Refund idempotency (P2):
- RefundRequest gains an optional client idempotency_key: two DISTINCT equal
partial refunds of one payment no longer collide on the amount-derived key
(the second was silently swallowed as a dedup)
- Extract resumeManualPendingRefund: resumes a pending refund with the row's
OWN stored key, so Square's key dedup returns the original refund if the
prior attempt completed — never issues a second
- (payment, amount) pending fallback: when the exact-key lookup misses (admin
reopened the modal, new UUID), resume the matching pending row instead of
creating a second pending row the sweep would double-process
- 409 in-flight guard: if a pending refund exists for the payment but no
same-amount row matches, reject a different-amount refund (money state at
Square is unknown — no new refund is safe until it resolves)
- Frontend (EditBookingModal): UUID per refund attempt, reused on retry,
mirroring the tip flow
Terminal completion (P3):
- GetCheckoutStatus serializes on pg_advisory_lock('crussell:terminal:' ||
SquarePayID) on a pinned connection — concurrent polls of the same checkout
can no longer both pass the dedup SELECT and race the UNIQUE constraint
Card-on-file / doc-only:
- Document why CreateCardOnFile is NOT rolled back on payment failure
(deterministic sha256 retry returns the same card; deletion breaks it)
- Document HasCompletedPayment's deliberate 'tip' exclusion
Regression tests:
- TestRefund_TwoEqualPartialRefunds_ClientKeyDisambiguates
- TestRefund_PendingResume_NewKeyAfterModalReopen (proves stored-key resume)
- TestRefund_PendingResume_DifferentAmountRejected (409 + no second row)
- TestRefund_GuardCountsPendingRefunds updated: 400 -> 409 (in-flight guard
fires first — strictly safer, blocks before any Square attempt)
- TestGetCheckoutStatus_ConcurrentPolls_SingleRecord (real two-goroutine race)
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