Use async_sequence in ReadableStream.from

[MattiasBuelens]

Replace our manual async iterator logic with the new Web IDL async_sequence<T> type and its associated operations (added in whatwg/webidl#1397).

This comes with one behavioral change: ReadableStream.from("abc") used to iterate over the Unicode code points of the string (per String.prototype[Symbol.iterator]), but this will now throw a TypeError instead. This is covered by the new WPTs.

Developers who want to create a ReadableStream from a string should update their code to either:

• ReadableStream.from(["abc"]) for a stream with a single chunk "abc"
• ReadableStream.from([..."abc"]) for a stream with three chunks "a", "b", "c"

Supersedes #1310. (Thanks to @lucacasonato for getting this started!)

• At least two implementers are interested (and none opposed):
  • WebKit: Use async_sequence in ReadableStream.from #1372 (review)
  • Gecko: Use async_sequence in ReadableStream.from #1372 (review)
  • Chromium: Use async_sequence in ReadableStream.from #1372 (comment)
• Tests are written and can be reviewed and commented upon at:
  • Reject strings in ReadableStream.from web-platform-tests/wpt#59594
• Implementation bugs are filed:
  • Chromium: https://issues.chromium.org/issues/531056906
  • Gecko: https://bugzilla.mozilla.org/show_bug.cgi?id=2052661
  • WebKit: https://bugs.webkit.org/show_bug.cgi?id=318598
  • Deno: Streams: use Web IDL async_sequence in ReadableStream.from denoland/deno#35777
  • Node.js: Streams: use Web IDL async_sequence in ReadableStream.from nodejs/node#64288
• MDN issue is filed: …
• The top of this comment includes a clear commit message to use.

(See WHATWG Working Mode: Changes for more details.)

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Preview | Diff

[MattiasBuelens]

The spec text doesn't build yet, because I'm waiting for a new release of Bikeshed that includes speced/bikeshed#3297.

You can already build it locally by manually updating the widlparser dependency:

pipx install bikeshed
pipx inject bikeshed --force widlparser==1.5.0
make local

[MattiasBuelens]

@lucacasonato Would this work for whatwg/fetch#1291?

[annevk]

Modulo green CI. WebKit supports this change. cc @youennf

[MattiasBuelens]

CI is (finally) green. Turns out Bikeshed needed to be updated across quite a few tools and services. 😅

Does Chromium and/or Gecko support this change? @ricea perhaps?

[lucacasonato]

LGTM!

[noamr]

@ricea for chromium
This seems like a good change to me, WDYT?

[saschanaz]

Gecko supports this.

[ricea]

Chromium supports this.

I haven't had time to check in detail whether this has any unexpected semantic side effects, but the approach looks fine.

[MattiasBuelens]

Thanks for the responses. I'll start opening implementation bugs.

I haven't had time to check in detail whether this has any unexpected semantic side effects, but the approach looks fine.

The async_sequence<T> machinery in Web IDL was largely based on the original semantics for ReadableStream.from(), so the behavior should be unchanged. Well, except for strings, but that's explained above and tested in web-platform-tests/wpt#59594. (By the way, reviews welcome! 😉)