I have just filed bugs on the two problems discovered here:
QTBUG-86874
QTBUG-86875
I have not found a way to get menu item relocation in the global menubar to
work properly on macOS without triggering a crash on quit as a side effect, and
I think I’m going to give up on it for the time being. Oh well.
Cheers,
-B.
Benjamin C. Haller
Messer Lab
Cornell University
> On Sep 23, 2020, at 12:40 PM, Ben Haller via Interest
> <interest@qt-project.org> wrote:
>
> Sorry for all the emails; this is proving to be quite a difficult problem and
> I’m really hoping somebody has insight into it.
>
> This code also produces the same crash as the static QFont usage I posted in
> my previous email, and this code seems very clearly innocent:
>
> ————————
>
> QFont tickFont;
> #ifdef __APPLE__
> tickFont.setPointSize(9);
> #else
> tickFont.setPointSize(7);
> #endif
> painter.setFont(tickFont);
>
> ————————
>
> So now I’m starting to think this may actually be a Qt bug, perhaps related
> to caching of fonts internally in QOpenGLWidget.
>
> But here’s the interesting bit: there is one other change in my app that is
> also causal. I’m trying to get my app on macOS to stay running after the
> last window has closed, and to get the global menu bar to work properly in
> that mode of operation, I need to keep my main window alive after it’s closed
> (see the previous discussion thread on Qt-Interest). To achieve that, I’m
> commenting out the call to:
>
> setAttribute(Qt::WA_DeleteOnClose);
>
> in my main window’s initialization. With that call commented out, I can
> close the window, the global menubar remains correct because the main window
> is still alive in the background, BUT the app crashes on quit due to the font
> issue above. With that setAttribute() call in place (not commented out), I
> can close the window, the app does not crash on quit, BUT the global menubar
> is screwed up because there’s no main window (see previous thread).
>
> My closeEvent() just calls event->accept(); and returns. With the
> setAttribute() call commented out, the window does not get deleted any more,
> it just closes (I have a qDebug() in its destructor to confirm that). But
> then quitting crashes. Putting the setAttribute() call back in fixes the
> crash; OR, commenting out the use of QFont in my QOpenGLWidget subclass fixes
> the crash.
>
> Anybody know what’s going on, and how I might fix it or work around it? I
> need to change the font in my widget, of course; and I seem to need to avoid
> deleting my main window, in order to keep the global menubar working when the
> last main window closes. So I’m stuck.
>
> Cheers,
> -B.
>
> Benjamin C. Haller
> Messer Lab
> Cornell University
>
>
>> On Sep 23, 2020, at 12:04 PM, Ben Haller via Interest
>> <interest@qt-project.org> wrote:
>>
>> Hi all. I’m starting a new thread since the focal question has completely
>> changed. I’ve tracked down the cause of the crash that I discussed in the
>> previous thread, to a specific bit of code:
>>
>> ————————
>>
>> static QFont *tickFont = nullptr;
>>
>> if (!tickFont)
>> {
>> tickFont = new QFont();
>> tickFont->setPointSize(9);
>> }
>> painter.setFont(*tickFont);
>>
>> ————————
>>
>> I do this inside a QOpenGLWidget subclass, in my paintGL() override. If I
>> comment this code out, no more crash on quit. I figured this out because
>> the backtrace for my crash seemed to involve private font cache teardown in
>> QApplication, so I figured maybe I was doing something naughty involved
>> QFonts, so I commented out every usage of QFont in my app, and then started
>> uncommenting them one by one; when I uncommented this one, it crashes on
>> quit. (I got AddressSanitizer working, but it didn’t provide any additional
>> info; I’m still working on getting Valgrind working on macOS 10.15.)
>>
>> So. What is wrong with the pattern above? I’m surprised that this crashes.
>> My intent is simply to keep this QFont instance permanently allocated, so
>> that whatever overhead is associated with making a new one doesn’t occur
>> each time my view repaints (which happens at ~60Hz, ideally). That is
>> perhaps premature optimization, which is the root of all evil, I realize.
