Global static QPixmapCache in Qt internals

[kshegunov]

@J-Hilk said in QApplication in std::thread:

@JKSH this actually crashes for me on btn.show()

That's expected. MacOS does not support plugging into the event loop from a thread different from main(). Should work on linux and windows, though.

[JKSH]

@kshegunov said in QApplication in std::thread:

Did you run this with a release Qt or with -developer-build? :)
(The address is rather odd, though)

Ah, right... I thought the Debug build would suffice. Don't have a dev build right now.

Can't reproduce:

...

What's your kit?
I have 5.15.2 from the repo with gcc 10.2.1.

Same result for me on Windows 10 20H2, with all official builds of:

• Qt 5.12.10 MSVC 2017 64-bit (using MSVC 2019 compiler)
• Qt 5.14.2 MinGW 7.3.0 32-bit
• Qt 5.15.2 MSVC 2019 64-bit

Might be Windows-specific.

[kshegunov]

@JKSH said in QApplication in std::thread:

Ah, right... I thought the Debug build would suffice. Don't have a dev build right now.

Not once in a universe lifetime.

@JKSH said in QApplication in std::thread:

Might be Windows-specific.

Possibly. Unfortunately I don't have windows on hand (or rather I'm too lazy now to boot into it). Maybe, I'll check it later.

[J.Hilk]

@kshegunov Apple does a lot of funky stuff, in that regard, let me fire up my vm

[JKSH]

@kshegunov said in QApplication in std::thread:

Not once in a universe lifetime.

Duly noted :-D

But before we get too sidetracked: @Suthiro's main issue isn't the message about timers. Rather, it's the crash which I believe is caused by delete m_qPlotsControl; (see https://stackoverflow.com/questions/67257329/weird-qpushbutton-bug ).

I believe it is fixable with m_qPlotsControl->deleteLater();... but that needs to be done before QApplication::exec() returns, because QDeferredDeleteEvent won't be processed without an event loop.

[J.Hilk]

@JKSH Ok I can reproduce it as well, (on the VM) Qt 15.3 & MSVC2019

[kshegunov]

@JKSH said in QApplication in std::thread:

I believe it is fixable with m_qPlotsControl->deleteLater();... but that needs to be done before QApplication::exec() returns, because QDeferredDeleteEvent won't be processed without an event loop.

I still don't understand this. For what reason should that be the case, and why would this generate a crash? I'm pretty sure deferred deletes are processed after the event loop has exited (see for example the QThread::finished -> QObject::deleteLater), but even if that weren't the case the QPushButton's destructor is going to be run before the thread actually finishes and before the application object's destructor's run (there's no difference between main() and any other function from c++'s point of view).

[kshegunov]

@JKSH said in QApplication in std::thread:

The code below prints "Bye" but not "Widget destroyed", proving that the Widget's destructor is not run:

I meant connecting to the aboutToQuit signal, but anyhow it's not that important.

Since the widget creates a timer in the background, then it's probably receiving background events/signals. delete could cause the event/signal handler to act on a dangling pointer -- that's how it could generate a crash.

I think this shouldn't happen, but can't investigate it currently.

Anyway, I was trying to say that @Suthiro can't simply replace delete m_qPlotsControl; with m_qPlotsControl->deleteLater(); in-place at https://stackoverflow.com/questions/67257329/weird-qpushbutton-bug . Some re-shuffling is required.

I suppose, but that timer message should go away, otherwise the code can break at any moment (this applies to your code too - the one you mentioned where you get the warning).

[Chris Kawa]

I reproduced this on Windows and the message about timers comes from a destructor of a static QPixmapCache object that happens on the main() thread. That's not something the user's code does. It's in Qt's internals.

Right, so remember the rule that no QObject should outlive the application object and this means no static QObjects? Apparently Qt itself doesn't respect that rule and also creates static objects that run code in the main thread no matter where the application object is created :/

I would consider this a Qt bug and expect it to crash all over the place.

EDIT: I missed that JKSH already mentioned this, but yeah, this seems to be the culprit.

[kshegunov]

@Chris-Kawa said in QApplication in std::thread:

I reproduced this on Windows and the message about timers comes from a destructor of a static QPixmapCache object that happens on the main() thread. That's not something the user's code does. It's in Qt's internals.

Hm, interesting. Do you have a woboq location?

Right, so remember the rule that no QObject should outlive the application object and this means no static QObjects?
Apparently Qt itself doesn't respect that rule and also creates static objects that run code in the main thread no matter where the application object is created :/

Yeah. Not living up to its own expectations it seems. :(

I would consider this a Qt bug and expect it to crash all over the place.

Me too, which is what I wrote somewhere upstairs - this should be fixed at the vendor side.

[Chris Kawa]

@kshegunov said in QApplication in std::thread:

Hm, interesting. Do you have a woboq location?

