[gdm-list] reauthentication breaks XDMCP

[Gordon Messmer <yinyang eburg com>]

Yesterday I installed Fedora 18's beta and configured it as an XDMCP 
server for an X11 terminal running CentOS 6.  The terminal will display 
the GDM login screen, but entering a username causes the interface to 
stop working.  The username will remain displayed, and the buttons will 
animate when clicked, but no login is apparently attempted.

It looks like the problem is caused by the reauthentication support 
added in commit 2853ce5812aa8f03f677516e64d5dcff1e6e01ab.  GDM logs an 
exception and the error "Error getting session ids from systemd: No such 
file or directory".

I think there are three problems here.  First, glib is warning that 
GError isn't being set correctly.  GDM's exception and subsequent 
unresponsiveness may be a result of that, or a bug of it's own.  I'm not 
sure.  The big problem, though, is that as of that commit, GDM seems to 
require multiseat information from the X11 servers.  That requirement 
doesn't seem to make sense in XDMCP mode, but no fallback appears to be 
present.

Assuming that is the case, could someone either implement an alternate 
path for XDMCP so that the greeter doesn't crash, or disable XDMCP 
support when the system is built WITH_SYSTEMD?  If XDMCP isn't going to 
work, it'd be far preferable for GDM to simply log that it cannot enable 
XDMCP when built WITH_SYSTEMD than it is to enable XDMCP and 
subsequently fail.

Thanks.


Version:
gdm-3.6.2-5.fc18.x86_64

Commit:
https://mail.gnome.org/archives/commits-list/2012-July/msg04426.html

Logs:
==> messages <==
Dec 23 17:38:05 herald gdm[2173]: gdm-binary[2173]: GLib-WARNING: GError 
set over the top of a previous GError or uninitialized memory.
Dec 23 17:38:05 herald gdm[2173]: This indicates a bug in someone's 
code. You must ensure an error is NULL before it's set.
Dec 23 17:38:05 herald gdm[2173]: The overwriting error message was: 
Error getting session ids from systemd: No such file or directory
Dec 23 17:38:05 herald gdm-binary[2173]: GLib-WARNING: GError set over 
the top of a previous GError or uninitialized memory.

==> gdm/192.168.89.1:0-greeter.log <==
    JS ERROR: !!!   Exception was: TypeError: Object 0x7fdf06b41ec8 is 
not a subclass of (null), it's a GLib_Error
    JS ERROR: !!!     message = '"Object 0x7fdf06b41ec8 is not a 
subclass of (null), it's a GLib_Error"'
    JS ERROR: !!!     fileName = '"/usr/share/gnome-shell/js/gdm/util.js"'
    JS ERROR: !!!     lineNumber = '159'
    JS ERROR: !!!     stack = '"([object _private_Gdm_Client],[object 
_private_Gio_SimpleAsyncResult])@/usr/share/gnome-shell/js/gdm/util.js:159
wrapper([object _private_Gdm_Client],[object 
_private_Gio_SimpleAsyncResult])@/usr/share/gjs-1.0/lang.js:204"'