QThread & Lost Queued-Signals

[J.Hilk]

Hello everyone,

I have the situation, that somehow signals from a different thread are lost, and I'm unsure why that is the case.

The situation:
I have a class SslSocket that is only depending on QObject

class SslSocket : public QObject
{
    Q_OBJECT

public:
    SslSocket(QString host, int port, SmtpClient::ConnectionType connection);
    ~SslSocket();

    enum errorType{
        connectError,
        loginError,
        sendError
    };
    Q_ENUM(errorType)
...
...
}

In it, I'm using the asynchronus QSslSocket approch to communicate with a server. Even so I use readyRead and bytesWritten instead of the blocking functions I decided to wrap the class up in a seperated thread.

    SslSocket *smtp = new SslSocket("smtp.gmail.com",465,SmtpClient::SslConnection);
    QThread *t = new QThread();
    smtp->moveToThread(t);

    connect(t, &QThread::started,   smtp, &SslSocket::initSslSocket);
    connect(t, &QThread::finished,  smtp, &SslSocket::deleteLater);
    connect(t, &QThread::finished,  t,    &QThread::deleteLater);

    connect(this, &SmtpEmail::destroyed, t, &QThread::quit);
    connect(smtp, &SslSocket::errorMsg,         this, &SmtpEmail::errorManagement/*,Qt::DirectConnection*/); //<= Problem
...
...
t->start();

It is working as indetend, but while testing for potential Errors, I set the server to refuse the SSL-Handshake.
Using qDebug I see that the SIGNAL errorMsg is emitted, but the SLOT errorManagement is not called, ever.

But when I change the connection type from Qt::QueuedConnection to Qt::DirectConnection, the slot is called! The slot is also called, when I do not move smtp to the thread, but let it life in the main thread.

Because the object lifes in a different thread I rather would not use DirectConnection if I can avoid it.

So, anyone any idea what is going on ?

Thanks

[J.Hilk]

I'm an Idiot.

I overwrote QWidget::event to emit a signal in certain situations.

bool SmtpEmail::event(QEvent *e)
{
    if(e->type() == QEvent::MouseButtonPress ||
            e->type() == QEvent::MouseButtonRelease ||
            e->type() == QEvent::MouseMove ){
        QMouseEvent *ev =(QMouseEvent *)e;
        emit mouseEvent(ev->type(),mapToGlobal(ev->pos()));
    }
    return false;
}

what I should have written:

bool SmtpEmail::event(QEvent *e)
{
    if(e->type() == QEvent::MouseButtonPress ||
            e->type() == QEvent::MouseButtonRelease ||
            e->type() == QEvent::MouseMove ){
        QMouseEvent *ev =(QMouseEvent *)e;
        emit mouseEvent(ev->type(),mapToGlobal(ev->pos()));
    }
    return QFrame::event(e);
}

returning false als the default value aparently interrupt the QueuedConnection mechanic.

[VRonin]

This looks like a typical case in which you don't have an event loop running or the event loop in the main thread is blocked (stuck in a while(true) or something similar)

[J.Hilk]

@VRonin hi,

I'm quite sure that I'have a event loop and that it is running. I do call QThread::start() , queded-connections inside the threaded class are executed without problem as well. Also QThread::isrunning() returns true:

I added the following to test:

QTimer *timer = new QTimer();
    timer->setInterval(500);
    connect(timer, &QTimer::timeout, timer, [=]{
        qDebug() << "Thread"<< QTime::currentTime().toString("ss.zzz"); 
        emit errorMsg(QTime::currentTime().toString("ss.zzz"));
    });
    timer->start();

connect(smtp, &SslSocket::errorMsg,         this, &SmtpEmail::errorManagement);
connect(smtp, &SslSocket::errorMsg, smtp,[=](QString s){qDebug()<< "Queued:"<< s;},Qt::QueuedConnection );
    connect(smtp, &SslSocket::errorMsg, smtp,[=](QString s){qDebug()<< "Direct:"<< s;},Qt::DirectConnection );
connect(smtp, &SslSocket::errorMsg, this, &SmtpEmail::msg, Qt::QueuedConnection);
    connect(smtp, &SslSocket::errorMsg, this, &SmtpEmail::msg, Qt::DirectConnection);

void SmtpEmail::msg(QString s)
{
    qDebug() << "Slot"<< s;
}

the result is:

Thread "14.433"
Direct: "14.433"
Slot "14.433"
Queued: "14.433"

The queued connections, except the lambda one, are missing. And I'm not sure you can actually queue lambdas!?

...

I don't think I blocked the main thread.

• I don't use while loops in my main thread. Howerver there is one in the SslSocket class that I use to read the data, but it should not becalled before the readyread signal

if(socket->canReadLine()){
        while (socket->canReadLine())
        ...
}

• I can resize the window without a problem
• I do get information from my other worker thread, that is using a serialport

[J.Hilk]

OK, just made a clean project with this code:

    QTimer *timer = new QTimer();
    QThread *thread = new QThread();
    timer->moveToThread(thread);
    timer->setInterval(500);
    connect(timer, &QTimer::timeout, this, &MainWindow::mySlot);
    connect(thread, &QThread::started, timer, QOverload<>::of(&QTimer::start));
    thread->start();

void MainWindow::mySlot()
{
    qDebug() << "my Slot"<< QTime::currentTime().toString("mm.zzz");
}

It is calling the slot, like it should. If I copy and paste that one to my class where I have the problems, and adjust mySlot,

the slot is not called....

I'll have to look into it more. I defenitly messed up somewhere...

[VRonin]

could you post the code of SmtpEmail?

[J.Hilk]

@VRonin

I have the code here, I took the liberty to obscure some strings and add sslsocket as well.

[J.Hilk]

I'm an Idiot.

I overwrote QWidget::event to emit a signal in certain situations.

bool SmtpEmail::event(QEvent *e)
{
    if(e->type() == QEvent::MouseButtonPress ||
            e->type() == QEvent::MouseButtonRelease ||
            e->type() == QEvent::MouseMove ){
        QMouseEvent *ev =(QMouseEvent *)e;
        emit mouseEvent(ev->type(),mapToGlobal(ev->pos()));
    }
    return false;
}

what I should have written:

bool SmtpEmail::event(QEvent *e)
{
    if(e->type() == QEvent::MouseButtonPress ||
            e->type() == QEvent::MouseButtonRelease ||
            e->type() == QEvent::MouseMove ){
        QMouseEvent *ev =(QMouseEvent *)e;
        emit mouseEvent(ev->type(),mapToGlobal(ev->pos()));
    }
    return QFrame::event(e);
}

returning false als the default value aparently interrupt the QueuedConnection mechanic.