Add HTTP 103 Early Hints support

[otbutz]

RFC 8297 is only a draft, but is now supported by Chrome 103:

https://caniuse.com/mdn-http_status_103

and Mozilla also intends to support it:

https://bugzilla.mozilla.org/show_bug.cgi?id=1407355

Apache already implements it: https://httpd.apache.org/docs/2.4/howto/http2.html#earlyhints

It would be nice if caddy would add (experimental) support for it. Following the RFC recommendations, this should only be added for HTTP/2 and HTTP/3:

Therefore, a server might refrain from sending 103 (Early Hints) responses over HTTP/1.1 unless the client is known to handle informational responses correctly.

[otbutz]

The very first step would be to make sure that caddy processes HTTP 103 properly while acting as a reverse proxy. That may already be the case.

[francislavoie]

@dunglas has been working on adding support for early hints to the Go stdlib, I'm sure he'll follow up soon to make sure Caddy supports it sufficiently.

[dunglas]

Indeed it's on my todo list. We'll have to wait for Go 1.19 first.

[bt90]

@dunglas does this also add support for HTTP3 based connections?

[dunglas]

@bt90 yes! I also patched the HTTP/3 library: quic-go/quic-go#3047

[pc-erin]

Is there any documentation on how use this feature now that it's implemented?

If we do respond 103 does that send hints but still maintain the connection for the main response?

[mholt]

@pc-erin HTTP 103 is for applications to emit. The only "application" Caddy really offers (at least, built-in) is the static file server; I have been considering building a feature that will parse HTML and send 103 responses accordingly (like for CSS, JS, and image resources), but maybe at a future date.

For now, this relays 103s from proxy backends down to the client.

What's your use case?

[bt90]

Using it in a reverse proxy scenario might also be neat. Add a matcher for the main page and tell the clients to also load files X, Y and Z.

[pc-erin]

I was working on a PHP application (which apparently doesn't support 103) and also buffers the full output, so no streaming the html head down first.

To mitigate this I thought it would be convenient to manually put some 103 hints in path handlers so that browsers could start downloading resources while PHP is building the response.

Something like:

{
  respond 103 {
    hint </css/common-styles.css>; rel=preload; as=style; nopush
    hint </js/common-scripts.js>; rel=preload; as=script; nopush
  }

  handle /page1 {
    respond 103 {
      hint </css/page1-styles.css>; rel=preload; as=style; nopush
      hint </js/page1-scripts.js>; rel=preload; as=script; nopush
    }
  }

  php_fastcgi...
}

[mholt]

Proposal opened at #5005