Streams: tests for Transformer.cancel

[lucacasonato]

PR: whatwg/streams#1283

[MattiasBuelens]

Hmm, I see. Since TransformStreamDefaultSourceCancelAlgorithm takes at least one microtask to run transformer.cancel() (even if it does not exist), the TypeError from terminate() now wins the race. 🤔

I suppose that's fine.

[debadree25]

hello @MattiasBuelens i am trying to implement this for node this test presently fails my knowledge of WPTs is little limited hence asking here so is cancelPromise also expected to throw in this test? I am seeing that running this test standalone throws TypeError but unable to understand why the WPT says this uncaught

[lucacasonato]

This test will fail if you have not implanted the changes from the streams spec PR (yet).

[lucacasonato]

This test passes against the reference implementation, so if it fails it may be a bug in your implementation.

[debadree25]

Could be a bug no doubt i just wnated to know this test is basically equalt to the following script right:

import { TransformStream } from 'stream/web';

let controller;
const ts = new TransformStream({
  start(c) {
    controller = c;
  }
});
const cancelReason = { name: 'cancelReason' };
const cancelPromise = ts.readable.cancel(cancelReason);
controller.terminate();

try {
  await Promise.all([cancelPromise, ts.writable.getWriter().closed]);
} catch (err) {
  console.log(err); // TypeError!
}

[MattiasBuelens]

@debadree25 Yes, that looks script right. Ideally, you want to assert that the TypeError originates from .closed and not from cancelPromise, so in Node you could use assert.rejects():

import assert from 'node:assert/strict';

await Promise.all([
  cancelPromise,
  assert.rejects(ts.writable.getWriter().closed, TypeError)
]);

so is cancelPromise also expected to throw in this test?

No, cancelPromise should always resolve.

The only change is that writer.closed now becomes errored by controller.terminate(), whereas previously it would be errored by ts.readable.cancel(cancelReason). This is because readable.cancel() now needs to go through the (possibly asynchronous) transformer.cancel() method first before it can error the writable, whereas controller.terminate() always errors the writable synchronously (without going through a transformer method).

[debadree25]

Thank you so much @MattiasBuelens for the detail response i guess i have found the issue with my implementation!

[domenic]

I guess since this was marked as draft, I shouldn't go ahead and merge it immediately. @lucacasonato, can you confirm this is up-to-date with all the latest changes in whatwg/streams#1283, and is ready to merge?

(I took it out of draft state anyway so we could see CI results. If those fail, the first thing to try is rebasing; WPT CI is very finicky and constantly getting patched.)

[lucacasonato]

@domenic Yes, this is up to date.

[domenic]

Looks like we have a lint failure due to trailing whitespace.