Created attachment 170881 [details]
New crash information added by DrKonqi

DrKonqi auto-attaching complete backtrace.

Created attachment 170882 [details]
WAYLAND_DEBUG=1 logs from KRunner just before the crash

KRunner backtrace:

#0  QPlatformScreen::screen (this=this@entry=0x0) at /usr/src/debug/dev-qt/qtbase-6.7.2/qtbase-everywhere-src-6.7.2/src/gui/kernel/qplatformscreen.cpp:115
#1  0x00007ffff4aca191 in QtWaylandClient::QWaylandWindow::handleScreensChanged (this=0x555556a7b830) at /usr/src/debug/dev-qt/qtwayland-6.7.2/qtwayland-everywhere-src-6.7.2/src/client/qwaylandwindow.cpp:1410
#2  0x00007ffff59e9ab5 in QtPrivate::QSlotObjectBase::call (this=0x555555ae6ed0, r=<optimized out>, a=0x7fffffffc2b8) at /usr/src/debug/dev-qt/qtbase-6.7.2/qtbase-everywhere-src-6.7.2/src/corelib/kernel/qobjectdefs_impl.h:469
#3  doActivate<false> (sender=0x555555b08c50, signal_index=3, argv=0x7fffffffc2b8) at /usr/src/debug/dev-qt/qtbase-6.7.2/qtbase-everywhere-src-6.7.2/src/corelib/kernel/qobject.cpp:4086
#4  0x00007ffff59e9ab5 in QtPrivate::QSlotObjectBase::call (this=0x555555b037e0, r=<optimized out>, a=0x7fffffffc400) at /usr/src/debug/dev-qt/qtbase-6.7.2/qtbase-everywhere-src-6.7.2/src/corelib/kernel/qobjectdefs_impl.h:469
#5  doActivate<false> (sender=0x7fffffffcd70, signal_index=9, argv=0x7fffffffc400) at /usr/src/debug/dev-qt/qtbase-6.7.2/qtbase-everywhere-src-6.7.2/src/corelib/kernel/qobject.cpp:4086
#6  0x00007ffff59a1857 in QMetaObject::activate (sender=<optimized out>, m=m@entry=0x7ffff66a1ca0 <QGuiApplication::staticMetaObject>, local_signal_index=local_signal_index@entry=1, argv=argv@entry=0x7fffffffc400) at /usr/src/debug/dev-qt/qtbase-6.7.2/qtbase-everywhere-src-6.7.2/src/corelib/kernel/qobject.cpp:4146
#7  0x00007ffff5f351b6 in QGuiApplication::screenAdded (this=<optimized out>, _t1=<optimized out>) at /usr/src/debug/dev-qt/qtbase-6.7.2/qtbase-everywhere-src-6.7.2_build/src/gui/Gui_autogen/include/moc_qguiapplication.cpp:421
#8  0x00007ffff5fabb5e in QWindowSystemInterface::handleScreenAdded (platformScreen=<optimized out>, isPrimary=isPrimary@entry=false) at /usr/src/debug/dev-qt/qtbase-6.7.2/qtbase-everywhere-src-6.7.2/src/gui/kernel/qwindowsysteminterface.cpp:727
#9  0x00007ffff4aeeafd in QtWaylandClient::QWaylandDisplay::handleScreenInitialized (this=0x5555555a4d30, screen=<optimized out>, screen@entry=0x7fffdc005e10) at /usr/src/debug/dev-qt/qtwayland-6.7.2/qtwayland-everywhere-src-6.7.2/src/client/qwaylanddisplay.cpp:601
#10 0x00007ffff4aeece3 in QtWaylandClient::QWaylandScreen::maybeInitialize (this=0x7fffdc005e10) at /usr/src/debug/dev-qt/qtwayland-6.7.2/qtwayland-everywhere-src-6.7.2/src/client/qwaylandscreen.cpp:74
#11 QtWaylandClient::QWaylandScreen::maybeInitialize (this=0x7fffdc005e10) at /usr/src/debug/dev-qt/qtwayland-6.7.2/qtwayland-everywhere-src-6.7.2/src/client/qwaylandscreen.cpp:65
#12 0x00007ffff366d38a in ffi_call_unix64 () at /usr/src/debug/dev-libs/libffi-3.4.6/libffi-3.4.6/src/x86/unix64.S:104
#13 0x00007ffff3669481 in ffi_call_int (cif=cif@entry=0x7fffffffc600, fn=<optimized out>, rvalue=<optimized out>, avalue=<optimized out>, closure=closure@entry=0x0) at /usr/src/debug/dev-libs/libffi-3.4.6/libffi-3.4.6/src/x86/ffi64.c:673
#14 0x00007ffff366c746 in ffi_call (cif=cif@entry=0x7fffffffc600, fn=<optimized out>, rvalue=rvalue@entry=0x0, avalue=avalue@entry=0x7fffffffc6d0) at /usr/src/debug/dev-libs/libffi-3.4.6/libffi-3.4.6/src/x86/ffi64.c:710
#15 0x00007ffff518303f in wl_closure_invoke (closure=closure@entry=0x7fffe00025b0, target=target@entry=0x5555555b24b0, opcode=opcode@entry=2, data=<optimized out>, flags=1) at ../wayland-1.23.0/src/connection.c:1228
#16 0x00007ffff5183889 in dispatch_event (display=display@entry=0x5555555a4f60, queue=queue@entry=0x5555555a5058) at ../wayland-1.23.0/src/wayland-client.c:1670
#17 0x00007ffff5183c63 in dispatch_queue (display=0x5555555a4f60, queue=0x5555555a5058) at ../wayland-1.23.0/src/wayland-client.c:1816
