Ability to pass NIX_PATH to executors

[utdemir]

Hi,

I am trying to to build my dotfiles repository with Hercules CI.

I have nixpkgs pinned, but still some of my dependencies, use NIX_PATH to lookup some trivial files from nixpkgs. So, I currently get these kind of errors error when used with Hercules CI:

Exception: file 'nixpkgs/nixos/modules/misc/assertions.nix' was not found in the Nix search path (add it using $NIX_PATH or -I), at /nix/store/k1pc4wx9f69l56kl8g0sxkzpwviw4x0i-source/modules/modules.nix:159:17; type: nix::ThrownError

error: file 'nixpkgs/nixos/modules/installer/scan/not-detected.nix' was not found in the Nix search path (add it using $NIX_PATH or -I), at /etc/nixos/hardware-configuration.nix:8:7

I was able to get things working by patching things and adding conditionals for CI, but it isn't pretty. Ideally, I would prefer having NIX_PATH available on executors, since there's still tooling using it. I know that it makes the builds less deterministic, so maybe having an option with a warning might be a better option.

Another alternative would be to have a way to set NIX_PATH on ci.nix. Maybe with some special key like:

# ci.nix:
{
  ...
  "hercules-ci-meta": {
    NIX_PATH = "nixpkgs=${(import ./sources.nix).nixpkgs}";
  }
}

[domenkozar]

Yeah, we discussed that supporting NIX_PATH as an opt-in makes sense.

[utdemir]

Thank you @domenkozar .

I just wanted to note that what I was asking for will (probably) also be possible with the recent "recursive nix" feature. After a new Nix version is released and if the agent supports it, I should be able to do something like:

runCommand "build" {} ''
  export NIX_PATH=nixpkgs="${sources.nix}"
  nix-build ./default.nix
''

[roberth]

We weren't able to prioritize this at the time and it seems that the window for this to be useful is closing, as the ecosystem has been moving away from NIX_PATH, accelerated by the adoption of tools like niv and the advent of Flakes. With version 0.9, the agent is in a good position to support this, as it splits the evaluation of project configuration and the actual evaluation for building. Do you still need this?

[utdemir]

Hi @roberth. I haven't been using Hercules CI for a while, so I do not need it.

And I also agree that NIX_PATH is something we should move away from, so I'm happy to close it. Thanks!