feat: Pass through individual channel configuration

[lpchaim]

While looking for a way to achieve per host nixpkgs configurations within snowfall a while ago (see #106), I found out it (very understandably) wasn't a supported use case. I've since familiarized myself some more with both nix and snowfall, so I figured I'd try and give solving that issue cleanly a shot. Hopefully the solution I came up with makes sense!

Essentially, channel-specific configuration is now passed through to flake-utils-plus.mkFlake, allowing channels to have distinct settings and making so individual hosts can have their channel changed. Additionally, the same overlays that were once applied only to the default nixpkgs channel are now also applied to any other user-specified channels automatically.

As a demonstration, the following is a minimal working snippet from my personal flake showcasing how I manage to enable cuda support for my one host with a NVIDIA GPU without affecting the other hosts.

{
  inputs = {
    nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
    nixpkgs-cuda.follows = "nixpkgs";
    snowfall-lib.url = "github:snowfallorg/lib";
  };

  outputs = inputs @ {self, ...}:
    snowfallLib.mkFlake {
      inherit inputs;
      src = ./.;

      channels-config = {
        allowUnfree = true;
      };
      channels.nixpkgs-cuda.config = {
        allowUnfree = true;
        cudaSupport = true;
      };

      systems.hosts.desktop.channelName = "nixpkgs-cuda";
    }
}

[jakehamilton]

Excellent! One quick question, otherwise this looks good to go pending some testing :)

[jakehamilton]

Do we need the default nixpkgs here if we are allowing fully dynamic channel configuration? I think it is possible that the user has channels other than one named nixpkgs. The previous version of Snowfall Lib made the assumption that inputs.nixpkgs would always exist, but maybe we don't need to anymore.

[lpchaim]

I'd say it's still desirable as a fallback. As is, removing it means no inputs are processed at all if the user doesn't go out of their way to configure specific channels, which breaks some valid configurations.

I tried to automatically discover nixpkgs inputs to apply the overlays to all of them out of the box and found some existing solutions but since they tended to feel quite hacky, I went with this because it's backwards compatible with the original behavior while still allowing for user customization. I figured nixpkgs + a handful of oddball channels would fit the vast majority of use cases, so it didn't sound worth the trouble.

[RuRo]

Hi @lpchaim I was wondering if you ever managed to make this work?

I think the current implementation mostly works with one minor annoyance - it seems that the pkgs argument that is passed to the modules/nixos/**/default.nix modules is still hardcoded to use the nixpkgs channel here:

lib/snowfall-lib/module/default.nix

Line 66 in 4054bdc

pkgs = user-inputs.self.pkgs.${system}.nixpkgs;

It seems that the "correct" version of nixpkgs is available in the channel parameter, but IMHO, pkgs should ideally also refer to the correct instance of nixpkgs.

I am actually not 100% sure, why that line is even needed. I tried commenting it out and the default pkgs (passed by fup? or maybe by lib.nixosSystem? I am not sure) was already seemingly the correct instance of nixpkgs.