// The one rule `web/worker.js` exists to keep: one frame at a time, in arrival
// order. No DOM, no worker globals — context/code/clients.md.

/** Chain every arrival onto one tail promise, starting from `head`.
 *
 *  An `async` listener does NOT serialize: the event loop is free to start the
 *  next message while an earlier one is suspended at an `await`, so two
 *  handler runs interleave and a later frame's replies can be posted before an
 *  earlier frame's — which the client's fold answers by DROPPING the overtaken
 *  update, not by replaying it. Chaining is also what makes "frames that
 *  arrived before `start()` finished" an ordinary case of this rule rather
 *  than a drain loop with the race in it twice over. */
export function serialize(head, handler, onError) {
  let tail = head.catch(() => {});
  return (value) => {
    tail = tail.then(() => handler(value)).catch((err) => onError(err, value));
    return tail;
  };
}