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:

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

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
https://github.com/NixOS/nixpkgs/blob/9c9eb1ce75c2e956ab7cec4db1b925a3f33b7424/nixos/lib/eval-config.nix#L50
to be evaluated to [] in this case.
Next, I’m parsing that this
https://github.com/NixOS/nixpkgs/blob/9c9eb1ce75c2e956ab7cec4db1b925a3f33b7424/nixos/modules/misc/nixpkgs.nix#L62
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?