Observer sales_order_paymentCancel and integrity constraint violation

[luigifab]

In Mage_SalesRule_Model_Observer::sales_order_paymentCancel($observer) we have encoutered some Integrity constraint violation. Perhaps our database is little fucked, I have not investigated more, but I do the following to not raise an error.

diff --git a/web/app/code/core/Mage/SalesRule/Model/Observer.php b/web/app/code/core/Mage/SalesRule/Model/Observer.php
index aac4f9327..ad9bc2744 100644
--- a/web/app/code/core/Mage/SalesRule/Model/Observer.php
+++ b/web/app/code/core/Mage/SalesRule/Model/Observer.php
@@ -158,7 +158,8 @@ class Mage_SalesRule_Model_Observer
                     Mage::getResourceModel('salesrule/coupon_usage')->updateCustomerCouponTimesUsed($customerId, $coupon->getId(), true);
 
                     // Decrement rule times_used
-                    if ($customerCoupon = Mage::getModel('salesrule/rule_customer')->loadByCustomerRule($customerId, $coupon->getRuleId())) {
+                    $customerCoupon = Mage::getModel('salesrule/rule_customer')->loadByCustomerRule($customerId, $coupon->getRuleId());
+                    if (!empty($customerCoupon) && !empty($customerCoupon->getId())) {
                         $customerCoupon->setTimesUsed($customerCoupon->getTimesUsed() - 1);
                         $customerCoupon->save();
                     }

It seems that the customerCoupon was not found.

Preconditions

1. OpenMage 20.0.3 / 20.0.5
2. PHP 7.2.31 / 7.4.6
3. A pending order with a coupon code.

Steps to reproduce

1. Unknown.

Expected result

1. No error.

Actual result

1. The following fatal error:

SQLSTATE[23000]: Integrity constraint violation: 1452 Cannot add or update a child row:
a foreign key constraint fails (`shop`.`salesrule_customer`, CONSTRAINT 
`FK_SALESRULE_CUSTOMER_RULE_ID_SALESRULE_RULE_ID` FOREIGN KEY (`rule_id`) 
REFERENCES `salesrule` (`rule_id`) ON DELETE CASCADE ON UPDATE CASCA),
query was: INSERT INTO `salesrule_customer` (`times_used`) VALUES (?)