Desktop manager "none" is getting replaced by desktopManager.default

[primeos]

Issue description

I'm using the SDDM display manager which calls the xsession script with the following parameter ($1): 'none + i3'.

Expected behavior:

• desktop manager: none
• display manager: i3

Actual behavior:

• desktop manager: xterm (desktopManager.default)
• display manager: i3

The bug should be caused by the following file: nixos/modules/services/x11/display-managers/default.nix

Update: The following snippet works as intended.

Unfortunately I don't understand all the Bash hacks but it should be caused by the following code snippet:

      # The session type is "<desktop-manager> + <window-manager>", so
      # extract those.
      windowManager="''${sessionType##* + }"
      : ''${windowManager:=${cfg.windowManager.default}}
      desktopManager="''${sessionType% + *}"
      : ''${desktopManager:=${cfg.desktopManager.default}}

This one might be relevant as well:

      # Handle being called by SDDM.
      if test "''${1:0:1}" = / ; then eval exec $1 $2 ; fi

http://wiki.bash-hackers.org/syntax/pe - This might help understanding the Bash hacks. And can we please better document these hacks?

Additional questions:

• If "''${1:0:1}" checks if / is the first character of $1: This shouldn't be required for SDDM anymore (or should it? - 8a1fcaf that commit is pretty new...). (Explained here: Fix sessions with sddm. #22723)
• The syntax for $1 is desktopManager + windowManager -> can we please use that order in the code?

Steps to reproduce

Technical details

• System: (NixOS: nixos-version, Ubuntu/Fedora: lsb_release -a, ...)
• Nix version: (run nix-env --version)
• Nixpkgs version: (run nix-instantiate --eval '<nixpkgs>' -A lib.nixpkgsVersion)

[primeos]

(Temporarily) closing this due to missing/wrong information (just noticed I was looking at the wrong place... :o). I might be able to solve this on my own.