generic-linux: add option extraXdgDataDirs

PR #1486

modules/misc/news.nix

@@ -1649,6 +1649,21 @@ in
           A new module is available: 'programs.ncmpcpp'
         '';
       }
+
+      {
+        time = "2020-09-11T10:06:47+00:00";
+        condition = hostPlatform.isLinux && config.targets.genericLinux.enable;
+        message = ''
+          A new option 'targets.genericLinux.extraXdgDataDirs' is available
+          to setup the user environment with the OS's data files.
+
+          This is useful for example to get Bash completion for
+          'systemctl' which shouldn't be installed through Home Manager.
+
+          This is also useful to have non Home Manager applications
+          available in menus.
+        '';
+      }
     ];
   };
 }

modules/targets/generic-linux.nix

@@ -14,14 +14,24 @@ in {
         GNU/Linux distributions other than NixOS.
       '';
     };
+
+    extraXdgDataDirs = mkOption {
+      type = types.listOf types.str;
+      default = [ ];
+      example = [ "/usr/share" "/usr/local/share" ];
+      description = ''
+        List of directory names to add to <envar>XDG_DATA_DIRS</envar>.
+      '';
+    };
   };
 
   config = mkIf config.targets.genericLinux.enable {
     home.sessionVariables = let
       profiles =
         [ "\${NIX_STATE_DIR:-/nix/var/nix}/profiles/default" profileDirectory ];
-      dataDirs =
+      dataDirs = concatStringsSep ":"
-        concatStringsSep ":" (map (profile: "${profile}/share") profiles);
+        (map (profile: "${profile}/share") profiles
+          ++ config.targets.genericLinux.extraXdgDataDirs);
     in { XDG_DATA_DIRS = "${dataDirs}\${XDG_DATA_DIRS:+:}$XDG_DATA_DIRS"; };
 
     home.sessionVariablesExtra = ''

tests/modules/targets-linux/generic-linux.nix

@@ -4,13 +4,16 @@ with lib;
 
 {
   config = {
-    targets.genericLinux.enable = true;
+    targets.genericLinux = {
+      enable = true;
+      extraXdgDataDirs = [ "/foo" ];
+    };
 
     nmt.script = ''
       assertFileExists home-path/etc/profile.d/hm-session-vars.sh
       assertFileContains \
         home-path/etc/profile.d/hm-session-vars.sh \
-        'export XDG_DATA_DIRS="''${NIX_STATE_DIR:-/nix/var/nix}/profiles/default/share:/home/hm-user/.nix-profile/share''${XDG_DATA_DIRS:+:}$XDG_DATA_DIRS"'
+        'export XDG_DATA_DIRS="''${NIX_STATE_DIR:-/nix/var/nix}/profiles/default/share:/home/hm-user/.nix-profile/share:/foo''${XDG_DATA_DIRS:+:}$XDG_DATA_DIRS"'
       assertFileContains \
         home-path/etc/profile.d/hm-session-vars.sh \
         '. "${pkgs.nix}/etc/profile.d/nix.sh"'