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.

9 files changed, 225 insertions, 201 deletions

diff --git a/nixos/modules/services/x11/desktop-managers/default.nix b/nixos/modules/services/x11/desktop-managers/default.nix
index 7240358c361..534551c0c4a 100644
--- a/nixos/modules/services/x11/desktop-managers/default.nix
+++ b/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.
+        '';
       };
 
     };
diff --git a/nixos/modules/services/x11/desktop-managers/pantheon.nix b/nixos/modules/services/x11/desktop-managers/pantheon.nix
index 3722bf365ba..e07d5b5eaad 100644
--- a/nixos/modules/services/x11/desktop-managers/pantheon.nix
+++ b/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 = ''
diff --git a/nixos/modules/services/x11/display-managers/account-service-util.nix b/nixos/modules/services/x11/display-managers/account-service-util.nix
new file mode 100644
index 00000000000..1dbe703b566
--- /dev/null
+++ b/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
+  '';
+}
diff --git a/nixos/modules/services/x11/display-managers/default.nix b/nixos/modules/services/x11/display-managers/default.nix
index 1a46e7c02e5..2d809b5cc9f 100644
--- a/nixos/modules/services/x11/display-managers/default.nix
+++ b/nixos/modules/services/x11/display-managers/default.nix
@@ -27,16 +27,7 @@ let
     Xft.hintstyle: hintslight
   '';
 
-  mkCases = session:
-    concatStrings (
-      mapAttrsToList (name: starts: ''
-                       (${name})
-                         ${concatMapStringsSep "\n  " (n: n.start) starts}
-                         ;;
-                     '') (lib.groupBy (n: n.name) session)
-    );
-
-  # file provided by services.xserver.displayManager.session.wrapper
+  # file provided by services.xserver.displayManager.sessionData.wrapper
   xsessionWrapper = pkgs.writeScript "xsession-wrapper"
     ''
       #! ${pkgs.bash}/bin/bash
@@ -116,79 +107,19 @@ let
           # Run the supplied session command. Remove any double quotes with eval.
           eval exec "$@"
       else
-          # Fall back to the default window/desktopManager
-          exec ${cfg.displayManager.session.script}
+          # TODO: Do we need this? Should not the session always exist?
+          echo "error: unknown session $1" 1>&2
+          exit 1
       fi
     '';
 
-  # file provided by services.xserver.displayManager.session.script
-  xsession = wm: dm: pkgs.writeScript "xsession"
-    ''
-      #! ${pkgs.bash}/bin/bash
-
-      # Legacy session script used to construct .desktop files from
-      # `services.xserver.displayManager.session` entries. Called from
-      # `sessionWrapper`.
-
-      # Expected parameters:
-      #   $1 = <desktop-manager>+<window-manager>
-
-      # The first argument of this script is the session type.
-      sessionType="$1"
-      if [ "$sessionType" = default ]; then sessionType=""; fi
-
-      # The session type is "<desktop-manager>+<window-manager>", so
-      # extract those (see:
-      # http://wiki.bash-hackers.org/syntax/pe#substring_removal).
-      windowManager="''${sessionType##*+}"
-      : ''${windowManager:=${cfg.windowManager.default}}
-      desktopManager="''${sessionType%%+*}"
-      : ''${desktopManager:=${cfg.desktopManager.default}}
-
-      # Start the window manager.
-      case "$windowManager" in
-        ${mkCases wm}
-        (*) echo "$0: Window manager '$windowManager' not found.";;
-      esac
-
-      # Start the desktop manager.
-      case "$desktopManager" in
-        ${mkCases dm}
-        (*) echo "$0: Desktop manager '$desktopManager' not found.";;
-      esac
-
-      ${optionalString cfg.updateDbusEnvironment ''
-        ${lib.getBin pkgs.dbus}/bin/dbus-update-activation-environment --systemd --all
-      ''}
-
-      test -n "$waitPID" && wait "$waitPID"
-
-      ${config.systemd.package}/bin/systemctl --user stop graphical-session.target
-
-      exit 0
-    '';
-
-  # Desktop Entry Specification:
-  # - https://standards.freedesktop.org/desktop-entry-spec/latest/
-  # - https://standards.freedesktop.org/desktop-entry-spec/latest/ar01s06.html
-  mkDesktops = names: pkgs.runCommand "desktops"
+  installedSessions = pkgs.runCommand "desktops"
     { # trivial derivation
       preferLocalBuild = true;
       allowSubstitutes = false;
     }
     ''
-      mkdir -p "$out/share/xsessions"
-      ${concatMapStrings (n: ''
-        cat - > "$out/share/xsessions/${n}.desktop" << EODESKTOP
-        [Desktop Entry]
-        Version=1.0
-        Type=XSession
-        TryExec=${cfg.displayManager.session.script}
-        Exec=${cfg.displayManager.session.script} "${n}"
-        Name=${n}
-        Comment=
-        EODESKTOP
-      '') names}
+      mkdir -p "$out/share/"{xsessions,wayland-sessions}
 
