Handle use-xdg-base-directories for profile link

[balsoft]

Motivation

#5588 broke the PATH setting logic in profile.sh: #5588 (comment)

This PR attempts to fix that by checking the use-xdg-base-directories config option and deciding whether to use the XDG-compliant location or the old one based on that.

Context

#5588 introduced a use-xdg-base-directories option which, when enabled, makes Nix use the new locations instead of ~/.nix-profile, ~/.nix-channel and ~/.nix-defexpr.
However, the accompanying profile.sh change was not quite correct: it chose the new location if the old one didn't exist.
Checking the use-xdg-base-directories should enable us to handle this properly (only adding the relevant profile link to $PATH).

Checklist for maintainers

Maintainers: tick if completed or explain if not relevant

• agreed on idea
• agreed on implementation strategy
• tests, as appropriate
  • functional tests - tests/**.sh
  • unit tests - src/*/tests
  • integration tests - tests/nixos/*
• documentation in the manual
• code and comments are self-explanatory
• commit message explains why the change was made
• new feature or incompatible change: updated release notes

[edolstra]

Ideally we shouldn't call nix (or any non-shell builtin) inside profile scripts since it slows down every shell invocation...

[balsoft]

Hm, true. Each invocation of nix show-config use-xdg-base-directories takes about 17 ms, which is quite a lot :(

[balsoft]

Would it be an acceptable compromise to just add both locations to $PATH then?

[edolstra]

I guess 17 ms is fine for login shells. That's probably preferrable over polluting $PATH with non-existent paths.

[Ericson2314]

Yeah I think it is fine for login shells.

[cole-h]

I'm slightly meh on the idea of needing to invoke nix on shell startup (even if it's "only" for login shells), but if this is decided as the accepted solution, we need to make sure to pass --extra-experimental-features (or use a legacy CLI invocation, if one exists...?).

[fricklerhandwerk]

Triaged in the Nix team meeting 2023-03-31:

• @Ericson2314: we need a plan to make XDG the default
  • it's a lot of work maintaining both
• to discuss

[nixos-discourse]

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

https://discourse.nixos.org/t/2023-03-31-nix-team-meeting-minutes-45/27002/1

[fricklerhandwerk]

Discussed in the Nix team meeting 2023-04-03:

• The team agrees we don't want to invoke Nix to check the config option, but no other decision was recorded in the meeting notes

• @Ericson2314 thinks the full solution will be getting rid of this config option altogether

  • then we are clearly going towards one version (e.g. check per XDG dir if they exist, or neither exists)
  • concretely, this would mean that just checking the state of the filesystem is actually correct, rather than mostly correct.

[nixos-discourse]

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

https://discourse.nixos.org/t/2023-04-03-nix-team-meeting-minutes-46/27008/1

[thufschmitt]

Re-discussed during the Nix team meeting. Since this isn't urgent any more (a less-correct but acceptable fix having being merged) @Ericson2314 will try an alternative approach that doesn't involve calling nix (with --extra-experimental-features 😱 ) in the init script. Closing that one in the meantime.

Discussion log

Not that urgent since another fix for the original issue landed
    That one is more principled, though
    But involves calling Nix in the shell startup script, which is meh
    @regnat: Also uses xp features in the init script, which is a no-go
    @ericson2314: We could make the usage of XDG dirs dependant on whether the corresponding files exist or not, which would make the current (master) solution totally correct, and would sidestep the problem altogether
        @ericson2314 will try doing that
        Will also make sure the default value indicates this.
    Will close that PR in the meantime

[nixos-discourse]

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

https://discourse.nixos.org/t/2023-07-24-nix-team-meeting-minutes-74/31116/1