>> But I want to understand *why* this pattern is bad, and what I might do
>> instead, if the font creation were to indeed be a bottleneck. Obviously
>> there’s something about memory management in Qt that I don’t understand,
>> because I don’t see anything wrong with this code! :-> Thanks for any help.
>>
>> Cheers,
>> -B.
>>
>> Benjamin C. Haller
>> Messer Lab
>> Cornell University
>>
>>
>>> On Sep 23, 2020, at 7:50 AM, Ben Haller via Interest
>>> <interest@qt-project.org> wrote:
>>>
>>> Hi all. I’m trying to figure out what appears to be a double dealloc
>>> problem of some sort. I’m sure the actual bug is in my code (I’m trying to
>>> get a window with complex subsidiary windows, etc., to disassemble and free
>>> the things it owns, without freeing itself), but the backtrace at the point
>>> that the crash happens doesn’t have any of my code in it at all. I’ve
>>> appended the full backtrace below my signature, in case it is informative;
>>> maybe somebody has seen this pattern before and knows what I’m likely to be
>>> doing wrong. My code does not use QFontCache or QScopedPointer at all, so
>>> it’s hard to guess where my bug might be.
>>>
>>> My question, though, is bigger: how would folks here recommend debugging a
>>> double dealloc in a Qt-based app, running under Qt Creator? I’m coming
>>> from the Cocoa/Xcode world, and in that world there’s a very nice tool
>>> called NSZombie: you set a flag, and then whenever an object would be
>>> deallocated, it instead turns into an NSZombie object that produces nice
>>> console logs if anybody ever tries to use that memory as an object again.
>>> But that’s for NSObject. Is there something similar in the Qt world, such
>>> that I could get a console log or a break in the debugger at the point
>>> where a C++/Qt object that has been dealloced gets touched again? Or if
>>> not, how would you recommend approaching this? Thanks for any help.
>>>
>>> Cheers,
>>> -B.
>>>
>>> Benjamin C. Haller
>>> Messer Lab
>>> Cornell University
>>>
>>>
>>> 1 QScopedPointer<QObjectData,
>>> QScopedPointerDeleter<QObjectData>>::operator->() const
>>>
>>>
>>> qscopedpointer.h 118
>>> 0x10201b594
>>> 2 decltype(fp.operator->()) qGetPtrHelper<QScopedPointer<QObjectData,
>>> QScopedPointerDeleter<QObjectData>> const>(QScopedPointer<QObjectData,
>>> QScopedPointerDeleter<QObjectData>> const&)
>>>
>>> qglobal.h 1133 0x10201b594
>>> 3 QObject::d_func() const
>>>
>>>
>>> qobject.h
>>> 132 0x10201b594
>>> 4 QObject::thread() const
>>>
>>>
>>> qobject.cpp
>>> 1512 0x10201b594
>>> 5 QOpenGLVertexArrayObjectPrivate::destroy()
>>>
>>>
>>>
>>> qopenglvertexarrayobject.cpp 212 0x101b21481
>>> 6 QOpenGLVertexArrayObject::destroy()
>>>
>>>
>>>
>>> qopenglvertexarrayobject.cpp 424 0x101b217cc
>>> 7 QOpenGLVertexArrayObject::~QOpenGLVertexArrayObject()