#18 wl_display_dispatch_queue_pending (display=0x5555555a4f60, queue=0x5555555a5058) at ../wayland-1.23.0/src/wayland-client.c:2058
#19 0x00007ffff4adefd5 in QtWaylandClient::EventThread::dispatchQueuePending (this=<optimized out>) at /usr/src/debug/dev-qt/qtwayland-6.7.2/qtwayland-everywhere-src-6.7.2/src/client/qwaylanddisplay.cpp:227
#20 QtWaylandClient::EventThread::readAndDispatchEvents (this=<optimized out>) at /usr/src/debug/dev-qt/qtwayland-6.7.2/qtwayland-everywhere-src-6.7.2/src/client/qwaylanddisplay.cpp:109
#21 QtWaylandClient::EventThread::readAndDispatchEvents (this=<optimized out>) at /usr/src/debug/dev-qt/qtwayland-6.7.2/qtwayland-everywhere-src-6.7.2/src/client/qwaylanddisplay.cpp:94
#22 QtWaylandClient::QWaylandDisplay::flushRequests (this=<optimized out>) at /usr/src/debug/dev-qt/qtwayland-6.7.2/qtwayland-everywhere-src-6.7.2/src/client/qwaylanddisplay.cpp:503
#23 0x00007ffff597d6cb in QObject::event (this=0x5555555a4d30, e=0x7fffe00019f0) at /usr/src/debug/dev-qt/qtbase-6.7.2/qtbase-everywhere-src-6.7.2/src/corelib/kernel/qobject.cpp:1452
#24 0x00007ffff6e44535 in QApplicationPrivate::notify_helper (this=<optimized out>, receiver=0x5555555a4d30, e=0x7fffe00019f0) at /usr/src/debug/dev-qt/qtbase-6.7.2/qtbase-everywhere-src-6.7.2/src/widgets/kernel/qapplication.cpp:3287
#25 0x00007ffff5a02658 in QCoreApplication::notifyInternal2 (receiver=0x5555555a4d30, event=0x7fffe00019f0) at /usr/src/debug/dev-qt/qtbase-6.7.2/qtbase-everywhere-src-6.7.2/src/corelib/kernel/qcoreapplication.cpp:1142
#26 0x00007ffff5a0270d in QCoreApplication::sendEvent (receiver=<optimized out>, event=<optimized out>) at /usr/src/debug/dev-qt/qtbase-6.7.2/qtbase-everywhere-src-6.7.2/src/corelib/kernel/qcoreapplication.cpp:1583
#27 0x00007ffff5a2d7fd in QCoreApplicationPrivate::sendPostedEvents (receiver=0x0, event_type=0, data=0x5555555994c0) at /usr/src/debug/dev-qt/qtbase-6.7.2/qtbase-everywhere-src-6.7.2/src/corelib/kernel/qcoreapplication.cpp:1940
#28 0x00007ffff5a2da6c in QCoreApplication::sendPostedEvents (receiver=<optimized out>, event_type=<optimized out>) at /usr/src/debug/dev-qt/qtbase-6.7.2/qtbase-everywhere-src-6.7.2/src/corelib/kernel/qcoreapplication.cpp:1797
#29 0x00007ffff578b0f7 in postEventSourceDispatch (s=0x5555555a1a70) at /usr/src/debug/dev-qt/qtbase-6.7.2/qtbase-everywhere-src-6.7.2/src/corelib/kernel/qeventdispatcher_glib.cpp:244
#30 0x00007ffff41df335 in g_main_dispatch (context=0x7fffe8000ef0) at ../glib-2.78.6/glib/gmain.c:3476
#31 0x00007ffff424b618 in g_main_context_dispatch_unlocked (context=0x7fffe8000ef0) at ../glib-2.78.6/glib/gmain.c:4284
#32 g_main_context_iterate_unlocked.isra.0 (context=0x7fffe8000ef0, block=<optimized out>, dispatch=1, self=<optimized out>) at ../glib-2.78.6/glib/gmain.c:4349
#33 0x00007ffff41de650 in g_main_context_iteration (context=0x7fffe8000ef0, may_block=1) at ../glib-2.78.6/glib/gmain.c:4414
#34 0x00007ffff5788ee3 in QEventDispatcherGlib::processEvents (this=0x5555555b5a70, flags=...) at /usr/src/debug/dev-qt/qtbase-6.7.2/qtbase-everywhere-src-6.7.2/src/corelib/kernel/qeventdispatcher_glib.cpp:394
#35 0x00007ffff5a3411a in QEventLoop::exec (this=this@entry=0x7fffffffcc40, flags=..., flags@entry=...) at /usr/src/debug/dev-qt/qtbase-6.7.2/qtbase-everywhere-src-6.7.2/src/corelib/global/qflags.h:34
#36 0x00007ffff5a34538 in QCoreApplication::exec () at /usr/src/debug/dev-qt/qtbase-6.7.2/qtbase-everywhere-src-6.7.2/src/corelib/global/qflags.h:74
#37 0x00007ffff5f36590 in QGuiApplication::exec () at /usr/src/debug/dev-qt/qtbase-6.7.2/qtbase-everywhere-src-6.7.2/src/gui/kernel/qguiapplication.cpp:1926
#38 0x00007ffff6d9f309 in QApplication::exec () at /usr/src/debug/dev-qt/qtbase-6.7.2/qtbase-everywhere-src-6.7.2/src/widgets/kernel/qapplication.cpp:2555
#39 0x000055555555def3 in main (argc=<optimized out>, argv=<optimized out>) at /usr/src/debug/kde-plasma/plasma-workspace-6.1.0/plasma-workspace-6.1.0/krunner/main.cpp:147

