actual-server/util/async.js

function sequential(fn) {
  let sequenceState = {
    running: null,
    queue: []
  };

  function pump() {
    if (sequenceState.queue.length > 0) {
      const next = sequenceState.queue.shift();
      run(next.args, next.resolve, next.reject);
    } else {
      sequenceState.running = null;
    }
  }

  function run(args, resolve, reject) {
    sequenceState.running = fn(...args);

    sequenceState.running.then(
      (val) => {
        pump();
        resolve(val);
      },
      (err) => {
        pump();
        reject(err);
      }
    );
  }

  return (...args) => {
    if (!sequenceState.running) {
      return new Promise((resolve, reject) => {
        return run(args, resolve, reject);
      });
    } else {
      return new Promise((resolve, reject) => {
        sequenceState.queue.push({ resolve, reject, args });
      });
    }
  };
}

module.exports = { sequential };