From d362df6682d3506e17c069f95cb39b10e516d123 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Tor=20Hedin=20Br=C3=B8nner?= <torhedinbronner@gmail.com>
Date: Sat, 12 Jan 2019 08:00:31 +0100
Subject: [PATCH 1/6] nixos/display-manager: extraSessionFilePackages ->
 sessionPackages

---
 nixos/modules/programs/sway.nix                          | 2 +-
 nixos/modules/services/x11/desktop-managers/default.nix  | 2 +-
 nixos/modules/services/x11/desktop-managers/gnome3.nix   | 4 ++--
 nixos/modules/services/x11/desktop-managers/pantheon.nix | 2 +-
 .../services/x11/desktop-managers/surf-display.nix       | 2 +-
 nixos/modules/services/x11/display-managers/default.nix  | 9 +++++----
 6 files changed, 11 insertions(+), 10 deletions(-)

diff --git a/nixos/modules/programs/sway.nix b/nixos/modules/programs/sway.nix
index 9a15c773463..f5c66cbd663 100644
--- a/nixos/modules/programs/sway.nix
+++ b/nixos/modules/programs/sway.nix
@@ -88,7 +88,7 @@ in {
     fonts.enableDefaultFonts = mkDefault true;
     programs.dconf.enable = mkDefault true;
     # To make a Sway session available if a display manager like SDDM is enabled:
-    services.xserver.displayManager.extraSessionFilePackages = [ swayJoined ];
+    services.xserver.displayManager.sessionPackages = [ swayJoined ];
   };
 
   meta.maintainers = with lib.maintainers; [ gnidorah primeos colemickens ];
diff --git a/nixos/modules/services/x11/desktop-managers/default.nix b/nixos/modules/services/x11/desktop-managers/default.nix
index 671a959cdde..ed2b09cb21c 100644
--- a/nixos/modules/services/x11/desktop-managers/default.nix
+++ b/nixos/modules/services/x11/desktop-managers/default.nix
@@ -101,7 +101,7 @@ in
               These are the known valid session names:
                 ${concatMapStringsSep "\n  " (w: "services.xserver.desktopManager.default = \"${w.name}\";") cfg.session.list}
               It's also possible the default can be found in one of these packages:
-                ${concatMapStringsSep "\n  " (p: p.name) config.services.xserver.displayManager.extraSessionFilePackages}
+                ${concatMapStringsSep "\n  " (p: p.name) config.services.xserver.displayManager.sessionPackages}
             '' defaultDM;
       };
 
diff --git a/nixos/modules/services/x11/desktop-managers/gnome3.nix b/nixos/modules/services/x11/desktop-managers/gnome3.nix
index 6725595e1cf..6d9bd284bc7 100644
--- a/nixos/modules/services/x11/desktop-managers/gnome3.nix
+++ b/nixos/modules/services/x11/desktop-managers/gnome3.nix
@@ -144,7 +144,7 @@ in
       services.gnome3.core-shell.enable = true;
       services.gnome3.core-utilities.enable = mkDefault true;
 
-      services.xserver.displayManager.extraSessionFilePackages = [ pkgs.gnome3.gnome-session ];
+      services.xserver.displayManager.sessionPackages = [ pkgs.gnome3.gnome-session ];
 
       environment.extraInit = ''
         ${concatMapStrings (p: ''
@@ -171,7 +171,7 @@ in
     })
 
     (mkIf flashbackEnabled {
-      services.xserver.displayManager.extraSessionFilePackages =  map
+      services.xserver.displayManager.sessionPackages =  map
         (wm: pkgs.gnome3.gnome-flashback.mkSessionForWm {
           inherit (wm) wmName wmLabel wmCommand;
         }) (optional cfg.flashback.enableMetacity {
diff --git a/nixos/modules/services/x11/desktop-managers/pantheon.nix b/nixos/modules/services/x11/desktop-managers/pantheon.nix
index 99db5b17b64..614f4638cf5 100644
--- a/nixos/modules/services/x11/desktop-managers/pantheon.nix
+++ b/nixos/modules/services/x11/desktop-managers/pantheon.nix
@@ -69,7 +69,7 @@ in
 
   config = mkIf cfg.enable {
 
-    services.xserver.displayManager.extraSessionFilePackages = [ pkgs.pantheon.elementary-session-settings ];
+    services.xserver.displayManager.sessionPackages = [ pkgs.pantheon.elementary-session-settings ];
 
