Process singleton guard with lockfile

Skill kjuhwa/skills-hub/skills/resilience/process-singleton-guard-with-lockfile

Prevent multiple daemon instances using exclusive lockfile creation and PID validationFrom its SKILL.md

Install

npx -y skills add kjuhwa/skills-hub --skill process-singleton-guard-with-lockfile

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One thing to look at

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SKILL.md

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Singleton daemon guard via exclusive lockfile

Acquire a singleton lock by writing the current PID to a file with exclusive-create ({ flag: 'wx' }). If the file exists, read the PID and probe liveness with process.kill(pid, 0). Dead PID → stale lock, overwrite. Live PID → abort.

Mechanism

function acquireLock(path) {
  try {
    fs.writeFileSync(path, String(process.pid), { flag: 'wx' });
    return true;
  } catch (e) {
    if (e.code !== 'EEXIST') throw e;
    const pid = parseInt(fs.readFileSync(path, 'utf8'), 10);
    try { process.kill(pid, 0); return false; } // alive → abort
    catch { fs.writeFileSync(path, String(process.pid)); return true; } // stale → take
  }
}
process.on('exit', () => fs.unlinkSync(path));

When to reuse

Background workers, evolution loops, schedulers, anything where parallel invocation would corrupt shared state.

What ships with it

Read from the repository