I am not sure whether the report includes all necessary symbols, but I can create a new backtrace if you tell me which packages are missing.
I already clicked on the Install Required Symbols button in the crash reporter, but it kept complaining the backtrace might be useless.

That looks correct to me, it's not showing any ?? which usually indicates missing symbols.

We are crashing in Phonon:

#8  Phonon::VLC::SurfacePainter::handlePaint(QPaintEvent*) (event=0x7ffcc3e58300, this=0x56245db35310) at ./src/video/videowidget.cpp:57
#9  Phonon::VLC::VideoWidget::paintEvent(QPaintEvent*) (this=<optimized out>, event=0x7ffcc3e58300) at ./src/video/videowidget.cpp:448

Unfortunately the crash is actually in qtgui and precisely those frames are missing debug symbols. Please try to get a new backtrace with the debug symbols for qtgui installed https://community.kde.org/Guidelines_and_HOWTOs/Debugging/How_to_create_useful_crash_reports#Backtraces

Somehow the crash reporter (drkonqi) does not start anymore:
```
Unable to start Dr. Konqi
org.kde.drkonqi: The specified process does not exist.
```
How can I fix this, or can I also create the new backtrace with gdb?

(In reply to Manuel Geißer from comment #5)
> or can I also create the new backtrace with gdb?

sure

New backtrace - I hope it's more useful now:

Reading symbols from /usr/bin/dolphin...
Reading symbols from /usr/lib/debug/.build-id/ce/2deed90dc691ba2e343a422e00e67b5680dcdf.debug...
(gdb) run
Starting program: /usr/bin/dolphin 
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".
[New Thread 0x7ffff046b700 (LWP 1518)]
[New Thread 0x7fffecc13700 (LWP 1519)]
[New Thread 0x7fffe7fff700 (LWP 1520)]
[New Thread 0x7fffe77fe700 (LWP 1521)]
[New Thread 0x7fffe6ffd700 (LWP 1522)]
[New Thread 0x7fffe67fc700 (LWP 1523)]
[Detaching after fork from child process 1525]
[Detaching after fork from child process 1527]
[Detaching after fork from child process 1528]
[Detaching after fork from child process 1529]
[Detaching after fork from child process 1530]
[Detaching after fork from child process 1531]
[New Thread 0x7fffe4877700 (LWP 1532)]
[New Thread 0x7fffd3fff700 (LWP 1533)]
[New Thread 0x7fffd35db700 (LWP 1534)]
[Thread 0x7fffd35db700 (LWP 1534) exited]
[New Thread 0x7fffd35db700 (LWP 1535)]
[New Thread 0x7fffd2dda700 (LWP 1536)]
QGestureManager::deliverEvent: could not find the target for gesture
QGestureManager::deliverEvent: could not find the target for gesture
[New Thread 0x7fbf98367700 (LWP 1537)]
[New Thread 0x7fbf97b66700 (LWP 1538)]
[New Thread 0x7fbf97365700 (LWP 1539)]
[New Thread 0x7fbf96b64700 (LWP 1540)]
[New Thread 0x7fbf96363700 (LWP 1541)]
[New Thread 0x7fbf95b62700 (LWP 1542)]
[New Thread 0x7fbf95791700 (LWP 1543)]