       ${concatMapStrings (pkg: ''
         for n in ${concatStringsSep " " pkg.providedSessions}; do
@@ -204,12 +135,16 @@ let
           ${xorg.lndir}/bin/lndir ${pkg}/share/xsessions $out/share/xsessions
         fi
         if test -d ${pkg}/share/wayland-sessions; then
-          mkdir -p "$out/share/wayland-sessions"
           ${xorg.lndir}/bin/lndir ${pkg}/share/wayland-sessions $out/share/wayland-sessions
         fi
       '') cfg.displayManager.sessionPackages}
     '';
 
+  dmDefault = cfg.desktopManager.default;
+  wmDefault = cfg.windowManager.default;
+
+  defaultSessionFromLegacyOptions = concatStringsSep "+" (filter (s: s != null) ([ dmDefault ] ++ optional (wmDefault != "none") wmDefault));
+
 in
 
 {
@@ -315,47 +250,48 @@ in
           inside the display manager with the desktop manager name
           followed by the window manager name.
         '';
-        apply = list: rec {
-          wm = filter (s: s.manage == "window") list;
-          dm = filter (s: s.manage == "desktop") list;
-          names = flip concatMap dm
-            (d: map (w: d.name + optionalString (w.name != "none") ("+" + w.name))
-              (filter (w: d.name != "none" || w.name != "none") wm));
-          desktops = mkDesktops names;
-          script = xsession wm dm;
+      };
+
+      sessionData = mkOption {
+        description = "Data exported for display managers’ convenience";
+        internal = true;
+        default = {};
+        apply = val: {
           wrapper = xsessionWrapper;
+          desktops = installedSessions;
+          sessionNames = concatMap (p: p.providedSessions) cfg.displayManager.sessionPackages;
+          # We do not want to force users to set defaultSession when they have only single DE.
+          autologinSession =
+            if cfg.displayManager.defaultSession != null then
+              cfg.displayManager.defaultSession
+            else if cfg.displayManager.sessionData.sessionNames != [] then
+              head cfg.displayManager.sessionData.sessionNames
+            else
+              null;
         };
       };
 
       defaultSession = mkOption {
-        type = with types; str // {
+        type = with types; nullOr str // {
           description = "session name";
-          check = d: let
-            sessionNames = cfg.displayManager.session.names ++
-              (concatMap (p: p.providedSessions) cfg.displayManager.sessionPackages);
-          in
-            assertMsg (str.check d && (d == "none" || (elem d sessionNames))) ''
+          check = d:
+            assertMsg (d != null -> (str.check d && elem d cfg.displayManager.sessionData.sessionNames)) ''
                 Default graphical session, '${d}', not found.
                 Valid names for 'services.xserver.displayManager.defaultSession' are:
-                  ${concatStringsSep "\n  " sessionNames}
+                  ${concatStringsSep "\n  " cfg.displayManager.sessionData.sessionNames}
               '';
         };
-        default = let
-            dmDefault = cfg.desktopManager.default;
-            wmDefault = cfg.windowManager.default;
-            defaultPackage =
-              if cfg.displayManager.sessionPackages != [] then
-                (head (head cfg.displayManager.sessionPackages).providedSessions)
-              else
-                null;
-            defaultDmWm = dmDefault + optionalString (wmDefault != "none") ("+" + wmDefault);
-          in
-            if defaultDmWm == "none" && isString defaultPackage then
-              defaultPackage
-            else
-              defaultDmWm;
+        default =
+          if dmDefault != null || wmDefault != null then
+            defaultSessionFromLegacyOptions
+          else
+            null;
         example = "gnome";
-        description = "Default graphical session (only effective for LightDM and SDDM).";
+        description = ''
+          Graphical session to pre-select in the session chooser (only effective for GDM and LightDM).
+
+          On GDM, LightDM and SDDM, it will also be used as a session for auto-login.
+        '';
       };
 
