xsession: make updating DBus environment optional

nixos/modules/services/x11/desktop-managers/gnome3.nix

@@ -165,6 +165,8 @@ in {
         '';
       };
 
+    services.xserver.updateDbusEnvironment = true;
+
     environment.variables.GIO_EXTRA_MODULES = [ "${gnome3.dconf}/lib/gio/modules"
                                                 "${gnome3.glib_networking.out}/lib/gio/modules"
                                                 "${gnome3.gvfs}/lib/gio/modules" ];

nixos/modules/services/x11/desktop-managers/xfce.nix

@@ -42,10 +42,13 @@ in
             # Set GTK_DATA_PREFIX so that GTK+ can find the Xfce themes.
             export GTK_DATA_PREFIX=${config.system.path}
 
-            exec ${pkgs.stdenv.shell} ${pkgs.xfce.xinitrc}
+            ${pkgs.stdenv.shell} ${pkgs.xfce.xinitrc} &
+            waitPID=$!
           '';
       };
 
+    services.xserver.updateDbusEnvironment = true;
+
     environment.systemPackages =
       [ pkgs.gtk # To get GTK+'s themes.
         pkgs.hicolor_icon_theme

nixos/modules/services/x11/display-managers/default.nix

@@ -126,7 +126,7 @@ let
         (*) echo "$0: Desktop manager '$desktopManager' not found.";;
       esac
 
-      ${optionalString cfg.startDbusSession ''
+      ${optionalString (cfg.startDbusSession && cfg.updateDbusEnvironment) ''
         ${pkgs.glib}/bin/gdbus call --session \
           --dest org.freedesktop.DBus --object-path /org/freedesktop/DBus \
           --method org.freedesktop.DBus.UpdateActivationEnvironment \

nixos/modules/services/x11/xserver.nix

@@ -233,6 +233,15 @@ in
         '';
       };
 
+      updateDbusEnvironment = mkOption {
+        type = types.bool;
+        default = false;
+        description = ''
+          Whether to update the DBus activation environment after launching the
+          desktop manager.
+        '';
+      };
+
       layout = mkOption {
         type = types.str;
         default = "us";