nixos/lightdm: use systemd.tmpfiles

[worldofpeace]

Motivation for this change

A while ago I noticed with elementary that the data dir /var/lib/lightdm-data/<username> didn't have the correct permissions for their wallpaper switcher to set the greeters wallpaper (that is create a file there called wallpaper).

So this is a general improvement that came out of that.

For Clarity

I deleted --log-dir=/var/log --run-dir=/run in execCmd because these will be set to /var/log/lightdm and /var/run/lightdm by default.

Also the correct home location is /var/lib/lightdm.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• 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 nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Fits CONTRIBUTING.md.

---

[xeji]

We can test lightdm via the gnome3 test:
@GrahamcOfBorg test gnome3

[GrahamcOfBorg]

No attempt on aarch64-linux (full log)

The following builds were skipped because they don't evaluate on aarch64-linux: tests.gnome3

Partial log (click to expand)

a) For `nixos-rebuild` you can set
  { nixpkgs.config.allowUnsupportedSystem = true; }
in configuration.nix to override this.

b) For `nix-env`, `nix-build`, `nix-shell` or any other Nix command you can add
  { allowUnsupportedSystem = true; }
to ~/.config/nixpkgs/config.nix.

[GrahamcOfBorg]

Success on x86_64-linux (full log)

Attempted: tests.gnome3

Partial log (click to expand)

machine: exit status 1
syncing
machine: running command: sync
machine: exit status 0
test script finished in 59.02s
cleaning up
killing machine (pid 597)
vde_switch: EOF on stdin, cleaning up and exiting
vde_switch: Could not remove ctl dir '/build/vde1.ctl': Directory not empty
/nix/store/igiarq24l7xws3ckkpkgzp5ag9y0b7f8-vm-test-run-gnome3

[xeji]

Also ran nixos/tests/lightdm locally. LGTM.

[xeji]

backported to 18.09: 03f1f35

[worldofpeace]

Thanks ❇️ @xeji