user-units conflict with packages

[jtojnar]

When a package contains a directory in one of the systemd directories (like flatpak does), it is symlinked into the *-units derivation:

nixpkgs/nixos/modules/system/boot/systemd-lib.nix

Line 139 in 64965e8

ln -s $fn $out/

Then later, the derivation will try to create the directory:

nixpkgs/nixos/modules/system/boot/systemd-lib.nix

Line 154 in 64965e8

mkdir $out/$fn.d

which will fail:

$ nixos-rebuild build-vm -I nixpkgs=https://github.com/NixOS/nixpkgs/archive/afac8c61e9542ad04cd9da0cd53f2e2aaaf1aeab.tar.gz -I nixos-config=dbus-collision.nix
building Nix...
building the system configuration...
these derivations will be built:
  /nix/store/q7iwii64xvgalffh5hc6ri61zr5q04yl-user-units.drv
  /nix/store/jz8761r0akvbja2sqgjxhla2cb5yjdkj-etc.drv
  /nix/store/xq3ldd72j8678icjxc1ja01h1bidkic0-nixos-system-nixos-18.03pre-git.drv
  /nix/store/gl6w1408n7bm37h01hwz85qq4c17a8hh-closure-info.drv
  /nix/store/5pskg6ybx5pflm23ih8x8vbkpvc0jd5p-run-nixos-vm.drv
  /nix/store/d8jiyxbay3v1g8vvmacvvzkv3cq2gmxc-nixos-vm.drv
building '/nix/store/q7iwii64xvgalffh5hc6ri61zr5q04yl-user-units.drv'...
mkdir: cannot create directory '/nix/store/pf3vkdl7nwhh1fvd7cy420jjinqvan5n-user-units/dbus.service.d': File exists
builder for '/nix/store/q7iwii64xvgalffh5hc6ri61zr5q04yl-user-units.drv' failed with exit code 1
cannot build derivation '/nix/store/jz8761r0akvbja2sqgjxhla2cb5yjdkj-etc.drv': 1 dependencies couldn't be built
cannot build derivation '/nix/store/xq3ldd72j8678icjxc1ja01h1bidkic0-nixos-system-nixos-18.03pre-git.drv': 1 dependencies couldn't be built
cannot build derivation '/nix/store/gl6w1408n7bm37h01hwz85qq4c17a8hh-closure-info.drv': 1 dependencies couldn't be built
cannot build derivation '/nix/store/5pskg6ybx5pflm23ih8x8vbkpvc0jd5p-run-nixos-vm.drv': 2 dependencies couldn't be built
cannot build derivation '/nix/store/d8jiyxbay3v1g8vvmacvvzkv3cq2gmxc-nixos-vm.drv': 2 dependencies couldn't be built
error: build of '/nix/store/d8jiyxbay3v1g8vvmacvvzkv3cq2gmxc-nixos-vm.drv' failed

You can reproduce it with the following configuration:

{ pkgs, ... }:
let
  conflictingDerivation = pkgs.stdenv.mkDerivation {
    name = "conflicting-derivation";
    phases = [ "installPhase" ];
    installPhase = ''
      mkdir -p "$out/lib/systemd/user/dbus.service.d"
    '';
  };
in {
  systemd.packages = [ conflictingDerivation ];
}