nixos: add --specialisation to nixos-rebuild

[Patryk27]

Description of changes

This commit fixes a papercut in nixos-rebuild where people wanting to switch to a specialisation (or test one) were forced to manually figure out the specialisation's path and run its activation script - since now, there's a dedicated option to do just that.

This is a backwards-compatible change which doesn't affect the existing behavior, which - to be fair - might still be considered sus by some people, the painful scenario here being:

• you boot into specialisation foo,
• you run nixos-rebuild switch,
• whoops, you're no longer at specialisation foo, but you're rather brought back to the base system.

(it's especially painful for cases where specialisation is used to load extra drivers, e.g. Nvidia, since then launching nixos-rebuild switch, while forgetting that you're inside a specialisation, can cause some parts of your system to get accidentally unloaded.)

I've tried to mitigate that by improving specialisations so that they create a dedicated file somewhere in /run/current-system containing the specialisation's name (which nixos-rebuild could then use as the default value for --specialisation), but I haven't been able to come up with anything working (plus it would be a breaking change then).

Closes #174065

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.

[RaitoBezarius]

Could you provide with the failed NixOS test so we can maybe help to fix it together?

[Patryk27]

Could you provide with the failed NixOS test so we can maybe help to fix it together?

I've gotten stuck with the basic idea of the test, since it looks like I can't just run nixos-rebuild switch there (because VM's /nix/store is read-only + it doesn't have any channels configured); I've taken a look at at nixos/tests/installer.nix and nixos/tests/os-prober.nix, but I've failed to understand the magic that makes them work 😅

So I guess something very basic could be:

import ./make-test-python.nix {
  name = "nixos-rebuild";

  nodes =  {
    machine = { ... }: {
      # 
    };
  };

  testScript = ''
    machine.wait_for_unit("default.target")

    # TODO create /etc/configuration.nix (+ hardward-configuration.nix?) with
    #      multiple specializations; make following work:
    machine.succeed("nixos-rebuild switch --specialisation-name foo")
  '';
}

[RaitoBezarius]

Could you provide with the failed NixOS test so we can maybe help to fix it together?

I've gotten stuck with the basic idea of the test, since it looks like I can't just run nixos-rebuild switch there (because VM's /nix/store is read-only + it doesn't have any channels configured); I've taken a look at at nixos/tests/installer.nix and nixos/tests/os-prober.nix, but I've failed to understand the magic that makes them work sweat_smile

So I guess something very basic could be:

import ./make-test-python.nix {
  name = "nixos-rebuild";

  nodes =  {
    machine = { ... }: {
      # 
    };
  };

  testScript = ''
    machine.wait_for_unit("default.target")

    # TODO create /etc/configuration.nix (+ hardward-configuration.nix?) with
    #      multiple specializations; make following work:
    machine.succeed("nixos-rebuild switch --specialisation-name foo")
  '';
}

If I provide you with a working baseline for the test, can you write the Python script? You can check systemd-boot.nix test which is interesting for your usecases.

[Patryk27]

I know how those tests work in general (and I've written a few before), I'm just having a problem with this particular setup - so if you manage to provide some foundation, I should be able to work on that 🙂

[ncfavier]

but I've failed to understand the magic that makes them work

I think the magic you're looking for is installation-device.nix? That should at least take care of channels.

nixpkgs/nixos/tests/installer.nix

Lines 286 to 287 in ead8d4e

	../modules/profiles/installation-device.nix
	../modules/profiles/base.nix

[Patryk27]

Okie, thanks, I'll look into it 🙂

[Patryk27]

Okie, I've changed --specialisation-name into just --specialisation and added a test 🎉

The test still needs a minor clean-up (I've just copy-pasted system.extraDependencies from os-prober.nix and I feel like they could get shortened), but I think it's mostly ready.

[Patryk27]

kk, test minimized 🙂

[RaitoBezarius]

@ofborg test nixos-rebuild-specialisations

[RaitoBezarius]

The aarch64 failure seems unrelated to this PR.

[RaitoBezarius]

Great job!

[RaitoBezarius]

@Patryk27 I think this PR is good to go, there's some style changes to be done on it (trailing whitespace), can I let you take care of them?

[Patryk27]

Oh my, fixed! 😇

[donovanglover]

Would it be possible to make --specialisation work for build-vm as well? I assume some logic needs to be added to nixBuild/nixFlakeBuild?

[Patryk27]

On the top of my head, I think it should be possible; I myself have no time to prepare such change now, though 😢

[nh2]

When passing a specialisation that doesn't exist, there is a lack of error message:

• nixos-rebuild --specialisation nonexistent (wrong name( does not print any error message #412749