niv can't be used to pin NixOS

[hyperfekt]

When pinning NixOS by setting nixpkgs in the $NIX_PATH to a file that imports nixpkgs from sources.nix, an infinite recursion will be entered because niv believes it has nixpkgs available and tries to use fetchers from the nixpkgs version it is supposed to fetch.

[nmattia]

This is a [never](#39 ending issue, and I still do not have a good solution unfortunately :(

However I've used it successfully with NixOS; can you share your setup?

[hyperfekt]

You probably had that file in the same directory as sources.nix, which is not the case for me since it is part of a larger config. I might be wrong about this, but I think with sources.nix being in the nix subdirectory the hasThisAsNixpkgsPath function will almost never return true.
I might be wrong about this, but I think to evaluate NixOS a number of files are directly accessed in different subdirectories, which all have to import nixpkgs, so it doesn't seem possible to put them all in the same (super)directory as/of sources.nix anyhow.

[hyperfekt]

Here is how I pin NixOS: nixpkgs in the $NIX_PATH points to this folder.
I've changed hasThisAsNixpkgsPath in the sources.nix to always return true for now.

[nmattia]

The assumption is that "most of the time" there will be a ./nix/default.nix next to the ./nix/sources.nix, maybe we should provide a config file that specifies

1. where the source.nix is
2. whether or not to import <nixpkgs> for the fetchers
   ?

[leotaku]

@nmattia I think having an additional config file wouldn't be the greatest thing. What I instead would like to see is a well-commented and easily understandable (for newcomers to Nix) sources.nix file.

Possibly the "logic" of the file could also be moved to a different .nix file, so that a new user can just set some simple parameters (like with a traditional config file), while advanced users still have full access to the power of Nix.

[michaelpj]

You can explicitly use builtin fetchers now. I recommend doing this for your nixpkgs pins especially.

[hyperfekt]

You can explicitly use builtin fetchers now. I recommend doing this for your nixpkgs pins especially.

Good point, seems I mistraced the variable dependencies of the functions used to call the builtins inside sources.nix. I guess that solves the issue.

[nmattia]

@michaelpj ok I'm lost, how does one enable the builtins nixpkgs?

[michaelpj]

Just change the type of the nixpkgs pin to builtin-tarball instead of tarball and it will work fine. I set up an example like the one described in the issue and I get infinite recursion with tarball (for the reasons explained) and it works fine with builtin-tarball.