nix.conf: create function for creating this file

[FRidh]

Issue description

The nix-daemon module creates a nix.conf file. The derivation creating this file is useful also for non-NixOS users and should therefore be factored out.

The main motivation is generating the paths for "extra-sandbox-paths".

Steps to reproduce

Technical details

Please run nix-shell -p nix-info --run "nix-info -m" and paste the
results.

[7c6f434c]

Should we factor out the generation of nix.conf or should we have official support for exporting /etc/ entries from a NixOS system?

Currently I use

  etcSelectPrefix = filenamePrefix: config:
    let
      nixosInstance = nixos {configuration = config;};
      selected = (pkgs.lib.filterAttrs
                           (k: v: pkgs.lib.hasPrefix filenamePrefix v.target)
                           nixosInstance.config.environment.etc);
    in (pkgs.lib.mapAttrs'
             (k: v: let source = v.source; in
                      { 
                        value = (if pkgs.lib.isString source then source else source.outPath);
                        name = v.target;
                      })
             selected);

and select the nix/ prefix with a configuration containing only { nix= {…}; } part.

[FRidh]

Should we factor out the generation of nix.conf or should we have official support for exporting /etc/ entries from a NixOS system?

It would need a different approach and is of a much larger scope, but why not.

Does your nixosInstance need a kernel configuration? I suppose not, unless a module that you request needs it.

[7c6f434c]

nixos=import <nixpkgs/nixos> is the typical context.

A typical invocation is etcSelectPrefix "nix/" { nix = { useSandbox = true; }; }

My current helper library is https://github.com/7c6f434c/lang-os/blob/master/use-from-nixos.nix and its use is https://github.com/7c6f434c/lang-os/blob/master/system-etc-pieces.nix and https://github.com/7c6f434c/lang-os/blob/master/local/test-system.nix (etcSelect… calls)

As of scope — I think the main question is «can we agree to keep /etc, and /etc/nix/ there, for a longer time than a module system; and does Nixpkgs as a project commit not to do completely crazy scoping that makes the entries invisible from outside». Because if the answer is «Yes there is a commitment.», an implementation against the current NixOS code is small. And I need to maintain it for a comfortable environment anyway.

Of course, adding makeNixConf to Nixpkgs further normalizes splitting the implementation from coordination/configuration as implemented by NixOS module system which does sound nice…

[infinisil]

See #44322 (comment)

Feel free to close this issue if you feel like that's good enough

[stale]

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[FRidh]

I have no need for this any longer and the suggestion in #44322 (comment) seems adequate as well.