In the handleScreensChanged function, newScreen is a null pointer at this point:

     1406 void QWaylandWindow::handleScreensChanged()
     1407 {
     1408     QPlatformScreen *newScreen = calculateScreenFromSurfaceEvents();
     1409
  >  1410     if (newScreen->screen() == window()->screen())
     1411         return;
     1412
     1413     QWindowSystemInterface::handleWindowScreenChanged(window(), newScreen->QPlatformScreen::screen());
     1414
     1415     if (fixedToplevelPositions && !QPlatformWindow::parent() && window()->type() != Qt::Popup
     1416         && window()->type() != Qt::ToolTip
     1417         && geometry().topLeft() != newScreen->geometry().topLeft()) {
     1418         auto geometry = this->geometry();
     1419         geometry.moveTo(newScreen->geometry().topLeft());
     1420         setGeometry(geometry);
     1421     }
     1422
     1423     updateScale();
     1424     updateBufferTransform();
     1425 }

... as mSurface->oldestEnteredScreen and QPlatformWindow::screen are both null pointers at that point.  Also, 

(gdb) p mSurface.d->m_screens.d.size
$33 = 0
(gdb) 

... where mSurface is the member in QWaylandWindow (checked during the execution of QWaylandWindow::calculateScreenFromSurfaceEvents)

I re-ran this with WAYLAND_DEBUG=1.  Log attached.  There might be a long pause in it due to a breakpoint.

I also forgot to state precise, high level, reproduction steps.  Here they are:

1. Lock the screen.
2. Hit Esc
3. Wait for ten seconds (for all screens to turn off fully, presumably)
4. Wake the screens up.  The locker will have crashed at this point.  I've used loginctl unlock-session via KRunner to unlock
5. Observe that there is no desktop or KRunner

*** This bug has been marked as a duplicate of bug 473020 ***

>... as mSurface->oldestEnteredScreen being null is allowed and fine

 QPlatformWindow::screen being null is a big surprise. I want to understand that rather than just lazily add a guard.

The wayland backend always ensures there's a screen as far as Qt is concerned. If there's no screen added we'll make a placeholder screen. 
QGuiApplication:;primaryScreen() should always be valid.

QWindow caches the screen pointer. When a window is added it starts on a primary screen. Whenever a screen is removed (QWindowSystemInterface::handleScreenRemoved) we loop through all windows and assign any windows now not on a screen to the primary screen. If application code calls QWindow::setScreen(null) it gets put on the primary screen.

Reading the wayland log:
 - we start with two screens (bound locally to IDs 24 and 26)
 - then screen 26 gets removed
(a delay)
 - screen 24 moves
 - screen 26 gets added again

Ah, I had the duplicate wrong, I'm pretty sure this is actually Bug 489180.

*** This bug has been marked as a duplicate of bug 489180 ***

*** Bug 489635 has been marked as a duplicate of this bug. ***

*** Bug 489414 has been marked as a duplicate of this bug. ***