fix: don't reevaluate nixpkgs in nixosConfigurations

[yajo]

Before this fix, pkgs was reevaluated in NixOS configurations. If any nixpkgs.config or nixpkgs.overlays was set, it got lost. Besides, this caused a performance penalty.

Besides, it was impossible to redefine nixpkgs.pkgs downstream because perSystemModule depended on pkgs. Any redefinition caused an infinite loop. Now it depends on nixpkgs.hostPlatform, which must be set in each machine configuration, so the loop is gone.

@moduon MT-9339

[zimbatm]

Sounds good once the CI is fixed. hostPlatform can also be a string sometimes.

[yajo]

hostPlatform can also be a string sometimes

I find that weird. Even if I set it as a string, when reading it, it's always an attrset here. Do you know why?

[phaer]

@yajo CI LGTM now, it's just the formatter who isn't pleased yet. Would fix it myself don't have write access to the branch and opening another PR seems like too much overhead :)

[yajo]

Thanks! Fixed.

[phaer]

Thank you! :)

[chermnyx]

After this PR was merged I can no longer use nixpkgs.config nixos option:

       Failed assertions:
       - Your system configures nixpkgs with an externally created instance.
       `nixpkgs.config` options should be passed when creating the instance instead.

[chermnyx]

After this PR was merged I can no longer use nixpkgs.config nixos option:

       Failed assertions:
       - Your system configures nixpkgs with an externally created instance.
       `nixpkgs.config` options should be passed when creating the instance instead.

The problem is caused by nixpkgs.pkgs = lib.mkDefault systemArgs.${config.nixpkgs.hostPlatform.system}.pkgs; being defined in this PR which causes the assertions https://github.com/NixOS/nixpkgs/blob/nixos-unstable/nixos/modules/misc/nixpkgs.nix#L407 in nixpkgs to fail

[delliottxyz]

This seems to cause an infinite recursion when used with nixos-facter?

I get this infinite recursion

       … in the argument of the not operator
         at /nix/store/bl7i7hafp2a4y5j043hi5ngqvbz0fl9q-source/modules/nixos/system.nix:8:24:
            7| {
            8|   nixpkgs = lib.mkIf (!options.nixpkgs.pkgs.isDefined) {
             |                        ^
            9|     hostPlatform = lib.mkIf (config.facter.report.system or null != null) (

       … while evaluating the attribute 'nixpkgs.pkgs.isDefined'
         at /nix/store/alzxn3hjisc84hrlv44x6hni48crww26-source/lib/modules.nix:932:23:
          931|         definitionsWithLocations = res.defsFinal;
          932|         inherit (res) isDefined;
             |                       ^
          933|         # This allows options to be correctly displayed using `${options.path.to.it}`

       error: infinite recursion encountered
       at /nix/store/alzxn3hjisc84hrlv44x6hni48crww26-source/lib/modules.nix:932:23:
          931|         definitionsWithLocations = res.defsFinal;
          932|         inherit (res) isDefined;
             |                       ^
          933|         # This allows options to be correctly displayed using `${options.path.to.it}`

where /nix/store/bl7i7hafp2a4y5j043hi5ngqvbz0fl9q-source/modules/nixos/system.nix is this file in nixos-facter-modules: https://github.com/nix-community/nixos-facter-modules/blob/60f8b8f3f99667de6a493a44375e5506bf0c48b1/modules/nixos/system.nix

[yajo]

After this PR was merged I can no longer use nixpkgs.config nixos option:

You should apply the config like explained here: https://github.com/numtide/blueprint/blob/main/docs/configuration.md#nixpkgsconfig

You'll get the same result, but with better performance.

[chermnyx]

After this PR was merged I can no longer use nixpkgs.config nixos option:

You should apply the config like explained here: https://github.com/numtide/blueprint/blob/main/docs/configuration.md#nixpkgsconfig

You'll get the same result, but with better performance.

I can't do that because I need to apply this conditionally (for example overriding some packages & enabling cuda only for systems with Nvidia GPU)

Is there a way I can avoid nixpkgs.pkgs being set?

[yajo]

What you can do in that case is to make the override conditional.

I have something similar here. You have to somehow declare the machine has NVIDIA, right? So this just reads that config and applies the wanted override (in this case just for a package) based on that condition. You don't really need to set nixpkgs.pkgs for that.

[chermnyx]

What you can do in that case is to make the override conditional.

I have something similar here. You have to somehow declare the machine has NVIDIA, right? So this just reads that config and applies the wanted override (in this case just for a package) based on that condition. You don't really need to set nixpkgs.pkgs for that.

I cannot apply overrides or use nixpkgs.config (for example to set cudaSupport = true;) inside nixos modules because nixpkgs.pkgs is being set by default in this module

Modifying every package that may need to use cuda or every package that depend on a patched package (when the patch is only needed for some systems) without overrides seem to require too much of boilerplate code compared to a single override

[zimbatm]

There is some tension between nixpkgs.pkgs and nixpkgs.config as both are mutually exclusive. Upstream decided to side with nixpkgs.pkgs and ignore nixpkgs.config if both are set.

For this use-case, we might want the reverse: upstream a nixpkgs.defaultPkgs to NixOS that gets picked as the default value for nixpkgs.pkgs, unless nixpkgs.config (and overlays) is set.

That would allow satisfying both use-cases here.

[JumpIn-Git]

There is some tension between nixpkgs.pkgs and nixpkgs.config as both are mutually exclusive. Upstream decided to side with nixpkgs.pkgs and ignore nixpkgs.config if both are set.

For this use-case, we might want the reverse: upstream a nixpkgs.defaultPkgs to NixOS that gets picked as the default value for nixpkgs.pkgs, unless nixpkgs.config (and overlays) is set.

That would allow satisfying both use-cases here.

maybe you could just set nixpkgs.pkgs to lib.mkIf (config.nixpkgs.config == {} && config.nixpkgs.overlays == []) <pkgs>! (mkIf may not work, just giving a idea) edit: mkIf should work since this is imported into the nixos system, where mkIf is supported