Add transformer.cancel method to TransformStream constructor

[MattiasBuelens]

Summary

The transformer object passed to the TransformStream constructor can now also have a cancel method, which is called whenever the readable side is cancelled or when the writable side is aborted. See the specification and the spec change for context.

Test results and supporting details

This method is not yet supported in any browser. However, server runtimes such as Node.js and Deno already support it:

• Node.js: implemented in nodejs/node@3dd96f1, shipped in version 20.14.0
• Deno: implemented in denoland/deno@6450334, shipped in version 1.38.0

[caugner]

LGTM overall, but I'm just not sure about the subfeature name and description.

[caugner]

Given that cancel is the key in the parameter object passed to the TransformStream constructor, it confuses me slightly that this is called a method (even if the spec uses this terminology). How about:

Suggested change

	"cancel": {
	"__compat": {
	"description": "`transformer.cancel` method",
	"transformer_cancel_callback": {
	"__compat": {
	"description": "`transformer.cancel` callback",

[MattiasBuelens]

The MDN articles for the ReadableStream/WritableStream/TransformStream constructors as well as the streams guides also talk about "methods", so I tried to be consistent with those. Although the specification does use the name TransformerCancelCallback for the type of that method... 😅

I think I'll stick with "method" for now. I'll make the feature name a bit more specific though, like you suggested.

[caugner]

Thank you, LGTM!