nixos/desktop-manager/none: add option to run XDG autostart files

`fcitx5` and `service.earlyoom` rely on use XDG autostart files to start.
But for X session with only window manager and no desktop manager
(`none` is used), no one can start them.

This options is added to run these autostart files for sessions without
desktop manager to make other services just work.

nixos/doc/manual/from_md/release-notes/rl-2205.section.xml

···
       </listitem>
       <listitem>
         <para>
+          The option
+          <link linkend="opt-services.xserver.desktopManager.runXdgAutostartIfNone">services.xserver.desktopManager.runXdgAutostartIfNone</link>
+          was added in order to automatically run XDG autostart files
+          for sessions without a desktop manager. This replaces helpers
+          like the <literal>dex</literal> package.
+        </para>
+      </listitem>
+      <listitem>
+        <para>
           A new module was added for the Envoy reverse proxy, providing
           the options <literal>services.envoy.enable</literal> and
           <literal>services.envoy.settings</literal>.

nixos/doc/manual/release-notes/rl-2205.section.md

···
 
 - The `services.stubby` module was converted to a [settings-style](https://github.com/NixOS/rfcs/blob/master/rfcs/0042-config-option.md) configuration.
 
+- The option
+  [services.xserver.desktopManager.runXdgAutostartIfNone](#opt-services.xserver.desktopManager.runXdgAutostartIfNone)
+  was added in order to automatically run XDG autostart files for sessions without a desktop manager.
+  This replaces helpers like the `dex` package.
+
 - A new module was added for the Envoy reverse proxy, providing the options `services.envoy.enable` and `services.envoy.settings`.
 
 - The option `services.duplicati.dataDir` has been added to allow changing the location of duplicati's files.

nixos/modules/services/x11/desktop-managers/none.nix

···
+{ config, lib, pkgs, ... }:
+with lib;
+let
+  runXdgAutostart = config.services.xserver.desktopManager.runXdgAutostartIfNone;
+in
 {
-  services.xserver.desktopManager.session =
-    [ { name = "none";
-        start = "";
-      }
-    ];
+  options = {
+    services.xserver.desktopManager.runXdgAutostartIfNone = mkOption {
+      type = types.bool;
+      default = false;
+      description = ''
+        Whether to run XDG autostart files for sessions without a desktop manager
+        (with only a window manager), these sessions usually don't handle XDG
+        autostart files by default.
+
+        Some services like <option>i18n.inputMethod</option> and
+        <option>service.earlyoom</option> use XDG autostart files to start.
+        If this option is not set to <literal>true</literal> and you are using
+        a window manager without a desktop manager, you need to manually start
+        them or running <package>dex</package> somewhere.
+      '';
+    };
+  };
+
+  config = mkMerge [
+    {
+      services.xserver.desktopManager.session = [
+        {
+          name = "none";
+          start = optionalString runXdgAutostart ''
+            /run/current-system/systemd/bin/systemctl --user start xdg-autostart-if-no-desktop-manager.target
+          '';
+        }
+      ];
+    }
+    (mkIf runXdgAutostart {
+      systemd.user.targets.xdg-autostart-if-no-desktop-manager = {
+        description = "Run XDG autostart files";
+        # From `plasma-workspace`, `share/systemd/user/plasma-workspace@.target`.
+        requires = [ "xdg-desktop-autostart.target" "graphical-session.target" ];
+        before = [ "xdg-desktop-autostart.target" "graphical-session.target" ];
+        bindsTo = [ "graphical-session.target" ];
+      };
+    })
+  ];
 }

nixos/tests/all-tests.nix

···
   xautolock = handleTest ./xautolock.nix {};
   xfce = handleTest ./xfce.nix {};
   xmonad = handleTest ./xmonad.nix {};
+  xmonad-xdg-autostart = handleTest ./xmonad-xdg-autostart.nix {};
   xrdp = handleTest ./xrdp.nix {};
   xss-lock = handleTest ./xss-lock.nix {};
   xterm = handleTest ./xterm.nix {};

nixos/tests/xmonad-xdg-autostart.nix

···
+import ./make-test-python.nix ({ lib, ... }: {
+  name = "xmonad-xdg-autostart";
+  meta.maintainers = with lib.maintainers; [ oxalica ];
+
+  nodes.machine = { pkgs, config, ... }: {
+    imports = [ ./common/x11.nix ./common/user-account.nix ];
+    test-support.displayManager.auto.user = "alice";
+    services.xserver.displayManager.defaultSession = "none+xmonad";
+    services.xserver.windowManager.xmonad.enable = true;
+    services.xserver.desktopManager.runXdgAutostartIfNone = true;
+
+    environment.systemPackages = [
+      (pkgs.writeTextFile {
+        name = "test-xdg-autostart";
+        destination = "/etc/xdg/autostart/test-xdg-autostart.desktop";
+        text = ''
+          [Desktop Entry]
+          Name=test-xdg-autoatart
+          Type=Application
+          Terminal=false
+          Exec=${pkgs.coreutils}/bin/touch ${config.users.users.alice.home}/xdg-autostart-executed
+        '';
+      })
+    ];
+  };
+
+  testScript = { nodes, ... }:
+    let
+      user = nodes.machine.config.users.users.alice;
+    in
+    ''
+      machine.wait_for_x()
+      machine.wait_for_file("${user.home}/xdg-autostart-executed")
+    '';
+})