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ADR-004-flock-pid-locking.md

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ADR-004-flock-pid-locking.md

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ADR-004: flock(2) for Atomic PID Locking

Status

Accepted

Context

The TypeScript version uses check-then-write for PID locking: read PID file, check if process exists, write new PID. This has a TOCTOU race condition (MEDIUM severity) where two processes can both pass the check and both write their PID.

Decision

Use flock(2) via the nix crate for atomic PID file locking. The lock file is held open for the daemon's lifetime. flock(LOCK_EX | LOCK_NB) fails immediately if another process holds the lock, eliminating the race.

Consequences

  • Positive: Atomic, race-free, kernel-enforced locking; auto-released on process crash
  • Negative: Linux/macOS only (no Windows support, which we don't need)
  • Note: The lock fd must be kept open for the daemon's lifetime; dropping it releases the lock