Motivation for this change

• current state isn't very reproducible when stateDir deviates from the expected value (/var/lib/mpd)... also we don't have the ability to understand what the filesystem looks in order to make the right choices
  • maybe the user has network mounted drives we don't know about
  • maybe the user is using btrfs subvolumes
  • maybe something else you never considered
• modules make too many assumptions about stateDir, resulting in various permissions issues:
  • postgresql fails to start if dataDir is not default  #80827
  • Cassandra home path permission not set correctly during installation if homeDir is different from default #81910
  • https://discourse.nixos.org/t/nixpkgs-policy-as-for-systemd-prestart-setup-scripts-vs-systemd-tmpfiles/5839/7
• when given arbitrary paths systemd.tmpfiles.rules generally doesn't behave as most users would expect
  • [ "d /a foo foo" "d /a/b/c foo foo" ] produces an error (rightfully so) because /a/b would be owned by root:root
• in conclusion: it is great when NixOS "just works" because of defaults, but in the case of user customization we should push some of the work onto the user for maximum flexibility

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/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.