caddyhttp: Implement placeholders in templates

[bagbag]

(Feature request out of https://caddy.community/t/use-placeholders-in-templates/7697)

I would like to see the ability of using placeholders like {http.error.status_code} in templates. This would allow us, for example, to create a single custom error page in nginx style. For now we have to create a page for every status code we would like to support (maybe using respond with include and arguments would work too, but I haven't tested that).

Instead of adding an extra layer of complexity to substitute placeholders in templates, my suggestion is to either implement a new template function like {{.Placerholder "http.error.status_code"}} (with a better name than .Placeholder) or if variables in Go templates work the way I guess they work, populate them (if that makes sense, I have no idea about Go templates yet). But I think a function is the better solution.

When this is implemented, caddys template functions .RemoteIP and .Host could be deprecated and later removed, as the placeholders includes them with http.request.remote.host and http.request.host.

I guess this feature would fit for caddy v2.1. I have no idea what the release schedule for caddy looks like, but if it takes a few days or more from now until the release of 2.1, I would like to try to implement this feature, as it is probably quite easy to implement and therefore a good candidate for me to get started with Go and contribute to this great project (hints welcomed).

[francislavoie]

When this is implemented, caddys template functions .RemoteIP and .Host could be deprecated and later removed

I don't think this is necessary, shorter/easier APIs are beneficial.

I guess this feature would fit for caddy v2.1. I have no idea what the release schedule for caddys looks like, but if it takes a few days or more from now until the release of 2.1, I would like to try to implement this feature, as it is probably quite easy to implement and therefore a good candidate for me to get started with Go and contribute to this great project (hints welcomed).

v2.1 is still a while away, v2.0 stable isn't even out yet! But yes, this would be the right version to target for this change.

The files you're looking for are in /modules/caddyhttp/templates/ 👍 Good luck!

[mholt]

I assume you mean .Req.RemoteIP and .Req.Host?

We can't really get rid of those since they're on the standard lib's http.Request struct.

We could expose a way to access replacer values from the templates instead; it just involves getting a context value out of the request, type-asserting it, then calling Get() on the Replacer type.

[francislavoie]

No he means .RemoteIP and .Host as documented here: https://caddyserver.com/docs/modules/http.handlers.templates

[mholt]

Oh, I think that's pretty old docs. It'll be updated at 2.0.

[mholt]

My bad -- those are actually different.