check_password took a permit from CHECKS in the async function and
ran Argon2 in spawn_blocking. A client that hung up dropped the
future and with it the permit, while the blocking check ran on, so a
flood of sign-ins that disconnected at once ran unbounded Argon2 work,
the very thing CHECKS was added for (#137).
The permit is now owned (CHECKS is an Arc) and moves into the blocking
closure, so it is freed when the check ends, whoever stopped waiting.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
rays
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