Fetch - document known differences from the spec

[Aprillion]

As of 2022-02-20, https://developers.cloudflare.com/workers/runtime-apis/fetch only points to MDN and does not mention any differences from the fetch specification.

I found 2 issues so far (while working on https://github.com/Aprillion/stampy-ui/commits/master), both worked fine in miniflare (which uses undici), but not after deployment to CF workers:

• cache options in the init parameter are not supported (originally reported in Warning about unsupported fetch options remix-run/remix#2015):

  await fetch(..., {cache: 'force-cache'})

• newlines are not removed from URL as per https://url.spec.whatwg.org/#concept-basic-url-parser

  (await fetch(' \n\n\nhttps://\t\texa\nmple\n.com\n ')).url  // should be https://example.com

Please let me know if these are bugs to be fixed (and if so, where can I report more issues in the future, please), or if these differences happen on purpose and should be documented (if so, I can work on a PR, but let me also know whether to open a sister issue in miniflare to make sure our local dev environment will match all these fetch quirks, debugging these differences in production was not ideal TBH)...

[Aprillion]

This sounds related: https://developers.cloudflare.com/workers/platform/compatibility-dates#new-url-parser-implementation, but it's not enabled by default (and not sure if it affects the first fetch parameter too), so the default DX might still be confusing (something works locally and breaks after deployment => it's open to hard to debug issues non-reproducible locally).

[hansede]

This is related: cloudflare/miniflare#183
Undici's fetch removes set-cookie headers from the response, but Cloudflare's doesn't.

[hansede]

Another way in which Undici's fetch behaves differently from Cloudflare's fetch is that Cloudflare adds the X-Forwarded-For header, but Undici does not.

[jasnell]

Hey all, yeah there are a couple of places where the Workers fetch() does diverge from the standard. The goal will be to close those gaps fairly soon (no specific timeline yet but they're on my roadmap).

[jkup]

Hey @Aprillion! Thanks for reporting this. For context, @jasnell has been leading the charge on identifying and fixing these divergences. As he mentioned, we don't have a specific timeline for getting fetch() to 100% compliance but it's definitely on our list and we should be getting to it soon! Thanks again for reporting.