problems with PATH order, when using as nixos-module.

[maralorn]

Since I upgraded my nixos to 18-09 I have the following problem:
My PATH contains the following 3 entries in this order:

... /home/maralorn/.nix-profile/bin: ... :/run/current-system/sw/bin:/etc/profiles/per-user/maralorn/bin

So when using home-manager manually user installed packages take precedence over system packages because they land in .nix-profile. But when using home-manager as a system module, the system binaries take precedence.
There are of course easy workarounds for this. (Manually changing the PATH.)
But I am not sure what the best solution to this problem is and how to fix it in home-manager. Is this maybe a sign that we shouldn't install the user-environment to /etc/profiles/per-user/?

I'd be glad for input. Thanks for home-manager. It's awesome!

[rycee]

Thanks for the kind words!

I think this is a bug in NixOS and there is not much Home Manager can do to solve this in an elegant manner.

The /etc/profiles/per-user environment profile is set in users-groups.nix: https://github.com/NixOS/nixpkgs/blob/b5a42132d57f26b954d4c6d42f07fcb60dbff799/nixos/modules/config/users-groups.nix#L567

while the others are defined together in environment.nix: https://github.com/NixOS/nixpkgs/blob/b5a42132d57f26b954d4c6d42f07fcb60dbff799/nixos/modules/programs/environment.nix#L26-L30

The PATH variable is assembled from these profiles. Unfortunately since they are defined separately they don't show up in the "logical order" but in the module system "merge order". I don't know the reason for separating the definitions, it seems to me possible to simply have

environment.profiles =
  [ "$HOME/.nix-profile"
    "/etc/profiles/per-user/$USER"
    "/nix/var/nix/profiles/default"
    "/run/current-system/sw"
];

in environment.nix and not set it at all in users-groups.nix.

@oxij Found you in git blame where you made a cleanup in the users-groups.nix file. Can you think of a reason why we couldn't do the above change?

[Shados]

@rycee "merge order" for list options is not arbitrary; you can manipulate it with mkOrder (or the simple wrappers mkBefore and mkAfter, none of which are really documented, so see <nixpkgs/lib/modules.nix> for details).

You could PR a nixpkgs change to do something like make the users-groups.nix line environment.profiles = mkBefore [ "$HOME/.nix-profile" "/etc/profiles/per-user/$USER" ]; and remove the first from environment.nix. This would seem reasonable to me (it is surprising to have more-specific per-user packages overriden by generic system-wide packages!).

You could also add that same definition in home-manager's nixos module. Which would work fine, but would mean those two directories would appear in $PATH twice :). That could slightly increase the time taken for shells to resolve binaries, I guess, but two paths are unlikely to have much impact, and I suspect some shells may be both deduplicating $PATH entries and building an internal hash of name -> binary path entries anyway.

[Gerschtli]

As far as I can tell, this issue will be resolved as soon as my PR in nixpkgs gets merged into the release channel. So I think we can close this issue, as there is nothing more to do then waiting.

[Gerschtli]

The fix is now available in nixos-19.03 channel. Issue can be closed.

[Gerschtli]

@rycee @maralorn This issue can be closed :)

[maralorn]

I don‘t have this problem anymore. But I changed my setup anyways.