Nix copying a store-path into the store

[getchoo]

@fzakaria

  # Put nixpkgs into /etc/nixpkgs for convenience
  environment.etc.nixpkgs.source = inputs.nixpkgs;
  # Point nixpath to that nixpkgs so that the system uses the same nix
  nix = {
    # nixpkgs has been pinned to 2.18 for a long time since newer versions have
    # been buggy. Let's try newer versions and be on the bleeding eedge
    # Should be 2.23.2 as of 2021-07-12
    package = pkgs.unstable.nixVersions.latest;

    nixPath = ["nixpkgs=/etc/nixpkgs" "nixos-config=/etc/nixos/configuration.nix"];

This snippet should no longer be required on versions of Nix >= 2.24.14. Copying of path type inputs like these have been fixed as of create cache entry for paths already in the nix store by Mic92 · Pull Request #12911 · NixOS/nix · GitHub, and I can’t replicate any kind of issue with them even on the latest Nix versions

…but it seems similar behavior has appeared recently elsewhere:

nix copies inputs already in the nix store

fetchTree attempts download despite narHash existing in store

@fzakaria

I’ve never heard of doing nix-path = nixpkgs=flake:nixpkgs is that “more correct” ?

The main idea behind it is the indirection. As opposed to passing a store path directly, using something like the Flake registry or a symlink allows for the same NIX_PATH to immediately being resolving to the new paths (when otherwise services, shells, etc. would need to restart to get the new NIX_PATH with new paths applied). The Flake registry is just one way to do this

It’s been recommended against to use /etc as a location for symlinks, though, as it’s a bit more stateful. So if you want to add your system’s Nixpkgs to your NIX_PATH without using Flakes (and the bugs associated with them), this snippet would probably be a bit better

@getchoo

{
  lib,
  pkgs,
  inputs,
  ...
}:

let
  flakeInputs = pkgs.linkFarm "flake-inputs" (
    lib.mapAttrs (lib.const (flake: flake.outPath)) inputs
  );
in

{
  nix = {
    nixPath = lib.mkForce (
      lib.mapAttrsToList (name: lib.const "${name}=/run/current-system/inputs/${name}") inputs
    );
  };

  system.extraSystemBuilderCmds = ''
    ln -s ${flakeInputs} $out/inputs
  '';
}