"QObject::~QObject: Timers cannot be stopped from another thread" message in Qt 6.8.2 plugins

Serg Mueller

Hello!
In Qt 6.8.2, any timer created in a plugin by calling the startTimer() function, when the program terminates, displays the console message: "QObject::~QObject: Timers cannot be stopped from another thread". In Qt 6.8.1, there is no such message. Is this a Qt 6.8.2 bug or is there something wrong with my code?

Test example:
file plugin/testplugininterface.h

#ifndef TESTPLUGININTERFACE_H
#define TESTPLUGININTERFACE_H

#include <QtPlugin>

class TestPluginInterface
{
public:
    virtual ~TestPluginInterface() {}
};

#define TestPluginInterface_iid "com.MyCompany.TestPlugin.TestPluginInterface"
Q_DECLARE_INTERFACE(TestPluginInterface, TestPluginInterface_iid)

#endif // TESTPLUGININTERFACE_H

file plugin/testplugin.h

#ifndef TESTPLUGIN_H
#define TESTPLUGIN_H

#include <QObject>
#include <QtDebug>
#include "testplugininterface.h"

class TestObject : public QObject
{
    Q_OBJECT
public:
    TestObject(QObject *parent = nullptr) : QObject(parent)
    {
	qDebug() << "TestObject::TestObject()";
	startTimer(1000);
    }
    virtual ~TestObject()
    {
	qDebug() << "TestObject::~TestObject()";
    }

protected:
    virtual void timerEvent(QTimerEvent *event)
    {
	qDebug() << "TestObject::timerEvent";
    }
};

class TestPlugin : public QObject, TestPluginInterface

{
    Q_OBJECT
    Q_PLUGIN_METADATA(IID "com.MyCompany.TestPlugin.TestPluginInterface")
    Q_INTERFACES(TestPluginInterface)

public:
    TestPlugin() : QObject()
    {
	TestObject *object = new TestObject(this);
	startTimer(1000);
    }
    virtual ~TestPlugin()
    {
	qDebug() << "~TestPlugin()";
    }

protected:
    virtual void timerEvent(QTimerEvent *event)
    {
	qDebug() << "TestPlugin::timerEvent";
    }
};

#endif // TESTPLUGIN_H

file plugin/CMakeLists.txt

set(TARGET testplugin)

add_library(${TARGET} SHARED
    testplugin.h
)

target_link_libraries(${TARGET} PRIVATE
    Qt::Core
)

file testapp/main.cpp

#include <QApplication>

#include "testwidget.h"

int main(int args, char **argv)
{
    QApplication app(args, argv);

    TestWidget w;
    w.show();

    return app.exec();
};

file testapp/testwidget.h

#ifndef TESTWIDGET_H
#define TESTWIDGET_H

#include <QWidget>
#include <QPluginLoader>
#include <QtDebug>
#include "testplugin.h"

class TestWidget : public QWidget
{
	Q_OBJECT
public:
	TestWidget(QWidget *parent = nullptr, Qt::WindowFlags f = Qt::WindowFlags())
	:QWidget(parent, f)
	{
	    QPluginLoader loader("../plugin/testplugin.so");
	    TestPluginInterface *pluginInterface = qobject_cast<TestPluginInterface *>(loader.instance());
	    if (!pluginInterface)
	    {
    		qDebug() << "Plugin is not loaded:" << loader.errorString();
	    }
	}
	virtual ~TestWidget(){}
};

#endif //TESTWIDGET_H

file testapp/CMakeLists.txt

set(TARGET testapp)

add_executable(${TARGET}
    main.cpp
    testwidget.h
)

target_include_directories(${TARGET} PRIVATE
    ../plugin
)

target_link_libraries(${TARGET} PRIVATE
    Qt::Widgets
)

root CMakeLists.txt

cmake_minimum_required(VERSION 3.9)
project(Tests LANGUAGES CXX)

set(CMAKE_AUTOMOC ON)

find_package(Qt6 REQUIRED COMPONENTS
    Core
    Widgets
)

add_subdirectory(plugin)
add_subdirectory(testapp)

Axel Spoerl

QPluginLoader doesn't use a separate thread.
It's unclear to me, where exactly the error occurs, i.e. which debug statements are hit before and after.
Both objects start a timer upon construction, in the thread that constructs them.
You're not playing around with threads, so it looks to me as if the error message is bogus.
I can't reproduce it, however (macOS arm64).
Which OS are you using?

Christian Ehrlicher

Run your app in a debugger with QT_FATAL_WARNINGS env var set and look at the backtrace to see where it comes from.

Serg Mueller

@Axel-Spoerl said in "QObject::~QObject: Timers cannot be stopped from another thread" message in Qt 6.8.2 plugins:

Which OS are you using?

Linux (Ubuntu and Debian).

Thanks for the answers. I reported the bug and am waiting for a solution.
https://bugreports.qt.io/browse/QTBUG-134080

clopez

QT has a thread that processes the events, and this error indicates that your program is forcing QT to reparse the events.
Why does your class extend public QObject? And you decorated the class with Q_OBJECT.
If I were you, I would remove the public QObject and re-test