去 读 [stream 的 API 文档]( https://nodejs.org/api/stream.html
> Readable streams have two "modes": a flowing mode and a paused mode.
> When in flowing mode, data is read from the underlying system and provided to
> your program as fast as possible. In paused mode, you must explicitly call
> stream.read() to get chunks of data out. Streams start out in paused mode.