nixos/displayManager: deprecate separate options for default wm/dm

The upstream session files display managers use have no concept of sessions being composed from
desktop manager and window manager. To be able to set upstream session files as default
session, we need a single option. Having two different ways to set default session would be confusing,
though, so we decided to deprecate the old method.

We also created separate script for each session, just like we already had a separate desktop
file for each one, and started using displayManager.sessionPackages mechanism to make the
session handling more uniform.

nixos/doc/manual/configuration/x-windows.xml

@@ -83,8 +83,7 @@
   desktop environment. If you wanted no desktop environment and i3 as your your
   window manager, you'd define:
 <programlisting>
-<xref linkend="opt-services.xserver.desktopManager.default"/> = "none";
-<xref linkend="opt-services.xserver.windowManager.default"/> = "i3";
+<xref linkend="opt-services.xserver.displayManager.defaultSession"/> = "none+i3";
 </programlisting>
   And, finally, to enable auto-login for a user <literal>johndoe</literal>:
 <programlisting>

nixos/doc/manual/configuration/xfce.xml

@@ -7,9 +7,8 @@
  <para>
   To enable the Xfce Desktop Environment, set
 <programlisting>
-<link linkend="opt-services.xserver.desktopManager.default">services.xserver.desktopManager</link> = {
-  <link linkend="opt-services.xserver.desktopManager.xfce.enable">xfce.enable</link> = true;
-  <link linkend="opt-services.xserver.desktopManager.default">default</link> = "xfce";
+<xref linkend="opt-services.xserver.desktopManager.xfce.enable" /> = true;
+<xref linkend="opt-services.xserver.displayManager.defaultSession" /> = "xfce";
 };
 </programlisting>
  </para>

nixos/doc/manual/release-notes/rl-2003.xml

@@ -55,6 +55,19 @@
       and adding a <option>--all</option> option which prints all options and their values.
     </para>
    </listitem>
+   <listitem>
+    <para>
+     <option>services.xserver.desktopManager.default</option> and <option>services.xserver.windowManager.default</option> options were replaced by a single <xref linkend="opt-services.xserver.displayManager.defaultSession"/> option to improve support for upstream session files. If you used something like:
+<programlisting>
+services.xserver.desktopManager.default = "xfce";
+services.xserver.windowManager.default = "icewm";
+</programlisting>
+     you should change it to:
+<programlisting>
+services.xserver.displayManager.defaultSession = "xfce+icewm";
+</programlisting>
+    </para>
+   </listitem>
   </itemizedlist>
  </section>
 

nixos/lib/testing-python.nix

@@ -262,9 +262,8 @@ in rec {
           virtualisation.memorySize = 1024;
           services.xserver.enable = true;
           services.xserver.displayManager.auto.enable = true;
-          services.xserver.windowManager.default = "icewm";
+          services.xserver.displayManager.defaultSession = "none+icewm";
           services.xserver.windowManager.icewm.enable = true;
-          services.xserver.desktopManager.default = "none";
         };
     in
       runInMachine ({

nixos/lib/testing.nix

@@ -249,9 +249,8 @@ in rec {
           virtualisation.memorySize = 1024;
           services.xserver.enable = true;
           services.xserver.displayManager.auto.enable = true;
-          services.xserver.windowManager.default = "icewm";
+          services.xserver.displayManager.defaultSession = "none+icewm";
           services.xserver.windowManager.icewm.enable = true;
-          services.xserver.desktopManager.default = "none";
         };
     in
       runInMachine ({

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

@@ -86,23 +86,14 @@ in
       };
 
       default = mkOption {
-        type = types.str;
-        default = "";
+        type = types.nullOr types.str;
+        default = null;
         example = "none";
-        description = "Default desktop manager loaded if none have been chosen.";
-        apply = defaultDM:
-          if defaultDM == "" && cfg.session.list != [] then
-            (head cfg.session.list).name
-          else if any (w: w.name == defaultDM) cfg.session.list then
-            defaultDM
-          else
-            throw ''
-              Default desktop manager (${defaultDM}) not found.
-              Probably you want to change
-                services.xserver.desktopManager.default = "${defaultDM}";
-              to one of
-                ${concatMapStringsSep "\n  " (w: "services.xserver.desktopManager.default = \"${w.name}\";") cfg.session.list}
-            '';
+        description = ''
+          <emphasis role="strong">Deprecated</emphasis>, please use <xref linkend="opt-services.xserver.displayManager.defaultSession"/> instead.
+
+          Default desktop manager loaded if none have been chosen.
+        '';
       };
 
     };

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

@@ -81,6 +81,8 @@ in
 
     services.xserver.displayManager.lightdm.greeters.pantheon.enable = mkDefault true;
 
+    # Without this, Elementary LightDM greeter will pre-select non-existent `default` session
+    # https://github.com/elementary/greeter/issues/368
     services.xserver.displayManager.defaultSession = "pantheon";
 
     services.xserver.displayManager.sessionCommands = ''

nixos/modules/services/x11/display-managers/account-service-util.nix

@@ -0,0 +1,39 @@
+{ accountsservice
+, glib
+, gobject-introspection
+, python3
+, wrapGAppsHook
+}:
+
+python3.pkgs.buildPythonApplication {
+  name = "set-session";
+
+  format = "other";
+
+  src = ./set-session.py;
+
+  dontUnpack = true;
+
+  strictDeps = false;
+
+  nativeBuildInputs = [
+    wrapGAppsHook
+    gobject-introspection
+  ];
+
+  buildInputs = [
+    accountsservice
+    glib
+  ];
+
+  propagatedBuildInputs = with python3.pkgs; [
+    pygobject3
+    ordered-set
+  ];
+
+  installPhase = ''
+    mkdir -p $out/bin
+    cp $src $out/bin/set-session
+    chmod +x $out/bin/set-session
+  '';
+}