Merge pull request #31268 from Ma27/x11-defaults

services.xserver: fix defaults of X11

nixos/doc/manual/release-notes/rl-1803.xml

@@ -100,6 +100,14 @@ following incompatible changes:</para>
       to connect to hidden networks.
     </para>
   </listitem>
+  <listitem>
+    <para>
+      The option <option>services.xserver.desktopManager.default</option> is now <literal>none</literal> by default.
+      An assertion failure is thrown if WM's and DM's default are <literal>none</literal>.
+      To explicitly run a plain X session without and DM or WM, the newly introduced option <option>services.xserver.plainX</option>
+      must be set to true.
+    </para>
+  </listitem>
 </itemizedlist>
 
 </section>

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

@@ -87,11 +87,11 @@ in
 
       default = mkOption {
         type = types.str;
-        default = "";
+        default = "none";
-        example = "none";
+        example = "plasma5";
         description = "Default desktop manager loaded if none have been chosen.";
         apply = defaultDM:
-          if defaultDM == "" && cfg.session.list != [] then
+          if defaultDM == "none" && cfg.session.list != [] then
             (head cfg.session.list).name
           else if any (w: w.name == defaultDM) cfg.session.list then
             defaultDM

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

@@ -61,7 +61,9 @@ in
         example = "wmii";
         description = "Default window manager loaded if none have been chosen.";
         apply = defaultWM:
-          if any (w: w.name == defaultWM) cfg.session then
+          if defaultWM == "none" && cfg.session != []  then
+            (head cfg.session).name
+          else if any (w: w.name == defaultWM) cfg.session then
             defaultWM
           else
             throw "Default window manager (${defaultWM}) not found.";

nixos/modules/services/x11/xserver.nix

@@ -161,6 +161,15 @@ in
         '';
       };
 
+      plainX = mkOption {
+        type = types.bool;
+        default = false;
+        description = ''
+          Whether the X11 session can be plain (without DM/WM) and
+          the Xsession script will be used as fallback or not.
+        '';
+      };
+
       autorun = mkOption {
         type = types.bool;
         default = true;
@@ -552,6 +561,11 @@ in
                 + "${toString (length primaryHeads)} heads set to primary: "
                 + concatMapStringsSep ", " (x: x.output) primaryHeads;
       })
+      { assertion = cfg.desktopManager.default == "none" && cfg.windowManager.default == "none" -> cfg.plainX;
+        message = "Either the desktop manager or the window manager shouldn't be `none`! "
+                + "To explicitly allow this, you can also set `services.xserver.plainX` to `true`. "
+                + "The `default` value looks for enabled WMs/DMs and select the first one.";
+      }
     ];
 
     environment.etc =