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, 3 insertions, 5 deletions

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