fix: correctly serve ISR pages with trailing slash set to 'always'

[teemingc]

Fixed #12556

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[pkg-svelte-dev]

Install the latest version of @sveltejs/kit from 8ab1c3e:

pnpm add https://pkg.svelte.dev/@sveltejs/kit/c/8ab1c3eafee04b2d06867377a7805e828e4754e0

Open in pkg.svelte.dev: https://pkg.svelte.dev/repos/kit/pr/16903

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Preview: https://svelte-dev-git-preview-kit-16903-svelte.vercel.app/

[teemingc]

@Nic-Polumeyv can you explain how the fix works? What happens to ISR routes with dynamic segments? Are they disallowed now or do they just still work?

[Nic-Polumeyv]

No problem, took me a couple hours to figure out!

Vercel resolves a request in three steps (first match wins):

1. routes listed before `handle: filesystem`
2. filesystem: a file or function at exactly this path? (/foo and /foo/ both count)
3. routes listed after `handle: filesystem`

adapter-vercel put every ISR src route in step 3. Only a src route attaches __pathname; a filesystem hit passes nothing, so the function only ever sees its own file path.

/isr/[slug] never gets caught by step 2. Its function is the file isr/[slug].func and no request has that path, so it falls through to step 3 and the src route captures the URL. /isr-trailing-slash does get caught by step 2, because its function is the file isr-trailing-slash.func, which is its URL:

GET /isr-trailing-slash/
  step 2: isr-trailing-slash.func exists → run it, no __pathname
  function sees its own path: /isr-trailing-slash (slash lost)
  kit: trailingSlash 'always', no slash → 308 to /isr-trailing-slash/
GET /isr-trailing-slash/
  same again

The (/)?, %2F and (/?) commits changed the src regex for /isr-trailing-slash in config.json. That regex is in step 3. Vercel never ran it, because step 2 had already served the request.

The fix puts ISR routes with no dynamic segments in step 1. Dynamic ones stay in step 3, since a [...rest] ISR route in step 1 would swallow every static asset.

Separately, dest no longer rebuilds the pathname from route segments. The src regex is wrapped in one capture group and dest is ?__pathname=$1, so the function gets the path as requested. That is what deletes get_pathname and the // cleanup in the wrapper.

Dynamic ISR routes still work, nothing about their placement changed. The one difference is /isr/alpha/ now 308s to /isr/alpha instead of being served, same as the other adapters.

[Rich-Harris]

brilliant!