Content negotiation fails with fetch from __layout

[CaptainCodeman]

Describe the bug

The normal content negotiation ability to be served html or json data based on the http accept header seems to break if the request comes from __layout.svelte. Instead of the expected content, it returns a 404 response.

Reproduction

example repo

Request root with accept: application/json header and expected data is returned
Request root with browser and error is produced (see log below - status from fetch is 404)

Changing url fetched in __layout to /__data.json OR renaming index.svelte "fixes" it (of course, you likely want a home page so the latter isn't really a fix, just thought it may help identify the cause).

Using __data.json works so it's not a show-stopper, but in case someone else comes across it this will hopefully be findable.

Logs

Unexpected token N in JSON at position 0
SyntaxError: Unexpected token N in JSON at position 0
    at JSON.parse (<anonymous>)
    at Proxy.<anonymous> (file:///Users/simon/dev/open-source/sveltekit-shadow-layout/.svelte-kit/runtime/server/index.js:2559:21)
    at async load (__layout.svelte:9:14)
    at async load_node (file:///Users/simon/dev/open-source/sveltekit-shadow-layout/.svelte-kit/runtime/server/index.js:2585:22)
    at async respond$1 (file:///Users/simon/dev/open-source/sveltekit-shadow-layout/.svelte-kit/runtime/server/index.js:2986:15)
    at async resolve (file:///Users/simon/dev/open-source/sveltekit-shadow-layout/.svelte-kit/runtime/server/index.js:3456:11)
    at async respond (file:///Users/simon/dev/open-source/sveltekit-shadow-layout/.svelte-kit/runtime/server/index.js:3392:20)
    at async file:///Users/simon/dev/open-source/sveltekit-shadow-layout/node_modules/.pnpm/@[email protected][email protected][email protected]/node_modules/@sveltejs/kit/dist/vite.js:1555:22

System Info

System:
    OS: macOS 12.4
    CPU: (6) x64 Intel(R) Core(TM) i5-8500B CPU @ 3.00GHz
    Memory: 29.02 MB / 32.00 GB
    Shell: 5.8.1 - /bin/zsh
  Binaries:
    Node: 16.14.0 - /usr/local/bin/node
    npm: 8.3.1 - /usr/local/bin/npm
  Browsers:
    Brave Browser: 102.1.39.111
    Chrome: 103.0.5060.134
    Chrome Canary: 106.0.5204.0
    Edge: 103.0.1264.71
    Firefox: 102.0.1
    Safari: 15.5
    Safari Technology Preview: 16.0
  npmPackages:
    @sveltejs/adapter-auto: next => 1.0.0-next.64 
    @sveltejs/kit: next => 1.0.0-next.396 
    svelte: ^3.44.0 => 3.49.0 
    vite: ^3.0.0 => 3.0.3

Severity

annoyance

Additional Information

The scenario is having the root endpoint be the data for the site, in a SaaS setup where the sites are served based on host name.

[mustofa-id]

it seem json parse is fail in ssr, try to check response status before returning value:

if (resp.ok) {
...

don't forget load is runs both during ssr and in the client.

[CaptainCodeman]

The point is it shouldn't fail, it should work. It works when calling the same endpoint from other places, or by calling the different (unnormalized? non-canonical?) form (__data.json). Just not calling '/' from __layout.svelte

The reason JSON parse fails is because the response is a 404, which is what's wrong. It should get the JSON data. The exact same code inside index.svelte works, so it's a "surprise".

[CaptainCodeman]

endpoints and loading are both going to change, so this is likely going to be obsolete