// Simple PID-based lock file. Atomic via O_EXCL.
// If a stale lock is found (PID no longer alive), it is removed.

import { closeSync, openSync, readFileSync, unlinkSync, writeSync } from 'node:fs';

function isAlive(pid) {
  try {
    process.kill(pid, 0);
    return true;
  } catch (e) {
    return e.code === 'EPERM';
  }
}

export function acquireLock(lockPath) {
  for (let attempt = 0; attempt < 2; attempt++) {
    try {
      const fd = openSync(lockPath, 'wx');
      writeSync(fd, String(process.pid));
      closeSync(fd);
      const release = () => {
        try {
          unlinkSync(lockPath);
        } catch {
          /* ignore */
        }
      };
      process.on('exit', release);
      process.on('SIGINT', () => {
        release();
        process.exit(130);
      });
      process.on('SIGTERM', () => {
        release();
        process.exit(143);
      });
      return release;
    } catch (err) {
      if (err.code !== 'EEXIST') throw err;
      // Lock exists — check if owner is still alive
      let pid = 0;
      try {
        pid = parseInt(readFileSync(lockPath, 'utf8').trim(), 10);
      } catch {
        /* unreadable */
      }
      if (pid && isAlive(pid)) {
        throw new Error(`Cron already running (PID ${pid}, lock ${lockPath})`);
      }
      // Stale lock — remove and retry
      console.log(`Removing stale lock (PID ${pid || '?'} no longer alive): ${lockPath}`);
      try {
        unlinkSync(lockPath);
      } catch {
        /* ignore */
      }
    }
  }
  throw new Error(`Could not acquire lock after retries: ${lockPath}`);
}