vm test not respecting disko.tests.extraConfig disko overwrites

[BerriJ]

@Lassulus @Enzime as discussed at 39C3:

I created this simple flake to demonstrate the problem.

To reproduce

1. Clone flake
2. Run:

❯ nix run github:nix-community/nixos-anywhere -- \
  --flake .#test \
  --vm-test

The disko config has a luks partition and usually it uses passwordFile to provide a password that must be provided during boot to unlock.

For the VM-Test, we need non-interactive unlocking. So I:

• Wrote a systemd service that writes /tmp/secret.key during boot (I need that since I use systemd in stage 1).
• Set disko.devices.disk.main.content.partitions.main.content.settings.keyFile = "/tmp/secret.key"; in disko.tests.extraConfig

Like this (see here:

disko.tests.extraConfig = {

  # This does not work
  disko.devices.disk.main.content.partitions.main.content.settings.keyFile = "/tmp/secret.key";

  boot.initrd = {
    systemd = {
      services = {
        createLuksKeyFileFile = {
          enable = lib.mkDefault true;
          description = "Create LUKS password file for Disko";
          wantedBy = [ "sysinit.target" ];
          before = [
            "sysinit.target"
            "systemd-cryptsetup.service"
          ];
          path = [ pkgs.coreutils ]; # Makes echo, tee, etc. available
          script = "umask 077; echo -n 'secretsecret' > /tmp/secret.key;";
          unitConfig = {
            DefaultDependencies = "no";
          };
          serviceConfig = {
            Type = "oneshot";
          };
        };
      };
    };
    luks.devices = {
      main = {
        # Works if I do the overwrite here
        # keyFile = "/tmp/secret.key";
      };
    };
  };
};

But it turns out, this does not work. A workaround is to set the keyFile path directly in boot.initrd.luks.devices but if we want to overwrite other disko options (like a partition size for example) we do not have that workaround available.

Interestingly nix repl shows the correct value in both cases (disko overwrite vs. boot overwrite):

❯ nix repl .
Nix 2.31.2+1
Type :? for help.
warning: Git tree '/home/jonathan/git/DSEE/NIXOS/disko-reprex' is dirty
Loading installable 'git+file:///home/jonathan/git/DSEE/NIXOS/disko-reprex#'...
Added 1 variables.
nixosConfigurations
nix-repl> nixosConfigurations.test.config.virtualisation.vmVariantWithDisko.boot.initrd.luks.devices.main.keyFile
"/tmp/secret.key"

When you use the disko overwrite, it will simply ask for a password during boot of the VM. So it seems the keyFile option is not properly propagated although nix repl shows it correctly.

With the help of Claude Opus 4.5 I was able to craft a PR to fix this: #1184 but honestly I do not understand it fully so I'd be thankful for a review on that.

If you want to try my PR, just use my disko as input of the flake config:

    disko = {
      url = "github:berrij/disko";
      inputs.nixpkgs.follows = "nixpkgs";
    };