Feature detecting streaming requests

[jakearchibald]

It would be nice to have a synchronous feature detect for streaming requests. This currently works with the Chrome implementation:

const supportsRequestStreams = (() => {
  let duplexAccessed = false;

  const hasContentType = new Request('', {
    body: new ReadableStream(),
    method: 'POST',
    get duplex() {
      duplexAccessed = true;
      return 'half';
    },
  }).headers.has('Content-Type');

  return duplexAccessed && !hasContentType;
})();

This tests that:

• duplex is implemented as part of RequestInit
• body can be a ReadableStream. Otherwise, it's converted to a string and gets a text/plain content type.

However, it's possible that an implementation could support these two things, but reject when that request is passed to fetch() because it doesn't support request streams. In fact, Safari already behaves like this, but it doesn't implement duplex so the test still works.

Is it reasonable to expect the above feature detect to mean "fetch supports streaming requests"? Should this be enforced with spec text and/or a wpt?

[yutakahirano]

Does Request.prototype.hasOwnProperty('body') work?

[jakearchibald]

Nah, that's true in Safari.

The main problem here is that Safari supports requests with stream bodies, but it rejects when they're passed to fetch. Here was the old feature detect I used to work around the Safari issue:

const supportsRequestStreamsP = (async () => {
  const supportsStreamsInRequestObjects = !new Request('', {
    body: new ReadableStream(),
    method: 'POST',
  }).headers.has('Content-Type');

  if (!supportsStreamsInRequestObjects) return false;

  return fetch('data:a/a;charset=utf-8,', {
    method: 'POST',
    body: new ReadableStream(),
  }).then(() => true, () => false);
})();

// Note: supportsRequestStreamsP is a promise.
if (await supportsRequestStreamsP) {
  // …
} else {
  // …
}

…but I'm hoping we won't have to do that now we have duplex.

[yutakahirano]

• https://wpt.fyi/results/fetch/api/request/request-init-stream.any.html (Request construction)
• https://wpt.fyi/results/fetch/api/basic/request-upload.h2.any.html (fetching)

Interesting.

How about asking for their plan at WebKit/standards-positions#24?

[jakearchibald]

Done! WebKit/standards-positions#24

[yutakahirano]

Thanks!

[annevk]

I'm going to close this as there is test coverage for this. But let me explicitly copy @youennf here so he knows to watch out for this when WebKit gets around to the feature.