Regression: infinite recursion when passing a function to nixpkgs' config arg

[me-and]

As of #437723, and in particular f5deefd, it is no longer possible to use a function as the config argument when importing nixpkgs.

As a minimal example, consider the following:

{
  pkgs ? import <nixpkgs> {
    config =
      { pkgs, ... }:
      {
        allowlistedLicenses = [ pkgs.lib.licenses.unfree ];
      };
  },
}:
pkgs.runCommandLocal "demo" { } "touch $out"

This builds just fine prior to f5deefd, but from that commit onwards, I see the below output:

$ nix-build -I nixpkgs="$HOME"/vcs/nixpkgs
error:
       … while evaluating a branch condition
         at /home/adamdinwoodie/vcs/nixpkgs/pkgs/stdenv/booter.nix:107:9:
          106|       thisStage =
          107|         if args.__raw or false then
             |         ^
          108|           args'

       … in the right operand of the update (//) operator
         at /home/adamdinwoodie/vcs/nixpkgs/pkgs/stdenv/booter.nix:90:44:
           89|     # for the final stage.
           90|     { allowCustomOverrides = index == 1; } // (stageFun prevStage)
             |                                            ^
           91|   ) (lib.lists.reverseList stageFuns);

       … while evaluating the option `allowlistedLicenses':

       … while evaluating definitions from `nixpkgs.config':

       (stack trace truncated; use '--show-trace' to show the full, detailed trace)

       error: infinite recursion encountered
       at /home/adamdinwoodie/vcs/demo/default.nix:6:33:
            5|       {
            6|         allowlistedLicenses = [ pkgs.lib.licenses.unfree ];
             |                                 ^
            7|       };

This use case seems to be the intent of the following code, essentially added at e1c1ecf:

nixpkgs/pkgs/top-level/default.nix

Lines 109 to 112 in 1a19c69

	# Allow both:
	# { /* the config */ } and
	# { pkgs, ... } : { /* the config */ }
	config1 = if lib.isFunction config0 then config0 { inherit pkgs; } else config0;

Tagging @ConnorBaker.

[me-and]

Possibly I'm being unfair in asserting that this was the intent of making config a function, or at least in the implication that this usage is entirely broken. In particular, pkgs.lib.getName seems to still work just fine in the definition of allowUnfreePredicate. It's just my use case with referencing a license from pkgs.lib.licenses in allowlistedLicenses that's not working.

[ConnorBaker]

I thought I had fixed this...

From what I remember, setting the type to types.raw instead of types.listOf (types.attrsOf types.anything) allowed evaluation to succeed, as did removing the options entirely.

I'll make a PR to change the type back to types.raw in the interim (which is what it would be without an option; see the freeformType attribute in pkgs/top-level/config.nix.

CC @infinisil -- any idea why this would cause infinite recursion? Presence or absence of packages at the top level of the package set shouldn't be conditioned on something like license, right?

[ConnorBaker]

@me-and would you test #457038?