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When implementing cross-process synchronization with filesystem primitives (e.g. mkdir as a mutex), avoid time-of-check-to-time-of-use races between detecting a stale lock and acting on it. Treat lock recovery as its own critical section:

  1. Record enough owner identity in the lock (PID plus process start time, not just PID) so staleness checks aren’t fooled by PID reuse.
  2. Never reclaim a lock based on a staleness check alone — a legitimate new owner may acquire it between your check and your reclaim. Instead, enter the specific lock directory and atomically claim a nested “reclaim” marker (e.g. mkdir lock/reclaim) so only one contender can attempt recovery, then re-read and re-verify the owner identity after the claim and immediately before any destructive action.
  3. Resolve all paths (install dirs, lock paths) to absolute paths before constructing lock paths or cd-ing into them for recovery; verify directory identity with something like [[ "$lock" -ef . ]] before deleting, since relative paths can silently resolve to the wrong location once the working directory changes.
  4. Always register cleanup via trap ... EXIT INT TERM to release owned locks and remove partial/temporary artifacts, and add regression tests that simulate concurrent contenders racing to acquire/reclaim the same lock to catch these races before they ship.

Example pattern for a safe reclaim:

if [[ "$stale" == true ]]; then
    (
        cd "$lock" 2>/dev/null || exit 0
        mkdir reclaim 2>/dev/null || exit 0   # atomic claim to recover
        current=$(cat owner 2>/dev/null || true)
        # re-verify the owner is still abandoned AND we're still looking
        # at the same directory before destroying anything
        [[ "$current" == "$previous" && "$lock" -ef . ]] && rm -rf "$lock"
    )
fi

This pattern — atomic claim, re-verify, then act — generalizes to any manually-implemented lock/mutex and should be applied whenever recovery or cleanup logic runs concurrently with other potential lock holders.

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