locked nixpkgs index is redownloaded every hour

[edrex]

Describe the bug
Expected behavior: the locked index should be cached as long as the GC root is pinned.
Observed: [155.8 MiB DL] downloading 'https://github.com/NixOS/nixpkgs/archive/ae5c332cbb5827f6b1f02572496b141021de335f.zip' after an hour.

I think this is an upstream issue, maybe the one discussed in https://discourse.nixos.org/t/pinned-nixpkgs-keeps-getting-garbage-collected/12912, but as honestly this is a huge usability papercut for devenv (I'd like to use it broadly, but these frequent refetches are too much) it seems wise to track here to move towards a solution. LMK if this is a dupe or something peculiar to my nix config. My devenv.yaml looks like:

inputs:
  nixpkgs:
    # use nixpkgs from system registry.
    url: nixpkgs

[domenkozar]

Should be fixed in #745

[edrex]

Ok, I'll start testing the python impl and report back. Thanks Domen!

[domenkozar]

Let me know if it does :)

[edrex]

Sorry for leaving this open for so long.

This issue is with nix itself, and isn't devenv-specific. Absent a link to a proper writeup, for anyone else struggling with frequent nixpkgs index refreshes, here's what to do:

• Assuming you're using a flake to build the output that configures nix itself (either nixos, home-manager).
• Use a direct path (skip the registry) for the nixpkgs input to that flake, like nixpkgs.url = "github:nixos/nixpkgs/nixos-unstable";
• Set nix.registry.nixpkgs.flake = inputs.nixpkgs; in your system config module (nixos, hm, darwin, nix-on-droid, etc). This says "use the locked version in the current flake at build time as the system registry nixpkgs". It results in a registry entry (nix registry list) that points at a store path, which ensures no network traffic.

There is a helper nix.generateRegistryFromInputs in the flake-utils-plus module that will do this for all your system flake inputs, which is handy but I haven't implemented it yet.