Propagate reason argument to resolved promise of cancel algorithm

[kennethmyhra]

When the user of the API does no supply a cancel algorithm make sure to
forward the reason argument to the resolved promise.

This changes step 4. of algorithm
SetUpTransformStreamDefaultControllerFromTransformer making sure to
forward the reason argument to the resolved promise. With this change
the reason argument in step 7. 'React to cancelPromise' of algorithm
TransformStreamDefaultSourceCancel will no longer be undefined.

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I work on the Ladybird browser and think this PR might resolve a spec bug. We have some failing WPT tests in transform-streams/backpressure/any.html, specifically:

• writer.closed should resolve after readable is canceled during start
• writer.closed should resolve after readable is canceled with backpressure
• writer.closed should resolve after readable is canceled with no backpressure

Correcting step 4 in SetUpTransformStreamDefaultControllerFromTransformer as described above so that the reason argument is propagated to the resolved promise resolves these failing tests.

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Preview | Diff

[domenic]

The reference implementation manages to pass these tests despite following the current spec:

streams/reference-implementation/lib/abstract-ops/transform-streams.js

Line 134 in 4d33866

let cancelAlgorithm = () => promiseResolvedWith(undefined);

. Are you able to explain the discrepancy?

[kennethmyhra]

Right, looking through the reference implementation the incoming reason argument for TransformStreamDefaultSourceCancelAlgorithm is the one used by the onFulfilled lambda.

streams/reference-implementation/lib/abstract-ops/transform-streams.js

Line 338 in 4d33866

WritableStreamDefaultControllerErrorIfNeeded(writable._controller, reason);

Then this is a bug in our implementation and not a spec issue. Thanks for the prompt reply!