type Job<T> = () => Promise<T>;

let tail = Promise.resolve();
let pendingJobs = 0;

export function queueSequential<T>(job: Job<T>): Promise<T> {
  pendingJobs += 1;

  const run = tail.then(async () => {
    try {
      return await job();
    } finally {
      pendingJobs -= 1;
    }
  });

  tail = run.then(
    () => undefined,
    () => undefined
  );

  return run;
}

export function queuedJobCount() {
  return pendingJobs;
}