systemd: do not install systemd files when user is root (#2454)

For the user root, there are no user services provided by systemd.
Therefore, these files will never be used.

modules/systemd.nix

@@ -258,7 +258,8 @@ in {
 
     # If we run under a Linux system we assume that systemd is
     # available, in particular we assume that systemctl is in PATH.
-    (mkIf pkgs.stdenv.isLinux {
+    # Do not install any user services if username is root.
+    (mkIf (pkgs.stdenv.isLinux && config.home.username != "root") {
       xdg.configFile = mkMerge [
         (lib.listToAttrs ((buildServices "service" cfg.services)
           ++ (buildServices "slices" cfg.slices)

tests/modules/systemd/default.nix

@@ -1,5 +1,6 @@
 {
   systemd-services = ./services.nix;
+  systemd-services-disabled-for-root = ./services-disabled-for-root.nix;
   systemd-session-variables = ./session-variables.nix;
   systemd-timers = ./timers.nix;
 }

tests/modules/systemd/services-disabled-for-root.nix

@@ -0,0 +1,23 @@
+{ config, lib, pkgs, ... }:
+
+with lib;
+
+{
+  config = {
+    home.username = mkForce "root";
+
+    systemd.user.services."test-service@" = {
+      Unit = { Description = "A basic test service"; };
+
+      Service = {
+        Environment = [ "VAR1=1" "VAR2=2" ];
+        ExecStart = ''/some/exec/start/command --with-arguments "%i"'';
+      };
+    };
+
+    nmt.script = ''
+      serviceFile=home-files/.config/systemd/user/test-service@.service
+      assertPathNotExists $serviceFile
+    '';
+  };
+}