Comment 3 for bug 690239

Alright, after some discussions with Ryan, I had a look into this again, and I have now fixed the problem in compiz. Some notes about this:

 * DBUS_SESSION_BUS_ADDRESS in the program's env isn't mandatory, since dbus can scrape that information from the root window property if it has X11 support
 * DBUS_SESSION_BUS_ADDRESS *won't* appear in the program env if it was launched from a VT or SSH or something
 * Compiz issues an UngrabServer request right after doing XQueryTree, and then initializes plugin screens right afterwards (before initializing every window).
 * UngrabServer isn't a synchronous operation, so there's a race for the X Server to process that in time
 * On CcpScreen's ctor, it will initialize libcompizconfig which loads backends (gconf)
 * GConf initializes and starts DBus to get a connection with the gconf server
 * GDbus sees that DBUS_SESSION_BUS_ADDRESS isn't set and calls dbus-launch --autolaunch to try and get the bus address
 * dbus-launch --autolaunch calls get_bus_address_platform_specific (X11 backend) which calls XOpenDisplay () and XGetWindowProperty () to try and get the session bus address from the window prop
 * The UngrabServer request hasn't been processed yet, so XOpenDisplay instantly fails
 * GDbus is using g_spawn_sync incorrectly and doesn't restore normal child handling, so when the child process dbus-launch fails to return an address (very rare case), it just blocks forever until glib times out the request. This is what causes the apparent hang, then crash 2 minutes later

All of the items above the last one have been fixed compiz-side so this is no longer an issue (though it does pose an interesting problem that trying to get the session bus address while a server grab is active will fail - that is probably a bad thing and should be resolved, maybe a temp socket file should be created in the user's homedir?). Anyways, I believe that while the last item is essentially a corner-case, it still prints an interesting debug message if the command failed to get the bus address and should be fixed.