Lightdm fails to undo quoting for Exec key

According to spec/documentation, "implementations must undo quoting before expanding field codes and before passing the argument to the executable program."

https://developer.gnome.org/desktop-entry-spec/#exec-variables

lightdm fails to do so and passes the double-quote characters to the executable program.

Consider the following files:

myecho:
#!/bin/bash
echo "$1" > /tmp/cmd

/usr/share/xsessions/test.desktop:
[Desktop Entry]
Name=Test
Comment=This is a test session
Type=Application
Exec=myecho "argument"

myecho is to be located somewhere in the PATH.

When trying to start a desktop session defined by test.desktop, as a result of running myecho, the first command-line argument from the Exec is written to /tmp/cmd.

Here is where lightdm is wrong: Starting the 'test' session under lightdm causes '"argument"' (without the surrounding single quotes) to be written to /tmp/cmd. The correct string should be 'argument' (without the surrounding single quotes). If the double-quotes are removed from the Exec line, lightdm behaves normally and 'argument' is written.

A practical consequence of this bug is that users can't use "bash -c" to run multiple commands when special characters (including spaces) are needed in the argument after "-c". It seems that the user must resort to a wrapper script because of this bug.

$ lsb_release -rd
Description: Ubuntu 19.04
Release: 19.04

$ apt-cache policy lightdm
lightdm:
  Installed: 1.28.0-0ubuntu1
  Candidate: 1.28.0-0ubuntu1
  Version table:
 *** 1.28.0-0ubuntu1 500
        500 http://mirrors.yun-idc.com/ubuntu disco/universe amd64 Packages
        100 /var/lib/dpkg/status