Qt bug? Signal emitted once, slot called multiple times

[MattPhillips]

Hi,

I'm getting this really strange situation, on multiple (unix-based) platforms, whereby a signal that I emit once results in multiple calls to the same slot. The slot does time-critical inter-process communication via socket connections, so this is pretty debilitating. Here's the signal:

class MyClass
{
    Q_OBJECT
    ...
    signals:
        void WriteToProc(const QByteArray& qb, int* num_bytes = NULL);
};
 
MyClass::SendData(...)
{
    QByteArray qb;
    ... //Fill up qb;
    qDebug() << "Emitting signal";
    emit WriteToProc(qb);
}

class MyClass
{
    Q_OBJECT
    ...
    signals:
        void WriteToProc(const QByteArray& qb, int* num_bytes = NULL);
};

MyClass::SendData(...)
{
    QByteArray qb;
    ... //Fill up qb;
    qDebug() << "Emitting signal";
    emit WriteToProc(qb);
}

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Here's the slot (has the same name):

class MyWorkerThread : public QThread
{
    Q_OBJECT
    ...
    public slots:
      	void WriteToProc(const QByteArray& qb, int* num_bytes);
    private:
        QLocalSocket* socket_out;
};
 
void MyWorkerThread::WriteToProc(const QByteArray& qb, int* num_bytes)
{
    if (num_bytes==NULL) socket_out->write(qb);
    else *num_bytes = socket_out->write(qb);
    qDebug() << "MyWorkerThread::WriteToProc";
}

class MyWorkerThread : public QThread
{
    Q_OBJECT
    ...
    public slots:
      	void WriteToProc(const QByteArray& qb, int* num_bytes);
    private:
        QLocalSocket* socket_out;
};

void MyWorkerThread::WriteToProc(const QByteArray& qb, int* num_bytes)
{
    if (num_bytes==NULL) socket_out->write(qb);
    else *num_bytes = socket_out->write(qb);
    qDebug() << "MyWorkerThread::WriteToProc";
}

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There's some stuff in there that's not directly relevant to the point that I include for completeness. SendData() and WriteToProc are in different threads, and the connection type is default (Qt::QueuedConnection). Anyway, the output is e.g.

Emitting signal
MyWorkerThread::WriteToProc
MyWorkerThread::WriteToProc
MyWorkerThread::WriteToProc

Moreover, looking at the output at the other end, I can see that the QByteArray is getting sent 3 (in this case; it can vary) times. I've verified that only SendData() is calling WriteToProc at that point. It's so weird--what could be going on?

Best,
Matt