nixos/systemd: disable systemd-gpt-auto-generator by default

[Artturin]

it is not declarative and breaks our declarative configuration sometimes

See: #4002

Motivation for this change

Closes #4002

Things done

tested in a vm

{
  inputs = {
    #nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
    nixpkgs.url = "/home/artturin/nixgits/my-nixpkgs";
  };

  outputs = inputs@{ self, nixpkgs }: {

    nixosConfigurations.vm = nixpkgs.lib.nixosSystem {
      system = "x86_64-linux";
      specialArgs = { inherit inputs; };
      modules = [
        ({ pkgs, lib, config, ... }: {
          # try without the comment and with
          #systemd.generators = { systemd-gpt-auto-generator = "${config.systemd.package}/lib/systemd/system-generators/systemd-gpt-auto-generator"; };
        })

        ({ pkgs, lib, config, modulesPath, ... }: {
          documentation.enable = false;
          users.mutableUsers = false;
          users.users.root = {
            password = "root";
          };
          users.users.user = {
            password = "user";
            isNormalUser = true;
            extraGroups = [ "wheel" ];
          };

          imports = [
            #(modulesPath + "/profiles/minimal.nix")
          ];
        })
      ];
    };
    # So that we can just run 'nix run' instead of
    # 'nix build ".#nixosConfigurations.vm.config.system.build.vm" && ./result/bin/run-nixos-vm'
    defaultPackage.x86_64-linux = self.nixosConfigurations.vm.config.system.build.vm;
    defaultApp.x86_64-linux = {
      type = "app";
      program = "${self.defaultPackage.x86_64-linux}/bin/run-nixos-vm";
    };
  };
}

tested on my system

  disabledModules = [ "system/boot/systemd.nix" ];
  
  imports = let
    pkgsReview = builtins.fetchTarball {
      url = "https://github.com/artturin/nixpkgs/archive/systemd-gpt-autogen-disable.tar.gz";
    };
  in [
    (import "${pkgsReview}/nixos/modules/system/boot/systemd.nix")
  ];

• 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/)
• 21.11 Release Notes (or backporting 21.05 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
• Fits CONTRIBUTING.md.

[dasJ]

Umm why not expose an option to opt into this very useful feature? Also I'm not really a fan of doing this a week before branchoff

[Artturin]

it can be reenabled with (i put it in the example value)

systemd.generators = { systemd-gpt-auto-generator = "${config.systemd.package}/lib/systemd/system-generators/systemd-gpt-auto-generator"; };

lets wait at least until branchoff before merging

[Artturin]

@dasJ do you or anyone you know use this feature? I think there's most likely no one using this on nixos since it's undeclarative and breaks declarative mounting sometimes

[dasJ]

Just self merge it again, idc really

[Artturin]

setting root= cmdline var disables the root logic
https://www.freedesktop.org/software/systemd/man/systemd-gpt-auto-generator.html#root=
https://github.com/systemd/systemd/blob/master/src/gpt-auto-generator/gpt-auto-generator.c#L822

however that will interact with https://www.freedesktop.org/software/systemd/man/systemd-fstab-generator.html#root=
if the fstab generator is enabled, i also do not know what the root= should be for zfs volumes

so i'd rather disable this for now at least until we use systemd for init

pinging @ElvishJerricco to make him aware of this pr due to #120015