fix: handle relative assets paths in server fetch correctly

[mastermakrela]

The Problem

Fixes #12081 (and maybe helps with #3850).

TL;DR:

// +server.ts
import asset from "$lib/image.png";

export const GET = async ({ fetch }) => { return fetch(asset); };

works in dev and preview, but always returns 404 with adapter-cloudflare and adapter-node.

More details and exhaustive reproduction in #12081 and here

The Solution

I've extended all places that check manifest.assets to also check manifest._.server_assets.
This has two benefits:

1. the adapters can return the file directly without calling the SvelteKit server
2. server_fetch (in packages/kit/src/runtime/server/fetch.js) can return the file earlier (previously it fell through to response)

This should always work because since #11649 all server assets should be copied to the output folder (source).

Other improvements

If read implementation is available, server_fetch will use it to get the files directly — should be theoretically faster than a fetch.

This PR is similar to what #3850 proposed, but because state.read/options.read expects a function that returns a buffer and read_implementation returns a stream, they aren't compatible. Also, it still wouldn't work with Cloudflare, because there the “file read” is an async fetch.

But this PR brings us as close as possible to #3850 as currently possible, because:

• node will use read — skips overhead of fetch
• Cloudflare will fetch static asset directly

and both of them (and maybe more adapters that I didn't test) will work with relative paths on the server.

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🦋 Changeset detected

Latest commit: e01efa1

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 4 packages

Name	Type
@sveltejs/adapter-cloudflare-workers	Patch
@sveltejs/adapter-cloudflare	Patch
@sveltejs/adapter-netlify	Patch
@sveltejs/kit	Patch

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[mastermakrela]

@eltigerchino thank you for the review :)

I've corrected all the issues you've noted and tested the Cloudflare adapter again.

I see some tests in CI have failed, but as far as I understand, two of them should fail (Vercel ones)
and the third one (/prerendering/mutative-endpoint) does not seam to be connected with my changes.

Is there anything else I could/should do?

[teemingc]

Thank you! This is a really good contribution. Don't worry about some of the tests which are flaky and pass when re-run.

cc: @dummdidumm do you mind taking another look at this?

[dummdidumm]

Fantastic work, and great PR description, was really easy to review because of that 👍