[Xcb] Bug#579813: ../../src/xcb_io.c:445: _XReply: Assertion `!dpy->xcb->reply_data' failed.

[Jamey Sharp]

Hi! Just to be clear, this certainly isn't a bug in libxcb: _XReply is
in libx11, not libxcb, and I believe Xlib compiled without XCB support
should have failed in this case too. But maybe it's just as well you
assigned it to libxcb1 (however briefly) so that I'd notice it.

Another instance of this symptom was a server bug:

https://bugzilla.redhat.com/show_bug.cgi?id=417821

That was a bug squashed in the X.org implementation of GLX sometime in
the 6.8 series, but the broken code has lingered in other places. We
can tell if that's the cause, if you can provide an xtrace or
Wireshark dump of the X protocol traffic between the failing
application and the server.

The most important other piece of information would be a stack trace
from the failing application. That may not actually help, as this
assertion can only detect that something went wrong earlier, but it's
worth trying anyway.

This assertion means that some extension library (could be libGL, I
don't know) didn't finish reading the last response from the server
before asking for the next one. Before XCB, that kind of failure
should have permanently mangled the X connection, unless I've missed
some magic recovery mechanism Xlib had for covering up bugs (which has
happened before).

Hope this helps,
Jamey