caddyfile: add handle_response blocks to reverse_proxy

[maxatome]

Caddyfile support for the handle_response option of reverse_proxy would be nice.

See the JSON configuration here:

https://caddyserver.com/docs/json/apps/http/servers/errors/routes/handle/reverse_proxy/handle_response/

Maybe it could look something like this in the Caddyfile:

reverse_proxy ... {
	...
	handle_response header X-Accel-Redirect {
		... 
	}
	handle_response status 4xx {
		...
	}
	handle_response {
		...
	}
}

(Responses can be matched by headers or status code. Just showing a few examples here.)

Originally posted by @mholt in #3347 (comment)

[maxatome]

@mholt do you have an idea of how to represent "routes" list in handle_response block?

handle_response [header <field>|status <status>] {
    status <status>
    routes?
}

I didn't find an other place where it is UnmarshalCaddyfiled so I prefer to ask before doing it the wrong way...

[francislavoie]

I think it's tricky to have status <status> inside of the block, because it's not a real handler directive (but there are ways around this, which I'll explain below).

But for "routes", take a look at how the handle directive is parsed, i.e. parseHandle -> ParseSegmentAsSubroute:

caddy/caddyconfig/httpcaddyfile/directives.go

Lines 268 to 278 in 24f3478

	// ParseSegmentAsSubroute parses the segment such that its subdirectives
	// are themselves treated as directives, from which a subroute is built
	// and returned.
	func ParseSegmentAsSubroute(h Helper) (caddyhttp.MiddlewareHandler, error) {
	allResults, err := parseSegmentAsConfig(h)
	if err != nil {
	return nil, err
	}
	return buildSubroute(allResults, h.groupCounter)
	}

There are ways around what I said above about status, like reading and deleting tokens before passing them through to ParseSegmentAsSubroute. The php_fastcgi directive uses this approach to allow for its own options intermixed with reverse_proxy directive options, while consuming the php_fastcgi options, and passing through the rest to reverse_proxy. You can get an idea of how that looks here:

caddy/modules/caddyhttp/reverseproxy/fastcgi/caddyfile.go

Lines 154 to 177 in 24f3478

	// read the subdirectives that we allow as overrides to
	// the php_fastcgi shortcut
	// NOTE: we delete the tokens as we go so that the reverse_proxy
	// unmarshal doesn't see these subdirectives which it cannot handle
	for dispenser.Next() {
	for dispenser.NextBlock(0) {
	switch dispenser.Val() {
	case "root":
	if !dispenser.NextArg() {
	return nil, dispenser.ArgErr()
	}
	fcgiTransport.Root = dispenser.Val()
	dispenser.Delete()
	dispenser.Delete()
	case "split":
	extensions = dispenser.RemainingArgs()
	dispenser.Delete()
	for range extensions {
	dispenser.Delete()
	}
	if len(extensions) == 0 {
	return nil, dispenser.ArgErr()
	}

That approach is super tedious and finnicky though, so I don't super recommend it. But it could work if handle_response must support other options than the ones that exist in other directives.

[maxatome]

Thanks @francislavoie, what do you think of #3712?
I am not sure I did it the right way, so tell me...

[mholt]

Done in #4021.