Flake inputs.*.follows doesn't change lockfile

[LunNova]

Describe the bug

When using inputs.*.follows, the input's lockfile in the nix store still has the old locked version.

This means that if you set .follows on an input and then add that to the registry with nix.registry.myregistryentry.flake = my-input in a nixos configuration it will not have the updated input.

The same applies for adding it to the nixPath with nix.nixPath = [ "mychannel=${my-input}" ]; or if the flake reads its own lock file for any other reason.

Steps To Reproduce

Here is a flake which uses nixpkgs as an input:
https://github.com/LunNova/nixos-configs/tree/nixpkgs-unfree-test

Use this flake as your own input and make its nixpkgs input follow a different revision:

{
  inputs.nixpkgs.url = "github:nixos/nixpkgs/nixos-21.11";
  inputs.nixpkgs-unfree-test.url = "github:lunnova/nixos-configs/nixpkgs-unfree-test";
  inputs.nixpkgs-unfree-test.inputs.nixpkgs.follows = "nixpkgs";

  outputs = { self, nixpkgs-unfree-test, ... }:
    let lockFile = builtins.fromJSON (builtins.readFile "${nixpkgs-unfree-test}/flake.lock");
    in
    { 
      test = {
        lockFile = lockFile.nodes.nixpkgs.locked.rev;
        runtime = nixpkgs-unfree-test.inputs.nixpkgs.rev;
      };
    };
}

$ nix eval github:lunnova/nixos-configs/nixpkgs-unfree-test-user#test
{ lockFile = "545a0831fa918b6a1c065c03fce907c05646416d"; runtime = "573095944e7c1d58d30fc679c81af63668b54056"; }

Expected behavior

$ nix eval github:lunnova/nixos-configs/nixpkgs-unfree-test-user#test
{ lockFile = "573095944e7c1d58d30fc679c81af63668b54056"; runtime = "573095944e7c1d58d30fc679c81af63668b54056"; }

When follows is used, a new copy of the flake could be created (I assume it can be hardlinked so this doesn't waste space?) in the nix store with its lockfile updated to match the follows.

nix-env --version output

Tested with stable and unstable.

$ nix-env --version
nix-env (Nix) 2.4

$ nix-env --version
nix-env (Nix) 2.5.0pre20211206_d1aaa7e

Workaround

I was able to work around this for my particular case by creating a new flake from a template and substituting the hashes, rather than using the follows mechanism.

      nixpkgs-unfree-relocked = pkgs.stdenv.mkDerivation {
        name = "nixpkgs-unfree-relocked";
        outputs = [ "out" ];
        dontUnpack = true;
        fixupPhase = "";
        installPhase = ''
          mkdir -p $out
          cp -t $out ${nixpkgs-unfree-path}/{flake.nix,flake.lock,default.nix}
          substituteInPlace $out/default.nix --replace "nixpkgs = null" 'nixpkgs = "${args.nixpkgs}"'
          substituteInPlace $out/flake.lock --replace \
            "%REV%" "${lock.nodes.nixpkgs.locked.rev}" --replace \
            "%HASH%" "${lock.nodes.nixpkgs.locked.narHash}"
          substituteInPlace $out/flake.nix --replace \
            "%REV%" "${lock.nodes.nixpkgs.locked.rev}"
        '';
      };

[LunNova]

The reason for wanting this is similar to #4891

[stale]

I marked this as stale due to inactivity. → More info

[tejing1]

Related: #6894