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When using KWin Wayland session in Debian testing with AMD Vega 56 GPU (connected over DisplayPort), it cores right after log-in and falls back into sddm. I managed to produce a core backtrace - it's happening in the interaction between kwin_wayland and wayland-client (see below). Mesa developers recommended opening kwin and wayland-client bug since it's likely a problem with either wayland-client or kwin_wayland (see also corresponding KDE bug and corresponding Mesa bug).

Configuration:

• Current Debian testing Linux (x86_64) (kernel: 4.19.0-rc3), KDE Plasma 5.13.5.
• kwin-wayland 5.13.5
• libwayland-client0 1.16.0
• GPU: Sapphire Pulse Vega 56 (amdgpu), DisplayPort connection.
• Mesa: 18.1.7.

Core was generated by `/usr/bin/kwin_wayland --xwayland --libinput --exit-with-session=/usr/lib/x86_64'.
Program terminated with signal SIGSEGV, Segmentation fault.
#0  0x00007eff59760f30 in wl_closure_init (message=message@entry=0x7, size=size@entry=52, num_arrays=num_arrays@entry=0x7eff5140858c, args=args@entry=0x0) at ../src/connection.c:562
562     ../src/connection.c: No such file or directory.
[Current thread is 1 (Thread 0x7eff51409700 (LWP 7249))]
(gdb) bt
#0  0x00007eff59760f30 in wl_closure_init (message=message@entry=0x7, size=size@entry=52, num_arrays=num_arrays@entry=0x7eff5140858c, args=args@entry=0x0) at ../src/connection.c:562
#1  0x00007eff59761aa0 in wl_connection_demarshal (connection=0x7eff440053e0, size=size@entry=52, objects=objects@entry=0x7eff440052e8, message=0x7) at ../src/connection.c:698
#2  0x00007eff5975fae8 in queue_event (len=52, display=0x7eff44005270) at ../src/wayland-client.c:1364
#3  read_events (display=0x7eff44005270) at ../src/wayland-client.c:1466
#4  wl_display_read_events (display=display@entry=0x7eff44005270) at ../src/wayland-client.c:1549
#5  0x00007eff59760169 in wl_display_dispatch_queue (display=0x7eff44005270, queue=0x7eff44005338) at ../src/wayland-client.c:1788
#6  0x00007eff5d123933 in KWayland::Client::ConnectionThread::Private::<lambda()>::operator() (__closure=0x7eff44009550) at ./src/client/connection_thread.cpp:129
#7  QtPrivate::FunctorCall<QtPrivate::IndexesList<>, QtPrivate::List<>, void, KWayland::Client::ConnectionThread::Private::setupSocketNotifier()::<lambda()> >::call (arg=<optimized out>, 
    f=...) at /usr/include/x86_64-linux-gnu/qt5/QtCore/qobjectdefs_impl.h:128
#8  QtPrivate::Functor<KWayland::Client::ConnectionThread::Private::setupSocketNotifier()::<lambda()>, 0>::call<QtPrivate::List<>, void> (arg=<optimized out>, f=...)
    at /usr/include/x86_64-linux-gnu/qt5/QtCore/qobjectdefs_impl.h:238
#9  QtPrivate::QFunctorSlotObject<KWayland::Client::ConnectionThread::Private::setupSocketNotifier()::<lambda()>, 0, QtPrivate::List<>, void>::impl(int, QtPrivate::QSlotObjectBase *, QObject *, void **, bool *) (which=<optimized out>, this_=0x7eff44009540, r=<optimized out>, a=<optimized out>, ret=<optimized out>)
    at /usr/include/x86_64-linux-gnu/qt5/QtCore/qobjectdefs_impl.h:421
#10 0x00007eff5e606910 in QtPrivate::QSlotObjectBase::call (a=0x7eff514087d0, r=0x564a80be84f0, this=0x7eff44009540) at ../../include/QtCore/../../src/corelib/kernel/qobjectdefs_impl.h:376
#11 QMetaObject::activate(QObject*, int, int, void**) () at kernel/qobject.cpp:3754
#12 0x00007eff5e606dd7 in QMetaObject::activate (sender=sender@entry=0x7eff44009440, m=m@entry=0x7eff5e863c60 <QSocketNotifier::staticMetaObject>, 
    local_signal_index=local_signal_index@entry=0, argv=argv@entry=0x7eff514087d0) at kernel/qobject.cpp:3633
#13 0x00007eff5e611ff9 in QSocketNotifier::activated (this=this@entry=0x7eff44009440, _t1=<optimized out>, _t2=...) at .moc/moc_qsocketnotifier.cpp:136
#14 0x00007eff5e612341 in QSocketNotifier::event (this=0x7eff44009440, e=0x7eff51408a30) at kernel/qsocketnotifier.cpp:266
#15 0x00007eff5e9cb4a1 in QApplicationPrivate::notify_helper(QObject*, QEvent*) () from /lib/x86_64-linux-gnu/libQt5Widgets.so.5
#16 0x00007eff5e9d2ae0 in QApplication::notify(QObject*, QEvent*) () from /lib/x86_64-linux-gnu/libQt5Widgets.so.5
#17 0x00007eff5e5dd579 in QCoreApplication::notifyInternal2(QObject*, QEvent*) () at ../../include/QtCore/5.11.1/QtCore/private/../../../../../src/corelib/thread/qthread_p.h:307
#18 0x00007eff5e62fe4a in QCoreApplication::sendEvent (event=0x7eff51408a30, receiver=<optimized out>) at ../../include/QtCore/../../src/corelib/kernel/qcoreapplication.h:234
#19 socketNotifierSourceDispatch(_GSource*, int (*)(void*), void*) () at kernel/qeventdispatcher_glib.cpp:106
#20 0x00007eff5a647287 in g_main_context_dispatch () from /lib/x86_64-linux-gnu/libglib-2.0.so.0
#21 0x00007eff5a6474c0 in ?? () from /lib/x86_64-linux-gnu/libglib-2.0.so.0
#22 0x00007eff5a64754c in g_main_context_iteration () from /lib/x86_64-linux-gnu/libglib-2.0.so.0
#23 0x00007eff5e62f223 in QEventDispatcherGlib::processEvents (this=0x7eff44000b20, flags=...) at kernel/qeventdispatcher_glib.cpp:423
#24 0x00007eff5e5dc24b in QEventLoop::exec(QFlags<QEventLoop::ProcessEventsFlag>) () at ../../include/QtCore/../../src/corelib/global/qflags.h:140
#25 0x00007eff5e42b176 in QThread::exec() () at ../../include/QtCore/../../src/corelib/global/qflags.h:120
#26 0x00007eff5e434d47 in QThreadPrivate::start(void*) () at thread/qthread_unix.cpp:367
#27 0x00007eff5efb5f2a in start_thread (arg=0x7eff51409700) at pthread_create.c:463
#28 0x00007eff5e0fdedf in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:95