Thread 1 "dolphin" received signal SIGSEGV, Segmentation fault.
0x00007ffff601f210 in qt_scale_image_32bit<Blend_RGB32_on_RGB32_NoAlpha> (
    destPixels=0x7fffe5694000 "\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377"..., dbpl=5120, 
    srcPixels=0x7fbf952f0028 <error: Cannot access memory at address 0x7fbf952f0028>, sbpl=5120, srch=<optimized out>, targetRect=..., srcRect=..., 
    clip=..., blender=...) at painting/qblendfunctions_p.h:223
223     painting/qblendfunctions_p.h: Datei oder Verzeichnis nicht gefunden.
(gdb) backtrace full
#0  0x00007ffff601f210 in qt_scale_image_32bit<Blend_RGB32_on_RGB32_NoAlpha>(unsigned char*, int, unsigned char const*, int, int, QRectF const&, QRectF const&, QRect const&, Blend_RGB32_on_RGB32_NoAlpha)                                                                                                             
    (destPixels=0x7fffe5694000 "\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377"..., dbpl=5120, srcPixels=0x7fbf952f0028 <error: Cannot access memory at address 0x7fbf952f0028>, sbpl=5120, srch=<optimized out>, targetRect=..., srcRect=..., clip=..., blender=...) at painting/qblendfunctions_p.h:223
        src = 0x7fbf95639428
        srcx = <optimized out>
        x = <optimized out>
        sx = 7.191011235955056
        sy = 7.2000000000000002
        ix = 471270
        iy = 471859
        tr = {x1 = 1094, y1 = 158, x2 = 1271, y2 = 257}
        tx1 = <optimized out>
        ty1 = <optimized out>
        h = 6
        w = <optimized out>
        basex = 235635
        srcy = 44118816
        dst = 0x7fffe57ced18
        ystart = <optimized out>
        xstart = <optimized out>
        yend = <optimized out>
        xend = <optimized out>
#1  0x00007ffff6103a7d in QRasterPaintEngine::drawImage(QRectF const&, QImage const&, QRectF const&, QFlags<Qt::ImageConversionFlag>) (this=
    0x555555db0100, r=..., img=..., sr=...) at ../../include/QtGui/5.15.2/QtGui/private/../../../../../src/gui/painting/qpaintengine_raster_p.h:446
        sourceRect2x = <optimized out>
        scale2x = <optimized out>
        func = 0x7ffff601bd00 <qt_scale_image_rgb32_on_rgb32(unsigned char*, int, unsigned char const*, int, int, QRectF const&, QRectF const&, QRect const&, int)>
        targetBounds = {xp = 1094, yp = 158, w = 178, h = 100}
        offs = <optimized out>
        m = 
          {affine = {_m11 = 6.9533558071792804e-310, _m12 = 6.9533558071654466e-310, _m21 = 4.6355709067866939e-310, _m22 = -4.5033391262525684e-263, _dx = 4.6355711882163671e-310, _dy = 6.9533475721928474e-310}, m_13 = 5.5335352334219613e-322, m_23 = 4.9406564584124654e-324, m_33 = 4.635571188197395e-310, m_type = 9, m_dirty = 28, d = 0x555555cc6390}
        exceedsPrecision = false
        copy = 
          {affine = {_m11 = 6.9533558071452887e-310, _m12 = 4.6355706630415814e-310, _m21 = 4.6355709051594393e-310, _m22 = 5.2370958459172134e-322, _dx = 5.3754342267527624e-321, _dy = 6.9533478050938118e-310}, m_13 = 4.6355707809782131e-310, m_23 = 6.9533478050890194e-310, m_33 = 4.6355710078776633e-310, m_type = 0, m_dirty = 24, d = 0x55e64730}
        path = {d_ptr = {d = 0x7fffffffc8e0}}
        d = 0x555555db49d0
        s = <optimized out>
        stretch_sr = true
        clip = <optimized out>
#2  0x00007ffff611f861 in QPainter::drawImage(QRectF const&, QImage const&, QRectF const&, QFlags<Qt::ImageConversionFlag>)
    (this=this@entry=0x7fffffffcb78, targetRect=..., image=..., sourceRect=..., flags=flags@entry=...)
    at ../../include/QtCore/../../src/corelib/tools/qrect.h:644
        d = 0x5555561dd540
        x = 0
--Type <RET> for more, q to quit, c to continue without paging--c
        y = 46
        w = 178
        h = 100
        sx = <optimized out>
        sy = 0
        sw = <optimized out>
        sh = 720
        imageScale = <optimized out>
#3  0x00007fffec06a74f in QPainter::drawImage(QRect const&, QImage const&) (image=..., r=..., this=0x7fffffffcb78) at /usr/include/x86_64-linux-gnu/qt5/QtCore/qrect.h:269
        painter = {static staticMetaObject = {d = {superdata = {direct = 0x0}, stringdata = 0x7ffff63162c0 <qt_meta_stringdata_QPainter>, data = 0x7ffff63161e0 <qt_meta_data_QPainter>, static_metacall = 0x0, relatedMetaObjects = 0x0, extradata = 0x0}}, d_ptr = {d = 0x5555561dd540}}
#4  Phonon::VLC::SurfacePainter::handlePaint(QPaintEvent*) (event=0x7fffffffcf30, this=0x5555561b5bd0) at ./src/video/videowidget.cpp:57
        painter = {static staticMetaObject = {d = {superdata = {direct = 0x0}, stringdata = 0x7ffff63162c0 <qt_meta_stringdata_QPainter>, data = 0x7ffff63161e0 <qt_meta_data_QPainter>, static_metacall = 0x0, relatedMetaObjects = 0x0, extradata = 0x0}}, d_ptr = {d = 0x5555561dd540}}
#5  Phonon::VLC::VideoWidget::paintEvent(QPaintEvent*) (this=<optimized out>, event=0x7fffffffcf30) at ./src/video/videowidget.cpp:448
#6  0x00007ffff658cc06 in QWidget::event(QEvent*) () at /lib/x86_64-linux-gnu/libQt5Widgets.so.5
#7  0x00007ffff6549dc3 in QApplicationPrivate::notify_helper(QObject*, QEvent*) () at /lib/x86_64-linux-gnu/libQt5Widgets.so.5
#8  0x00007ffff6552bb8 in QApplication::notify(QObject*, QEvent*) () at /lib/x86_64-linux-gnu/libQt5Widgets.so.5
#9  0x00007ffff5a726da in QCoreApplication::notifyInternal2(QObject*, QEvent*) () at /lib/x86_64-linux-gnu/libQt5Core.so.5
#10 0x00007ffff658500a in QWidgetPrivate::sendPaintEvent(QRegion const&) () at /lib/x86_64-linux-gnu/libQt5Widgets.so.5
#11 0x00007ffff6585832 in QWidgetPrivate::drawWidget(QPaintDevice*, QRegion const&, QPoint const&, QFlags<QWidgetPrivate::DrawWidgetFlag>, QPainter*, QWidgetRepaintManager*) () at /lib/x86_64-linux-gnu/libQt5Widgets.so.5
#12 0x00007ffff655c427 in  () at /lib/x86_64-linux-gnu/libQt5Widgets.so.5
#13 0x00007ffff655cf44 in  () at /lib/x86_64-linux-gnu/libQt5Widgets.so.5
#14 0x00007ffff65866d5 in QWidgetPrivate::syncBackingStore() () at /lib/x86_64-linux-gnu/libQt5Widgets.so.5
#15 0x00007ffff658d5fc in QWidget::event(QEvent*) () at /lib/x86_64-linux-gnu/libQt5Widgets.so.5
#16 0x00007ffff66a58c8 in QMainWindow::event(QEvent*) () at /lib/x86_64-linux-gnu/libQt5Widgets.so.5
#17 0x00007ffff71ebc73 in KMainWindow::event(QEvent*) () at /lib/x86_64-linux-gnu/libKF5XmlGui.so.5
#18 0x00007ffff723713b in KXmlGuiWindow::event(QEvent*) () at /lib/x86_64-linux-gnu/libKF5XmlGui.so.5
#19 0x00007ffff7ee19c8 in DolphinMainWindow::event(QEvent*) (this=<optimized out>, event=<optimized out>) at ./src/dolphinmainwindow.cpp:2428
#20 0x00007ffff6549dc3 in QApplicationPrivate::notify_helper(QObject*, QEvent*) () at /lib/x86_64-linux-gnu/libQt5Widgets.so.5
#21 0x00007ffff6552bb8 in QApplication::notify(QObject*, QEvent*) () at /lib/x86_64-linux-gnu/libQt5Widgets.so.5
#22 0x00007ffff5a726da in QCoreApplication::notifyInternal2(QObject*, QEvent*) () at /lib/x86_64-linux-gnu/libQt5Core.so.5
#23 0x00007ffff5a74fe1 in QCoreApplicationPrivate::sendPostedEvents(QObject*, int, QThreadData*) () at /lib/x86_64-linux-gnu/libQt5Core.so.5
#24 0x00007ffff5acd987 in  () at /lib/x86_64-linux-gnu/libQt5Core.so.5
#25 0x00007ffff378817d in g_main_context_dispatch () at /lib/x86_64-linux-gnu/libglib-2.0.so.0
#26 0x00007ffff3788400 in  () at /lib/x86_64-linux-gnu/libglib-2.0.so.0
#27 0x00007ffff37884a3 in g_main_context_iteration () at /lib/x86_64-linux-gnu/libglib-2.0.so.0
#28 0x00007ffff5accfe2 in QEventDispatcherGlib::processEvents(QFlags<QEventLoop::ProcessEventsFlag>) () at /lib/x86_64-linux-gnu/libQt5Core.so.5
#29 0x00007ffff5a711eb in QEventLoop::exec(QFlags<QEventLoop::ProcessEventsFlag>) () at /lib/x86_64-linux-gnu/libQt5Core.so.5
#30 0x00007ffff5a79394 in QCoreApplication::exec() () at /lib/x86_64-linux-gnu/libQt5Core.so.5
#31 0x00007ffff7ed9fa2 in kdemain(int, char**) (argc=<optimized out>, argv=<optimized out>) at ./src/main.cpp:222
        app = <unvollständiger Typ>
        migrate = {d = std::unique_ptr<Kdelibs4ConfigMigratorPrivate> = {get() = 0x555555633370}}
        aboutData = {static staticMetaObject = {d = {superdata = {direct = 0x0}, stringdata = 0x7ffff6cfc940, data = 0x7ffff6cfc840, static_metacall = 0x7ffff6c94d00, relatedMetaObjects = 0x0, extradata = 0x0}}, d = std::unique_ptr<KAboutDataPrivate> = {get() = 0x555555631110}}
        parser = {d = 0x5555555bec20}
        splitView = <optimized out>
        openFiles = <optimized out>
        args = {<QList<QString>> = {<QListSpecialMethods<QString>> = {<No data fields>}, {p = {static shared_null = {ref = {atomic = {_q_value = {<std::__atomic_base<int>> = {static _S_alignment = 4, _M_i = -1}, <No data fields>}}}, alloc = 0, begin = 0, end = 0, array = {0x0}}, d = 0x7ffff5b61490 <QListData::shared_null>}, d = 0x7ffff5b61490 <QListData::shared_null>}}, <No data fields>}
        urls = {<QListSpecialMethods<QUrl>> = {<No data fields>}, {p = {static shared_null = {ref = {atomic = {_q_value = {<std::__atomic_base<int>> = {static _S_alignment = 4, _M_i = -1}, <No data fields>}}}, alloc = 0, begin = 0, end = 0, array = {0x0}}, d = 0x55555562db40}, d = 0x55555562db40}}
        startedWithURLs = <optimized out>
        mainWindow = <optimized out>
        dolphinDBusService = <unvollständiger Typ>
        interface = {<QObject> = {<No data fields>}, static staticMetaObject = {d = {superdata = {direct = 0x7ffff5d03ae0 <QObject::staticMetaObject>}, stringdata = 0x7ffff7f580e0 <qt_meta_stringdata_DBusInterface>, data = 0x7ffff7f58020 <qt_meta_data_DBusInterface>, static_metacall = 0x7ffff7ed7af0 <DBusInterface::qt_static_metacall(QObject*, QMetaObject::Call, int, void**)>, relatedMetaObjects = 0x0, extradata = 0x0}}, m_isDaemon = false}
#32 0x00007ffff7cbb0b3 in __libc_start_main (main=0x555555555060 <main>, argc=1, argv=0x7fffffffddf8, init=<optimized out>, fini=<optimized out>, rtld_fini=<optimized out>, stack_end=0x7fffffffdde8) at ../csu/libc-start.c:308
        result = <optimized out>
        unwind_buf = {cancel_jmp_buf = {{jmp_buf = {93824992235888, -7378732541055937274, 93824992235632, 140737488346608, 0, 0, 7378732540072219910, 7378750303349298438}, mask_was_saved = 0}}, priv = {pad = {0x0, 0x0, 0x1, 0x7fffffffddf8}, data = {prev = 0x0, cleanup = 0x0, canceltype = 1}}}
        not_first_call = <optimized out>
#33 0x000055555555509e in _start ()

Strangely the source refs are wrong in the frames below the top one,

> #2  0x00007ffff611f861 in QPainter::drawImage(QRectF const&, QImage const&, QRectF const&, QFlags<Qt::ImageConversionFlag>)
    (this=this@entry=0x7fffffffcb78, targetRect=..., image=..., sourceRect=..., flags=flags@entry=...)
    at ../../include/QtCore/../../src/corelib/tools/qrect.h:644

qrect.h most definitely doesn't contain QPainter::drawImage.