     # Ensure lightdm is used when Pantheon is enabled
     # Without it screen locking will be nonfunctional because of the use of lightlocker
diff --git a/nixos/modules/services/x11/desktop-managers/surf-display.nix b/nixos/modules/services/x11/desktop-managers/surf-display.nix
index 140dde828da..9aeb0bbd2a8 100644
--- a/nixos/modules/services/x11/desktop-managers/surf-display.nix
+++ b/nixos/modules/services/x11/desktop-managers/surf-display.nix
@@ -118,7 +118,7 @@ in {
   };
 
   config = mkIf cfg.enable {
-    services.xserver.displayManager.extraSessionFilePackages = [
+    services.xserver.displayManager.sessionPackages = [
       pkgs.surf-display
     ];
 
diff --git a/nixos/modules/services/x11/display-managers/default.nix b/nixos/modules/services/x11/display-managers/default.nix
index c252c3c90c9..4a75787f715 100644
--- a/nixos/modules/services/x11/display-managers/default.nix
+++ b/nixos/modules/services/x11/display-managers/default.nix
@@ -194,14 +194,15 @@ let
         if test -d ${pkg}/share/xsessions; then
           ${xorg.lndir}/bin/lndir ${pkg}/share/xsessions $out/share/xsessions
         fi
-      '') cfg.displayManager.extraSessionFilePackages}
+      '') cfg.displayManager.sessionPackages}
+
 
       ${concatMapStrings (pkg: ''
         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.extraSessionFilePackages}
+      '') cfg.displayManager.sessionPackages}
     '';
 
 in
@@ -261,11 +262,11 @@ in
         '';
       };
 
-      extraSessionFilePackages = mkOption {
+      sessionPackages = mkOption {
         type = types.listOf types.package;
         default = [];
         description = ''
-          A list of packages containing xsession files to be passed to the display manager.
+          A list of packages containing x11 or wayland session files to be passed to the display manager.
         '';
       };
 

From dd7144b8601958562d78f795abcab4668404d15d Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Tor=20Hedin=20Br=C3=B8nner?= <torhedinbronner@gmail.com>
Date: Sat, 12 Jan 2019 08:33:05 +0100
Subject: [PATCH 2/6] nixos/sessionPackages: provide session names in passthru

We want access to the valid session names at evaluation time.
---
 nixos/modules/programs/sway.nix                   |  1 +
 .../services/x11/display-managers/default.nix     | 15 ++++++++++++++-
 .../gnome-3/core/gnome-session/default.nix        |  1 +
 .../gnome-3/misc/gnome-flashback/default.nix      |  2 ++
 .../elementary-session-settings/default.nix       |  1 +
 pkgs/desktops/surf-display/default.nix            |  4 ++++
 6 files changed, 23 insertions(+), 1 deletion(-)

diff --git a/nixos/modules/programs/sway.nix b/nixos/modules/programs/sway.nix
index f5c66cbd663..d685a525932 100644
--- a/nixos/modules/programs/sway.nix
+++ b/nixos/modules/programs/sway.nix
@@ -24,6 +24,7 @@ let
   swayJoined = pkgs.symlinkJoin {
     name = "sway-joined";
     paths = [ swayWrapped swayPackage ];
+    passthru.providedSessions = [ "sway" ];
   };
 in {
   options.programs.sway = {
diff --git a/nixos/modules/services/x11/display-managers/default.nix b/nixos/modules/services/x11/display-managers/default.nix
index 4a75787f715..4dc06b30c7a 100644
--- a/nixos/modules/services/x11/display-managers/default.nix
+++ b/nixos/modules/services/x11/display-managers/default.nix
@@ -263,7 +263,20 @@ in
       };
 
