shopper/framework: Race condition on Discount.usage_limit allows silent over-redemption

Impact

CreateOrderFromCartAction::execute previously created the Order row before checking and incrementing the discount's total_use counter. Under concurrent checkout pressure (Black Friday, flash sale, viral coupon), the global usage_limit was silently exceeded: orders were committed with the discount fully applied to price_amount while the counter blocked at usage_limit. The merchant had no signal that an over-redemption had occurred.

A second related bug: usage_limit_per_user was effectively a no-op because the counter it relied on (DiscountDetail.total_use) was never incremented anywhere in the codebase. The per-user check therefore always saw 0 uses and validation passed regardless of how many times the same customer had previously redeemed the coupon. For eligibility = Everyone the per-user limit could not fire at all because the underlying DiscountDetail row only exists for eligibility = Customers.

Direct financial loss: each over-redemption is a discount the merchant did not intend to grant.

Patches

Fixed in v2.8.0. CreateOrderFromCartAction now:

• Reserves the discount slot atomically before the order row is created, inside the same DB::transaction with lockForUpdate and a compare-and-swap on total_use.
• Throws DiscountLimitReachedException::global and rolls back the transaction when the global limit was exhausted between cart validation and commit. No order is committed.
• Throws DiscountLimitReachedException::perUser and rolls back when the discount is restricted to one use per customer and the customer has already redeemed it.
• Snapshots discount_id, discount_code, discount_type, discount_value_at_apply and discount_currency_code onto the orders table for resilience against later discount edits or deletions.

DiscountValidator was updated to perform the same Order-based per-user check at cart-apply time so the rejection is surfaced before checkout.