The top one may be correct though and point at a problem with the memory calculation for the image so I'm going with that theory, not that I have any idea what could possibly be wrong there.

(In reply to Harald Sitter from comment #8)
> qrect.h most definitely doesn't contain QPainter::drawImage.
> 
> The top one may be correct though and point at a problem with the memory
> calculation for the image so I'm going with that theory, not that I have any
> idea what could possibly be wrong there.

Strange, but I can only paste what the debugger throws out...

I have a theory and I have a plan for fixing it, but I'd like to do some more testing. Are you able to build phonon-vlc with a patch?

I think yes, provided there are some instructions on how to do it.

I have just tried building latest phonon-vlc from git and was able to do it. However, how do I 'tell' dolphin to use the self-built version of phonon, and not the system-provided one?

Since you are on neon I've just thrown the patch into the package.
You'll want to edit /etc/apt/sources.list.d/neon.list and change /user to /release, then update the package with a command like `pkcon refresh && pkcon update phonon4qt5-backend-vlc`
Once done you can switch back to /user

The patch introduces a number of different image calculation branches controlled through environment variables. Please make note which variants crashed or not:

- make sure all dolphins are closed in between steps
- run dolphin with `P_OUR_MATHS=1 dolphin --new-window`
- try to get it to crash
- run dolphin with `P_QIMAGE=1 dolphin --new-window`
- try to get it to crash
- run dolphin with `P_PIC_ALIGN=1 dolphin --new-window`
- try to get it to crash
- run dolphin with `P_PIC_VIS=1 dolphin --new-window`
- try to get it to crash

Comment on the bug which variants crashed and which did not.

Please also grab the console output of at least one run. Specifically I'm intrested in the lines that look like this:

Phonon::VLC::SurfacePainter::formatCallback: -------- 1920 1088 V
Phonon::VLC::SurfacePainter::formatCallback: rgb32 7680 1088 8355840
Phonon::VLC::SurfacePainter::formatCallback: V rgb32 7680 1090 8371200
Phonon::VLC::SurfacePainter::formatCallback: :: V rgb32 [ 0 ] 7680 1090 7680 1088 4
Phonon::VLC::VideoMemoryStream::formatCallbackInternal: FORMAT[chroma: RV32, w: 1920, h: 1088, pitch: 7680, lines: 7680, size: 58982400]

P_OUR_MATHS=1 dolphin --new-window
-> sometimes crashes, but perhaps less than before

P_QIMAGE=1 dolphin --new-window
-> there was one crash when I was very fast with starting the video

P_PIC_ALIGN=1 dolphin --new-window
-> I was not able to make it crash at all

P_PIC_VIS=1 dolphin --new-window
-> crashes, but I also need to be fast

The command-line output lines you wrote about did not turn up - I don't know why

All output I get basically consists of this:
```
[h264 @ 0x7ec244c9b280] get_buffer() failed
[h264 @ 0x7ec244c9b280] thread_get_buffer() failed
[h264 @ 0x7ec244c9b280] decode_slice_header error
[h264 @ 0x7ec244c9b280] no frame!
```

Interesting.

I've put another build into the /release repo, if you could update and try the P_QIMAGE case again, please. It should now also produce some output like I've described.

It'd be super awesome if you could also check if that qimage crash indeed has the same backtrace, I would rather have expected qimage to never crash based on the files I have tired.

(In reply to Harald Sitter from comment #16)
> Interesting.
> 
> I've put another build into the /release repo, if you could update and try
> the P_QIMAGE case again, please. It should now also produce some output like
> I've described.
I've updated, but still don't have the output.
However, `P_QIMAGE=1 dolphin --new-window` now crashes much more often, virtually always, which I find peculiar.

> It'd be super awesome if you could also check if that qimage crash indeed
> has the same backtrace, I would rather have expected qimage to never crash
> based on the files I have tired.
Yes. I will post the backtrace in the next comment. It looks fairly similar to me.

manuel@lx2020:~$ export P_QIMAGE=1
manuel@lx2020:~$ gdb --args dolphin --new-window
GNU gdb (Ubuntu 9.2-0ubuntu1~20.04) 9.2
Copyright (C) 2020 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.
Type "show copying" and "show warranty" for details.
This GDB was configured as "x86_64-linux-gnu".
Type "show configuration" for configuration details.
For bug reporting instructions, please see:
<http://www.gnu.org/software/gdb/bugs/>.
Find the GDB manual and other documentation resources online at:
    <http://www.gnu.org/software/gdb/documentation/>.

For help, type "help".
Type "apropos word" to search for commands related to "word"...
Reading symbols from dolphin...
Reading symbols from /usr/lib/debug/.build-id/ce/2deed90dc691ba2e343a422e00e67b5680dcdf.debug...
(gdb) run
Starting program: /usr/bin/dolphin --new-window
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".
[New Thread 0x7ffff046b700 (LWP 6669)]
[New Thread 0x7fffecc13700 (LWP 6670)]
[New Thread 0x7fffe7fff700 (LWP 6671)]
[New Thread 0x7fffe77fe700 (LWP 6672)]
[New Thread 0x7fffe6ffd700 (LWP 6673)]
[New Thread 0x7fffe67fc700 (LWP 6674)]
[Detaching after fork from child process 6684]
[Detaching after fork from child process 6690]
[Detaching after fork from child process 6692]
[Detaching after fork from child process 6693]
[Detaching after fork from child process 6694]
[Detaching after fork from child process 6695]
[New Thread 0x7fbfcbfff700 (LWP 6696)]
[Thread 0x7fbfcbfff700 (LWP 6696) exited]
[New Thread 0x7fbfcbfff700 (LWP 6697)]
[New Thread 0x7fffe4600700 (LWP 6698)]
[New Thread 0x7fbfa1366700 (LWP 6699)]
[New Thread 0x7fbfa0b65700 (LWP 6700)]
[New Thread 0x7fbfa0364700 (LWP 6701)]
[New Thread 0x7fbf9fb63700 (LWP 6702)]
[New Thread 0x7fbf9f362700 (LWP 6703)]
[New Thread 0x7fbf9eb61700 (LWP 6704)]
[New Thread 0x7fbf9e795700 (LWP 6705)]

