export class Mutex {
  constructor () {
    this._locked = false
    this._waiting = []
  }

  lock () {
    const unlock = () => {
      const next = this._waiting.shift()
      if (next) {
        next(unlock)
      } else {
        this._locked = false
      }
    }

    if (this._locked) {
      return new Promise(resolve => this._waiting.push(resolve)).then(() => unlock)
    } else {
      this._locked = true
      return Promise.resolve(unlock)
    }
  }
}