Should WebTransportSendStream and WebTransportReceiveStream be transferable?

[jan-ivar]

For almost a year, between #307 and #395 (nevermind #423), the following streams were transferable streams:

let {readable, writable} = await wt.createBidirectionalStream();

writable = await wt.createUnidirectionalStream();

for await (readable of wt.incomingUnidirectionalStreams)

But right now they're not, since "Platform objects can be transferable objects if their primary interface is decorated with the [Transferable] IDL extended attribute."

Q: Should they be? This would allow .getStats() to be called where data is read and written, as well as make this allowed:

const wt = new WebTransport(url);
const {readable, writable} = await wt.createBidirectionalStream();
sendWorker.postMessage({writable}, [writable]);
receiveWorker.postMessage({readable}, [readable]);

If we don't then the above will fail, and apps would have to work around it like this instead instead:

const wt = new WebTransport(url);
const wts = await wt.createBidirectionalStream();
const sendable = new TransformStream(), receivable = new TransformStream();
sendable.pipeTo(wts.writable);
wts.readable.pipeTo(receivable.writable);
sendWorker.postMessage({sendable}, [sendable]);
receiveWorker.postMessage({receivable}, [receivable]);

[aboba]

I was hoping to avoid the pipeTo workarounds you describe. For example, being able to transfer the readable stream wt.incomingUnidirectionalStreams would be helpful.

[aboba]

Attempting to transfer wt.incomingUndirectionalStreams doesn't appear to work, as you say. When getReader() is called on the transferred readable stream of streams, the error is A ReadableStream could not be cloned because it was not transferred.

On main thread it does seem possible to construct a WebTransport, build a 'complex' readable or writable stream based on it, and then transfer those streams to (distinct) workers. However, so far the performance seems quite bad and it crashes the tab.

Sample (which reliably crashes the tab after pressing stop): https://webrtc.internaut.com/wc/wtSender6/

[ricea]

When getReader() is called on the transferred readable stream of streams, the error is A ReadableStream could not be cloned because it was not transferred.

Sorry about this. Currently when a stream is transferred, the chunks that are passed through the stream are always cloned, never transferred. Since incomingUnidirectionalStreams is a stream of streams, each chunk is a stream, and streams can only be transferred, not cloned.

There was a plan to also support transferring the chunks (see Future Work) but without a concrete use case it never proceeded to the design stage.

[jan-ivar]

Meeting:

• Seems fine. No objections