Thread 1 "dolphin" received signal SIGSEGV, Segmentation fault.
0x00007ffff601f210 in qt_scale_image_32bit<Blend_RGB32_on_RGB32_NoAlpha> (
    destPixels=0x7fffe5694000 "\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377"..., dbpl=5120, 
    srcPixels=0x7fbf9e2f4028 <error: Cannot access memory at address 0x7fbf9e2f4028>, sbpl=5120, srch=<optimized out>, targetRect=..., srcRect=..., 
    clip=..., blender=...) at painting/qblendfunctions_p.h:223
223     painting/qblendfunctions_p.h: Datei oder Verzeichnis nicht gefunden.
(gdb) bt
#0  0x00007ffff601f210 in qt_scale_image_32bit<Blend_RGB32_on_RGB32_NoAlpha>(unsigned char*, int, unsigned char const*, int, int, QRectF const&, QRectF const&, QRect const&, Blend_RGB32_on_RGB32_NoAlpha)                                                                                                             
    (destPixels=0x7fffe5694000 "\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377"..., dbpl=5120, srcPixels=0x7fbf9e2f4028 <error: Cannot access memory at address 0x7fbf9e2f4028>, sbpl=5120, srch=<optimized out>, targetRect=..., srcRect=..., clip=..., blender=...) at painting/qblendfunctions_p.h:223
#1  0x00007ffff6103a7d in QRasterPaintEngine::drawImage(QRectF const&, QImage const&, QRectF const&, QFlags<Qt::ImageConversionFlag>) (this=
    0x555555db3820, r=..., img=..., sr=...) at ../../include/QtGui/5.15.2/QtGui/private/../../../../../src/gui/painting/qpaintengine_raster_p.h:446
#2  0x00007ffff611f861 in QPainter::drawImage(QRectF const&, QImage const&, QRectF const&, QFlags<Qt::ImageConversionFlag>)
    (this=this@entry=0x7fffffffcb38, targetRect=..., image=..., sourceRect=..., flags=flags@entry=...)
    at ../../include/QtCore/../../src/corelib/tools/qrect.h:644
#3  0x00007fffec06a74f in QPainter::drawImage(QRect const&, QImage const&) (image=..., r=..., this=0x7fffffffcb38)
    at /usr/include/x86_64-linux-gnu/qt5/QtCore/qrect.h:269
#4  Phonon::VLC::SurfacePainter::handlePaint(QPaintEvent*) (event=0x7fffffffcef0, this=0x555556176430) at ./src/video/videowidget.cpp:57
#5  Phonon::VLC::VideoWidget::paintEvent(QPaintEvent*) (this=<optimized out>, event=0x7fffffffcef0) at ./src/video/videowidget.cpp:448
#6  0x00007ffff658cc06 in QWidget::event(QEvent*) () at /lib/x86_64-linux-gnu/libQt5Widgets.so.5
#7  0x00007ffff6549dc3 in QApplicationPrivate::notify_helper(QObject*, QEvent*) () at /lib/x86_64-linux-gnu/libQt5Widgets.so.5
#8  0x00007ffff6552bb8 in QApplication::notify(QObject*, QEvent*) () at /lib/x86_64-linux-gnu/libQt5Widgets.so.5
#9  0x00007ffff5a726da in QCoreApplication::notifyInternal2(QObject*, QEvent*) () at /lib/x86_64-linux-gnu/libQt5Core.so.5
#10 0x00007ffff658500a in QWidgetPrivate::sendPaintEvent(QRegion const&) () at /lib/x86_64-linux-gnu/libQt5Widgets.so.5
#11 0x00007ffff6585832 in QWidgetPrivate::drawWidget(QPaintDevice*, QRegion const&, QPoint const&, QFlags<QWidgetPrivate::DrawWidgetFlag>, QPainter*, QWidgetRepaintManager*) () at /lib/x86_64-linux-gnu/libQt5Widgets.so.5
#12 0x00007ffff655c427 in  () at /lib/x86_64-linux-gnu/libQt5Widgets.so.5
#13 0x00007ffff655cf44 in  () at /lib/x86_64-linux-gnu/libQt5Widgets.so.5
#14 0x00007ffff65866d5 in QWidgetPrivate::syncBackingStore() () at /lib/x86_64-linux-gnu/libQt5Widgets.so.5
#15 0x00007ffff658d5fc in QWidget::event(QEvent*) () at /lib/x86_64-linux-gnu/libQt5Widgets.so.5
#16 0x00007ffff66a58c8 in QMainWindow::event(QEvent*) () at /lib/x86_64-linux-gnu/libQt5Widgets.so.5
#17 0x00007ffff71ebc73 in KMainWindow::event(QEvent*) () at /lib/x86_64-linux-gnu/libKF5XmlGui.so.5
#18 0x00007ffff723713b in KXmlGuiWindow::event(QEvent*) () at /lib/x86_64-linux-gnu/libKF5XmlGui.so.5
#19 0x00007ffff7ee19c8 in DolphinMainWindow::event(QEvent*) (this=<optimized out>, event=<optimized out>) at ./src/dolphinmainwindow.cpp:2428
#20 0x00007ffff6549dc3 in QApplicationPrivate::notify_helper(QObject*, QEvent*) () at /lib/x86_64-linux-gnu/libQt5Widgets.so.5
#21 0x00007ffff6552bb8 in QApplication::notify(QObject*, QEvent*) () at /lib/x86_64-linux-gnu/libQt5Widgets.so.5
#22 0x00007ffff5a726da in QCoreApplication::notifyInternal2(QObject*, QEvent*) () at /lib/x86_64-linux-gnu/libQt5Core.so.5
#23 0x00007ffff5a74fe1 in QCoreApplicationPrivate::sendPostedEvents(QObject*, int, QThreadData*) () at /lib/x86_64-linux-gnu/libQt5Core.so.5
#24 0x00007ffff5acd987 in  () at /lib/x86_64-linux-gnu/libQt5Core.so.5
#25 0x00007ffff378817d in g_main_context_dispatch () at /lib/x86_64-linux-gnu/libglib-2.0.so.0
#26 0x00007ffff3788400 in  () at /lib/x86_64-linux-gnu/libglib-2.0.so.0
#27 0x00007ffff37884a3 in g_main_context_iteration () at /lib/x86_64-linux-gnu/libglib-2.0.so.0
#28 0x00007ffff5accfe2 in QEventDispatcherGlib::processEvents(QFlags<QEventLoop::ProcessEventsFlag>) () at /lib/x86_64-linux-gnu/libQt5Core.so.5
#29 0x00007ffff5a711eb in QEventLoop::exec(QFlags<QEventLoop::ProcessEventsFlag>) () at /lib/x86_64-linux-gnu/libQt5Core.so.5
#30 0x00007ffff5a79394 in QCoreApplication::exec() () at /lib/x86_64-linux-gnu/libQt5Core.so.5
#31 0x00007ffff7ed9fa2 in kdemain(int, char**) (argc=<optimized out>, argv=<optimized out>) at ./src/main.cpp:222
#32 0x00007ffff7cbb0b3 in __libc_start_main (main=
    0x555555555060 <main>, argc=2, argv=0x7fffffffddb8, init=<optimized out>, fini=<optimized out>, rtld_fini=<optimized out>, stack_end=0x7fffffffdda8)
    at ../csu/libc-start.c:308
#33 0x000055555555509e in _start ()

(In reply to Manuel Geißer from comment #17)
> I've updated, but still don't have the output.
> However, `P_QIMAGE=1 dolphin --new-window` now crashes much more often,
> virtually always, which I find peculiar.
I clarify: It really always crashes. It didn't even work once (also outside gdb).

It is profoundly strange that the debug output still doesn't show, it's now being sent to stderr directly. Seeing as there is only a single option that doesn't ever crash, even without the debug output, the solution is clear.
Thanks!

I've been thinking about this some more...

It is suspicious that some of the cases only crash when you are quick enough. That sounds an awful lot like a race condition in the code.

I've pushed yet another build to neon. Please update and simply try to crash dolphin, `dolphin --new-window` (no environment variable necessary).

Assuming my theory is correct this shouldn't crash. If it does, please check if the backtrace is still the same.

I've tried the new version - in fact, it always crashes at the very beginning. It didn't even work once.