       job = {
@@ -406,6 +342,27 @@ in
   };
 
   config = {
+    assertions = [
+      {
+        assertion = cfg.desktopManager.default != null || cfg.windowManager.default != null -> cfg.displayManager.defaultSession == defaultSessionFromLegacyOptions;
+        message = "You cannot use both services.xserver.displayManager.defaultSession option and legacy options (services.xserver.desktopManager.default and services.xserver.windowManager.default).";
+      }
+    ];
+
+    warnings =
+      mkIf (dmDefault != null || wmDefault != null) [
+        ''
+          The following options are deprecated:
+            ${concatStringsSep "\n  " (map ({c, t}: t) (filter ({c, t}: c != null) [
+            { c = dmDefault; t = "- services.xserver.desktopManager.default"; }
+            { c = wmDefault; t = "- services.xserver.windowManager.default"; }
+            ]))}
+          Please use
+            services.xserver.displayManager.defaultSession = "${concatStringsSep "+" (filter (s: s != null) [ dmDefault wmDefault ])}";
+          instead.
+        ''
+      ];
+
     services.xserver.displayManager.xserverBin = "${xorg.xorgserver.out}/bin/X";
 
     systemd.user.targets.graphical-session = {
@@ -414,6 +371,67 @@ in
         StopWhenUnneeded = false;
       };
     };
+
+    # Create desktop files and scripts for starting sessions for WMs/DMs
+    # that do not have upstream session files (those defined using services.{display,desktop,window}Manager.session options).
+    services.xserver.displayManager.sessionPackages =
+      let
+        dms = filter (s: s.manage == "desktop") cfg.displayManager.session;
+        wms = filter (s: s.manage == "window") cfg.displayManager.session;
+
+        # Script responsible for starting the window manager and the desktop manager.
+        xsession = wm: dm: pkgs.writeScript "xsession" ''
+          #! ${pkgs.bash}/bin/bash
+
+          # Legacy session script used to construct .desktop files from
+          # `services.xserver.displayManager.session` entries. Called from
+          # `sessionWrapper`.
+
+          # Start the window manager.
+          ${wm.start}
+
+          # Start the desktop manager.
+          ${dm.start}
+
+          ${optionalString cfg.updateDbusEnvironment ''
+            ${lib.getBin pkgs.dbus}/bin/dbus-update-activation-environment --systemd --all
+          ''}
+
+          test -n "$waitPID" && wait "$waitPID"
+
+          ${config.systemd.package}/bin/systemctl --user stop graphical-session.target
+
+          exit 0
+        '';
+      in
+        # We will generate every possible pair of WM and DM.
+        concatLists (
+          crossLists
+            (dm: wm: let
+              sessionName = "${dm.name}${optionalString (wm.name != "none") ("+" + wm.name)}";
+              script = xsession dm wm;
+            in
+              optional (dm.name != "none" || wm.name != "none")
+                (pkgs.writeTextFile {
+                  name = "${sessionName}-xsession";
+                  destination = "/share/xsessions/${sessionName}.desktop";
+                  # Desktop Entry Specification:
+                  # - https://standards.freedesktop.org/desktop-entry-spec/latest/
+                  # - https://standards.freedesktop.org/desktop-entry-spec/latest/ar01s06.html
+                  text = ''
+                    [Desktop Entry]
+                    Version=1.0
+                    Type=XSession
+                    TryExec=${script}
+                    Exec=${script}
+                    Name=${sessionName}
+                  '';
+                } // {
+                  providedSessions = [ sessionName ];
+                })
+            )
+            [dms wms]
+          );
   };
 
   imports = [
@@ -421,6 +439,7 @@ in
      "The option is no longer necessary because all display managers have already delegated lid management to systemd.")
     (mkRenamedOptionModule [ "services" "xserver" "displayManager" "job" "logsXsession" ] [ "services" "xserver" "displayManager" "job" "logToFile" ])
     (mkRenamedOptionModule [ "services" "xserver" "displayManager" "logToJournal" ] [ "services" "xserver" "displayManager" "job" "logToJournal" ])
+    (mkRenamedOptionModule [ "services" "xserver" "displayManager" "extraSessionFilesPackages" ] [ "services" "xserver" "displayManager" "sessionPackages" ])
   ];
 
