nixos/filesystems: condition mount-pstore.service on unmounted /sys/fs/pstore

[ghost]

Motivation for this change

For unknown reasons, switching to a system that first introduces this service has it fail with /sys/fs/pstore already having been mounted (see discussion in #85073).

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Added a release notes entry if the change is major or breaking
• Fits CONTRIBUTING.md.

[zhaofengli]

Also tagging #123888.

[teto]

had the issue on rebuild too

[ckiee]

Just ran into this too, logs: https://i.ronthecookie.me/26958.txt.

I'll test this PR soon.

[nixos-discourse]

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

https://discourse.nixos.org/t/prs-ready-for-review/3032/532

[ghost]

Also needs a backport to 21.05 before the release.

[symphorien]

can you open the backport PR?

[ghost]

Yes, give me three minutes.

[ajs124]

I'm getting this, after upgrading to 21.05:

mount[729]: mount: /sys/fs/pstore: mount point does not exist.

Edit: This might be because of some custom kernel config?

[ghost]

The modprobe@pstore.service requirement should be guarding against a missing pstore module causing that particular failure. Is there any other info you can provide? I'll look into if there are any circumstances under which the pstore module will NOT create that path.

[ajs124]

That kernel does in fact not have the pstore module built.

If the executable path is prefixed with "-", an exit code of the command normally considered a failure (i.e. non-zero exit status or abnormal exit due to signal) is recorded, but has no further effect and is considered equivalent to success.

ExecStart=-/nix/store/3zkp71zhmdx4akmghd6nby1ibdy5sns5-kmod-27/sbin/modprobe -abq %I

Sooooo… I guess that doesn't work?

[ghost]

Oh. So much for that. Have to admit I never noticed that little dash. I'll create another PR - 3 is a good number. :D