Backtrace (seems similar to me):
Reading symbols from dolphin...
Reading symbols from /usr/lib/debug/.build-id/ce/2deed90dc691ba2e343a422e00e67b5680dcdf.debug...
(gdb) run
Starting program: /usr/bin/dolphin --new-window
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".
[New Thread 0x7ffff046b700 (LWP 8335)]
[New Thread 0x7fffecc13700 (LWP 8336)]
[New Thread 0x7fffe7fff700 (LWP 8337)]
[New Thread 0x7fffe77fe700 (LWP 8338)]
[New Thread 0x7fffe6ffd700 (LWP 8339)]
[New Thread 0x7fffe67fc700 (LWP 8340)]
[Detaching after fork from child process 8342]
[Detaching after fork from child process 8343]
[Detaching after fork from child process 8344]
[Detaching after fork from child process 8345]
[Detaching after fork from child process 8346]
[New Thread 0x7fbfcbfff700 (LWP 8347)]
[Thread 0x7fbfcbfff700 (LWP 8347) exited]
[New Thread 0x7fbfcbfff700 (LWP 8348)]
[New Thread 0x7fffe4627700 (LWP 8349)]
[New Thread 0x7fbfa1382700 (LWP 8350)]
[New Thread 0x7fbfa0b81700 (LWP 8351)]
[New Thread 0x7fbfa0380700 (LWP 8352)]
[New Thread 0x7fbf9fb7f700 (LWP 8353)]
[New Thread 0x7fbf9f37e700 (LWP 8354)]
[New Thread 0x7fbf9eb7d700 (LWP 8355)]
[New Thread 0x7fbf9e7ac700 (LWP 8356)]

