nixos/displayManager: Create a common environment wrapper for all dms

This makes it easier to support a wider variety of .desktop session files. In
particular this makes it possible to use both the «legacy» sessions and upstream
session files.

We separate `xsession` into two parts, `xsessionWrapper` and `xsession`.
`xsessionWrapper` sets up the correct environment and then lauches the session's
Exec command (from the .desktop file), falling back to launching the default
window/desktopManager through the `xsession` script (required by at least some
nixos tests).

`xsession` then _only_ handles launching desktop-managers/window-managers defined
through `services.xserver.desktopManager.session`.

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

@@ -27,55 +27,26 @@ let
     Xft.hintstyle: hintslight
   '';
 
-  # file provided by services.xserver.displayManager.session.script
+  # file provided by services.xserver.displayManager.session.wrapper
-  xsession = wm: dm: pkgs.writeScript "xsession"
+  xsessionWrapper = pkgs.writeScript "xsession-wrapper"
     ''
       #! ${pkgs.bash}/bin/bash
 
-      # Expected parameters:
+      # Shared environment setup for graphical sessions.
-      #   $1 = <desktop-manager>+<window-manager>
-
-      # Actual parameters (FIXME):
-      # SDDM is calling this script like the following:
-      #   $1 = /nix/store/xxx-xsession (= $0)
-      #   $2 = <desktop-manager>+<window-manager>
-      # SLiM is using the following parameter:
-      #   $1 = /nix/store/xxx-xsession <desktop-manager>+<window-manager>
-      # LightDM keeps the double quotes:
-      #   $1 = /nix/store/xxx-xsession "<desktop-manager>+<window-manager>"
-      # The fake/auto display manager doesn't use any parameters and GDM is
-      # broken.
-      # If you want to "debug" this script don't print the parameters to stdout
-      # or stderr because this script will be executed multiple times and the
-      # output won't be visible in the log when the script is executed for the
-      # first time (e.g. append them to a file instead)!
-
-      # All of the above cases are handled by the following hack (FIXME).
-      # Since this line is *very important* for *all display managers* it is
-      # very important to test changes to the following line with all display
-      # managers:
-      if [ "''${1:0:1}" = "/" ]; then eval exec "$1" "$2" ; fi
-
-      # Now it should be safe to assume that the script was called with the
-      # expected parameters.
 
       . /etc/profile
       cd "$HOME"
 
-      # The first argument of this script is the session type.
-      sessionType="$1"
-      if [ "$sessionType" = default ]; then sessionType=""; fi
-
       ${optionalString cfg.startDbusSession ''
         if test -z "$DBUS_SESSION_BUS_ADDRESS"; then
-          exec ${pkgs.dbus.dbus-launch} --exit-with-session "$0" "$sessionType"
+          exec ${pkgs.dbus.dbus-launch} --exit-with-session "$0" "$@"
         fi
       ''}
 
       ${optionalString cfg.displayManager.job.logToJournal ''
         if [ -z "$_DID_SYSTEMD_CAT" ]; then
           export _DID_SYSTEMD_CAT=1
-          exec ${config.systemd.package}/bin/systemd-cat -t xsession "$0" "$sessionType"
+          exec ${config.systemd.package}/bin/systemd-cat -t xsession "$0" "$@"
         fi
       ''}
 
@@ -101,6 +72,7 @@ let
       ${config.systemd.package}/bin/systemctl --user import-environment DISPLAY XAUTHORITY DBUS_SESSION_BUS_ADDRESS
 
       # Load X defaults.
+      # FIXME: Check XDG_SESSION_TYPE against x11
       ${xorg.xrdb}/bin/xrdb -merge ${xresourcesXft}
       if test -e ~/.Xresources; then
           ${xorg.xrdb}/bin/xrdb -merge ~/.Xresources
@@ -132,12 +104,33 @@ let
       # Allow the user to setup a custom session type.
       if test -x ~/.xsession; then
           exec ~/.xsession
-      else
-          if test "$sessionType" = "custom"; then
-              sessionType="" # fall-thru if there is no ~/.xsession
-          fi
       fi
 
+      if test "$1"; then
+          # Run the supplied session command. Remove any double quotes with eval.
+          eval exec "$@"
+      else
+          # Fall back to the default window/desktopManager
+          exec ${cfg.displayManager.session.script}
+      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).
@@ -194,7 +187,6 @@ let
         Type=XSession
         TryExec=${cfg.displayManager.session.script}
         Exec=${cfg.displayManager.session.script} "${n}"
-        X-GDM-BypassXsession=true
         Name=${n}
         Comment=
         EODESKTOP
@@ -292,6 +284,7 @@ in
               (filter (w: d.name != "none" || w.name != "none") wm));
           desktops = mkDesktops names;
           script = xsession wm dm;
+          wrapper = xsessionWrapper;
         };
       };
 

nixos/modules/services/x11/display-managers/gdm.nix

@@ -173,7 +173,7 @@ in
       ${optionalString cfg.gdm.debug "Enable=true"}
     '';
 
-    environment.etc."gdm/Xsession".source = "${pkgs.gnome3.gdm}/etc/gdm/Xsession";
+    environment.etc."gdm/Xsession".source = config.services.xserver.displayManager.session.wrapper;
 
     # GDM LFS PAM modules, adapted somehow to NixOS
     security.pam.services = {

nixos/modules/services/x11/display-managers/lightdm.nix

@@ -49,7 +49,7 @@ let
 
       [Seat:*]
       xserver-command = ${xserverWrapper}
-      session-wrapper = ${dmcfg.session.script}
+      session-wrapper = ${dmcfg.session.wrapper}
       ${optionalString cfg.greeter.enable ''
         greeter-session = ${cfg.greeter.name}
       ''}

nixos/modules/services/x11/display-managers/sddm.nix

@@ -49,7 +49,7 @@ let
     MinimumVT=${toString (if xcfg.tty != null then xcfg.tty else 7)}
     ServerPath=${xserverWrapper}
     XephyrPath=${pkgs.xorg.xorgserver.out}/bin/Xephyr
-    SessionCommand=${dmcfg.session.script}
+    SessionCommand=${dmcfg.session.wrapper}
     SessionDir=${dmcfg.session.desktops}/share/xsessions
     XauthPath=${pkgs.xorg.xauth}/bin/xauth
     DisplayCommand=${Xsetup}

nixos/modules/services/x11/display-managers/slim.nix

@@ -14,7 +14,7 @@ let
       default_xserver ${dmcfg.xserverBin}
       xserver_arguments ${toString dmcfg.xserverArgs}
       sessiondir ${dmcfg.session.desktops}/share/xsessions
-      login_cmd exec ${pkgs.runtimeShell} ${dmcfg.session.script} "%session"
+      login_cmd exec ${pkgs.runtimeShell} ${dmcfg.session.wrapper} "%session"
       halt_cmd ${config.systemd.package}/sbin/shutdown -h now
       reboot_cmd ${config.systemd.package}/sbin/shutdown -r now
       logfile /dev/stderr