Add custom lightdm-gtk module for default session

The upstream module does not allow for setting a default session,
although this is possible in lightdm-gtk-greeter for more than two years
now.

modules/boot-login.nix

@@ -1,5 +1,6 @@
 {
   pkgs,
+  lib,
   ...
 }: {
   boot = {
@@ -15,7 +16,8 @@
     xserver.displayManager = {
       lightdm = {
         enable = true;
-	greeters.gtk = {
+        greeters.gtk.enable = false;
+        greeters.gtk-custom = {
           enable = true;
           theme = {
             name = "Adwaita-dark";

modules/lightdm-gtk.nix

@@ -0,0 +1,167 @@
+{
+  config,
+  lib,
+  pkgs,
+  ...
+}:
+
+with lib;
+
+let
+
+  dmcfg = config.services.xserver.displayManager;
+  ldmcfg = dmcfg.lightdm;
+  xcfg = config.services.xserver;
+  cfg = ldmcfg.greeters.gtk-custom;
+
+  inherit (pkgs) writeText;
+
+  theme = cfg.theme.package;
+  icons = cfg.iconTheme.package;
+  cursors = cfg.cursorTheme.package;
+
+  gtkGreeterConf = writeText "lightdm-gtk-greeter.conf" ''
+    [greeter]
+    theme-name = ${cfg.theme.name}
+    icon-theme-name = ${cfg.iconTheme.name}
+    cursor-theme-name = ${cfg.cursorTheme.name}
+    cursor-theme-size = ${toString cfg.cursorTheme.size}
+    background = ${ldmcfg.background}
+    ${optionalString (dmcfg.defaultSession != null) "default-session = ${dmcfg.defaultSession}"}
+    ${optionalString (cfg.clock-format != null) "clock-format = ${cfg.clock-format}"}
+    ${optionalString (cfg.indicators != null) "indicators = ${concatStringsSep ";" cfg.indicators}"}
+    ${optionalString (xcfg.dpi != null) "xft-dpi=${toString xcfg.dpi}"}
+    ${cfg.extraConfig}
+  '';
+
+in
+{
+  options = {
+
+    services.xserver.displayManager.lightdm.greeters.gtk-custom = {
+
+      enable = mkOption {
+        type = types.bool;
+        default = true;
+        description = ''
+          Whether to enable lightdm-gtk-greeter as the lightdm greeter.
+        '';
+      };
+
+      theme = {
+
+        package = mkPackageOption pkgs "gnome-themes-extra" { };
+
+        name = mkOption {
+          type = types.str;
+          default = "Adwaita";
+          description = ''
+            Name of the theme to use for the lightdm-gtk-greeter.
+          '';
+        };
+
+      };
+
+      iconTheme = {
+
+        package = mkPackageOption pkgs "adwaita-icon-theme" { };
+
+        name = mkOption {
+          type = types.str;
+          default = "Adwaita";
+          description = ''
+            Name of the icon theme to use for the lightdm-gtk-greeter.
+          '';
+        };
+
+      };
+
+      cursorTheme = {
+
+        package = mkPackageOption pkgs "adwaita-icon-theme" { };
+
+        name = mkOption {
+          type = types.str;
+          default = "Adwaita";
+          description = ''
+            Name of the cursor theme to use for the lightdm-gtk-greeter.
+          '';
+        };
+
+        size = mkOption {
+          type = types.int;
+          default = 16;
+          description = ''
+            Size of the cursor theme to use for the lightdm-gtk-greeter.
+          '';
+        };
+      };
+
+      clock-format = mkOption {
+        type = types.nullOr types.str;
+        default = null;
+        example = "%F";
+        description = ''
+          Clock format string (as expected by strftime, e.g. "%H:%M")
+          to use with the lightdm gtk greeter panel.
+
+          If set to null the default clock format is used.
+        '';
+      };
+
+      indicators = mkOption {
+        type = types.nullOr (types.listOf types.str);
+        default = null;
+        example = [
+          "~host"
+          "~spacer"
+          "~clock"
+          "~spacer"
+          "~session"
+          "~language"
+          "~a11y"
+          "~power"
+        ];
+        description = ''
+          List of allowed indicator modules to use for the lightdm gtk
+          greeter panel.
+
+          Built-in indicators include "~a11y", "~language", "~session",
+          "~power", "~clock", "~host", "~spacer". Unity indicators can be
+          represented by short name (e.g. "sound", "power"), service file name,
+          or absolute path.
+
+          If set to null the default indicators are used.
+        '';
+      };
+
+      extraConfig = mkOption {
+        type = types.lines;
+        default = "";
+        description = ''
+          Extra configuration that should be put in the lightdm-gtk-greeter.conf
+          configuration file.
+        '';
+      };
+
+    };
+
+  };
+
+  config = mkIf (ldmcfg.enable && cfg.enable) {
+
+    services.xserver.displayManager.lightdm.greeter = mkDefault {
+      package = pkgs.lightdm-gtk-greeter.xgreeters;
+      name = "lightdm-gtk-greeter";
+    };
+
+    environment.systemPackages = [
+      cursors
+      icons
+      theme
+    ];
+
+    environment.etc."lightdm/lightdm-gtk-greeter.conf".source = gtkGreeterConf;
+
+  };
+}