       sessionPackages = mkOption {
-        type = types.listOf types.package;
+        type = with types; listOf (package // {
+          description = "package with provided sessions";
+          check = p: assertMsg
+            (package.check p && p ? providedSessions
+            && p.providedSessions != [] && all isString p.providedSessions)
+            ''
+              Package, '${p.name}', did not specify any session names, as strings, in
+              'passthru.providedSessions'. This is required when used as a session package.
+
+              The session names can be looked up in:
+                ${p}/share/xsessions
+                ${p}/share/wayland-sessions
+           '';
+        });
         default = [];
         description = ''
           A list of packages containing x11 or wayland session files to be passed to the display manager.
diff --git a/pkgs/desktops/gnome-3/core/gnome-session/default.nix b/pkgs/desktops/gnome-3/core/gnome-session/default.nix
index 8aaf68ea1e3..d63435ce2e9 100644
--- a/pkgs/desktops/gnome-3/core/gnome-session/default.nix
+++ b/pkgs/desktops/gnome-3/core/gnome-session/default.nix
@@ -55,6 +55,7 @@ stdenv.mkDerivation rec {
       packageName = "gnome-session";
       attrPath = "gnome3.gnome-session";
     };
+    providedSessions = [ "gnome" "gnome-xorg" ];
   };
 
   meta = with stdenv.lib; {
diff --git a/pkgs/desktops/gnome-3/misc/gnome-flashback/default.nix b/pkgs/desktops/gnome-3/misc/gnome-flashback/default.nix
index 808d36dab95..c1791ee5bbd 100644
--- a/pkgs/desktops/gnome-3/misc/gnome-flashback/default.nix
+++ b/pkgs/desktops/gnome-3/misc/gnome-flashback/default.nix
@@ -141,6 +141,8 @@ let
           Type=Application
           DesktopNames=GNOME-Flashback;GNOME;
         '';
+      } // {
+        providedSessions = [ "gnome-flashback-${wmName}" ];
       };
 
       mkSystemdTargetForWm = { wmName }:
diff --git a/pkgs/desktops/pantheon/desktop/elementary-session-settings/default.nix b/pkgs/desktops/pantheon/desktop/elementary-session-settings/default.nix
index ab7333212a0..07c9e724fc6 100644
--- a/pkgs/desktops/pantheon/desktop/elementary-session-settings/default.nix
+++ b/pkgs/desktops/pantheon/desktop/elementary-session-settings/default.nix
@@ -131,6 +131,7 @@ stdenv.mkDerivation rec {
       inherit repoName;
       attrPath = pname;
     };
+    providedSessions = [ "pantheon" ];
   };
 
   meta = with stdenv.lib; {
diff --git a/pkgs/desktops/surf-display/default.nix b/pkgs/desktops/surf-display/default.nix
index 47b7e117206..dd064486647 100644
--- a/pkgs/desktops/surf-display/default.nix
+++ b/pkgs/desktops/surf-display/default.nix
@@ -43,6 +43,10 @@ stdenv.mkDerivation rec {
 
   makeFlags = [ "PREFIX=${placeholder "out"}" ];
 
+  passthru = {
+    providedSessions = [ "surf-display" ];
+  };
+
   meta = with stdenv.lib; {
     description = "Kiosk browser session manager based on the surf browser";
     homepage = "https://code.it-zukunft-schule.de/cgit/surf-display/";

From 58e5290fcee60178973f4cc21011fbbf03497410 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Tor=20Hedin=20Br=C3=B8nner?= <torhedinbronner@gmail.com>
Date: Sat, 12 Jan 2019 08:43:24 +0100
Subject: [PATCH 3/6] nixos/displayManager: check for provided sessions in
 mkDesktops

---
 .../services/x11/display-managers/default.nix       | 13 +++++++++----
 1 file changed, 9 insertions(+), 4 deletions(-)

diff --git a/nixos/modules/services/x11/display-managers/default.nix b/nixos/modules/services/x11/display-managers/default.nix
index 4dc06b30c7a..f55bfbfd0c1 100644
--- a/nixos/modules/services/x11/display-managers/default.nix
+++ b/nixos/modules/services/x11/display-managers/default.nix
@@ -191,13 +191,18 @@ let
       '') names}
 
       ${concatMapStrings (pkg: ''
+        for n in ${concatStringsSep " " pkg.providedSessions}; do
+          if ! test -f ${pkg}/share/wayland-sessions/$n.desktop -o \
+                    -f ${pkg}/share/xsessions/$n.desktop; then
+            echo "Couldn't find provided session name, $n.desktop, in session package ${pkg.name}:"
+            echo "  ${pkg}"
+            return 1
+          fi
+        done
+
         if test -d ${pkg}/share/xsessions; then
           ${xorg.lndir}/bin/lndir ${pkg}/share/xsessions $out/share/xsessions
         fi
-      '') cfg.displayManager.sessionPackages}
-
-
-      ${concatMapStrings (pkg: ''
         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

