Connecting to GDM via XDMCP crashes it (GdmXdmcpDisplay shouldn't be abstract)

[Dana Goyette]

I've enabled XDMCP in GDM, and created an xinetd service for VNC via XDMCP.

Any time I try to connect via VNC, gdm crashes, taking down the local display as well.
After that, gdm won't start again until I kill the "orphaned" local display.

It looks like the fix is a single-line change: https://bugzilla.gnome.org/show_bug.cgi?id=757714

ProblemType: Crash
DistroRelease: Ubuntu 15.10
Package: gdm 3.16.2-0ubuntu2
Uname: Linux 4.4.0-040400rc4-generic x86_64
ApportVersion: 2.19.1-0ubuntu5
Architecture: amd64
CrashCounter: 1
Date: Fri Dec 11 15:24:52 2015
ExecutablePath: /usr/sbin/gdm
InstallationDate: Installed on 2011-07-12 (1613 days ago)
InstallationMedia: Ubuntu-Server 11.04 "Natty Narwhal" - Release amd64 (20110426)
ProcCmdline: /usr/sbin/gdm
ProcEnviron:
 LANG=en_US.UTF-8
 PATH=(custom, no user)
Signal: 5
SourcePackage: gdm
StacktraceTop:
 g_type_create_instance (type=14342800) at /build/glib2.0-ajuDY6/glib2.0-2.46.1/./gobject/gtype.c:1816
 g_object_constructor (type=<optimized out>, n_construct_properties=13, construct_params=0xdadb90) at /build/glib2.0-ajuDY6/glib2.0-2.46.1/./gobject/gobject.c:2064
 ()
 g_object_new_internal (n_params=6, params=0x7fff444e5fd0, class=0xdaa6a0) at /build/glib2.0-ajuDY6/glib2.0-2.46.1/./gobject/gobject.c:1697
 g_object_new_internal (class=class@entry=0xdaa6a0, params=params@entry=0x7fff444e5fd0, n_params=n_params@entry=6) at /build/glib2.0-ajuDY6/glib2.0-2.46.1/./gobject/gobject.c:1777
Title: gdm crashed with signal 5 in g_type_create_instance()
UpgradeStatus: Upgraded to wily on 2015-11-07 (34 days ago)
UserGroups: pulse-access
mtime.conffile..etc.gdm.custom.conf: 2015-12-11T15:11:59.227422

[Apport retracing service (apport)]

StacktraceTop:
 g_type_create_instance (type=14342800) at /build/glib2.0-ajuDY6/glib2.0-2.46.1/./gobject/gtype.c:1816
 g_object_constructor (type=<optimized out>, n_construct_properties=13, construct_params=0xdadb90) at /build/glib2.0-ajuDY6/glib2.0-2.46.1/./gobject/gobject.c:2064
 gdm_display_constructor (type=14342800, n_construct_properties=13, construct_properties=0xdadb90) at gdm-display.c:1110
 g_object_new_with_custom_constructor (n_params=6, params=0x7fff444e5fd0, class=0xdaa6a0) at /build/glib2.0-ajuDY6/glib2.0-2.46.1/./gobject/gobject.c:1697
 g_object_new_internal (class=class@entry=0xdaa6a0, params=params@entry=0x7fff444e5fd0, n_params=n_params@entry=6) at /build/glib2.0-ajuDY6/glib2.0-2.46.1/./gobject/gobject.c:1777