nix-profile: fix both profile links detection

[wh0]

Motivation

In #7925, using "new nix link exists" instead of "old nix link doesn't exist" mistakenly made it impossible to trigger the "oh no, both links exist" logic. This restores that.

Context

What I gathered from the discussion in March, pushing harder to use the new link is not a priority. But the logic is here, so I want to let it work.

Priorities

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[roberth]

Could you add a test case?

[wh0]

what invokes the tests?

[edolstra]

@wh0 Something like:

nix build .#hydraJobs.installerTests.ubuntu-22-04.x86_64-linux.{install-default,install-force-daemon,install-force-no-daemon}

[wh0]

come to think of it, Nix has already changed to using the new link when both exist, and no one has complained. I'm going to rework this to maintain that logic, only reinstating the both-links-exist diagnostic and updating the wording accordingly to reflect this new behavior.

analysis of the timeline:

1. nixos 23.05 ships with nix 2.13
2. nix 2.14 comes out with support for the new link (Follow XDG Base Directory standard #5588). I believe it creates the new link on new installs, from the if ! [ -e "$NIX_LINK" ]; then NIX_LINK="$NIX_LINK_NEW" logic. there is also a diagnostic that detects when both links are present, and it uses the old link in this case.
3. nix 2.14.1 comes out with a change back to using the old link for new installs, if [ -e "$NIX_LINK_NEW" ]; then NIX_LINK="$NIX_LINK_NEW". however, this changes the logic to use the new link when both links are present.
4. nixos 23.11 ships with nix 2.18, I think

updated goals:

• create the old link for new installs where neither link initially exists
• when both links exist, display the diagnostic message
• when both links exist, use the new link

[wh0]

@wh0 Something like:

nix build .#hydraJobs.installerTests.ubuntu-22-04.x86_64-linux.{install-default,install-force-daemon,install-force-no-daemon}

wow is that going to make me recompile the whole project each time I make a change to the test script? I don't think I'm going to be able to do this until a few more iterations of moore's law

[wh0]

oh phew, it recompiles nix for each of the install tests, but I don't have to rebuild it each time I change a test.

added an installer test case with both links present

[roberth]

This looks good now. Thank you, and sorry for the delay.

[wh0]

no worries, thanks for merging!

[nixos-discourse]

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

https://discourse.nixos.org/t/2024-06-03-nix-team-meeting-minutes-149/46582/1