 }
diff --git a/nixos/modules/services/x11/display-managers/gdm.nix b/nixos/modules/services/x11/display-managers/gdm.nix
index 095569fa08a..6630f012f04 100644
--- a/nixos/modules/services/x11/display-managers/gdm.nix
+++ b/nixos/modules/services/x11/display-managers/gdm.nix
@@ -31,44 +31,9 @@ let
     load-module module-position-event-sounds
   '';
 
-  dmDefault = config.services.xserver.desktopManager.default;
-  wmDefault = config.services.xserver.windowManager.default;
-  hasDefaultUserSession = dmDefault != "none" || wmDefault != "none";
-  defaultSessionName = dmDefault + optionalString (wmDefault != "none") ("+" + wmDefault);
-
-  setSessionScript = pkgs.python3.pkgs.buildPythonApplication {
-    name = "set-session";
-
-    format = "other";
-
-    src = ./set-session.py;
-
-    dontUnpack = true;
-
-    strictDeps = false;
-
-    nativeBuildInputs = with pkgs; [
-      wrapGAppsHook
-      gobject-introspection
-    ];
-
-    buildInputs = with pkgs; [
-      accountsservice
-      glib
-    ];
-
-    propagatedBuildInputs = with pkgs.python3.pkgs; [
-      pygobject3
-      ordered-set
-    ];
-
-    installPhase = ''
-      mkdir -p $out/bin
-      cp $src $out/bin/set-session
-      chmod +x $out/bin/set-session
-    '';
-  };
+  defaultSessionName = config.services.xserver.displayManager.defaultSession;
 
+  setSessionScript = pkgs.callPackage ./account-service-util.nix { };
 in
 
 {
@@ -186,7 +151,7 @@ in
         environment = {
           GDM_X_SERVER_EXTRA_ARGS = toString
             (filter (arg: arg != "-terminate") cfg.xserverArgs);
-          XDG_DATA_DIRS = "${cfg.session.desktops}/share/";
+          XDG_DATA_DIRS = "${cfg.sessionData.desktops}/share/";
         } // optionalAttrs (xSessionWrapper != null) {
           # Make GDM use this wrapper before running the session, which runs the
           # configured setupCommands. This relies on a patched GDM which supports
@@ -204,16 +169,19 @@ in
           cat - > /run/gdm/.config/gnome-initial-setup-done <<- EOF
           yes
           EOF
-        ''
-        # TODO: Make setSessionScript aware of previously used sessions
-        # + optionalString hasDefaultUserSession ''
-        #   ${setSessionScript}/bin/set-session ${defaultSessionName}
-        # ''
-        ;
+        '' + optionalString (defaultSessionName != null) ''
+          # Set default session in session chooser to a specified values – basically ignore session history.
+          ${setSessionScript}/bin/set-session ${cfg.sessionData.autologinSession}
+        '';
       };
 
-    # Because sd_login_monitor_new requires /run/systemd/machines
-    systemd.services.display-manager.wants = [ "systemd-machined.service" ];
+    systemd.services.display-manager.wants = [
+      # Because sd_login_monitor_new requires /run/systemd/machines
+      "systemd-machined.service"
+      # setSessionScript wants AccountsService
+      "accounts-daemon.service"
+    ];
+
     systemd.services.display-manager.after = [
       "rc-local.service"
       "systemd-machined.service"
@@ -329,7 +297,7 @@ in
       ${optionalString cfg.gdm.debug "Enable=true"}
     '';
 
-    environment.etc."gdm/Xsession".source = config.services.xserver.displayManager.session.wrapper;
+    environment.etc."gdm/Xsession".source = config.services.xserver.displayManager.sessionData.wrapper;
 