From d25365c3c13f2fcc4e5574f01106c061c493e63e Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Tor=20Hedin=20Br=C3=B8nner?= <torhedinbronner@gmail.com>
Date: Sat, 12 Jan 2019 10:31:37 +0100
Subject: [PATCH 4/6] nixos/displayManager: introduce defaultSession

There's two ways of providing graphical sessions now:
- `displayManager.session` via. `desktopManager.session` and
  `windowManager.session`
- `displayManager.sessionPackages`

`sessionPackages` doesn't make a distinction between desktop and window
managers. This makes selecting a session provided by a package using
`desktopManager.default` nonsensical.

We therefor introduce `displayManager.defaultSession` which can select a session
from either `displayManager.session` or `displayManager.sessionPackages`.

It will default to `desktopManager.default + windowManager.default` as before.
If the dm default is "none" it will select the first provided session from
`sessionPackages`.
---
 .../services/x11/desktop-managers/default.nix | 12 +++----
 .../x11/desktop-managers/pantheon.nix         |  4 +--
 .../services/x11/display-managers/default.nix | 31 +++++++++++++++++++
 .../services/x11/display-managers/lightdm.nix |  8 ++---
 .../services/x11/display-managers/sddm.nix    |  8 ++---
 nixos/tests/gnome3-xorg.nix                   |  2 +-
 6 files changed, 44 insertions(+), 21 deletions(-)

diff --git a/nixos/modules/services/x11/desktop-managers/default.nix b/nixos/modules/services/x11/desktop-managers/default.nix
index ed2b09cb21c..7240358c361 100644
--- a/nixos/modules/services/x11/desktop-managers/default.nix
+++ b/nixos/modules/services/x11/desktop-managers/default.nix
@@ -96,13 +96,13 @@ in
           else if any (w: w.name == defaultDM) cfg.session.list then
             defaultDM
           else
-            builtins.trace ''
-              Default desktop manager (${defaultDM}) not found at evaluation time.
-              These are the known valid session names:
+            throw ''
+              Default desktop manager (${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}
-              It's also possible the default can be found in one of these packages:
-                ${concatMapStringsSep "\n  " (p: p.name) config.services.xserver.displayManager.sessionPackages}
-            '' defaultDM;
+            '';
       };
 
     };
diff --git a/nixos/modules/services/x11/desktop-managers/pantheon.nix b/nixos/modules/services/x11/desktop-managers/pantheon.nix
index 614f4638cf5..3722bf365ba 100644
--- a/nixos/modules/services/x11/desktop-managers/pantheon.nix
+++ b/nixos/modules/services/x11/desktop-managers/pantheon.nix
@@ -81,9 +81,7 @@ in
 
     services.xserver.displayManager.lightdm.greeters.pantheon.enable = mkDefault true;
 
-    # If not set manually Pantheon session cannot be started
-    # Known issue of https://github.com/NixOS/nixpkgs/pull/43992
-    services.xserver.desktopManager.default = mkForce "pantheon";
+    services.xserver.displayManager.defaultSession = "pantheon";
 
     services.xserver.displayManager.sessionCommands = ''
       if test "$XDG_CURRENT_DESKTOP" = "Pantheon"; then
diff --git a/nixos/modules/services/x11/display-managers/default.nix b/nixos/modules/services/x11/display-managers/default.nix
index f55bfbfd0c1..1a46e7c02e5 100644
--- a/nixos/modules/services/x11/display-managers/default.nix
+++ b/nixos/modules/services/x11/display-managers/default.nix
@@ -327,6 +327,37 @@ in
         };
       };
 
+      defaultSession = mkOption {
+        type = with types; 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))) ''
+                Default graphical session, '${d}', not found.
+                Valid names for 'services.xserver.displayManager.defaultSession' are:
+                  ${concatStringsSep "\n  " 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;
+        example = "gnome";
+        description = "Default graphical session (only effective for LightDM and SDDM).";
+      };
+
       job = {
 
         preStart = mkOption {
diff --git a/nixos/modules/services/x11/display-managers/lightdm.nix b/nixos/modules/services/x11/display-managers/lightdm.nix
index cf4c05acbcc..71cd6079ea1 100644
--- a/nixos/modules/services/x11/display-managers/lightdm.nix
+++ b/nixos/modules/services/x11/display-managers/lightdm.nix
@@ -9,9 +9,8 @@ let
   xEnv = config.systemd.services.display-manager.environment;
   cfg = dmcfg.lightdm;
 
