Crash using QFuture with QML - c++

Question

I'm trying to create a simple Task class that will be exposed to QML to do asynchronous operations in C++. To do this, I want to avoid subclassing QObject and instead I'm using the Q_GADGET macro as follows:
#pragma once
#include <QFuture>
// Workaround to use a templated class with Q_GADGET
class TaskBase {
Q_GADGET
};
template <typename T>
class Task : public TaskBase
{
public:
explicit Task() {}
// Creates a task from a QFuture
explicit Task(QFuture<T> future) : m_future(future)
{
auto watcher = new QFutureWatcher<T>();
QObject::connect(watcher, &QFutureWatcher<T>::finished, [this, watcher](){
qDebug () << "Finished, result is" << result();
watcher->deleteLater();
});
watcher->setFuture(future);
}
// Returns the result of the task
Q_INVOKABLE T result() { return m_future.result(); }
private:
QFuture<T> m_future;
};
// Register classes to QML
Q_DECLARE_METATYPE(Task<qint64>)
Q_DECLARE_METATYPE(Task<QString>)
Q_DECLARE_METATYPE(Task<bool>)
Then I'm using the class as follows:
// main.cpp
#include <QGuiApplication>
#include <QQmlApplicationEngine>
#include <QtConcurrent>
#include "task.h"
class HeavyComputer : public QObject
{
Q_OBJECT
public:
HeavyComputer(QObject *parent = nullptr) : QObject(parent) {}
Q_INVOKABLE Task<qint64> compute() const
{
return Task<qint64>(QtConcurrent::run([](){
qint64 sum = 0;
for(int i = 0; i < 100000; ++i){
sum += i;
}
return sum;
}));
}
};
int main(int argc, char *argv[])
{
QGuiApplication app(argc, argv);
qRegisterMetaType<Task<qint64>>();
// Create a QML engine.
QQmlApplicationEngine engine;
// Register the HeavyComputer class
qmlRegisterType<HeavyComputer>("HeavyComputer", 1, 0, "HeavyComputer");
// Load the QML file.
const QUrl url(QStringLiteral("qrc:/qml/main.qml"));
QObject::connect(
&engine, &QQmlApplicationEngine::objectCreated,
&app, [url](QObject *obj, const QUrl &objUrl)
{
if (!obj && url == objUrl)
QCoreApplication::exit(-1); },
Qt::QueuedConnection);
engine.load(url);
// Return the application's exit code.s
return app.exec();
}
#include "main.moc"
Using it from QML:
import QtQuick 2.15
import QtQuick.Controls 2.15
import QtQuick.Window 2.15
import HeavyComputer 1.0
Window {
id: window
visible: true
width: 640
height: 480
title: qsTr("Hello World")
HeavyComputer {
id: computer
}
Button {
anchors.bottom: parent.bottom
text: "Compute"
onClicked: {
computer.compute()
}
}
}
Now unfortunately, if you run the example and click on the button multiple times the application crashes. The debugger points out that the problem is in this line due a write access violation:
qDebug () << "Finished, result is" << result();
The stack information says this:
1 std::_Atomic_storage<QMutexData *,8>::compare_exchange_strong atomic 737 0x7ffb1d1ddec2
2 std::atomic<QMutexData *>::compare_exchange_strong atomic 1600 0x7ffb1d1ddfcc
3 QAtomicOps<QMutexData *>::testAndSetAcquire<QMutexData *> qatomic_cxx11.h 308 0x7ffb1d1d985a
4 QBasicAtomicPointer<QMutexData>::testAndSetAcquire qbasicatomic.h 273 0x7ffb1d1e8354
5 QBasicMutex::fastTryLock qmutex.h 109 0x7ffb1d1df761
6 QMutex::lock qmutex.cpp 230 0x7ffb1d21fb28
7 QMutexLocker::QMutexLocker qmutex.h 234 0x7ffb1d1da5bd
8 QFutureInterfaceBase::waitForResult qfutureinterface.cpp 317 0x7ffb1d236099
9 QFuture<__int64>::result qfuture.h 209 0x7ff7aba58421
10 Task<__int64>::result task.h 27 0x7ff7aba58467
11 <lambda_37427196d3c8e94283745470a735a1b5>::operator() task.h 20 0x7ff7aba56ce1
12 QtPrivate::FunctorCall<QtPrivate::IndexesList<>,QtPrivate::List<>,void,<lambda_37427196d3c8e94283745470a735a1b5>>::call qobjectdefs_impl.h 146 0x7ff7aba575d9
13 QtPrivate::Functor<<lambda_37427196d3c8e94283745470a735a1b5>,0>::call<QtPrivate::List<>,void> qobjectdefs_impl.h 257 0x7ff7aba53e43
14 QtPrivate::QFunctorSlotObject<<lambda_37427196d3c8e94283745470a735a1b5>,0,QtPrivate::List<>,void>::impl qobjectdefs_impl.h 449 0x7ff7aba57d4e
15 QtPrivate::QSlotObjectBase::call qobjectdefs_impl.h 398 0x7ffb1d64e9d2
16 doActivate<0> qobject.cpp 3886 0x7ffb1d6a6bb8
17 QMetaObject::activate qobject.cpp 3947 0x7ffb1d6936d7
18 QFutureWatcherBase::finished moc_qfuturewatcher.cpp 274 0x7ffb1d23a5f3
19 QFutureWatcherBasePrivate::sendCallOutEvent qfuturewatcher.cpp 442 0x7ffb1d23af0e
20 QFutureWatcherBase::event qfuturewatcher.cpp 336 0x7ffb1d23a581
... <More>
EDIT: the problem also only occurs if HeavyComputer::compute() is called from QML. Doing this in C++:
HeavyComputer computer;
computer.compute();
works just fine with no crashes.