     # GDM LFS PAM modules, adapted somehow to NixOS
     security.pam.services = {
diff --git a/nixos/modules/services/x11/display-managers/lightdm-greeters/mini.nix b/nixos/modules/services/x11/display-managers/lightdm-greeters/mini.nix
index fa9445af32e..0025f9b3603 100644
--- a/nixos/modules/services/x11/display-managers/lightdm-greeters/mini.nix
+++ b/nixos/modules/services/x11/display-managers/lightdm-greeters/mini.nix
@@ -53,9 +53,8 @@ in
           Whether to enable lightdm-mini-greeter as the lightdm greeter.
 
           Note that this greeter starts only the default X session.
-          You can configure the default X session by
-          <option>services.xserver.desktopManager.default</option> and
-          <option>services.xserver.windowManager.default</option>.
+          You can configure the default X session using
+          <xref linkend="opt-services.xserver.displayManager.defaultSession"/>.
         '';
       };
 
diff --git a/nixos/modules/services/x11/display-managers/lightdm.nix b/nixos/modules/services/x11/display-managers/lightdm.nix
index a8642fec271..f7face0adb7 100644
--- a/nixos/modules/services/x11/display-managers/lightdm.nix
+++ b/nixos/modules/services/x11/display-managers/lightdm.nix
@@ -8,9 +8,9 @@ let
   dmcfg = xcfg.displayManager;
   xEnv = config.systemd.services.display-manager.environment;
   cfg = dmcfg.lightdm;
+  sessionData = dmcfg.sessionData;
 
-  defaultSessionName = dmcfg.defaultSession;
-  hasDefaultUserSession = defaultSessionName != "none";
+  setSessionScript = pkgs.callPackage ./account-service-util.nix { };
 
   inherit (pkgs) lightdm writeScript writeText;
 
@@ -44,22 +44,19 @@ let
         greeter-user = ${config.users.users.lightdm.name}
         greeters-directory = ${cfg.greeter.package}
       ''}
-      sessions-directory = ${dmcfg.session.desktops}/share/xsessions:${dmcfg.session.desktops}/share/wayland-sessions
+      sessions-directory = ${dmcfg.sessionData.desktops}/share/xsessions:${dmcfg.sessionData.desktops}/share/wayland-sessions
       ${cfg.extraConfig}
 
       [Seat:*]
       xserver-command = ${xserverWrapper}
-      session-wrapper = ${dmcfg.session.wrapper}
+      session-wrapper = ${dmcfg.sessionData.wrapper}
       ${optionalString cfg.greeter.enable ''
         greeter-session = ${cfg.greeter.name}
       ''}
       ${optionalString cfg.autoLogin.enable ''
         autologin-user = ${cfg.autoLogin.user}
         autologin-user-timeout = ${toString cfg.autoLogin.timeout}
-        autologin-session = ${defaultSessionName}
-      ''}
-      ${optionalString hasDefaultUserSession ''
-        user-session=${defaultSessionName}
+        autologin-session = ${sessionData.autologinSession}
       ''}
       ${optionalString (dmcfg.setupCommands != "") ''
         display-setup-script=${pkgs.writeScript "lightdm-display-setup" ''
@@ -197,11 +194,9 @@ in
           LightDM auto-login requires services.xserver.displayManager.lightdm.autoLogin.user to be set
         '';
       }
-      { assertion = cfg.autoLogin.enable -> hasDefaultUserSession;
+      { assertion = cfg.autoLogin.enable -> sessionData.autologinSession != null;
         message = ''
-          LightDM auto-login requires that services.xserver.desktopManager.default and
-          services.xserver.windowManager.default are set to valid values. The current
-          default session: ${defaultSessionName} is not valid.
+          LightDM auto-login requires that services.xserver.displayManager.defaultSession is set.
         '';
       }
       { assertion = !cfg.greeter.enable -> (cfg.autoLogin.enable && cfg.autoLogin.timeout == 0);
@@ -212,6 +207,20 @@ in
       }
     ];
 
