6-way concurrent redeem of one code 500'd on users_username_key (each request
generated a fresh user id but the same uuid-derived username), plus the rarer
two-codes-one-uuid race. Same drift family as VerifyLinkCode, which already
locks its code row and handles the conflict.
- SELECT ... FOR UPDATE the code row: same-code racers serialise; losers exit
as ErrLinkCodeInvalid (400 invalid_code), no user row is attempted.
- INSERT users ... ON CONFLICT (username) DO NOTHING + re-read by username:
cross-code racers converge on the winner's row (role checked, staff still
refused) instead of a unique-violation 500.
- account_links ON CONFLICT (mc_uuid) DO NOTHING for the same race.
Verified live: same-code x6 = 1x200 + 5x400; two-codes x2 = 2x200 same user;
db clean; zero unmapped errors.
Lemon-miaow
0414913bc7