404 and layout load fetch to prerendered endpoint returns internal server error

[teemingc]

Describe the bug

Both kit.svelte.dev and svelte.dev are not showing the 404 page correctly.
e.g., https://kit.svelte.dev/abc CC: PuruVJ

EDIT: See #10556 (comment)

Reproduction

https://github.com/gtm-nayan/kit-sandbox/tree/prerendered-endpoint-repro

1. Build the site.
2. Run pnpm preview
3. Navigate to a missing route e.g., /abc.

Expected: The +error.svelte page is rendered with a 404 status.

Actual: The fallback error.html page is used and the status is 500.

Logs

No response

System Info

n/a

Severity

annoyance

Additional Information

No response

[Conduitry]

I had thought this was a duplicate, but after searching, I think what I was thinking about was sveltejs/learn.svelte.dev#421 - so, not a duplicate, but definitely one of the underlying causes.

However, I also found #8751, which was not the issue I was looking for. Is this different than that?

[teemingc]

However, I also found #8751, which was not the issue I was looking for. Is this different than that?

I think #8751 strictly occurs for client-side navigation while this one is happening during SSR.

[gtm-nayan]

Doesn't seem specific to the site, it's choking on the fetch('/nav.json') (in the top level layout) with an Internal Server Error when it should probably just be serving the prerendered file.

Minimal repro here:
https://github.com/gtm-nayan/kit-sandbox/tree/prerendered-endpoint-repro

[gtm-nayan]

I think there should be a check for prerendered routes somewhere around here

kit/packages/kit/src/runtime/server/fetch.js

Lines 81 to 98 in 245cdc5

	const is_asset = manifest.assets.has(filename);
	const is_asset_html = manifest.assets.has(filename_html);
	if (is_asset || is_asset_html) {
	const file = is_asset ? filename : filename_html;
	if (state.read) {
	const type = is_asset
	? manifest.mimeTypes[filename.slice(filename.lastIndexOf('.'))]
	: 'text/html';
	return new Response(state.read(file), {
	headers: type ? { 'content-type': type } : {}
	});
	}
	return await fetch(request);
	}

but that list of prerendered stuff is only generated at the adapter level and the internal server doesn't know about them currently.