breaking: generate plaintext 404.html for Cloudflare Pages, instead of SPA-style fallback

[Rich-Harris]

Alternative to #9762. Closes #9386. Instead of generating an SPA-style fallback page, we render a plaintext Not Found page, which is sufficient for the case where a request is made for a missing asset that is covered by an exclude rule. For all other requests, we will hit the worker, which will generate the correct 404 page.

Today:

• /missing
• /_app/immutable/missing

With this PR:

• /missing (same as before)
• /_app/immutable/missing (plaintext 404)

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Latest commit: 97aaa63

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[teemingc]

Looks good! The issue has always been because of the wildcard exclude of our immutable assets and this solves that (albeit we don’t get the nice looking error page but it’s unlikely someone navigates to /_app/* anyway).

EDIT: For those using wildcards for their page routes, fallback should be opt-in as mentioned in #9762 (comment)

If someone was messing around with the _routes file they should be prepared to deal with the whole “generate fallback” and “if building” shenanigans anyway

[RowanAldean]

Looks functional, not ideal. Seems safe to merge.

Opt-in for fallback generation

[benmccann]

maybe a little weird to have one path that behaves differently, but really shouldn't matter. I'm fine with this if it helps simplify things