Thread 1 "dolphin" received signal SIGSEGV, Segmentation fault.
0x00007ffff601f210 in qt_scale_image_32bit<Blend_RGB32_on_RGB32_NoAlpha> (
    destPixels=0x7fffe4878000 "\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377"..., dbpl=5120, 
    srcPixels=0x7fbf9e30b028 <error: Cannot access memory at address 0x7fbf9e30b028>, sbpl=5120, srch=<optimized out>, targetRect=..., srcRect=..., 
    clip=..., blender=...) at painting/qblendfunctions_p.h:223
223     painting/qblendfunctions_p.h: Datei oder Verzeichnis nicht gefunden.
(gdb) bt full
#0  0x00007ffff601f210 in qt_scale_image_32bit<Blend_RGB32_on_RGB32_NoAlpha>(unsigned char*, int, unsigned char const*, int, int, QRectF const&, QRectF const&, QRect const&, Blend_RGB32_on_RGB32_NoAlpha)                                                                                                             
    (destPixels=0x7fffe4878000 "\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377\f\f\f\377"..., dbpl=5120, srcPixels=0x7fbf9e30b028 <error: Cannot access memory at address 0x7fbf9e30b028>, sbpl=5120, srch=<optimized out>, targetRect=..., srcRect=..., clip=..., blender=...) at painting/qblendfunctions_p.h:223
        src = 0x7fbf9e5a2c28
        srcx = <optimized out>
        x = <optimized out>
        sx = 8
        sy = 8
        ix = 524288
        iy = 524288
        tr = {x1 = 1112, y1 = 155, x2 = 1271, y2 = 244}
        tx1 = <optimized out>
        ty1 = <optimized out>
        h = 23
        w = <optimized out>
        basex = 262143
        srcy = 34865151
        dst = 0x7fffe498d560
        ystart = <optimized out>
        xstart = <optimized out>
        yend = <optimized out>
        xend = <optimized out>
#1  0x00007ffff6103a7d in QRasterPaintEngine::drawImage(QRectF const&, QImage const&, QRectF const&, QFlags<Qt::ImageConversionFlag>) (this=
    0x555555c0b600, r=..., img=..., sr=...) at ../../include/QtGui/5.15.2/QtGui/private/../../../../../src/gui/painting/qpaintengine_raster_p.h:446
        sourceRect2x = <optimized out>
        scale2x = <optimized out>
        func = 0x7ffff601bd00 <qt_scale_image_rgb32_on_rgb32(unsigned char*, int, unsigned char const*, int, int, QRectF const&, QRectF const&, QRect const&, int)>
        targetBounds = {xp = 1112, yp = 155, w = 160, h = 90}
        offs = <optimized out>
        m = 
          {affine = {_m11 = 6.9533558071776994e-310, _m12 = 1.8897412473187624e-116, _m21 = 4.6355708548983388e-310, _m22 = 1.8897412473187624e-116, _dx = 4.635571128515846e-310, _dy = 6.9533475721928474e-310}, m_13 = 5.5335352334219613e-322, m_23 = 4.9406564584124654e-324, m_33 = 4.6355711284968738e-310, m_type = 9, m_dirty = 28, d = 0x55555609b8b0}
        exceedsPrecision = false
        copy = 
          {affine = {_m11 = 6.9533558071437077e-310, _m12 = 1.8897412473187624e-116, _m21 = 6.9533558072176199e-310, _m22 = 6.9533558072176199e-310, _dx = 4.6355705476361469e-310, _dy = 6.9533484472143311e-310}, m_13 = 6.953355807228687e-310, m_23 = 6.9533428057777792e-310, m_33 = 6.9533558071433124e-310, m_type = 8, m_dirty = 10, d = 0x7fffffffc950}
        path = {d_ptr = {d = 0x7fffffffcf30}}
        d = 0x555555de9c90
        s = <optimized out>
        stretch_sr = true
        clip = <optimized out>
#2  0x00007ffff611f861 in QPainter::drawImage(QRectF const&, QImage const&, QRectF const&, QFlags<Qt::ImageConversionFlag>)
    (this=this@entry=0x7fffffffcb58, targetRect=..., image=..., sourceRect=..., flags=flags@entry=...)
    at ../../include/QtCore/../../src/corelib/tools/qrect.h:644
        d = 0x555555fba050
        x = 0
--Type <RET> for more, q to quit, c to continue without paging--c
        y = 43
        w = 160
        h = 90
        sx = <optimized out>
        sy = 0
        sw = <optimized out>
        sh = 720
        imageScale = <optimized out>
#3  0x00007fffec07a74f in QPainter::drawImage(QRect const&, QImage const&) (image=..., r=..., this=0x7fffffffcb58) at /usr/include/x86_64-linux-gnu/qt5/QtCore/qrect.h:269
        painter = {static staticMetaObject = {d = {superdata = {direct = 0x0}, stringdata = 0x7ffff63162c0 <qt_meta_stringdata_QPainter>, data = 0x7ffff63161e0 <qt_meta_data_QPainter>, static_metacall = 0x0, relatedMetaObjects = 0x0, extradata = 0x0}}, d_ptr = {d = 0x555555fba050}}
#4  Phonon::VLC::SurfacePainter::handlePaint(QPaintEvent*) (event=0x7fffffffcf10, this=0x55555608a590) at ./src/video/videowidget.cpp:57
        painter = {static staticMetaObject = {d = {superdata = {direct = 0x0}, stringdata = 0x7ffff63162c0 <qt_meta_stringdata_QPainter>, data = 0x7ffff63161e0 <qt_meta_data_QPainter>, static_metacall = 0x0, relatedMetaObjects = 0x0, extradata = 0x0}}, d_ptr = {d = 0x555555fba050}}
#5  Phonon::VLC::VideoWidget::paintEvent(QPaintEvent*) (this=<optimized out>, event=0x7fffffffcf10) at ./src/video/videowidget.cpp:448
#6  0x00007ffff658cc06 in QWidget::event(QEvent*) () at /lib/x86_64-linux-gnu/libQt5Widgets.so.5
#7  0x00007ffff6549dc3 in QApplicationPrivate::notify_helper(QObject*, QEvent*) () at /lib/x86_64-linux-gnu/libQt5Widgets.so.5
#8  0x00007ffff6552bb8 in QApplication::notify(QObject*, QEvent*) () at /lib/x86_64-linux-gnu/libQt5Widgets.so.5
#9  0x00007ffff5a726da in QCoreApplication::notifyInternal2(QObject*, QEvent*) () at /lib/x86_64-linux-gnu/libQt5Core.so.5
#10 0x00007ffff658500a in QWidgetPrivate::sendPaintEvent(QRegion const&) () at /lib/x86_64-linux-gnu/libQt5Widgets.so.5
#11 0x00007ffff6585832 in QWidgetPrivate::drawWidget(QPaintDevice*, QRegion const&, QPoint const&, QFlags<QWidgetPrivate::DrawWidgetFlag>, QPainter*, QWidgetRepaintManager*) () at /lib/x86_64-linux-gnu/libQt5Widgets.so.5
#12 0x00007ffff655c427 in  () at /lib/x86_64-linux-gnu/libQt5Widgets.so.5
#13 0x00007ffff655cf44 in  () at /lib/x86_64-linux-gnu/libQt5Widgets.so.5
#14 0x00007ffff65866d5 in QWidgetPrivate::syncBackingStore() () at /lib/x86_64-linux-gnu/libQt5Widgets.so.5
#15 0x00007ffff658d5fc in QWidget::event(QEvent*) () at /lib/x86_64-linux-gnu/libQt5Widgets.so.5
#16 0x00007ffff66a58c8 in QMainWindow::event(QEvent*) () at /lib/x86_64-linux-gnu/libQt5Widgets.so.5
#17 0x00007ffff71ebc73 in KMainWindow::event(QEvent*) () at /lib/x86_64-linux-gnu/libKF5XmlGui.so.5
#18 0x00007ffff723713b in KXmlGuiWindow::event(QEvent*) () at /lib/x86_64-linux-gnu/libKF5XmlGui.so.5
#19 0x00007ffff7ee19c8 in DolphinMainWindow::event(QEvent*) (this=<optimized out>, event=<optimized out>) at ./src/dolphinmainwindow.cpp:2428
#20 0x00007ffff6549dc3 in QApplicationPrivate::notify_helper(QObject*, QEvent*) () at /lib/x86_64-linux-gnu/libQt5Widgets.so.5
#21 0x00007ffff6552bb8 in QApplication::notify(QObject*, QEvent*) () at /lib/x86_64-linux-gnu/libQt5Widgets.so.5
#22 0x00007ffff5a726da in QCoreApplication::notifyInternal2(QObject*, QEvent*) () at /lib/x86_64-linux-gnu/libQt5Core.so.5
#23 0x00007ffff5a74fe1 in QCoreApplicationPrivate::sendPostedEvents(QObject*, int, QThreadData*) () at /lib/x86_64-linux-gnu/libQt5Core.so.5
#24 0x00007ffff5acd987 in  () at /lib/x86_64-linux-gnu/libQt5Core.so.5
#25 0x00007ffff378817d in g_main_context_dispatch () at /lib/x86_64-linux-gnu/libglib-2.0.so.0
#26 0x00007ffff3788400 in  () at /lib/x86_64-linux-gnu/libglib-2.0.so.0
#27 0x00007ffff37884a3 in g_main_context_iteration () at /lib/x86_64-linux-gnu/libglib-2.0.so.0
#28 0x00007ffff5accfe2 in QEventDispatcherGlib::processEvents(QFlags<QEventLoop::ProcessEventsFlag>) () at /lib/x86_64-linux-gnu/libQt5Core.so.5
#29 0x00007ffff5a711eb in QEventLoop::exec(QFlags<QEventLoop::ProcessEventsFlag>) () at /lib/x86_64-linux-gnu/libQt5Core.so.5
#30 0x00007ffff5a79394 in QCoreApplication::exec() () at /lib/x86_64-linux-gnu/libQt5Core.so.5
#31 0x00007ffff7ed9fa2 in kdemain(int, char**) (argc=<optimized out>, argv=<optimized out>) at ./src/main.cpp:222
        app = <unvollständiger Typ>
        migrate = {d = std::unique_ptr<Kdelibs4ConfigMigratorPrivate> = {get() = 0x555555625130}}
        aboutData = {static staticMetaObject = {d = {superdata = {direct = 0x0}, stringdata = 0x7ffff6cfc940, data = 0x7ffff6cfc840, static_metacall = 0x7ffff6c94d00, relatedMetaObjects = 0x0, extradata = 0x0}}, d = std::unique_ptr<KAboutDataPrivate> = {get() = 0x5555555bbe00}}
        parser = {d = 0x5555555f6210}
        splitView = <optimized out>
        openFiles = <optimized out>
        args = {<QList<QString>> = {<QListSpecialMethods<QString>> = {<No data fields>}, {p = {static shared_null = {ref = {atomic = {_q_value = {<std::__atomic_base<int>> = {static _S_alignment = 4, _M_i = -1}, <No data fields>}}}, alloc = 0, begin = 0, end = 0, array = {0x0}}, d = 0x7ffff5b61490 <QListData::shared_null>}, d = 0x7ffff5b61490 <QListData::shared_null>}}, <No data fields>}
        urls = {<QListSpecialMethods<QUrl>> = {<No data fields>}, {p = {static shared_null = {ref = {atomic = {_q_value = {<std::__atomic_base<int>> = {static _S_alignment = 4, _M_i = -1}, <No data fields>}}}, alloc = 0, begin = 0, end = 0, array = {0x0}}, d = 0x555555635880}, d = 0x555555635880}}
        startedWithURLs = <optimized out>
        mainWindow = <optimized out>
        dolphinDBusService = <unvollständiger Typ>
        interface = {<QObject> = {<No data fields>}, static staticMetaObject = {d = {superdata = {direct = 0x7ffff5d03ae0 <QObject::staticMetaObject>}, stringdata = 0x7ffff7f580e0 <qt_meta_stringdata_DBusInterface>, data = 0x7ffff7f58020 <qt_meta_data_DBusInterface>, static_metacall = 0x7ffff7ed7af0 <DBusInterface::qt_static_metacall(QObject*, QMetaObject::Call, int, void**)>, relatedMetaObjects = 0x0, extradata = 0x0}}, m_isDaemon = false}
#32 0x00007ffff7cbb0b3 in __libc_start_main (main=0x555555555060 <main>, argc=2, argv=0x7fffffffddd8, init=<optimized out>, fini=<optimized out>, rtld_fini=<optimized out>, stack_end=0x7fffffffddc8) at ../csu/libc-start.c:308
        result = <optimized out>
        unwind_buf = {cancel_jmp_buf = {{jmp_buf = {93824992235888, -9098853407025089255, 93824992235632, 140737488346576, 0, 0, 9098853405917099289, 9098835439382738201}, mask_was_saved = 0}}, priv = {pad = {0x0, 0x0, 0x2, 0x7fffffffddd8}, data = {prev = 0x0, cleanup = 0x0, canceltype = 2}}}
        not_first_call = <optimized out>
#33 0x000055555555509e in _start ()

Meh. New build. Please try again.

Yes, that one looks better, at least it does not crash directly on play. I'll try it out a little more and will report whether or not I was able to make it crash.

Good news: I was never able to crash this version, regardless of how quickly I started the video. The latest changes must have fixed the bug. Thanks very much!

Unfortunately, I have directly found a new way to crash/freeze dolphin now, related to preview video playback, but I'm certain it's different from the previous and not caused by phonon.
This happens if I click the pause button and then quickly afterwards resize the panel. I have only tried it on Wayland so far, maybe it doesn't happen on X, will need to check that.
I already have two backtraces, one with SIGABRT and one with SIG32. Should I open a new issue, or continue here?

Ah, probably that is really a Wayland problem. When on crash, the command-line output says "The Wayland connection broke. Did the Wayland compositor die?".

