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`.

1 files changed, 1 insertions, 1 deletions

diff --git a/nixos/tests/gnome3-xorg.nix b/nixos/tests/gnome3-xorg.nix
index eb4c376319be2..aa03501f6a550 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;
     };