fetch() does not work properly on the server when base path is set

[thet0ast3r]

Describe the bug

When using sveltekit on the same host as an external api, fetching like this in load functions works -
(using sveltekit's fetch)
fetch('/products/...')
that is, when in svelte.config.js paths.base is not set or set to ''.

If however the base path is set to something other than '', all server side requests (in load functions) to an 'external' path fail, making it impossible to ssr pages with calls to e.g. /api/... from /base/ .

Reproduction

https://github.com/thet0ast3r/kit-fetch-bug

Here is a minimal example.
Please note that this repo proxies requests to /products to dummyjson, to simulate an external api that has the same domain.

run npm run dev

click on 'navigate to someroute'
click on 'navigate to home/base'
reload home '/test' to prompt ssr

in svelte.config.js, change base to '', and repeat the last 3 steps.
Notice how ssr now works correctly.

Logs

No relevant logs. Request returns "Not Found" and 404 on the server side.

This is the response object:

Response {
  [Symbol(realm)]: { settingsObject: {} },
  [Symbol(state)]: {
    aborted: false,
    rangeRequested: false,
    timingAllowPassed: false,
    requestIncludesCredentials: false,
    type: 'default',
    status: 404,
    timingInfo: null,
    cacheState: '',
    statusText: '',
    headersList: HeadersList {
      cookies: null,
      [Symbol(headers map)]: [Map],
      [Symbol(headers map sorted)]: null
    },
    urlList: [],
    body: { stream: undefined, source: [Uint8Array], length: 9 }
  },
  [Symbol(headers)]: HeadersList {
    cookies: null,
    [Symbol(headers map)]: Map(1) { 'content-length' => [Object] },
    [Symbol(headers map sorted)]: null
  }
}

System Info

System:
    OS: macOS 14.1
    CPU: (10) arm64 Apple M1 Pro
    Memory: 56.72 MB / 16.00 GB
    Shell: 5.9 - /bin/zsh
  Binaries:
    Node: 18.8.0 - /usr/local/bin/node
    npm: 8.18.0 - /usr/local/bin/npm
  Browsers:
    Chrome: 119.0.6045.159
    Safari: 17.1
  npmPackages:
    @sveltejs/adapter-auto: ^2.0.0 => 2.1.1 
    @sveltejs/kit: ^1.27.4 => 1.27.6 
    svelte: ^4.0.5 => 4.2.5 
    vite: ^4.4.2 => 4.5.0

Severity

blocking all usage of SvelteKit

Additional Information

No response

[thet0ast3r]

Update:
I think i found where the problem occurs.

In /src/runtime/server/respond.js, line 91:

changing this snippet

	if (base && !state.prerendering?.fallback) {
		if (!decoded.startsWith(base)) {
			return text('Not found', { status: 404 });
		}
		decoded = decoded.slice(base.length) || '/';
	}

to this:

	if (base && !state.prerendering?.fallback) {
		/*
		if (!decoded.startsWith(base)) {
			return text('Not found', { status: 404 });
		}*/
		decoded = decoded.slice(base.length) || '/';
	}

solves the issue.
Ofc, this is not a fix, but the root cause of the problem.
A request lands here, it has a base, but then the request does not start with 'base' so just 'Not found' is returned, even though there might be another app living on that route.

This also explains why this works when base is the empty string: this part is never executed. So the only thing i see that this code accomplishes is it doesn't allow access to routes lower than 'base', which conflicts with the browser behavior.
A sveltekit fetch() in the browser will happily respond with whatever is at a route other than 'base', whereas the server's fetch replication will just respond with 404 'Not found'.

edit:
it probably should look something more like this:

	if (base && !state.prerendering?.fallback) {
		
		if (decoded.startsWith(base)) {
			decoded = decoded.slice(base.length) || '/';
		}
		
	}

[rlopez4]

I'm interested in seeing this fixed