Cannot set property duplex of #<Request> which has only a getter

[probablykasper]

Describe the bug

As of SvelteKit 1.0.0-next.406, I started getting this issue in server endpoints that used ky for making web reqeusts:

Cannot set property duplex of #<Request> which has only a getter

The issue stems from it setting request.duplex like this:

    const request = new Request('https://example.com')
    if (supportsRequestStreams) {
        // @ts-expect-error - Types are outdated.
        request.duplex = 'half';
    }

In SvelteKit 1.0.0-next.406, supportsRequestStreams started evaluating to true. It's defined as this:

const supportsRequestStreams = (() => {
    let duplexAccessed = false;
    let hasContentType = false;
    const supportsReadableStream = typeof globalThis.ReadableStream === 'function';
    if (supportsReadableStream) {
        hasContentType = new globalThis.Request('https://a.com', {
            body: new globalThis.ReadableStream(),
            method: 'POST',
            // @ts-expect-error - Types are outdated.
            get duplex() {
                duplexAccessed = true;
                return 'half';
            },
        }).headers.has('Content-Type');
    }
    return duplexAccessed && !hasContentType;
})();

For context, setting request.duplex seems to be needed for compatibility with Chrome 105: sindresorhus/ky@316fffe

I'm not sure entirely sure if this is an incorrect implementation in SvelteKit, or if the issue is with ky.

Ky issue: sindresorhus/ky#467

Reproduction

https://github.com/probablykasper/sveltekit-fetch-duplex-bug

Logs

No response

System Info

System:
    OS: macOS 10.15.7
    CPU: (8) x64 Intel(R) Core(TM) i7-7700K CPU @ 4.20GHz
    Memory: 820.51 MB / 32.00 GB
    Shell: 3.5.1 - /usr/local/bin/fish
  Binaries:
    Node: 16.15.0 - ~/Library/Caches/fnm_multishells/70304_1667344002694/bin/node
    Yarn: 1.22.15 - ~/Library/Caches/fnm_multishells/70304_1667344002694/bin/yarn
    npm: 8.12.1 - ~/Library/Caches/fnm_multishells/70304_1667344002694/bin/npm
  Browsers:
    Brave Browser: 97.1.34.81
    Chrome: 107.0.5304.87
    Firefox: 106.0.1
    Firefox Developer Edition: 107.0
    Safari: 15.6.1
  npmPackages:
    @sveltejs/adapter-auto: 1.0.0-next.86 => 1.0.0-next.86 
    @sveltejs/kit: 1.0.0-next.531 => 1.0.0-next.531 
    svelte: ^3.52.0 => 3.52.0 
    vite: ^3.2.2 => 3.2.2

Severity

serious, but I can work around it

Additional Information

No response

[dummdidumm]

This is probably a result of #7412 where we had to add the duplex property to prevent undici from erroring. From looking at the referenced Ky-code I don't see how this could have an impact on this though. Maybe it's also not related to our fix but the most recent undici version whose Response implementation throws this error? In that case Ky would need to work around that somehow.

[repsac-by]

This behavior of undici@5.12.0 is fully consistent with the specification.

readonly attribute RequestDuplex duplex;

Note that the duplex attribute is marked readonly.
This attribute can only be set by passing RequestInit.

[dummdidumm]

Then this is on Ky's end, because in the code that was linked they modify that property after initialising the request.

[vhscom]

I'm seeing OP described behavior in SvelteKit and I'm not using Ky. Kit 529 + Node Adapter 100. Occurs on POST requests only.

[repsac-by]

@vhscom please provide a minimal reproducible repo.

[vhscom]

For those who see this issue, ensure you're passing duplex: 'half' into any fetch calls using RequestInit like:

const res = await fetch('/api/listing', {
  method: 'POST',
  // @ts-ignore-next-line - Types outdated
  duplex: 'half',
  body: JSON.stringify(theBody)
});

If helpful, pin undici using overrides in the package manifest. 5.12.0 is the Unidici version which introduces duplex.