Function installMainThreadWatchdog

src/mcp/liveness-watchdog.ts:108–170

()

 * starting).
 */
export function installMainThreadWatchdog(): WatchdogHandle | null {
  if (isEnvTruthy(process.env.CODEGRAPH_NO_WATCHDOG)) return null;

  const timeoutMs = parseWatchdogTimeoutMs(process.env.CODEGRAPH_WATCHDOG_TIMEOUT_MS);
  const checkMs = deriveCheckIntervalMs(timeoutMs);

  let child: ChildProcess;
  try {
    // No execArgv inheritance (unlike Worker), so the child carries none of our
    // V8 flags — it runs no WASM and needs none. stderr inherits the parent's
    // fd 2 so the kill notice lands wherever the parent logs (daemon.log).
    child = spawn(
      process.execPath,
      ['-e', CHILD_SOURCE, String(process.pid), String(timeoutMs)],
      {
        stdio: ['pipe', 'ignore', 'inherit'],
        windowsHide: true,
        // The watchdog touches no files; keep its cwd off the project/temp dir
        // so it can't hold one open (Windows EPERM-on-cleanup, mirrors the
        // parse-worker quirk).
        cwd: os.tmpdir(),
      }
    );
  } catch (err) {
    debug(`spawn failed: ${err instanceof Error ? err.message : String(err)}`);
    return null;
  }

  const stdin = child.stdin;
  if (!stdin) {
    debug('child has no stdin pipe; not arming');
    try { child.kill(); } catch { /* ignore */ }
    return null;
  }
  // Writing after the child exits surfaces EPIPE on the stream — swallow it so
  // it can't escalate to the global handler (which now exits, #850).
  stdin.on('error', () => { /* child gone; heartbeat writes are best-effort */ });
  child.on('error', (err) => debug(`child error: ${err.message}`));

  // Heartbeat: a byte per tick. When the main thread wedges, these stop and the
  // child's timeout fires. unref'd so it never keeps the process alive itself.
  const heartbeat = setInterval(() => {
    try { stdin.write('\n'); } catch { /* child gone */ }
  }, checkMs);
  heartbeat.unref();

  // Neither the child nor its pipe should keep the parent alive past its work.
  child.unref();
  try { (stdin as unknown as { unref?: () => void }).unref?.(); } catch { /* ignore */ }

  debug(`armed (child pid ${child.pid ?? '?'}): timeoutMs=${timeoutMs} checkMs=${checkMs}`);

  let stopped = false;
  return {
    stop(): void {
      if (stopped) return;
      stopped = true;
      clearInterval(heartbeat);