* On crash

Backtrace 1:
Starting program: /usr/bin/dolphin --new-window
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".
[New Thread 0x7ffff046b700 (LWP 7677)]
[New Thread 0x7fffecc13700 (LWP 7678)]
[New Thread 0x7fffe7fff700 (LWP 7679)]
[New Thread 0x7fffe77fe700 (LWP 7680)]
[New Thread 0x7fffe6ffd700 (LWP 7681)]
[New Thread 0x7fffe67fc700 (LWP 7682)]
[Detaching after fork from child process 7692]
[Detaching after fork from child process 7698]
[Detaching after fork from child process 7700]
[Detaching after fork from child process 7701]
[Detaching after fork from child process 7702]
[New Thread 0x7fbfcbfff700 (LWP 7703)]
[Thread 0x7fbfcbfff700 (LWP 7703) exited]
[New Thread 0x7fbfcbfff700 (LWP 7704)]
[New Thread 0x7fffe4627700 (LWP 7705)]
[New Thread 0x7fbfa1382700 (LWP 7706)]
[New Thread 0x7fbfa0b81700 (LWP 7707)]
[New Thread 0x7fbfa0380700 (LWP 7708)]
[New Thread 0x7fbf9fb7f700 (LWP 7709)]
[New Thread 0x7fbf9f37e700 (LWP 7710)]
[New Thread 0x7fbf9eb7d700 (LWP 7711)]
[New Thread 0x7fbf9e7ac700 (LWP 7712)]
[h264 @ 0x7fbfac0e0fc0] get_buffer() failed
[h264 @ 0x7fbfac0e0fc0] thread_get_buffer() failed
[h264 @ 0x7fbfac0e0fc0] decode_slice_header error
[h264 @ 0x7fbfac0e0fc0] no frame!
[Thread 0x7fbf9eb7d700 (LWP 7711) exited]
[Thread 0x7fbfa1382700 (LWP 7706) exited]
[Thread 0x7fbfa0380700 (LWP 7708) exited]
[Thread 0x7fbfa0b81700 (LWP 7707) exited]
[Thread 0x7fbf9fb7f700 (LWP 7709) exited]
[Thread 0x7fbf9f37e700 (LWP 7710) exited]
[Thread 0x7fffe4627700 (LWP 7705) exited]
[New Thread 0x7fffe4627700 (LWP 7715)]
This plugin supports grabbing the mouse only for popup windows
This plugin supports grabbing the mouse only for popup windows
This plugin supports grabbing the mouse only for popup windows
This plugin supports grabbing the mouse only for popup windows
[New Thread 0x7fbf9f37e700 (LWP 7716)]
[New Thread 0x7fbf9fb7f700 (LWP 7717)]
[New Thread 0x7fbfa0380700 (LWP 7718)]
[New Thread 0x7fbfa0b81700 (LWP 7719)]
[New Thread 0x7fffe5ffb700 (LWP 7720)]
[New Thread 0x7fffe57fa700 (LWP 7721)]
The Wayland connection broke. Did the Wayland compositor die?

Thread 1 "dolphin" received signal SIGABRT, Aborted.
__GI_raise (sig=sig@entry=6) at ../sysdeps/unix/sysv/linux/raise.c:50
50      ../sysdeps/unix/sysv/linux/raise.c: Datei oder Verzeichnis nicht gefunden.
(gdb) bt full
#0  __GI_raise (sig=sig@entry=6) at ../sysdeps/unix/sysv/linux/raise.c:50
        set = 
            {__val = {2560, 0, 0, 0, 71777214294589695, 71777214294589695, 18303028646048598135, 18303028646048598135, 4635165809045603508, 0, 4601133428098110976, 0, 4650362439446036480, 0, 4636033603912859648, 0}}
        pid = <optimized out>
        tid = <optimized out>
#1  0x00007ffff7cb9859 in __GI_abort () at abort.c:79
        save_stage = 1
        act = 
          {__sigaction_handler = {sa_handler = 0x0, sa_sigaction = 0x0}, sa_mask = {__val = {0 <repeats 16 times>}}, sa_flags = 0, sa_restorer = 0x5555555e0a00}
        sigs = {__val = {32, 0 <repeats 15 times>}}
#2  0x00007ffff5851bf7 in  () at /lib/x86_64-linux-gnu/libQt5Core.so.5
#3  0x00007ffff34edd65 in  () at /lib/x86_64-linux-gnu/libQt5WaylandClient.so.5
#4  0x00007ffff34ff16c in QtWaylandClient::QWaylandDisplay::flushRequests() () at /lib/x86_64-linux-gnu/libQt5WaylandClient.so.5
#5  0x00007ffff5aa9730 in  () at /lib/x86_64-linux-gnu/libQt5Core.so.5
#6  0x00007ffff5accfb6 in QEventDispatcherGlib::processEvents(QFlags<QEventLoop::ProcessEventsFlag>) () at /lib/x86_64-linux-gnu/libQt5Core.so.5
#7  0x00007ffff5a711eb in QEventLoop::exec(QFlags<QEventLoop::ProcessEventsFlag>) () at /lib/x86_64-linux-gnu/libQt5Core.so.5
#8  0x00007ffff5a79394 in QCoreApplication::exec() () at /lib/x86_64-linux-gnu/libQt5Core.so.5
#9  0x00007ffff7ed9fa2 in kdemain(int, char**) (argc=<optimized out>, argv=<optimized out>) at ./src/main.cpp:222
        app = <unvollständiger Typ>
        migrate = {d = std::unique_ptr<class Kdelibs4ConfigMigratorPrivate> = {get() = 0x555555632840}}
        aboutData = 
            {static staticMetaObject = {d = {superdata = {direct = 0x0}, stringdata = 0x7ffff6cfc940, data = 0x7ffff6cfc840, static_metacall = 0x7ffff6c94d00, relatedMetaObjects = 0x0, extradata = 0x0}}, d = std::unique_ptr<class KAboutDataPrivate> = {get() = 0x5555555bc1a0}}
        parser = {d = 0x5555555f5e70}
        splitView = <optimized out>
        openFiles = <optimized out>
        args = 
              {<QList<QString>> = {<QListSpecialMethods<QString>> = {<No data fields>}, {p = {static shared_null = {ref = {atomic = {_q_value = {<std::__atomic_base<int>> = {static _S_alignment = 4, _M_i = -1}, <No data fields>}}}, alloc = 0, begin = 0, end = 0, array = {0x0}}, d = 0x7ffff5b61490 <QListData::shared_null>}, d = 0x7ffff5b61490 <QListData::shared_null>}}, <No data fields>}
        urls = 
            {<QListSpecialMethods<QUrl>> = {<No data fields>}, {p = {static shared_null = {ref = {atomic = {_q_value = {<std::__atomic_base<int>> = {static _S_alignment = 4, _M_i = -1}, <No data fields>}}}, alloc = 0, begin = 0, end = 0, array = {0x0}}, d = 0x555555630cc0}, d = 0x555555630cc0}}
        startedWithURLs = <optimized out>
        mainWindow = <optimized out>
        dolphinDBusService = <unvollständiger Typ>
        interface = 
          {<QObject> = {<No data fields>}, static staticMetaObject = {d = {superdata = {direct = 0x7ffff5d03ae0 <QObject::staticMetaObject>}, stringdata = 0x7ffff7f580e0 <qt_meta_stringdata_DBusInterface>, data = 0x7ffff7f58020 <qt_meta_data_DBusInterface>, static_metacall = 0x7ffff7ed7af0 <DBusInterface::qt_static_metacall(QObject*, QMetaObject::Call, int, void**)>, relatedMetaObjects = 0x0, extradata = 0x0}}, m_isDaemon = false}
#10 0x00007ffff7cbb0b3 in __libc_start_main (main=
    0x555555555060 <main>, argc=2, argv=0x7fffffffddd8, init=<optimized out>, fini=<optimized out>, rtld_fini=<optimized out>, stack_end=0x7fffffffddc8)
    at ../csu/libc-start.c:308
        result = <optimized out>
        unwind_buf = 
              {cancel_jmp_buf = {{jmp_buf = {93824992235888, 4479315001357509903, 93824992235632, 140737488346576, 0, 0, -4479315000248359665, -4479297304327609073}, mask_was_saved = 0}}, priv = {pad = {0x0, 0x0, 0x2, 0x7fffffffddd8}, data = {prev = 0x0, cleanup = 0x0, canceltype = 2}}}
        not_first_call = <optimized out>
#11 0x000055555555509e in _start ()

