This bug was initially created as a copy of Bug #2092450

I am copying this bug because the same patches are in el9 as well.

Description of problem:

Downstream patches prevent mutter / gnome-shell from working with XDMCP.

Version-Release number of selected component (if applicable):

mutter-40.9-1.el9

How reproducible:

Always

Steps to Reproduce:

1. Setup xdmcp on el9 (see https://access.redhat.com/solutions/2736)
2. Try to connect to the host using Xorg, Xnest or Xwayland

Actual results:

Nothing happens, no greeter is shown

Expected results:

GDM greeter appears

Additional info:

In the debug logs, we can see an XError:

org.gnome.Shell.desktop[24817]: X Error of failed request:  BadMatch (invalid parameter attributes)
org.gnome.Shell.desktop[24817]:   Major opcode of failed request:  131 (XInputExtension)
org.gnome.Shell.desktop[24817]:   Minor opcode of failed request:  22 (X_GetFeedbackControl)
org.gnome.Shell.desktop[24817]:   Serial number of failed request:  416
org.gnome.Shell.desktop[24817]:   Current serial number in output stream:  416


So this is XGetFeedbackControl() returning a BadMatch.

I spent quite some time trying to figure where that call to XGetFeedbackControl() could come from, as it was nowhere to be found in mutter, gnome-shell or even gnome-settings-daemon, even though gdb was telling me that was coming from libmutter.

…until I realized that's from a downstream patch actually:

  0001-backends-x11-Support-synaptics-configuration.patch

which does:

  states = XGetFeedbackControl (xdisplay, xdevice, &num_feedbacks);

Removing this patch (and a few other dependent ones) allows GDM greeter to work just fine with xdmcp.

The fix should be fairly trivial I reckon, those calls need to be within an X error trap push/pop.