>>>
>>>
>>>
>>> qopenglvertexarrayobject.cpp 392 0x101b217c3
>>> 8 QOpenGLVertexArrayObject::~QOpenGLVertexArrayObject()
>>>
>>>
>>>
>>> qopenglvertexarrayobject.cpp 391 0x101b217b9
>>> 9 QOpenGLTextureGlyphCache::~QOpenGLTextureGlyphCache()
>>>
>>>
>>>
>>> qopengltextureglyphcache.cpp 93 0x101b1cacd
>>> 10 QOpenGLTextureGlyphCache::~QOpenGLTextureGlyphCache()
>>>
>>>
>>>
>>> qopengltextureglyphcache.cpp 88 0x101b1cbce
>>> 11 QOpenGLTextureGlyphCache::~QOpenGLTextureGlyphCache()
>>>
>>>
>>>
>>> qopengltextureglyphcache.cpp 88 0x101b1cbc9
>>> 12
>>> QExplicitlySharedDataPointer<QFontEngineGlyphCache>::~QExplicitlySharedDataPointer()
>>>
>>>
>>> qshareddata.h
>>> 184 0x101883c92
>>> 13
>>> QExplicitlySharedDataPointer<QFontEngineGlyphCache>::~QExplicitlySharedDataPointer()
>>>
>>>
>>> qshareddata.h
>>> 184 0x101883c74
>>> 14 QFontEngine::GlyphCacheEntry::~GlyphCacheEntry()
>>>
>>>
>>> qfontengine.cpp
>>> 1570 0x101883c74
>>> 15 QFontEngine::GlyphCacheEntry::~GlyphCacheEntry()
>>>
>>>
>>> qfontengine.cpp
>>> 1569 0x101883c74
>>> 16 void
>>> std::allocator_traits<std::allocator<std::__list_node<QFontEngine::GlyphCacheEntry,
>>> void
>>> *>>>::__destroy<QFontEngine::GlyphCacheEntry>(std::integral_constant<bool,
>>> false>, std::allocator<std::__list_node<QFontEngine::GlyphCacheEntry, void
>>> *>>&, QFontEngine::GlyphCacheEntry *) memory
>>> 1732 0x101883c74
>>> 17 void
>>> std::allocator_traits<std::allocator<std::__list_node<QFontEngine::GlyphCacheEntry,
>>> void
>>> *>>>::destroy<QFontEngine::GlyphCacheEntry>(std::allocator<std::__list_node<QFontEngine::GlyphCacheEntry,
>>> void *>>&, QFontEngine::GlyphCacheEntry *)
>>> memory 1595 0x101883c74
>>> 18 std::__list_imp<QFontEngine::GlyphCacheEntry,
>>> std::allocator<QFontEngine::GlyphCacheEntry>>::clear()
>>>
>>>
>>> list 733 0x101883c74
>>> 19 std::__list_imp<QFontEngine::GlyphCacheEntry,
>>> std::allocator<QFontEngine::GlyphCacheEntry>>::~__list_imp()
>>>
>>>
>>> list 712 0x101883c24
>>> 20 std::list<QFontEngine::GlyphCacheEntry>::~list()
>>>
>>>
>>> list
>>> 805 0x101883c24
>>> 21 std::list<QFontEngine::GlyphCacheEntry>::~list()
>>>
>>>
>>> list
>>> 805 0x101883c24
>>> 22 QHashNode<void const *,
>>> std::list<QFontEngine::GlyphCacheEntry>>::~QHashNode()
>>>
>>>
>>> qhash.h 147
>>> 0x101883c24
>>> 23 QHashNode<void const *,
>>> std::list<QFontEngine::GlyphCacheEntry>>::~QHashNode()
>>>
>>>
>>> qhash.h 147
>>> 0x101883c24
>>> 24 QHash<void const *,
>>> std::list<QFontEngine::GlyphCacheEntry>>::deleteNode2(QHashData::Node *)
>>>
>>>
>>> qhash.h
>>> 563 0x101883c24
>>> 25 QHashData::free_helper(void ( *)(QHashData::Node *))
>>>
>>>
>>> qhash.cpp
>>> 572 0x101e5db39
>>> 26 QHash<void const *,
>>> std::list<QFontEngine::GlyphCacheEntry>>::freeData(QHashData *)
>>>
>>>