-  dmDefault = xcfg.desktopManager.default;
-  wmDefault = xcfg.windowManager.default;
-  hasDefaultUserSession = dmDefault != "none" || wmDefault != "none";
+  defaultSessionName = dmcfg.defaultSession;
+  hasDefaultUserSession = defaultSessionName != "none";
 
   inherit (pkgs) lightdm writeScript writeText;
 
@@ -71,7 +70,6 @@ let
       ${cfg.extraSeatDefaults}
     '';
 
-  defaultSessionName = dmDefault + optionalString (wmDefault != "none") ("+" + wmDefault);
 in
 {
   # Note: the order in which lightdm greeter modules are imported
@@ -199,7 +197,7 @@ in
           LightDM auto-login requires services.xserver.displayManager.lightdm.autoLogin.user to be set
         '';
       }
-      { assertion = cfg.autoLogin.enable -> dmDefault != "none" || wmDefault != "none";
+      { assertion = cfg.autoLogin.enable -> hasDefaultUserSession;
         message = ''
           LightDM auto-login requires that services.xserver.desktopManager.default and
           services.xserver.windowManager.default are set to valid values. The current
diff --git a/nixos/modules/services/x11/display-managers/sddm.nix b/nixos/modules/services/x11/display-managers/sddm.nix
index 24e120c4f2c..b51582d2ca5 100644
--- a/nixos/modules/services/x11/display-managers/sddm.nix
+++ b/nixos/modules/services/x11/display-managers/sddm.nix
@@ -71,11 +71,7 @@ let
     ${cfg.extraConfig}
   '';
 
-  defaultSessionName =
-    let
-      dm = xcfg.desktopManager.default;
-      wm = xcfg.windowManager.default;
-    in dm + optionalString (wm != "none") ("+" + wm);
+  defaultSessionName = dmcfg.defaultSession;
 
 in
 {
@@ -210,7 +206,7 @@ in
           SDDM auto-login requires services.xserver.displayManager.sddm.autoLogin.user to be set
         '';
       }
-      { assertion = cfg.autoLogin.enable -> elem defaultSessionName dmcfg.session.names;
+      { assertion = cfg.autoLogin.enable -> defaultSessionName != "none";
         message = ''
           SDDM auto-login requires that services.xserver.desktopManager.default and
           services.xserver.windowManager.default are set to valid values. The current
diff --git a/nixos/tests/gnome3-xorg.nix b/nixos/tests/gnome3-xorg.nix
index eb4c376319b..aa03501f6a5 100644
--- a/nixos/tests/gnome3-xorg.nix
+++ b/nixos/tests/gnome3-xorg.nix
@@ -16,7 +16,7 @@ import ./make-test.nix ({ pkgs, ...} : {
       services.xserver.displayManager.lightdm.autoLogin.enable = true;
       services.xserver.displayManager.lightdm.autoLogin.user = "alice";
       services.xserver.desktopManager.gnome3.enable = true;
-      services.xserver.desktopManager.default = "gnome-xorg";
+      services.xserver.displayManager.defaultSession = "gnome-xorg";
 
       virtualisation.memorySize = 1024;
     };

From 53ef29c1380424e4e697725c4f3df30f9acacebc Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Tor=20Hedin=20Br=C3=B8nner?= <torhedinbronner@gmail.com>
Date: Mon, 14 Jan 2019 14:17:40 +0100
Subject: [PATCH 5/6] nixos/lightdm: enable wayland sessions

