nixos: fix cross-compiling by using buildPackages for build-time lndir

[ju1m]

Motivation for this change

Cross-compiling.

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 via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all packages 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/)
• 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.

[Artturin]

how to test?

[ju1m]

how to test?

@Artturin, if ${host} is the name of a cross-compiled flake configuration enabling services.xserver, then:

nix -L build .#nixosConfigurations.${host}.config.services.xserver.displayManager.sessionData.desktops

[ju1m]

• Added another commit for systemd-lib to also use pkgs.buildPackages.xorg.lndir instead of pkgs.xorg.lndir in a runCommand.

Testing looks like:

nix -L build .#nixosConfigurations.${host}.config.environment.etc.'"systemd/system"'.source

[Artturin]

Could you send a cross compile flake, I haven't used cross compilation with flakes yet

[ju1m]

@Artturin, here's a minimal flake.nix:

{
inputs.nixpkgs.url = "github:NixOS/nixpkgs/4be96dbda035972f60f9e0f386aaec2abd6f1cdb";
outputs = inputs: let
  remoteNixpkgsPatches = [
    {
      meta.description = "nixos: fix cross-compiling by using buildPackages for build-time lndir";
      url = "https://github.com/NixOS/nixpkgs/pull/142273.diff";
      sha256 = "sha256-ZCDQ7SpGhH8JvAwWzdcyrc68RFEWHxxAj0M2+AvEzIg=";
    }
  ];
  originPkgs = inputs.nixpkgs.legacyPackages."x86_64-linux";
  nixpkgs = originPkgs.applyPatches {
    name = "nixpkgs-patched";
    src = inputs.nixpkgs;
    patches = map originPkgs.fetchpatch remoteNixpkgsPatches;
  };
  nixosSystem = import (nixpkgs + "/nixos/lib/eval-config.nix");
  # Uncomment to use a Nixpkgs without remoteNixpkgsPatches
  #nixosSystem = inputs.nixpkgs.lib.nixosSystem;
  in
  {
    nixosConfigurations.nixos = nixosSystem {
      system = "x86_64-linux";
      modules = [
        ({ pkgs, lib, config, ... }: {
          nixpkgs.crossSystem = lib.systems.examples.armv7l-hf-multiplatform;
          services.xserver = {
            enable = true;
            desktopManager.session = [
              { name = "home-manager";
                start = ''
                  ${pkgs.runtimeShell} $HOME/.hm-xsession &
                  waitPID=$!
                '';
                bgSupport = true;
              }
            ];
          };
        })
      ];
    };
  };
}

Test with:

nix -L build .#nixosConfigurations.nixos.config.services.xserver.displayManager.sessionData.desktops

Change nixosSystem if you want to test without the patch.

Note that testing the systemd-lib commit will need you to hack through other stuffs not cross-compiling (like mesa) wanted by:

nix -L build .#nixosConfigurations.nixos.config.environment.etc.'"systemd/system"'.source

Meaning having to disable stuffs or using an overlay to set failing packages to the dummy hello package.

[Artturin]

i added a modified version of your example to https://nixos.wiki/wiki/Nixpkgs/Reviewing_changes#Testing_the_cross-compilation_of_modules

[nixos-discourse]

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

https://discourse.nixos.org/t/how-to-try-a-pr/15410/12

[nixos-discourse]

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

https://discourse.nixos.org/t/best-pattern-to-include-multiples-patches-prs-to-nixpgs-in-flake-nix/40917/2