Global Channels and per-user profiles

[Ericson2314]

Describe the bug

I ran into trouble installing (a) Nix and then (b) home-manager on my friend's macOS machine. A bunch of similar issues have been reported, but I think this one is slightly different.

The problem is this:

nix/src/libexpr/eval.cc

Lines 2494 to 2495 in 19c1a46

	add(settings.nixStateDir + "/profiles/per-user/root/channels/nixpkgs", "nixpkgs");
	add(settings.nixStateDir + "/profiles/per-user/root/channels");

these two lines neglect the fact that with the XDG profile dirs, these will instead be in ~root/.local/state/profiles/channels.

Steps To Reproduce

1. Install Nix 2.14.1 on macOS in multi-user mode
2. Try to do anything involving <nixpkgs>
3. get errors because there is no nixpkgs key defined via the Nix Path.

Expected behavior

Global channels are usable with the default (no env var set) Nix path.

nix-env --version output

nix-env (Nix) 2.14.1

Additional context

A simple fix would be to get the the default Nix path to try to look in ~root/.local/state/nix/profiles/channels too, but I don't think that is the right solution. "per user" profiles (channels or otherwise) should be just for that user, which is why it is safe and correct to move them to ~/.local/state/nix/profiles. If channels are to be shared system wide, they are not a per-user concept, even a root user concept. Indeed, root-specific channels should be secret channels just for the root user.

We have precedent of this: the default profile which has long been $nixStateDir/profiles/default not $nixStateDir/profiles/per-user/root precisely for the same reason. We should simply reuse that same solution and have nixStateDir + profiles/default or similar. Neither of these are effected by the XDG stuff because they are not per user, and this is correct.

Priorities

Add 👍 to issues you find important.

[fricklerhandwerk]

Edit from @Ericson2314: This is discussion is about the general issue with the release, not this specific channels issue in particular. My fussing with the team agenda caused these notes to land here somewhat inadvertently.

---

Discussed in the Nix team meeting:

• @thufschmitt: was pushing for merging that, but it was really hard to have a proper compatibility layer due to how things are set up
  • wondering what's the right thing to do:
    • fix up quickly
    • revert and think it through
    • ignore backwards compatibility
• @Ericson2314: it's almost there, should change the installer to use the new directories
  • @thufschmitt: not enough to change the installer, need proper backwards compatibility for existing setups
• @fricklerhandwerk: would veto on ignoring home manager concerns, should rather rethink this
  • @thufschmitt: could help HM fixing forward, seems like an easy change
  • @Ericson2314: reverting would be demoralising
• @edolstra: this one feels more useful than the XDG one to me
  • if we revert this, would it break people who already rely on the change?
  • @Ericson2314: probably yes
  • @thufschmitt: will think about a fix forward
    • @Ericson2314: flip the default instead of doing more compat
• no decision

[nixos-discourse]

This issue has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/2023-03-06-nix-team-meeting-minutes-38/26056/1

[corneliusroemer]

I ran into exactly this issue as described, see https://stackoverflow.com/questions/75657834/fresh-install-of-nix-fails-on-m1-mac-error-attribute-nixpkgs-in-selection-pa

Can I suggest a quick fix @fricklerhandwerk? The most affected people by this seem to be people who install for the first time. They will usually use recommended curl -L https://nixos.org/nix/install | sh, this is mentioned everywhere as the way to install.

So why don't you change that install script to download 2.13 until you have figured out a way forward with 2.14?

By the way, one workaround seems to be nix-channel --add https://nixos.org/channels/nixpkgs-unstable, though this may have undesirable side effects. At least it results in an apparently working installation.

[fricklerhandwerk]

Yes, we talked about the more general solution of versioned installers recently, and I'm in favor of this. Need to prioritise it on the team though. @Ericson2314 do we have an issue for that?

[Ericson2314]

@fricklerhandwerk An issue for what, rolling back the default installer? I agree that is a good stop-gap.

[fricklerhandwerk]

No, for providing versioned installers.

We can always just roll back, but the interesting question is what to do next.

[corneliusroemer]

So have you already rolled back the default installer? This should be a few clicks and save lots of people quite some hassle. I don't know how many people install nix but it'll be dozens a day and they may well be lost by this.

[corneliusroemer]

Possibly related? #7992

[edolstra]

We've reverted the installer to 2.13.3 for now.

[Ericson2314]

Discussed in the Nix team Meeting:

• @Ericson2314: users should be able to pick up system channels, while ~root is not accessible
• @Theophane: global channels location is more public so we should be wary of using it, maybe keep /nix/var/nix/profile/per-user/root/channels?
• @Ericson2314: Worried about having that path as a special case where the directories no longer "mean" what they did before
• @thufschmitt: Could have a new path, and then the old path as a symlink to it
• @Ericson2314: Sounds great!
• Assigned to @thufschmitt

[nixos-discourse]

This issue has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/2023-03-10-nix-team-meeting-minutes-39/26279/1

[Ericson2314]

#8073 fixed this.

[Ericson2314]

#8147 builds on this, but is just icing on the cake.

[nixos-discourse]

This issue has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/sudo-nixos-rebuild-switch-upgrade-does-not-get-updates-anymore/27072/7