第 50 课：关闭成功后保持已释放状态

class Handle {
  private closing?: Promise<void>
  private closed: boolean = false
  private releaseFn: () => Promise<void>
  constructor(releaseFn: () => Promise<void>) { this.releaseFn = releaseFn }
  close(): Promise<void> {
    if (this.closed) return Promise.resolve()
    if (this.closing) return this.closing
    this.closing = this.releaseFn().then((): void => { this.closed = true })
      .finally((): void => { this.closing = undefined })
    return this.closing
  }
}