SWR Cache-Control Header Invalid

[mrfrase3]

Environment

N/A (logic goes back to nitro)

Reproduction

https://github.com/unjs/ocache/blob/main/src/http.ts#L156-L181

Describe the bug

Currently SWR attempts to set headers to guide a CDN like Cloudflare to do SWR in its caching layer, it does this by setting s-maxage= and stale-while-revalidate, under the assumption that s-maxage is a CDN-only maxage.

e.g.

{
  swr: true,
  cache: {
    maxAge: 3 * 60 * 60, // 3 hours
    staleMaxAge: 1 * 60 * 60, // 1 hour
  },
}

Produces: Cache-Control: s-maxage=10800, stale-while-revalidate=3600

As-per Cloudflare's documentation, setting s-maxage invalidates the stale-while-revalidate directive as it's against spec, so it is only setting maxage for the cache. (It is also missing the public directive)

CDN Caching directives should use the CDN-Cache-Control header, who then ignore and pass Cache-Control as-is to the browser.

E.g. a correct response would be:

Cache-Control: public, maxage=0, must-revalidate
CDN-Cache-Control: public, max-age=10800, stale-while-revalidate=3600

It might be a good idea to allow exposing the browser cache policy to the developer by adding a browserMaxAge field.

Additional context

https://developers.cloudflare.com/cache/concepts/revalidation/#directives-that-disable-stale-while-revalidate

https://developers.cloudflare.com/cache/concepts/cache-control/

https://developers.cloudflare.com/cache/concepts/cdn-cache-control/

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