Note: can't launch gnome on wayland due to duplicate entry names:
  https://github.com/CanonicalLtd/lightdm/issues/16
---
 nixos/modules/services/x11/display-managers/lightdm.nix | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/nixos/modules/services/x11/display-managers/lightdm.nix b/nixos/modules/services/x11/display-managers/lightdm.nix
index 71cd6079ea1..a8642fec271 100644
--- a/nixos/modules/services/x11/display-managers/lightdm.nix
+++ b/nixos/modules/services/x11/display-managers/lightdm.nix
@@ -44,7 +44,7 @@ let
         greeter-user = ${config.users.users.lightdm.name}
         greeters-directory = ${cfg.greeter.package}
       ''}
-      sessions-directory = ${dmcfg.session.desktops}/share/xsessions
+      sessions-directory = ${dmcfg.session.desktops}/share/xsessions:${dmcfg.session.desktops}/share/wayland-sessions
       ${cfg.extraConfig}
 
       [Seat:*]

From 8dc5ff7dcfd1c58c32004ffae25e6d31ed83d86c Mon Sep 17 00:00:00 2001
From: Jan Tojnar <jtojnar@gmail.com>
Date: Tue, 10 Dec 2019 15:10:30 +0100
Subject: [PATCH 6/6] 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  |   3 +-
 nixos/doc/manual/configuration/xfce.xml       |   5 +-
 nixos/doc/manual/release-notes/rl-2003.xml    |  13 +
 nixos/lib/testing-python.nix                  |   3 +-
 nixos/lib/testing.nix                         |   3 +-
 .../services/x11/desktop-managers/default.nix |  23 +-
 .../x11/desktop-managers/pantheon.nix         |   2 +
 .../display-managers/account-service-util.nix |  39 +++
 .../services/x11/display-managers/default.nix | 231 ++++++++++--------
 .../services/x11/display-managers/gdm.nix     |  62 ++---
 .../lightdm-greeters/mini.nix                 |   5 +-
 .../services/x11/display-managers/lightdm.nix |  33 ++-
 .../services/x11/display-managers/sddm.nix    |  16 +-
 .../services/x11/window-managers/default.nix  |  15 +-
 nixos/tests/common/x11.nix                    |   8 +-
 nixos/tests/i3wm.nix                          |   2 +-
 nixos/tests/lightdm.nix                       |   3 +-
 nixos/tests/plasma5.nix                       |   2 +-
 nixos/tests/sddm.nix                          |   6 +-
 nixos/tests/xmonad.nix                        |   4 +-
 20 files changed, 254 insertions(+), 224 deletions(-)
 create mode 100644 nixos/modules/services/x11/display-managers/account-service-util.nix

diff --git a/nixos/doc/manual/configuration/x-windows.xml b/nixos/doc/manual/configuration/x-windows.xml
index 9206f43ea39..55ad9fe6e65 100644
--- a/nixos/doc/manual/configuration/x-windows.xml
+++ b/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>
diff --git a/nixos/doc/manual/configuration/xfce.xml b/nixos/doc/manual/configuration/xfce.xml
index 6ac99c6b2be..027828bb936 100644
--- a/nixos/doc/manual/configuration/xfce.xml
+++ b/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>
diff --git a/nixos/doc/manual/release-notes/rl-2003.xml b/nixos/doc/manual/release-notes/rl-2003.xml
index 579b8d53744..fd9b2b49b8c 100644
--- a/nixos/doc/manual/release-notes/rl-2003.xml
+++ b/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>
 
diff --git a/nixos/lib/testing-python.nix b/nixos/lib/testing-python.nix
index d567d268765..c4eb9328b1d 100644
--- a/nixos/lib/testing-python.nix
+++ b/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 ({
diff --git a/nixos/lib/testing.nix b/nixos/lib/testing.nix
index a5f060a8d8e..ae8ecd6270c 100644
--- a/nixos/lib/testing.nix
+++ b/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 ({
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.
+        '';
       };
 
     };
diff --git a/nixos/tests/common/x11.nix b/nixos/tests/common/x11.nix
index c5a7c165d12..5ad0ac20fac 100644
--- a/nixos/tests/common/x11.nix
+++ b/nixos/tests/common/x11.nix
@@ -1,12 +1,12 @@
+{ lib, ... }:
+
 { services.xserver.enable = true;
 
