Only serve _app/immutable with immutable cache headers

[Rich-Harris]

Rather than serving everything under /_app with an immutable cache header (including /_app/version.json, which is definitely not immutable), this moves immutable assets into /_app/build so they can easily be given separate treatment.

Fixes #4837.

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🦋 Changeset detected

Latest commit: 257d2fd

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 6 packages

Name	Type
@sveltejs/adapter-cloudflare	Patch
@sveltejs/adapter-cloudflare-workers	Patch
@sveltejs/adapter-netlify	Patch
@sveltejs/adapter-node	Patch
@sveltejs/adapter-vercel	Patch
@sveltejs/kit	Patch

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[bjon]

Previous /_app/version.json files, can already be stuck in CDNs. Maybe we can bust them with ?v2, or something?

[Rich-Harris]

Having ?v2 in the codebase might be a bit unusual. One option for cache-busting would be to change appDir to something else (like _sveltekit or something), though not sure whether that means changing the default or just encouraging people to change it in their own apps.

It should be possible to just purge your CDN cache, though?

[bjon]

It should be possible to just purge your CDN cache, though?

Yes, but i was thinking about users that already downloaded the version.json.
But maybe that's not a big issue.

[Rich-Harris]

For users it's not an issue — the request is issued with a no-cache header. It's only CDNs that don't forward no-cache requests to the origin server that are a problem, AFAICT

[bjon]

Yes, you're right! At least in Chrome, its works today, without a CDN.

[benmccann]

I think you'll also need to update this line:

kit/packages/adapter-netlify/index.js

Line 67 in 4592b76

`\n\n/${builder.config.kit.appDir}/*\n cache-control: public\n cache-control: immutable\n cache-control: max-age=31536000\n`

[benmccann]

I wonder if build is the best name for this new directory because it seems like it'll be weird to have build/_app/build. How about immutable? That would also hopefully help in keeping anyone from accidentally sticking a mutable file there

[Rich-Harris]

Yeah, I like immutable. Will update

[benmccann]

lgtm. though I mainly reviewed the netlify and node adapters since I'm most familiar with them. might be good to get eyes on the cloudflare adapters as well

[Rich-Harris]

I took another run at the cloudflare adapters and noticed a bug — now fixed