "Invalid coupon code")))
+
+    coupon = coupon_result["coupon"]
+    updated_used = (coupon.used or 0) + 1
+    frappe.db.set_value("POS Coupon", coupon.name, "used", updated_used, update_modified=False)
+    coupon.used = updated_used
+    return coupon
+
+
+def increment_coupon_usage(coupon_code, customer=None, company=None):
+    """Backward-compatible wrapper around atomic coupon consumption."""
     try:
-        coupon = frappe.get_doc("POS Coupon", {"coupon_code": coupon_code.upper()})
-        coupon.used = (coupon.used or 0) + 1
-        coupon.db_set('used', coupon.used)
-        frappe.db.commit()
+        return consume_coupon_usage(coupon_code, customer=customer, company=company)
     except Exception as e:
         frappe.log_error(
             title="Coupon Usage Increment Failed",
             message=f"Failed to increment usage for coupon {coupon_code}: {str(e)}"
         )
+        raise
 
 
 def decrement_coupon_usage(coupon_code):

pos_next/pos_next/doctype/pos_coupon/test_pos_coupon.py

@@ -6,6 +6,7 @@
 
 from pos_next.pos_next.doctype.pos_coupon.pos_coupon import (
     _get_customer_coupon_usage_count,
+    consume_coupon_usage,
 )
 
 
@@ -53,3 +54,30 @@ def test_one_use_coupon_skips_doctypes_without_coupon_field(self, mock_db, mock_
             "Sales Invoice",
             filters={"customer": "Customer A", "coupon_code": "SAVE10", "docstatus": 1},
         )
+
+    @patch("pos_next.pos_next.doctype.pos_coupon.pos_coupon.check_coupon_code")
+    @patch("pos_next.pos_next.doctype.pos_coupon.pos_coupon.frappe.db")
+    def test_consume_coupon_usage_locks_and_updates_usage(self, mock_db, mock_check_coupon_code):
+        coupon = Mock()
+        coupon.name = "PROMO-0001"
+        coupon.used = 2
+
+        mock_db.sql.return_value = [{"name": coupon.name}]
+        mock_check_coupon_code.return_value = {"valid": True, "coupon": coupon}
+
+        result = consume_coupon_usage("promo10", customer="Customer A", company="Test Co")
+
+        self.assertEqual(result.used, 3)
+        mock_db.sql.assert_called_once()
+        mock_check_coupon_code.assert_called_once_with(
+            "PROMO10",
+            customer="Customer A",
+            company="Test Co",
+        )
+        mock_db.set_value.assert_called_once_with(
+            "POS Coupon",
+            "PROMO-0001",
+            "used",
+            3,
+            update_modified=False,
+        )