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  1. Qt
  2. QTBUG-93291

QFileSystemWatcher not safe wrt moveToThread

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Details

    • Bug
    • Resolution: Duplicate
    • P2: Important
    • None
    • 5.15.2
    • Core: I/O
    • None
    • Windows

    Description

      In a complex application I tried to push the QFileSystemWatcher off the main thread, as it does excessive stat syscalls which can lead to the UI being blocked for a long time when calling addPaths.

      To do that, I basically did this:

      auto watcher = new QFileSystemWatcher;
watcher->moveToThread(someBackgroundThread);
// connect to signals, etc.
// then interact with the normal API via thread safe events:
QMetaObject::invokeMethod(watcher, [watcher, paths]() {
    watcher->addPaths(path);
});

// once done with the watcher:
watcher->deleteLater();

      This worked fine on macOS and Linux, but on Windows I started to see strange crashes in our unit tests with this backtrace:

      ucrtbase.dll!abort() Unknown
  vcruntime140.dll!_purecall() Line 29 C++
> Qt5Core.dll!QAbstractEventDispatcher::filterNativeEvent(const QByteArray & eventType, void * message, long * result) Line 495 C++
  Qt5Core.dll!QEventDispatcherWin32::processEvents(QFlags<enum QEventLoop::ProcessEventsFlag> flags) Line 605 C++
  Qt5Core.dll!QCoreApplication::processEvents(QFlags<enum QEventLoop::ProcessEventsFlag> flags, int ms) Line 1316 C++
  Qt5Core.dll!QTest::qWait(int ms) Line 105 C++
  tst_dataset.exe!TestDataset::test_duplicateChannelFileExport() Line 121 C++

      I believe this comes from the fact that QFileSystemWatcher is not moveToThread safe:

      
QWindowsFileSystemWatcherEngine::QWindowsFileSystemWatcherEngine(QObject *parent)
    : QFileSystemWatcherEngine(parent)
{
#ifndef Q_OS_WINRT
    if (QAbstractEventDispatcher *eventDispatcher = QAbstractEventDispatcher::instance()) {
        m_driveListener = new QWindowsRemovableDriveListener(this);
        eventDispatcher->installNativeEventFilter(m_driveListener);

      this gets called on construction of the QFileSystemWatcher, but there is no handling of a QEvent::ThreadChange in that class, which should remove the event filter and then re-add it in the new thread.

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          duplicates

          Bug - A problem which impairs or prevents the functions of the product. QTBUG-67546 QFileSytemWatcher cannot have thread affinity changed

          • P1: Critical - Urgent and Important, will STOP the release if matched with set FixVersion field. This includes regressions from the last version that are not edge cases; Data loss; Build issues; All but the most unlikely crashes/lockups/hanging; Serious usability issues and embarrassing bugs & Bugs critical to a deliverable. This is the highest possible priority for requirements.
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              thiago Thiago Macieira
              milianw Milian Wolff
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              Dates

                Created:
                Updated:
                Resolved:

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