Caddy: Support HTTP/2 Server Push Link headers

[FiloSottile]

Caddy could send Link headers to use both browser prefetching and CloudFlare server push https://blog.cloudflare.com/announcing-support-for-http-2-server-push-2/

[wmark]

Could unconditional server push (indication?) become a problem, as in: browsers ignoring any pushes from hosts that they've learned use _push_ frivolously (repeatedly pushing what's already in cache)?

[FiloSottile]

There's no point in blindly ignoring something the browser already received, and the browser can kill push attempts early when it notices an unfulfilled promise that it does not need.

[Penagwin]

At least for telling cloudflare to server push multiple files, you currently can't.
See: https://github.com/mholt/caddy/issues/818

[brodock]

I was reading an article somewhere and they said Apache and AppEngine are using Link Header (https://tools.ietf.org/html/rfc5988) to intercept from your App Server and do the HTTP/2 server push, so this seems to became the default standard for that.

[KyleAMathews]

Something useful for static sites is a "push manifest". This is what GAE supports apparently https://www.npmjs.com/package/http2-push-manifest

[mholt]

This is now implemented, #1215!