   # Automatically log in.
   services.xserver.displayManager.auto.enable = true;
 
   # Use IceWM as the window manager.
-  services.xserver.windowManager.default = "icewm";
-  services.xserver.windowManager.icewm.enable = true;
-
   # Don't use a desktop manager.
-  services.xserver.desktopManager.default = "none";
+  services.xserver.displayManager.defaultSession = lib.mkDefault "none+icewm";
+  services.xserver.windowManager.icewm.enable = true;
 }
diff --git a/nixos/tests/i3wm.nix b/nixos/tests/i3wm.nix
index 8afa845f1e2..126178d1187 100644
--- a/nixos/tests/i3wm.nix
+++ b/nixos/tests/i3wm.nix
@@ -7,7 +7,7 @@ import ./make-test-python.nix ({ pkgs, ...} : {
   machine = { lib, ... }: {
     imports = [ ./common/x11.nix ./common/user-account.nix ];
     services.xserver.displayManager.auto.user = "alice";
-    services.xserver.windowManager.default = lib.mkForce "i3";
+    services.xserver.displayManager.defaultSession = lib.mkForce "none+i3";
     services.xserver.windowManager.i3.enable = true;
   };
 
diff --git a/nixos/tests/lightdm.nix b/nixos/tests/lightdm.nix
index ef30f7741e2..46c2ed7ccc5 100644
--- a/nixos/tests/lightdm.nix
+++ b/nixos/tests/lightdm.nix
@@ -8,9 +8,8 @@ import ./make-test-python.nix ({ pkgs, ...} : {
     imports = [ ./common/user-account.nix ];
     services.xserver.enable = true;
     services.xserver.displayManager.lightdm.enable = true;
-    services.xserver.windowManager.default = "icewm";
+    services.xserver.displayManager.defaultSession = "none+icewm";
     services.xserver.windowManager.icewm.enable = true;
-    services.xserver.desktopManager.default = "none";
   };
 
   enableOCR = true;
diff --git a/nixos/tests/plasma5.nix b/nixos/tests/plasma5.nix
index 6884f17aabb..2eccfdf47f5 100644
--- a/nixos/tests/plasma5.nix
+++ b/nixos/tests/plasma5.nix
@@ -12,8 +12,8 @@ import ./make-test-python.nix ({ pkgs, ...} :
     imports = [ ./common/user-account.nix ];
     services.xserver.enable = true;
     services.xserver.displayManager.sddm.enable = true;
+    services.xserver.displayManager.defaultSession = "plasma5";
     services.xserver.desktopManager.plasma5.enable = true;
-    services.xserver.desktopManager.default = "plasma5";
     services.xserver.displayManager.sddm.autoLogin = {
       enable = true;
       user = "alice";
diff --git a/nixos/tests/sddm.nix b/nixos/tests/sddm.nix
index 4bdcd701dcf..a145705250f 100644
--- a/nixos/tests/sddm.nix
+++ b/nixos/tests/sddm.nix
@@ -16,9 +16,8 @@ let
         imports = [ ./common/user-account.nix ];
         services.xserver.enable = true;
         services.xserver.displayManager.sddm.enable = true;
-        services.xserver.windowManager.default = "icewm";
+        services.xserver.displayManager.defaultSession = "none+icewm";
         services.xserver.windowManager.icewm.enable = true;
-        services.xserver.desktopManager.default = "none";
       };
 
       enableOCR = true;
@@ -52,9 +51,8 @@ let
             user = "alice";
           };
         };
-        services.xserver.windowManager.default = "icewm";
+        services.xserver.displayManager.defaultSession = "none+icewm";
         services.xserver.windowManager.icewm.enable = true;
-        services.xserver.desktopManager.default = "none";
       };
 
       testScript = { nodes, ... }: let
diff --git a/nixos/tests/xmonad.nix b/nixos/tests/xmonad.nix
index ab3888ca43f..ef711f8dcf6 100644
--- a/nixos/tests/xmonad.nix
+++ b/nixos/tests/xmonad.nix
@@ -4,10 +4,10 @@ import ./make-test-python.nix ({ pkgs, ...} : {
     maintainers = [ nequissimus ];
   };
 
-  machine = { lib, pkgs, ... }: {
+  machine = { pkgs, ... }: {
     imports = [ ./common/x11.nix ./common/user-account.nix ];
     services.xserver.displayManager.auto.user = "alice";
-    services.xserver.windowManager.default = lib.mkForce "xmonad";
+    services.xserver.displayManager.defaultSession = "none+xmonad";
     services.xserver.windowManager.xmonad = {
       enable = true;
       enableContribAndExtras = true;