Port locking infrastructure to writable streams

[domenic]

Writable streams need a similar way of being locked for exclusive writing, as readable streams do with their readers. This is notably crucial for off-main-thread piping (#97).

One problem that immediately presents itself, looking at the current list of writable stream properties and methods (closed/ready/state/abort()/close()/write()), is that pretty much all of them belong on the writer. Does that mean writable streams themselves become empty objects with getWriter() methods!?

I guess we could add a few pass-throughs (like ReadableStream.prototype.cancel acquiring, then releasing, a reader). Do we just add pass-through for everything? Well, probably not for ready/state/closed at least, as that causes problems. But if all we have is abort()/close()/write() passthroughs, doesn't that encourage the bad practice of ignoring backpressure signals? Maybe just abort() and close()?

[tyoshino]

become empty objects with getWriter() methods!?

Yeah, pipeTo etc. all belongs to the readable side.

What do you mean by pass-through? Keeping a method also on the WritableStream? As you just said, I also think we should basically limit them to WritableStreamWriter as much as possible.

just abort() and write()?

Maybe you meant abort() and close().

[domenic]

What do you mean by pass-through? Keeping a method also on the WritableStream?

I mean, like we do for ReadableStream.prototype.cancel, have versions of them on the WritableStream which throw if the WritableStream is locked.

Maybe you meant abort() and close().

Yes, editing OP.

[tyoshino]

OK. I slightly prefer not putting them on WritableStream. The same about .cancel(). Basically.

But given that we have .cancel() on ReadableStream, maybe it's better to make them consistent. I.e. have abort() or both abort() and close().

[domenic]

Plan:

class WritableStream {
  constructor(underlyingSink = {}, { size, highWaterMark = 0 } = {})

  get locked()

  getWriter()
  abort(reason)
  close()
}

class WritableStreamWriter {
  constructor(stream)

  get closed()
  get ready() // see #318

  abort(reason)
  close()
  releaseLock()
  write(chunk)
}

No need for WritableStreamController. Can eliminate error parameter to underlying sink start().

[tyoshino]

PR created: #462

[tyoshino]

Fixed.

#462 has been succeeded by #488.

Only pass-through for abort() has been added.