NixOS: how to import some configuration from GitLab? infinite recursion encountered - nix

Question

I'm trying to deploy a NixOS VM while storing its configuration on a private GitLab repository.
My configuration.nix looks like this (simplified to only include the relevant bits):
{ pkgs, ... }:
let
repo = pkgs.fetchFromGitLab { owner = "hectorj"; repo = "nix-fleet"; };
in {
imports = [
./hardware-configuration.nix
"${repo}/my-server-name/host.nix"
];
}
but it is giving me this error:
error: infinite recursion encountered
at /nix/var/nix/profiles/per-user/root/channels/nixos/lib/modules.nix:496:28:
495| builtins.addErrorContext (context name)
496| (args.${name} or config._module.args.${name})
| ^
497| ) (lib.functionArgs f);
I do not understand where the recursion is happening.
It doesn't seem like its even fetching the repo, as I can put any non-existing names in the args and get the same error.
I saw https://nixos.org/guides/installing-nixos-on-a-raspberry-pi.html doing something similar without issue:
imports = ["${fetchTarball "https://github.com/NixOS/nixos-hardware/archive/936e4649098d6a5e0762058cb7687be1b2d90550.tar.gz" }/raspberry-pi/4"];
And I can use that line on my VM and it will build fine.
What am I missing?

Answer 1

The recursion is as follows
Compute the configuration
Compute the config fixpoint of all modules
Find all modules
Compute "${repo}/my-server-name/host.nix"
Compute repo (pkgs.fetch...)
Compute pkgs
Compute config._module.args.pkgs (Nixpkgs can be configured by NixOS options)
Compute the configuration (= 1)
You can break the recursion at 6 by using builtins.fetchTarball instead.
Alternatively, you can break it around 7, by using a different "pkgs".
If you're using configuration.nix as part of a larger expression, you may be able to pass an invoked Nixpkgs to NixOS via specialArgs.pkgs2 = import nixpkgs { ... }. This creates a pkgs2 module argument that can't be configured by NixOS itself.
Otherwise, you could define pkgs2 in a let binding.
{ pkgs, ... }:
let
# pkgs2: An independent Nixpkgs that can be constructed before the NixOS
# imports are resolved.
pkgs2 = import <nixpkgs> {};
repo = pkgs2.fetchFromGitLab { owner = "hectorj"; repo = "nix-fleet"; };
in {
imports = [
./hardware-configuration.nix
"${repo}/my-server-name/host.nix"
];
}