Add a configuration option to disable transforming fetch request into direct function calls

[aloussase]

Describe the problem

As described here, fetch calls made during SSR can be replaced by function calls when the origin is the same as the application. This is good in many cases, but causes problems when the application is running behind a reverse-proxy and you are making an API call to another service proxied under a different path, but the same domain.

For example, I might have the following:

example.com/trivia-api -> backend api
example.com/trivia     -> svelte kit application

Requests in this case should go through the proxy in order to resolve correctly, instead of being transformed into function calls.

Describe the proposed solution

A configuration option to turn off this behaviour.

Alternatives considered

No response

Importance

would make my life easier

Additional Information

No response

[Conduitry]

See #10163/#10170. It was for this precise use case - of being behind a proxy - that I implemented and am using this feature.

[aloussase]

But how would that help me? I need to prevent the subRequest in the first place.

[Conduitry]

You don't need to prevent it, you just need to send a real request when your handler hook gets called with a sub-request, as described in the issue.

[aloussase]

You mean something like this?

	if (
		event.isSubRequest
	) {
		return event.fetch(new Request(event.request.url, event.request));
	}

[Conduitry]

Yes. You can even do return fetch(event.request) if you want to use the same URL - you don't need to create a new Request object.

[aloussase]

This doesn't seem to be working correctly. The application is still trying to invoke a function, which is causing my app to run out of memory.

With this code

if (event.isSubRequest) {
	return event.fetch(event.request);
}

The request enters the condition after the fetch, which makes me think isSubRequest is true for the second request as well.

[aloussase]

Ended up having to do it like this:

if (event.isSubRequest) {
	console.log('subRequest detected, making manual call');
	return await fetch(event.request.url, {
		method: event.request.method,
		headers: event.request.headers,
		credentials: 'include',
		body: await event.request.text()
	});
}

Honestly, it doesn't make sense to me why this would be a default with no option to opt out. It's not expected functionality at all.