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  1. Qt
  2. QTBUG-64171

QFileSystemWatcher leads to a crash when created in a background thread



    • Type: Bug
    • Status: Closed
    • Priority: P2: Important
    • Resolution: Done
    • Affects Version/s: 5.9.2
    • Fix Version/s: 5.9.4
    • Labels:
    • Environment:
      Windows OS
    • Commits:
      851c226247d692a608d78a1d7f9e621f4a82f40d (qtbase/5.9, 4.11.2017, 5.9.4)


      When QFileSystemWacther is created on OS Windows it installs a native event filter (QWindowsRemovableDriveListener) in  QCoreApplication::eventDispatcher(), see QWindowsFileSystemWatcherEngine::QWindowsFileSystemWatcherEngine(). When QFileSystemWacther is destroyed it removes the filter from QAbstractEventDispatcher::instance(), see QAbstractNativeEventFilter::~QAbstractNativeEventFilter(). For GUI thread that's the same, for a background thread it's different. So if you create QFileSystemWacther in a background thread and delete it there, there will be a dangling pointer in QCoreApplication::eventDispatcher() which will almost certainly lead to a crash when handling native events.


      It looks like a regression somewhere between Qt 5.5 and Qt 5.9.2, since I checked with Qt 5.4 it didn't crash (actually it used to work even for Qt 4). I attached a project for your convenience, just select "test" -> crash it!" from the application menu. But it's reproducible with the following line of code:

      QtConcurrent::run([](){ delete new QFileSystemWatcher; });

      EDIT: Added a backtrace in case someone googles by stack:

      Stack trace, VS2017

      > Qt5Cored.dll!QAbstractEventDispatcher::filterNativeEvent(const QByteArray & eventType, void * message, long * result) Line 467 C++
      qwindowsd.dll!QWindowsContext::windowsProc(HWND__ * hwnd, unsigned int message, QtWindows::WindowsEventType et, unsigned __int64 wParam, __int64 lParam, __int64 * result, QWindowsWindow * * platformWindowPtr) Line 899 C++
      qwindowsd.dll!qWindowsWndProc(HWND__ * hwnd, unsigned int message, unsigned __int64 wParam, __int64 lParam) Line 1332 C++
      user32.dll!UserCallWinProcCheckWow() Unknown
      user32.dll!DispatchClientMessage() Unknown
      user32.dll!__fnDWORD() Unknown
      ntdll.dll!KiUserCallbackDispatcherContinue() Unknown
      user32.dll!NtUserPeekMessage() Unknown
      user32.dll!PeekMessageW() Unknown
      Qt5Cored.dll!QEventDispatcherWin32::processEvents(QFlags<enum QEventLoop::ProcessEventsFlag> flags) Line 559 C++
      qwindowsd.dll!QWindowsGuiEventDispatcher::processEvents(QFlags<enum QEventLoop::ProcessEventsFlag> flags) Line 74 C++
      Qt5Cored.dll!QEventLoop::processEvents(QFlags<enum QEventLoop::ProcessEventsFlag> flags) Line 135 C++
      Qt5Cored.dll!QEventLoop::exec(QFlags<enum QEventLoop::ProcessEventsFlag> flags) Line 212 C++
      Qt5Cored.dll!QCoreApplication::exec() Line 1291 C++
      Qt5Guid.dll!QGuiApplication::exec() Line 1680 C++
      Qt5Widgetsd.dll!QApplication::exec() Line 2911 C++
      qfilesystemwatcher_crash.exe!main(int argc, char * * argv) Line 10 C++
      qfilesystemwatcher_crash.exe!WinMain(HINSTANCE__ * _formal, HINSTANCE_ * __formal, char * __formal, int __formal) Line 104 C++
      qfilesystemwatcher_crash.exe!invoke_main() Line 107 C++
      qfilesystemwatcher_crash.exe!__scrt_common_main_seh() Line 283 C++
      qfilesystemwatcher_crash.exe!__scrt_common_main() Line 326 C++
      qfilesystemwatcher_crash.exe!WinMainCRTStartup() Line 17 C++
      kernel32.dll!BaseThreadInitThunk() Unknown
      ntdll.dll!RtlUserThreadStart() Unknown


        1. qfilesystemwatcher_crash.zip
          2 kB
        2. qtbug64171_diag.diff
          4 kB
        3. qtbug64171_log.txt
          1.0 kB
        4. qtbug64171.zip
          2 kB
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            kleint Friedemann Kleint
            alex.butterworth Abs Mad
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