[frameworks] Fix astro default routes since it is not SPA

[lucleray]

The default routes for Astro are redirecting all non-existing files to index.html, which means that the 404.html page is not used, and instead index.html is shown for all not found pages.

Astro outputs files for each page (ie. about.html, blog.html, ...) so the {handle: 'filesystem'} route should be enough to route all existing pages correctly.

The missing part to ship this fix is to answer the following question: can we safely assume that Astro will always output a 404.html file?

Internal ref: https://github.com/vercel/customer-issues/issues/638

[okikio]

We need a way to have the redirects work but still have the 404 pages work, ideally leaving it to Astro to decide. Yesterday I did some research into it, and the problem is much larger in scope on the Astro side, I'll see if I can get a possible fix out, but if not this might be the only solution we have for now.

[EndangeredMassa]

@okikio @lucleray what's the path forward on this work?

[lucleray]

I'm going to close it for now. If it still makes sense to ship it, we can reopen.

[delucis]

Hi everyone, Astro core maintainer here 👋

I was about to make a PR more or less identical to this and thought it might be best to discuss here first — maybe we can re-open.

We currently document a weird hack in the Astro Vercel deploy guide, where setting "cleanUrls": true in vercel.json makes custom 404 pages display for static builds, but I’ve never understood why that would work. Would be cool if that wasn’t needed.

There seems to be a bit of confusion between “SPA” mode and “SSR” in the discussion above, Astro doesn’t have an SPA-mode where you would route everything to /index.html and let client-side routing take over, but does support SSR via its Vercel adapter.

In SSR mode, Astro’s adapter outputs routing configuration so I believe this defaultRoutes is ignored. In static mode, Astro builds an HTML file per page, so the filesystem routing should work, we just need to handle the special 404 case.

The missing part to ship this fix is to answer the following question: can we safely assume that Astro will always output a 404.html file?

The answer here is no: if a user does not create a src/pages/404.astro file, there won’t be a dist/404.html to route to. I assume in this case Vercel displays a generic 404 page?

[styfle]

I tried the example from the original PR #7747 as well as the latest example and both are SSG style, so I don't know why routes were added to begin with.

I discussed with @TooTallNate and we found that removing these routes (and fixing the asset header) would make astro work perfectly, thanks!

fixed