/**
 * Stopping a spawned process group without blocking the event loop.
 *
 * The test runner puts every suite command in its own process group so one
 * signal can take down the runner, every test file it forked, and anything
 * those forked. Ending that group has one non-obvious constraint: **the wait
 * for the child to die cannot be a poll on liveness.**
 *
 * A dead child stays a zombie until its parent reaps it, and the parent here is
 * this process, which reaps through libuv when the event loop runs. So a busy
 * wait on `kill(pid, 0)` is doubly wrong: it blocks the very loop that would do
 * the reaping, and it then reads the unreaped zombie as alive. The wait would
 * always burn its full grace period and always end in a needless SIGKILL. There
 * is no waitpid from JavaScript that would see through this, so the wait has to
 * be asynchronous and keyed on the child's own `exit` event instead.
 */

/** Signal the whole group, then the child itself in case it leads no group. */
export function signalGroup(child, sig) {
  try { process.kill(-child.pid, sig); } catch { /* group already gone */ }
  try { child.kill(sig); } catch { /* already gone */ }
}

/**
 * Wrap a spawned child so its exit is observable as both a flag and a promise.
 * Register this before any other `exit` listener so `hasExited` is already true
 * by the time the others run.
 *
 * @returns {{child: import('node:child_process').ChildProcess, exited: Promise<void>, hasExited: boolean}}
 */
export function trackChildExit(child) {
  const running = { child, hasExited: false, exited: null };
  running.exited = new Promise((resolve) => {
    child.once('exit', () => {
      running.hasExited = true;
      resolve();
    });
  });
  return running;
}

/**
 * SIGTERM the group, wait for the child to actually exit, and escalate to
 * SIGKILL only if it has not exited within `graceMs`. Returns as soon as the
 * child is gone, so a suite that dies promptly costs milliseconds rather than
 * the whole grace period.
 *
 * @returns {Promise<'exited'|'killed'|'already-gone'>}
 */
export async function stopGroup(running, { graceMs = 2000, killGraceMs = 500 } = {}) {
  if (!running || running.hasExited) return 'already-gone';

  signalGroup(running.child, 'SIGTERM');
  if (await raceExit(running, graceMs)) return 'exited';

  signalGroup(running.child, 'SIGKILL');
  await raceExit(running, killGraceMs);
  return 'killed';
}

/**
 * The last resort, for `process.on('exit')` where the loop is closed and
 * nothing can be awaited. It cannot wait, so it does not pretend to: SIGTERM
 * for anything that handles it, then SIGKILL for anything that does not.
 */
export function killGroupSync(running) {
  if (!running || running.hasExited) return false;
  signalGroup(running.child, 'SIGTERM');
  signalGroup(running.child, 'SIGKILL');
  return true;
}

function raceExit(running, ms) {
  return new Promise((resolve) => {
    const timer = setTimeout(() => resolve(false), ms);
    running.exited.then(() => {
      clearTimeout(timer);
      resolve(true);
    });
  });
}

/**
 * The runner's shutdown state machine: which group is ours, and how it ends.
 *
 * Kept here rather than as loose module state in the runner because the
 * interesting case is a state bug, not a signalling one. The first signal hands
 * the group off from `current` to `stopping` and then awaits the grace period.
 * A second signal has to be able to reach that same handle, or it kills nothing
 * and `exit` abandons the escalation still in flight, leaving a detached suite
 * running after the runner is gone. Clearing one reference without holding the
 * other is exactly how that happens.
 *
 * @param {object} [opts]
 * @param {(code: number) => void} [opts.exit]  injectable for tests; in
 *        production this never returns, so a shutdown that has been overtaken
 *        by a second signal simply stops there.
 */
export function createGroupShutdown({ exit = (code) => process.exit(code), graceMs = 2000 } = {}) {
  let current = null;
  let stopping = null;
  let shuttingDown = false;

  return {
    /** Adopt a freshly spawned group. */
    track(running) {
      current = running;
      return running;
    },

    /** The group finished on its own; nothing left to end. */
    release() {
      current = null;
    },

    get shuttingDown() {
      return shuttingDown;
    },

    /** A termination signal arrived. The second one stops waiting. */
    async onSignal(exitCode) {
      if (shuttingDown) {
        // Still inside the first shutdown's grace period. Do not wait it out:
        // kill the handle that shutdown is holding, which `current` no longer
        // is, and leave.
        killGroupSync(stopping);
        exit(exitCode);
        return;
      }
      shuttingDown = true;
      stopping = current;
      current = null;
      await stopGroup(stopping, { graceMs });
      stopping = null;
      exit(exitCode);
    },

    /** `process.on('exit')`: the last resort, and the one that cannot wait. */
    onExit() {
      const running = current || stopping;
      current = null;
      stopping = null;
      return killGroupSync(running);
    },
  };
}