Eval-config.nix waging war against nixpkgs.pkgs?

blaggacao · October 3, 2020, 5:19am

Why does <nixos/lib/eval-config.nix> not honor modularily set nixpkgs.pkgs for use in the pkgs arguments of modules?
Even though

github.com

NixOS/nixpkgs/blob/9c9eb1ce75c2e956ab7cec4db1b925a3f33b7424/nixos/lib/eval-config.nix#L15


      
          # !!! Please think twice before adding to this argument list!
          # Ideally eval-config.nix would be an extremely thin wrapper
          # around lib.evalModules, so that modular systems that have nixos configs
          # as subcomponents (e.g. the container feature, or nixops if network
          # expressions are ever made modular at the top level) can just use
          # types.submodule instead of using eval-config.nix
          { # !!! system can be set modularly, would be nice to remove
            system ? builtins.currentSystem
          , # !!! is this argument needed any more? The pkgs argument can
            # be set modularly anyway.
            pkgs ? null
          , # !!! what do we gain by making this configurable?
            baseModules ? import ../modules/module-list.nix
          , # !!! See comment about args in lib/modules.nix
            extraArgs ? {}
          , # !!! See comment about args in lib/modules.nix
            specialArgs ? {}
          , modules
          , # !!! See comment about check in lib/modules.nix
            check ? true
          , prefix ? []

is not set on the caller?

When evaluating a modularily set version of packages with an eval config of a different version, then I get an evaluation conflict, for example here:

github.com

NixOS/nixpkgs/blob/9c9eb1ce75c2e956ab7cec4db1b925a3f33b7424/nixos/modules/services/video/epgstation/default.nix#L51


      
                system = { appenders = [ "stdout" ]; level = "info"; };
                access = { appenders = [ "stdout" ]; level = "info"; };
                stream = { appenders = [ "stdout" ]; level = "info"; };
              };
            };
          
          
  defaultPassword = "INSECURE_GO_CHECK_CONFIGURATION_NIX\n";
          in
          {
            options.services.epgstation = {
              enable = mkEnableOption pkgs.epgstation.meta.description;
          
          
    usePreconfiguredStreaming = mkOption {
                type = types.bool;
                default = true;
                description = ''
                  Use preconfigured default streaming options.
          
          
        Upstream defaults:
                  <link xlink:href="https://github.com/l3tnun/EPGStation/blob/master/config/config.sample.json"/>
                '';

giving proof of en environment in which the module’s nixpkgs is based on the new version while pkgs is an old version.

If I use eval-config.nix off the same version, then there is no version mismatch.

I can’t see where eval-config does this magic (in the case that it’s own pkgs argument is null).

Any pointer?

I’m parsing this

github.com

NixOS/nixpkgs/blob/9c9eb1ce75c2e956ab7cec4db1b925a3f33b7424/nixos/lib/eval-config.nix#L50


      
                # this.  Since the latter defaults to the former, the former should
                # default to the argument. That way this new default could propagate all
                # they way through, but has the last priority behind everything else.
                nixpkgs.system = lib.mkDefault system;
          
          
      # Stash the value of the `system` argument. When using `nesting.children`
                # we want to have the same default value behavior (immediately above)
                # without any interference from the user's configuration.
                nixpkgs.initialSystem = system;
          
          
      _module.args.pkgs = lib.mkIf (pkgs_ != null) (lib.mkForce pkgs_);
              };
            };
          
          
in rec {
          
          
  # Merge the option definitions in all modules, forming the full
            # system configuration.
            inherit (lib.evalModules {
              inherit prefix check;
              modules = baseModules ++ extraModules ++ [ pkgsModule ] ++ modules;

to be evaluated to [] in this case.
Next, I’m parsing that this

github.com

NixOS/nixpkgs/blob/9c9eb1ce75c2e956ab7cec4db1b925a3f33b7424/nixos/modules/misc/nixpkgs.nix#L62


      
            pkgsType = mkOptionType {
              name = "nixpkgs";
              description = "An evaluation of Nixpkgs; the top level attribute set of packages";
              check = builtins.isAttrs;
            };
          
          
  defaultPkgs = import ../../.. {
              inherit (cfg) config overlays localSystem crossSystem;
            };
          
          
  finalPkgs = if opt.pkgs.isDefined then cfg.pkgs.appendOverlays cfg.overlays else defaultPkgs;
          
          
in
          
          
{
            options.nixpkgs = {
          
          
    pkgs = mkOption {
                defaultText = literalExample
                  ''import "''${nixos}/.." {
                      inherit (cfg) config overlays localSystem crossSystem;

would set it to my modularly set pkgs version.

But the error suggests there is at least one instantiation involved with a version mismatch. Why?

Is laziness broken?