Fail prerendering if a page is shadowed by an endpoint

[alexbjorlig]

Describe the bug

If you create a bob/index with a POST and bob/index.svelte --> posting will not work when deployed to Vercel.

Vercel throws error 405.

Reproduction

Deploy this repo to Vercel, and visit /bob and hit the submit button - and then the error will appear.

As long as it exists 😅, you can also try it here: https://sveltekit-p9hxntt45-jeddah.vercel.app/bob

Logs

No response

System Info

Latest version `next` of kit and vercel adapter.

Severity

serious, but I can work around it

Additional Information

I can simply work around by not enabling prerender=true on the sveltekit component.

Let me know if you need more information 😎

[Rich-Harris]

This is expected behaviour — prerendered pages always take priority over dynamic endpoints. The reason for that is that in most environments (Vercel included) there isn't even an opportunity to run SvelteKit if a matching file exists on the filesystem. The 405 you're seeing is coming from Vercel itself, which sees a request for /bob, sees that there's a matching bob/index.html file already prerendered, then says 'hang on a minute, you can't POST to a prerendered file'.

So you'll always need to disable prerendering for these pages. We could probably do a better job of diagnosing this at build time — if a page is shadowed by an endpoint, prerendering should fail. (When we implement #3532, we can change that to 'if a page is shadowed by an endpoint with non-GET handlers'.)

[alexbjorlig]

Hi @Rich-Harris - thanks for the detailed update, makes total sense. At least a got a decent error message from Vercel (the 405). When I deployed to Cloudflare (when I had no idea what caused the issue), I simply got no response at all 😱

Would be great with a simple check that caused the build/serve to fail.

[elliott-with-the-longest-name-on-github]

@benmccann

Can also take this one if you give me the go-ahead.

[elliott-with-the-longest-name-on-github]

@Rich-Harris

I was looking into this one, and I think you can just close it.

In the prerendering/basics test project, there's already a test verifying this behavior. The only reason the project can even build is because continueOnError === true for the test project. If it weren't, you'd see the error 'Cannot prerender pages that have endpoints with mutative methods' thrown from this line and build would fail.

So I think this one's a freebie. :)

[elliott-with-the-longest-name-on-github]

Here's a repo proving the point.

[elliott-with-the-longest-name-on-github]

Looking into it a bit further, the only case in which this doesn't work is when running in dev (because the current condition only checks state.prerender). If we wanted it to throw when running the dev server, we could change this line to node.module.prerender ?? state?.prerender?.default ?? false. prerender isn't used anywhere else inside of load_shadow_data, so it won't cause any unexpected behavior. All tests still pass (I checked). If we want to make that change, I'm happy to open a PR.

Personally, I would advocate for that change, as I hate getting slapped in the face by buildtime errors when they could've been caught during dev runtime.