Backtrace 2 (interestingly, this one is without Wayland warning):
Starting program: /usr/bin/dolphin --new-window
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".
[New Thread 0x7ffff046b700 (LWP 7896)]
[New Thread 0x7fffecc13700 (LWP 7897)]
[New Thread 0x7fffe7fff700 (LWP 7898)]
[New Thread 0x7fffe77fe700 (LWP 7899)]
[New Thread 0x7fffe6ffd700 (LWP 7900)]
[New Thread 0x7fffe67fc700 (LWP 7901)]
[Detaching after fork from child process 7911]
[Detaching after fork from child process 7917]
[Detaching after fork from child process 7919]
[Detaching after fork from child process 7921]
[Detaching after fork from child process 7922]
[New Thread 0x7fbfcbfff700 (LWP 7923)]
[New Thread 0x7fbfcbfff700 (LWP 7924)]
[Thread 0x7fbfcbfff700 (LWP 7923) exited]
[New Thread 0x7fffe4627700 (LWP 7925)]
This plugin supports grabbing the mouse only for popup windows
[New Thread 0x7fbfa1382700 (LWP 7926)]
[New Thread 0x7fbfa0b81700 (LWP 7927)]
[New Thread 0x7fbfa0380700 (LWP 7928)]
[New Thread 0x7fbf9fb7f700 (LWP 7929)]
[New Thread 0x7fbf9f37e700 (LWP 7930)]
[New Thread 0x7fbf9eb7d700 (LWP 7931)]
[New Thread 0x7fbf9e7ac700 (LWP 7932)]
This plugin supports grabbing the mouse only for popup windows
This plugin supports grabbing the mouse only for popup windows
This plugin supports grabbing the mouse only for popup windows

Thread 16 "dolphin" received signal SIG32, Real-time event 32.
[Switching to Thread 0x7fbf9eb7d700 (LWP 7931)]
futex_wait_cancelable (private=<optimized out>, expected=0, futex_word=0x7fbfac010d90) at ../sysdeps/nptl/futex-internal.h:183
183     ../sysdeps/nptl/futex-internal.h: Datei oder Verzeichnis nicht gefunden.
(gdb) bt full
#0  futex_wait_cancelable (private=<optimized out>, expected=0, futex_word=0x7fbfac010d90) at ../sysdeps/nptl/futex-internal.h:183
        __ret = -512
        oldtype = 0
        err = <optimized out>
        spin = 0
        buffer = {__routine = 0x7ffff420d050 <__condvar_cleanup_waiting>, __arg = 0x7fbf9eb7cc20, __canceltype = -1204620007, __prev = 0x0}
        cbuffer = {wseq = 12, cond = 0x7fbfac010d68, mutex = 0x7fbfac010d40, private = 0}
        err = <optimized out>
        g = 0
        flags = <optimized out>
        g1_start = <optimized out>
        signals = <optimized out>
        result = 0
        wseq = 12
        seq = 6
#1  __pthread_cond_wait_common (abstime=0x0, clockid=0, mutex=0x7fbfac010d40, cond=0x7fbfac010d68) at pthread_cond_wait.c:508
        spin = 0
        buffer = {__routine = 0x7ffff420d050 <__condvar_cleanup_waiting>, __arg = 0x7fbf9eb7cc20, __canceltype = -1204620007, __prev = 0x0}
        cbuffer = {wseq = 12, cond = 0x7fbfac010d68, mutex = 0x7fbfac010d40, private = 0}
        err = <optimized out>
        g = 0
        flags = <optimized out>
        g1_start = <optimized out>
        signals = <optimized out>
        result = 0
        wseq = 12
        seq = 6
#2  __pthread_cond_wait (cond=0x7fbfac010d68, mutex=0x7fbfac010d40) at pthread_cond_wait.c:638
#3  0x00007fffe47b700b in  () at /lib/x86_64-linux-gnu/libvlccore.so.9
#4  0x00007ffff4206609 in start_thread (arg=<optimized out>) at pthread_create.c:477
        ret = <optimized out>
        pd = <optimized out>
        unwind_buf = 
              {cancel_jmp_buf = {{jmp_buf = {140460978329344, -76074712189965297, 140737025041710, 140737025041711, 140737025042208, 140460978327104, 112175596410694671, 76088622723689487}, mask_was_saved = 0}}, priv = {pad = {0x0, 0x0, 0x0, 0x0}, data = {prev = 0x0, cleanup = 0x0, canceltype = 0}}}
        not_first_call = 0
#5  0x00007ffff7db6293 in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:95

The second case is much easier to reproduce. Crash 1 seems to have been luck, I wasn't able to trigger it a second time so far.

This is great news.

The first trace is indeed the compositor crashing, [1] might help debug that in the future but is unrelated to the bug at hand.
Your second trace isn't even a crash it's simply SIG32, which is sometimes used to synchronize threads, in particular somewhere deep in VLC. You can set `handle 32 pass nostop` in ~/.gdbinit then it won't stop gdb anymore in the future ;)

https://community.kde.org/Guidelines_and_HOWTOs/Debugging/How_to_create_useful_crash_reports#Retrieving_a_backtrace_using_coredumpctl

Git commit f263063d1a9c748f92fa8cb021d8d8a85bbace42 by Harald Sitter.
Committed on 23/03/2021 at 10:12.
Pushed by sitter into branch '0.11'.

fix surface painting format setup

originally the format callback was not locked, but since we intercept
qwidget paints that meant we'd be able to receive paints before the
callback/setup had run. this could then result in a race condition where
if the user was quick enough, or supposedly the system under sufficient
load, that paints would be done on random blobs of invalid data. in
particular when the paint would happen right as the other thread was in
the format setup callback.

to deal with this simply lock the format callback function and ensure we
only paint when the frame has been constructed with actual data (frame
is non-null after the callback since we feed it the plane as data,
making it non-null)

M  +7    -1    src/video/videowidget.cpp

https://invent.kde.org/libraries/phonon-vlc/commit/f263063d1a9c748f92fa8cb021d8d8a85bbace42

Git commit 1fe55de0df172829722bab864f05351f244c116e by Harald Sitter.
Committed on 23/03/2021 at 10:19.
Pushed by sitter into branch '0.11'.

stop using our helper function to calculate pitch and lines

the surface painter was more inefficient than it needed to be by
manually calculating a pitch and stride but then constructing a qimage
anyway. after some research it seems that we can simply construct our
qimage and pass its picture values to VLC with the assumption that VLC
will adjust the data as it gets passed into vmem output (i.e. our data
blob). this then also allow us to get rid of the intermediate plane
bytearray and simply use the memory the qimage allocates anyway.

M  +33   -15   src/video/videowidget.cpp

https://invent.kde.org/libraries/phonon-vlc/commit/1fe55de0df172829722bab864f05351f244c116e

I've also popped the final patches into the neon packaging, in case you want to give them a last try.

(In reply to Harald Sitter from comment #35)
> I've also popped the final patches into the neon packaging, in case you want
> to give them a last try.

I tested the final version, and it works alright.

(In reply to Harald Sitter from comment #32)
> Your second trace isn't even a crash it's simply SIG32, which is sometimes
> used to synchronize threads, in particular somewhere deep in VLC. You can
> set `handle 32 pass nostop` in ~/.gdbinit then it won't stop gdb anymore in
> the future ;)
Oh, sorry. Thanks for explaining how to make gdb ignore this.

(In reply to Manuel Geißer from comment #36)
> (In reply to Harald Sitter from comment #35)
> > I've also popped the final patches into the neon packaging, in case you want
> > to give them a last try.
> 
> I tested the final version, and it works alright.

Awesome. Thanks for you help debugging this :)