The Writer is not a concrete class. Any object that implements write(), end(), and abort() can be a writer making it easy to adapt existing APIs or create specialized implementations without subclassing. There's no complex UnderlyingSink protocol with start(), write(), close(), and abort() callbacks that must coordinate through a controller whose lifecycle and state are independent of the WritableStream it is bound to.
// block: Wait for space (unbounded pending queue)，详情可参考旺商聊官方下载
，详情可参考91视频
（一）包含两个以上涉及不同税率、征收率的业务；
k--; // 剩余删除名额减1（关键：控制删除位数，保证保留足够长度）。关于这个话题，WPS下载最新地址提供了深入分析