nixos/caddy: add support for Caddy v2

[Br1ght0ne]

Motivation for this change

Caddy v2 was recently released.
While v2 support is necessary, configs for v1 shouldn't break (at least until 20.09).

Things done

This PR adds support for Caddy v2, and uses it by default, while still allowing users to opt-in to using Caddy v1.

Added services.caddy.adapter for supporting config formats other than Caddyfile.
See https://caddyserver.com/docs/config-adapters.

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.

[Br1ght0ne]

@GrahamcOfBorg test caddy

[Br1ght0ne]

@xfix I removed the commit adding --validate and rebased onto latest master.

[sugar700]

I don't think we actually want caddy validate as I mentioned earlier (mostly because it verifies paths exist, and it's reasonable that they may not exist in build environment due to being outside nix store).

Suggested change

# TODO: validate with `caddy validate`?

[curbengh]

to add on, adapt validates the input config (check its syntax) by default and will return error 1 when encounter invalid input.

[sugar700]

This looks fine, it's mergeable after the TODO comment gets deleted.

[AluisioASG]

ExecReload has to be updated as well:

Signals are mostly the same, except USR1 and USR2 are no longer supported. Use the caddy reload command or the API instead to load new configuration.

[k2s]

There are 2 official upstream systemd service files with explanation at https://github.com/caddyserver/dist/tree/master/init.

In compare to v1 the Caddy v2 starts without configuration file and best practice in v2 is to use API to upload config. JSON is native format, but with simple curl POST it is possible to upload regular Caddyfile too. Caddy reloads last state after restart. All that is working with https://github.com/caddyserver/dist/blob/master/init/caddy-api.service.

In my opinion that this should be default in NixOS. The configuration file should be added to command line arguments only if additional parameters related to configuration source and provider are specified.

[sephii]

It would be really nice to have this in 20.09. :) can I help in some way?

[sugar700]

@filalex77 What is the status of this pull request? I really would like to have this in NixOS 20.09.

My opinion is that we don't need Caddy API support for 20.09. It could be implemented in the future, but I don't think it should block Caddy v2 support.

[sugar700]

ExecReload = "${pkgs.coreutils}/bin/kill -USR1 $MAINPID"; should be changed to use caddy reload --config ${path} instead for Caddy 2 as USR1 signal is no longer supported.

[sephii]

I've opened a new PR which addresses the remaining comments in the hope of getting this in 20.09. I've also rebased on current master. I still have problems with running tests though, any help is appreciated.

[Br1ght0ne]

Closed in favor of #97217 by @sephii.