nixos/nix-channel: fix subscribing to default channel

[nikstur]

Description of changes

Fixes #264602

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandboxing enabled in nix.conf? (See Nix manual)
  • sandbox = relaxed
  • sandbox = true
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 23.11 Release Notes (or backporting 23.05 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
• Fits CONTRIBUTING.md.

[RaitoBezarius]

LGTM.

[eclairevoyant]

Manual testing seems to confirm this is fixed, approving.

[nikstur]

I'll wait for ofborg eval, which might take a few hours, and merge then.

[nikstur]

@ofborg test activation-nix-channel

[nixos-discourse]

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

https://discourse.nixos.org/t/error-reading-symbolic-link-nix-var-nix-profiles-per-user-root-channels-no-such-file-or-directory/34838/3

[mhwombat]

Now that the PR has been merged, I have a new symptom that I think is related. I did the fix:

sudo nix-channel --remove nixos && sudo nix-channel --remove \"
sudo nix-channel --add https://channels.nixos.org/nixos-unstable nixos

So now I have:

✦ ❯ sudo nix-channel --list
nixos https://channels.nixos.org/nixos-unstable

But when I rebuild, I get this error:

✦ ❯ sudo nixos-rebuild switch --upgrade
[sudo] password for amy: 
warning: Nix search path entry '/nix/var/nix/profiles/per-user/root/channels/nixos' does not exist, ignoring
error:
       … <borked>

         at «none»:0: (source not available)

       … while calling the 'import' builtin

         at «string»:1:6:

            1| with import <nixpkgs/nixos> {}; config.system.build.nixos-rebuild
             |      ^

       (stack trace truncated; use '--show-trace' to show the full trace)

       error: file 'nixpkgs/nixos' was not found in the Nix search path (add it using $NIX_PATH or -I)

       at «none»:0: (source not available)
building Nix...
warning: Nix search path entry '/nix/var/nix/profiles/per-user/root/channels/nixos' does not exist, ignoring
error: file 'nixpkgs/nixos' was not found in the Nix search path (add it using $NIX_PATH or -I)

       at «none»:0: (source not available)
warning: Nix search path entry '/nix/var/nix/profiles/per-user/root/channels/nixos' does not exist, ignoring
error: file 'nixpkgs' was not found in the Nix search path (add it using $NIX_PATH or -I)

       at «none»:0: (source not available)
warning: Nix search path entry '/nix/var/nix/profiles/per-user/root/channels/nixos' does not exist, ignoring
error: file 'nixpkgs/nixos/modules/installer/tools/nix-fallback-paths.nix' was not found in the Nix search path (add it using $NIX_PATH or -I)

       at «none»:0: (source not available)
/tmp/nixos-rebuild.86Fpgf/nix
warning: Nix search path entry '/nix/var/nix/profiles/per-user/root/channels/nixos' does not exist, ignoring
error: file 'nixpkgs' was not found in the Nix search path (add it using $NIX_PATH or -I)

       at «none»:0: (source not available)
building the system configuration...
warning: Nix search path entry '/nix/var/nix/profiles/per-user/root/channels/nixos' does not exist, ignoring
error: file 'nixpkgs/nixos' was not found in the Nix search path (add it using $NIX_PATH or -I)

       at «none»:0: (source not available)

nixos on  main [!?] took 3s 

[eclairevoyant]

Did you run nix-channel --update as root? BTW I think how-to questions are better asked on say, discourse

[mhwombat]

@eclairevoyant Thank you; that solved the problem.

Apologies for asking here instead of on Discourse. I thought since this was a known issue, it would be helpful to others to have the full fix documented here. But now that I know the solution was nix-channel --update, I realise that Discourse was the best place to ask.