nixos/tools: generate an xserver config

The desktop configuration won't enable xserver.
If there's multiple desktops enabled they will now evaluate properly.

nixos/modules/installer/tools/nixos-generate-config.pl

@@ -585,6 +585,22 @@ EOF
     return $config;
 }
 
+sub generateXserverConfig {
+    my $xserverEnabled = "@xserverEnabled@";
+
+    my $config = "";
+    if ($xserverEnabled eq "1") {
+        $config = <<EOF;
+  # Enable the X11 windowing system.
+  services.xserver.enable = true;
+EOF
+    } else {
+        $config = <<EOF;
+  # Enable the X11 windowing system.
+  # services.xserver.enable = true;
+EOF
+    }
+}
 
 if ($showHardwareConfig) {
     print STDOUT $hwConfig;
@@ -630,6 +646,8 @@ EOF
 
         my $networkingDhcpConfig = generateNetworkingDhcpConfig();
 
+        my $xserverConfig = generateXserverConfig();
+
         (my $desktopConfiguration = <<EOF)=~s/^/  /gm;
 @desktopConfiguration@
 EOF

nixos/modules/installer/tools/tools.nix

@@ -36,6 +36,7 @@ let
     path = lib.optionals (lib.elem "btrfs" config.boot.supportedFilesystems) [ pkgs.btrfs-progs ];
     perl = "${pkgs.perl}/bin/perl -I${pkgs.perlPackages.FileSlurp}/${pkgs.perl.libPrefix}";
     inherit (config.system.nixos-generate-config) configuration desktopConfiguration;
+    xserverEnabled = config.services.xserver.enable;
   };
 
   nixos-option =
@@ -136,6 +137,8 @@ in
         #   keyMap = "us";
         # };
 
+      $xserverConfig
+
       $desktopConfiguration
         # Configure keymap in X11
         # services.xserver.layout = "us";

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

@@ -199,7 +199,6 @@ in
       # Seed our configuration into nixos-generate-config
       system.nixos-generate-config.desktopConfiguration = [''
         # Enable the GNOME 3 Desktop Environment.
-        services.xserver.enable = true;
         services.xserver.displayManager.gdm.enable = true;
         services.xserver.desktopManager.gnome3.enable = true;
       ''];

nixos/modules/services/x11/desktop-managers/plasma5.nix

@@ -186,7 +186,6 @@ in
       # Seed our configuration into nixos-generate-config
       system.nixos-generate-config.desktopConfiguration = [''
         # Enable the Plasma 5 Desktop Environment.
-        services.xserver.enable = true;
         services.xserver.displayManager.sddm.enable = true;
         services.xserver.desktopManager.plasma5.enable = true;
       ''];

nixos/tests/nixos-generate-config.nix

@@ -13,9 +13,8 @@ import ./make-test-python.nix ({ lib, ... } : {
 
     system.nixos-generate-config.desktopConfiguration = [''
       # DESKTOP
-      # services.xserver.enable = true;
+      services.xserver.displayManager.gdm.enable = true;
-      # services.xserver.displayManager.gdm.enable = true;
+      services.xserver.desktopManager.gnome3.enable = true;
-      # services.xserver.desktopManager.gnome3.enable = true;
     ''];
   };
   testScript = ''