+    # Set default session in session chooser to a specified values – basically ignore session history.
+    # Auto-login is already covered by a config value.
+    services.xserver.displayManager.job.preStart = optionalString (!cfg.autoLogin.enable && dmcfg.defaultSession != null) ''
+      ${setSessionScript}/bin/set-session ${dmcfg.defaultSession}
+    '';
+
+    # setSessionScript needs session-files in XDG_DATA_DIRS
+    services.xserver.displayManager.job.environment.XDG_DATA_DIRS = "${dmcfg.sessionData.desktops}/share/";
+
+    # setSessionScript wants AccountsService
+    systemd.services.display-manager.wants = [
+      "accounts-daemon.service"
+    ];
+
     # lightdm relaunches itself via just `lightdm`, so needs to be on the PATH
     services.xserver.displayManager.job.execCmd = ''
       export PATH=${lightdm}/sbin:$PATH
diff --git a/nixos/modules/services/x11/display-managers/sddm.nix b/nixos/modules/services/x11/display-managers/sddm.nix
index b51582d2ca5..4224c557ed6 100644
--- a/nixos/modules/services/x11/display-managers/sddm.nix
+++ b/nixos/modules/services/x11/display-managers/sddm.nix
@@ -50,8 +50,8 @@ let
     MinimumVT=${toString (if xcfg.tty != null then xcfg.tty else 7)}
     ServerPath=${xserverWrapper}
     XephyrPath=${pkgs.xorg.xorgserver.out}/bin/Xephyr
-    SessionCommand=${dmcfg.session.wrapper}
-    SessionDir=${dmcfg.session.desktops}/share/xsessions
+    SessionCommand=${dmcfg.sessionData.wrapper}
+    SessionDir=${dmcfg.sessionData.desktops}/share/xsessions
     XauthPath=${pkgs.xorg.xauth}/bin/xauth
     DisplayCommand=${Xsetup}
     DisplayStopCommand=${Xstop}
@@ -59,19 +59,19 @@ let
 
     [Wayland]
     EnableHidpi=${if cfg.enableHidpi then "true" else "false"}
-    SessionDir=${dmcfg.session.desktops}/share/wayland-sessions
+    SessionDir=${dmcfg.sessionData.desktops}/share/wayland-sessions
 
     ${optionalString cfg.autoLogin.enable ''
     [Autologin]
     User=${cfg.autoLogin.user}
-    Session=${defaultSessionName}.desktop
+    Session=${autoLoginSessionName}.desktop
     Relogin=${boolToString cfg.autoLogin.relogin}
     ''}
 
     ${cfg.extraConfig}
   '';
 
-  defaultSessionName = dmcfg.defaultSession;
+  autoLoginSessionName = dmcfg.sessionData.autologinSession;
 
 in
 {
@@ -206,11 +206,9 @@ in
           SDDM auto-login requires services.xserver.displayManager.sddm.autoLogin.user to be set
         '';
       }
-      { assertion = cfg.autoLogin.enable -> defaultSessionName != "none";
+      { assertion = cfg.autoLogin.enable -> autoLoginSessionName != null;
         message = ''
-          SDDM auto-login requires that services.xserver.desktopManager.default and
-          services.xserver.windowManager.default are set to valid values. The current
-          default session: ${defaultSessionName} is not valid.
+          SDDM auto-login requires that services.xserver.displayManager.defaultSession is set.
         '';
       }
     ];
diff --git a/nixos/modules/services/x11/window-managers/default.nix b/nixos/modules/services/x11/window-managers/default.nix
index c17f3830d0e..04a9fc46628 100644
--- a/nixos/modules/services/x11/window-managers/default.nix
+++ b/nixos/modules/services/x11/window-managers/default.nix
@@ -59,15 +59,14 @@ in
       };
 
       default = mkOption {
-        type = types.str;
-        default = "none";
+        type = types.nullOr types.str;
+        default = null;
         example = "wmii";
-        description = "Default window manager loaded if none have been chosen.";
-        apply = defaultWM:
-          if any (w: w.name == defaultWM) cfg.session then
-            defaultWM
-          else
-            throw "Default window manager (${defaultWM}) not found.";
+        description = ''
+          <emphasis role="strong">Deprecated</emphasis>, please use <xref linkend="opt-services.xserver.displayManager.defaultSession"/> instead.
+
+          Default window manager loaded if none have been chosen.
+        '';
       };
 
     };