>>> qhash.h
>>> 603 0x10187a47b
>>> 27 QHash<void const *, std::list<QFontEngine::GlyphCacheEntry>>::~QHash()
>>>
>>>
>>> qhash.h
>>> 250 0x10187a46f
>>> 28 QHash<void const *, std::list<QFontEngine::GlyphCacheEntry>>::~QHash()
>>>
>>>
>>> qhash.h
>>> 250 0x10187a44f
>>> 29 QFontEngine::~QFontEngine()
>>>
>>>
>>> qfontengine.cpp
>>> 277 0x10187a44f
>>> 30 QCoreTextFontEngine::~QCoreTextFontEngine()
>>>
>>>
>>>
>>> qfontengine_coretext.mm 215 0x105b7cf8e
>>> 31 QCoreTextFontEngine::~QCoreTextFontEngine()
>>>
>>>
>>>
>>> qfontengine_coretext.mm 215 0x105b7cf89
>>> 32 QFontEngineMulti::~QFontEngineMulti()
>>>
>>>
>>> qfontengine.cpp
>>> 1782 0x101880039
>>> 33 QFontEngineMulti::~QFontEngineMulti()
>>>
>>>
>>> qfontengine.cpp
>>> 1778 0x10188012e
>>> 34 QFontEngineMulti::~QFontEngineMulti()
>>>
>>>
>>> qfontengine.cpp
>>> 1778 0x101880129
>>> 35 QFontCache::clear()
>>>
>>>
>>> qfont.cpp
>>> 2861 0x1018762eb
>>> 36 QFontCache::~QFontCache()
>>>
>>>
>>> qfont.cpp
>>> 2817 0x10187603c
>>> 37 QFontCache::~QFontCache()
>>>
>>>
>>> qfont.cpp
>>> 2816 0x1018763ee
>>> 38 QFontCache::~QFontCache()
>>>
>>>
>>> qfont.cpp
>>> 2816 0x1018763e9
>>> 39 QThreadStorageData::set(void *)
>>>
>>>
>>> qthreadstorage.cpp
>>> 162 0x101e25777
>>> 40 QGuiApplicationPrivate::~QGuiApplicationPrivate()
>>>
>>>
>>>
>>> qguiapplication.cpp 1694 0x1017ca490
>>> 41 QApplicationPrivate::~QApplicationPrivate()
>>>
>>>
>>> qapplication.cpp
>>> 184 0x10116f81e
>>> 42 QApplicationPrivate::~QApplicationPrivate()
>>>
>>>
>>> qapplication.cpp
>>> 184 0x10116f819
>>> 43 QScopedPointerDeleter<QObjectData>::cleanup(QObjectData *)
>>>
>>>
>>> qscopedpointer.h
>>> 60 0x10201c06c
>>> 44 QScopedPointer<QObjectData,
>>> QScopedPointerDeleter<QObjectData>>::~QScopedPointer()
>>>
>>>
>>> qscopedpointer.h 107
>>> 0x10201c061
>>> 45 QScopedPointer<QObjectData,
>>> QScopedPointerDeleter<QObjectData>>::~QScopedPointer()
>>>
>>>
>>> qscopedpointer.h 105
>>> 0x10201c05d
>>> 46 QObject::~QObject()
>>>
>>>
>>> qobject.cpp
>>> 1119 0x10201c05d
>>> 47 QApplication::~QApplication()
>>>
>>>
>>> qapplication.cpp
>>> 837 0x101171b3d
>>> 48 main
>>>
>>>
>>> main.cpp
>>> 246 0x100012256
>>> 49 start
>>>
>>>
>>> (x86_64)
>>> /usr/lib/system/libdyld.dylib 0x7fff6b461cc9
>>>
>>> _______________________________________________
>>> Interest mailing list
>>> Interest@qt-project.org
>>> https://lists.qt-project.org/listinfo/interest
>>
>> _______________________________________________
>> Interest mailing list
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>> https://lists.qt-project.org/listinfo/interest
>
> _______________________________________________
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> https://lists.qt-project.org/listinfo/interest
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