Instance declaration is here:
https://code.woboq.org/qt5/qtbase/src/gui/image/qpixmapcache.cpp.html#pm_cache

And here's Windows with MSVC crash callstack on exit:

1   QPMCache::~QPMCache                                                                                   qpixmapcache.cpp 269
2   ``anonymous namespace'::Q_QGS_pm_cache::innerFunction'::`2'::Holder::~Holder                          Qt5Guid             
3   ``anonymous namespace'::Q_QGS_pm_cache::innerFunction'::`2'::`dynamic atexit destructor for 'holder'' Qt5Guid             
4   initterm_e                                                                                            ucrtbased           
5   initterm_e                                                                                            ucrtbased           
6   initterm_e                                                                                            ucrtbased           
7   execute_onexit_table                                                                                  ucrtbased           
8   __scrt_dllmain_uninitialize_c                                                                         utility.cpp      399
9   dllmain_crt_process_detach                                                                            dll_dllmain.cpp  182
10  dllmain_crt_dispatch                                                                                  dll_dllmain.cpp  220
11  dllmain_dispatch                                                                                      dll_dllmain.cpp  293
12  _DllMainCRTStartup                                                                                    dll_dllmain.cpp  335
13  RtlActivateActivationContextUnsafeFast                                                                ntdll               
14  LdrShutdownProcess                                                                                    ntdll               
15  RtlExitUserProcess                                                                                    ntdll               
16  FatalExit                                                                                             KERNEL32            
17  wassert                                                                                               ucrtbased           
18  wassert                                                                                               ucrtbased           
19  exit                                                                                                  ucrtbased           
20  __scrt_common_main_seh                                                                                exe_common.inl   297

I used this simplified code:

int main(int argc, char *argv[])
{
    std::thread thr([](int argc, char *argv[])
    {
        QApplication a(argc, argv);
        QPushButton btn("Push");
        btn.show();
        a.exec();
    }, argc, argv);

    thr.join();
}

[kshegunov]

Yes, this is constructed appropriately, however the deallocation should've been tied to the QCoreApplication::aboutToQuit. I doubt anyone is going to bother fixing it though, I'm not sure if it's not going to be discarded with "this is unsupported" even ...

[JKSH]

Good detective work, @Chris-Kawa!

I encountered this a few years ago but had forgotten most of the details.

@kshegunov said in Global static QPixmapCache in Qt internals:

Yes, this is constructed appropriately, however the deallocation should've been tied to the QCoreApplication::aboutToQuit. I doubt anyone is going to bother fixing it though, I'm not sure if it's not going to be discarded with "this is unsupported" even ...

Since a QPixmap requires a QGuiApplication, would a simple(?) and correct fix be to allocate the QPixmapCache in the QGuiApplication constructor, and deallocate it from the QGuiApplication destructor?

[kshegunov]

Possibly. Or it can be a pseudo-singleton (similarly to QCoreApplication) and be initialized on demand in the instance retrieving function (dropping the global static that it currently uses). You could poke Thiago, but as written, I wouldn't hold my breath.

[Christian Ehrlicher]

There are some reports about such an issue:
https://bugreports.qt.io/browse/QTBUG-48709
https://bugreports.qt.io/browse/QTBUG-21807

And it does not crash here for me on Linux and Windows (MSVC, debug-build) with 5.15.x

[kshegunov]

@Christian-Ehrlicher said in Global static QPixmapCache in Qt internals:

And it does not crash here for me on Linux and Windows (MSVC, debug-build) with 5.15.x

We are talking about the QTimers from another thread message. JKSH, split the threads. The other stuff (the supposed crash) contines to befuddle me.

[Christian Ehrlicher]

I also don't have a warning about the timer with the testcase on windows/msvc :)

[Suthiro]

@kshegunov

We are talking about the QTimers from another thread message. JKSH, split the threads. The other stuff (the supposed crash) contines to befuddle me.

I don't know how exactly the crash in the original thread is connected with this. However, the crash and the message about timers are appearing together. They appear after any interaction with simple QPushButton. Using deleteLater() as suggested by @JKSH solves the crash, but the message persists.

@Christian-Ehrlicher said in Global static QPixmapCache in Qt internals:

And it does not crash here for me on Linux and Windows (MSVC, debug-build) with 5.15.x

To clarify: crash appears only when one tries to unload a library with QApplication inside. The library in my case is loaded by lua.exe, but I doubt it is lua-related problem.

[jeremy_k]

@kshegunov said in Global static QPixmapCache in Qt internals:

Possibly. Or it can be a pseudo-singleton (similarly to QCoreApplication) and be initialized on demand in the instance retrieving function (dropping the global static that it currently uses). You could poke Thiago, but as written, I wouldn't hold my breath.

That sounds like a source and binary compatibility break, unless instance() is used from within the existing static functions.

A thread_local version would be another solution, that could be implemented without changing any user source code. Keep the global static interface object for binary compatibility, and move the actual QObject-based implementation to thread_local storage for the gui thread.

The QPixmapCache documentation already says that the cache is only usable from the "application's main thread".

[kshegunov]

@jeremy_k said in Global static QPixmapCache in Qt internals:

That sounds like a source and binary compatibility break, unless instance() is used from within the existing static functions.

Adding non-virtual functions doesn't brake API or ABI compatibility. Also this is an internal member, any global getter function could do, it's not really necessary to expose it to the user.

A thread_local version would be another solution, that could be implemented without changing any user source code. Keep the global static interface object for binary compatibility, and move the actual QObject-based implementation to thread_local storage for the gui thread.

The QPixmapCache documentation already says that the cache is only usable from the "application's main thread".

It's an internal class, I didn't intend to modify the QPixmapCache to begin with. (see the woboq link in Chris' post)