nixos/dhcpcd: optimize exitHook

[Izorkin]

Description of changes

Small optimize exit hook.
Fixed this message log:

dhcpcd[149086]: Failed to reload-or-try-restart ntpd.service: Unit ntpd.service not found.
dhcpcd[149086]: Failed to reload-or-try-restart openntpd.service: Unit openntpd.service not found.
dhcpcd[149086]: Failed to reload-or-try-restart chronyd.service: Unit chronyd.service not found.

cc @SuperSandro2000 @ncfavier @blitz @flokli

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandbox = true set in nix.conf? (See Nix manual)
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 23.05 Release Notes (or backporting 22.11 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
  • (Release notes changes) Ran nixos/doc/manual/md-to-db.sh to update generated release notes
• Fits CONTRIBUTING.md.

[rnhmjoj]

Uhm, apparently it's a syntax error having an empty then clause:

client # [   13.485210] dhcpcd[614]: /etc/dhcpcd.exit-hook: line 10: syntax error near unexpected token `fi'
client # [   13.498172] dhcpcd[614]: /etc/dhcpcd.exit-hook: line 10: `fi'

The easy way out is to put a noop : before the optionalStrings.

[rnhmjoj]

An alternative could be something like this:

let daemons = with config.services;
     lib.optional chrony.enable "chrony"
  ++ lib.optional ntp.enable "ntpd"
  ++ lib.optional openntpd.enable "openntpd";
in
pkgs.writeText "dhcpcd.exit-hook" ''
  if [ "$reason" = BOUND -o "$reason" = REBOOT ]; then
    # Restart ntpd. We need to restart it to make sure that it will actually do something:
    # if ntpd cannot resolve the server hostnames in its config file, then it will never do
    # anything ever again ("couldn't resolve ..., giving up on it"), so we silently lose
    # time synchronisation. This also applies to openntpd.
    for daemon in ${toString daemons}; do
      systemctl try-reload-or-restart "$daemon".service
    done
  fi

  ${cfg.runHook}
''

[Izorkin]

Returned a working variant.

[rnhmjoj]

The last two checks are still redundant, but it works now. So, it's fine by me.

[blitz]

I think we went full circle to the redundant checks again. What happened?

[Izorkin]

I think we went full circle to the redundant checks again. What happened?

#209506 (comment)

[SuperSandro2000]

I know to little about dhcpd and how it interacts with the other services to know if we should merge this or not.

[Izorkin]

Rebased PR.

[nixos-discourse]

This pull request has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/prs-ready-for-review/3032/1993

[Izorkin]

Rebased PR.

[Izorkin]

It is possible to include this PR in the 23.05 release?

[Izorkin]

Rebased PR.