A non-unique index on (doctor_id, slot_start) plus a count-then-insert check
left a TOCTOU race: two concurrent requests could both pass isSlotTaken and
both insert. wrapInTransaction alone doesn't stop the phantom under InnoDB
REPEATABLE-READ.
Add a nullable, unique active_slot_key on Appointment = "doctorId:slotStart"
while the booking occupies the slot (pending/confirmed — in lockstep with
isSlotTaken); NULL once expired/completed/no_show/cancelled (NULLs don't collide
in a MySQL unique index, so released slots rebook freely). bookAtomically now:
catches the unique violation -> SlotTakenException, and expires lapsed pendings
in-transaction so the ~1-min window before the expiry cron doesn't wrongly block
rebooking. All three booking paths (online / my / admin) routed through it.
Migration backfills one row per (doctor, slot) — the latest id — so the index
builds even on dirty historical data without destructively cancelling bookings.
(Backfill surfaced a real pre-existing double-booked slot in dev data.)
Regression: tests/Appointment/SlotUniquenessTest. Adjusted the expiry-service
test fixture to use distinct slots (one live booking per slot is now enforced).
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
