feat: add server endpoint fallback handler

[eltigerchino]

closes #9164

Adds a catch-all handler that handles all unimplemented valid server requests. This is done by exporting the fallback function from a +server.js file.

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[Conduitry]

Have we thought about the effects this might have on https://kit.svelte.dev/docs/routing#server-content-negotiation ? I haven't thought about that all the way through yet. Also, if we decided (as in #9780) to start adding Vary: Accept to responses for requests with sibling GET/POST endpoints and pages, would this make that more difficult?

[eltigerchino]

Have we thought about the effects this might have on kit.svelte.dev/docs/routing#server-content-negotiation ?

Content negotiation is still in play here, so the accept: text/html always connects the page.

Also, if we decided (as in #9780) to start adding Vary: Accept to responses for requests with sibling GET/POST endpoints and pages, would this make that more difficult?

I'm not sure myself. Will need to start reading the MDN docs on what Vary: Accept is.

[eltigerchino]

Also, if we decided (as in #9780) to start adding Vary: Accept to responses for requests with sibling GET/POST endpoints and pages, would this make that more difficult?

I'm not sure myself. Will need to start reading the MDN docs on what Vary: Accept is.

After reading MDN, it seems we would only need to return Vary: Accept, right? If so, it shouldn't impact this PR.

The server uses the Vary header to indicate which headers it actually used for content negotiation (or more precisely, the associated request headers), so that caches can work optimally.

https://developer.mozilla.org/en-US/docs/Web/HTTP/Content_negotiation#server-driven_content_negotiation

[benmccann]

@s3812497 it looks like this PR will need a rebase

[dominikg]

all is still misleading or ambiguous. If you have POST and all, which one is handling an incoming post request? Isn't clear from the word all, which implies all requests are handled by it.

[ghostdevv]

all is still misleading or ambiguous. If you have POST and all, which one is handling an incoming post request? Isn't clear from the word all, which implies all requests are handled by it.

I think it's probably fine, esp since the way people would learn about it is via docs which can explain it - however there could still be better words out there for it?

[karimfromjordan]

Other name suggestions were:

• fallback
• other
• using the default export

[eltigerchino]

Name suggestion: any - matches any request method not already exported.

[gtm-nayan]

I like fallback more for your description.

[matthewrobb]

Why not just support this?

/** @type {import('@sveltejs/kit').[Handle](https://kit.svelte.dev/docs/types#public-types-handle)} */
export async function handle({ event, resolve }) {
    if (event.url.pathname.startsWith('/custom')) {
        return new Response('custom response');
    }

    const response = await resolve(event);
    return response;
}

[eltigerchino]

Why not just support this?

/** @type {import('@sveltejs/kit').[Handle](https://kit.svelte.dev/docs/types#public-types-handle)} */
export async function handle({ event, resolve }) {
    if (event.url.pathname.startsWith('/custom')) {
        return new Response('custom response');
    }

    const response = await resolve(event);
    return response;
}

Handling this case in the handle hook is already supported but would be made easier by this PR (directly exporting the fallback in the server endpoint and not cluttering the handle hook).

[eltigerchino]

The export all handler has been renamed to fallback.

[matthewrobb]

Why not just support this?

/** @type {import('@sveltejs/kit').[Handle](https://kit.svelte.dev/docs/types#public-types-handle)} */
export async function handle({ event, resolve }) {
    if (event.url.pathname.startsWith('/custom')) {
        return new Response('custom response');
    }

    const response = await resolve(event);
    return response;
}

Handling this case in the handle hook is already supported but would be made easier by this PR (directly exporting the fallback in the server endpoint and not cluttering the handle hook).

Right, no my suggestion was to allow +server.js to export a handler hook that is only hit within that route scope. It can be used like an all OR a fallback OR an after etc. without any ambiguity compared to the other exports in that module.

Another benefit would be scenarios where you have a common resolution you want to do once up front for a whole routing scope, almost like what you get with layouts in view routes.

[eltigerchino]

Right, no my suggestion was to allow +server.js to export a handler hook that is only hit within that route scope. It can be used like an all OR a fallback OR an after etc. without any ambiguity compared to the other exports in that module.

Sorry, I misunderstood. I'd definitely be in favour of subdirectory handle hooks replacing this as also mentioned by Ben

[matthewrobb]

Right, no my suggestion was to allow +server.js to export a handler hook that is only hit within that route scope. It can be used like an all OR a fallback OR an after etc. without any ambiguity compared to the other exports in that module.

Sorry, I misunderstood. I'd definitely be in favour of subdirectory handle hooks replacing this as also mentioned by Ben

Ah I missed that earlier comment. I also don't see any replies to it. Is there some other location that subdirectory hooks has/is being discussed?

[matthewrobb]

Honestly after thinking about it a bit I think there's certainly room for both a fallback and subdirectory hooks. 👍 this PR.

[eltigerchino]

Ah I missed that earlier comment. I also don't see any replies to it. Is there some other location that subdirectory hooks has/is being discussed?

There doesn't seem to be from what I've seen (probably just talked about internally?). You may open an issue for it.

[karimfromjordan]

Astro 3.0 is moving to capitalized function names too. I forwarded some feedback from here and they may rename the ALL function to fallback in which case both Astro and SvelteKit would share the same API (if this PR gets merged).

[karimfromjordan]

Can this be... merged?

[eltigerchino]

I'm still not sure if we should go ahead with this if we're planning to implement per directory handle hooks. This is something @dominikg might be looking into soon.

[dominikg]

fallback is different from hooks as it would only be called if no other handler is defined. hooks would always be called. so i think both can make sense even if you could be able to implement fallback with a hook, it may be complicated to figure out which handlers are defined.