Crussell/backend/handlers/payments/giftcard_clawback.go

package payments

import (
	"context"
	"errors"
	"fmt"
	"log"
	"log/slog"

	"crussell/db"

	"github.com/jackc/pgx/v5"
)

// RevertGiftCardFunding undoes the gift-card funding performed earlier in the
// SAME till-sale request after a definitive Square charge rejection, matching
// the gift_card_transactions accounting: a created card is deleted (with its
// purchase transaction) and any immediate redeem-to-account credit reversed; a
// topped-up card has the amount subtracted back out and its top-up transaction
// removed. The clawback is claim-first: it atomically claims the till sale
// with a gating `status='pending'` UPDATE whose row lock serializes against
// the handler's completion UPDATE, then runs the card mutation + failed-mark
// in the same transaction so a late same-key retry cannot re-complete a sale
// whose gift card no longer exists.
//
// This is the SINGLE money-reversal implementation shared by the till handler
// (revertGiftCardFunding), the stale-pending sweep (sweep.go) and the Square
// webhook clawback (handlers/webhooks/square.go). Do not fork it: a divergent
// clawback is real money loss.
func RevertGiftCardFunding(ctx context.Context, action, giftCardID string, amount float64, redeemToUserID *string, tillSaleID string) error {
	tx, err := db.Conn.Begin(ctx)
	if err != nil {
		return fmt.Errorf("failed to begin clawback transaction: %w", err)
	}
	defer func() {
		if err := tx.Rollback(ctx); err != nil && !errors.Is(err, pgx.ErrTxClosed) {
			slog.Error("failed to rollback gift-card clawback transaction", "err", err)
		}
	}()

	// Claim the sale first: the row lock serializes against the handler's
	// completion UPDATE; a zero-row claim means the funding is not ours.
	tag, err := tx.Exec(ctx, `
		UPDATE till_sales SET status = 'failed', updated_at = NOW()
		WHERE id = $1 AND status = 'pending'`, tillSaleID)
	if err != nil {
		return fmt.Errorf("failed to claim till sale for clawback: %w", err)
	}
	if tag.RowsAffected() == 0 {
		return errTillSaleNotPending
	}

	if action == "create" {
		// A newly created card's transactions are scoped to THIS sale's
		// funding (reference_type='till_sale' AND reference_id=sale id) — never
		// a wholesale delete, which would destroy the value of a different
		// idempotency-keyed top-up sale that funded the same card before this
		// create resolved. Then remove the card itself.
		if _, err := tx.Exec(ctx, `DELETE FROM gift_card_transactions WHERE gift_card_id = $1 AND reference_type = 'till_sale' AND reference_id = $2`, giftCardID, tillSaleID); err != nil {
			return fmt.Errorf("failed to delete gift card transaction: %w", err)
		}
		if _, err := tx.Exec(ctx, `DELETE FROM gift_cards WHERE id = $1`, giftCardID); err != nil {
			return fmt.Errorf("failed to delete gift card: %w", err)
		}
		// If the card was immediately redeemed to a user balance in this
		// request, reverse that credit (guarded so it can never go negative).
		if redeemToUserID != nil && *redeemToUserID != "" {
			bTag, bErr := tx.Exec(ctx, `
				UPDATE user_giftcard_balances
				SET balance = user_giftcard_balances.balance - $1, updated_at = NOW()
				WHERE user_id = $2 AND balance >= $1
			`, amount, *redeemToUserID)
			if bErr != nil {
				return fmt.Errorf("failed to reverse redeemed gift card balance: %w", bErr)
			}
			if bTag.RowsAffected() == 0 {
				// The guard blocked the reversal because some of the credited
				// balance was already spent. The sale is still marked failed
				// below — do not fail the whole clawback tx — but the
				// un-reversed credit must be flagged for manual reconciliation
				// (mirrors the top-up branch).
				log.Printf("CRITICAL: ... MANUAL RECONCILIATION REQUIRED: create-with-redeem clawback for gift card %s could not fully reverse the £%.2f balance credited to user %s (balance < amount)", giftCardID, amount, *redeemToUserID)
			}
		}
	} else {
		// Top-up: subtract the amount back out of the card. The guard keeps
		// amount_remaining from ever going negative in the pathological case
		// where some of the top-up was already spent before the charge failed.
		tag, err := tx.Exec(ctx, `
			UPDATE gift_cards
			SET total_funds_added = total_funds_added - $1,
			    amount_remaining = amount_remaining - $1
			WHERE id = $2 AND amount_remaining >= $1
		`, amount, giftCardID)
		if err != nil {
			return fmt.Errorf("failed to reverse gift card top-up: %w", err)
		}
		if tag.RowsAffected() == 0 {
			// The guard blocked the reversal because some of the top-up was
			// already spent. The sale is still marked failed below — do not
			// fail the whole clawback tx — but the unreversed money must be
			// flagged for manual reconciliation.
			log.Printf("CRITICAL: ... MANUAL RECONCILIATION REQUIRED: top-up %v on gift card %s could not be fully reversed (amount_remaining < top-up)", amount, giftCardID)
		}
		// Remove only this request's top-up transaction (reference_id = till
		// sale) so prior sales' accounting on the same card is untouched.
		if _, err := tx.Exec(ctx, `
			DELETE FROM gift_card_transactions
			WHERE gift_card_id = $1 AND reference_type = 'till_sale' AND reference_id = $2
		`, giftCardID, tillSaleID); err != nil {
			return fmt.Errorf("failed to delete gift card top-up transaction: %w", err)
		}
	}

	if err := tx.Commit(ctx); err != nil {
		return fmt.Errorf("failed to commit clawback transaction: %w", err)
	}
	return nil
}

// IsTillSaleNotPending reports whether err is the claim-first sentinel
// (errTillSaleNotPending): the gating `status='pending'` UPDATE matched zero
// rows, so the till sale is no longer pending and its gift card must be left
// untouched. Exported so cross-package clawback callers (the Square webhook)
// can detect the sentinel without reaching into the unexported error value.
func IsTillSaleNotPending(err error) bool {
	return errors.Is(err, errTillSaleNotPending)
}