Comment 9 for bug 2005116

try failed when change --session in /usr/lib/gnome-flashback/gnome-flashback-metacity

//
found
/etc/X11/Xsession.d/00upstart use $DESKTOP_SESSION

so I trace $DESKTOP_SESSION variable
It set in lightdm in source https://github.com/canonical/lightdm

dmrc.c
dmrc_load
~/.dmrc

user-list.c
priv->session = g_key_file_get_string (dmrc, "Desktop", "Session", NULL);
const gchar *session = priv->session;

seat.c
session_set_env (session, "DESKTOP_SESSION", session_name);

build install to /usr/sbin/lightdm

I guess it may lightdm execute problem