Qt application crashes when making 2 network requests from 2 threads

Question

I have a Qt application that launches two threads from the main thread at start up. Both these threads make network requests using distinct instances of the QNetworkAccessManager object. My program keeps crashing about 50% of the times and I'm not sure which thread is crashing.

There is no data sharing or signalling occurring directly between the two threads. When a certain event occurs, one the threads signals the main thread, which may in turn signal the second thread. However, by printing logs, I am pretty certain the crash doesn't occur during the signalling.

The structure of both threads is as follows. There's hardly any difference between the threads except for the URL etc.

MyThread() : QThread() {
    moveToThread(this);
}

MyThread()::~MyThread() {
    delete m_manager;
    delete m_request;
}

MyThread::run() {
    m_manager = new QNetworkAccessManager();
    m_request = new QNetworkRequest(QUrl("..."));

    makeRequest();
    exec();
}

MyThread::makeRequest() {
    m_reply = m_manager->get(*m_request);
    connect(m_reply, SIGNAL(finished()), this, SLOT(processReply()));
    // my log line
}

MyThread::processReply() {
    if (!m_reply->error()) {
        QString data = QString(m_reply->readAll());
        emit signalToMainThread(data);
    }
    m_reply->deleteLater();
    exit(0);
}

Now the weird thing is that if I don't start one of the threads, the program runs fine, or at least doesn't crash in around 20 invocations. If both threads run one after the other, the program doesn't crash. The program only crashes about half the times if I start and run both the threads concurrently.

Another interesting thing I gathered from logs is that whenever the program crashes, the line labelled with the comment my log line is the last to be executed by both the threads. So I don't know which thread causes the crash. But it leads me to suspect that QNetworkAccessManager is somehow to blame.

I'm pretty blank about what's causing the crash. I will appreciate any suggestions or pointers. Thanks in advance.

Answer 1

First of all you're doing it wrong! Fix your threading first

// EDIT From my own experience with this pattern i know that it may lead to many unclear crashes. I would start from clearing this thing out, as it may straighten some things and make finding problem clear. Also I don't know how do you invoke makeRequest. Also about QNetworkRequest. It is only a data structure so you don't need to make it on heap. Stack construction would be enough. Also you should remember (or protect somehow) from overwriting m_reply pointer. Do you call makeRequest more than once? If you do, then it may lead to deleting currently processed request after previous request finished.

What does happen if you call makeRequest twice:

1. First call of makeRequest assigns m_reply pointer.
2. Second call of makeRequest assigns m_reply pointer second time (replacing assigned pointer but not deleting pointed object)
3. Second request finishes before first, so processReply is called. deleteLater is queued at second
4. Somewhere in eventloop second reply is deleted, so from now m_reply pointer is pointing at some random (deleted) memory.
5. First reply finishes, so another processReply is called, but it operates on m_reply that is pointing a garbage, so every call at m_reply produces crash.

It is one of possible scenarios. That's why you don't get crash every time.

I'm not sure why do you call exit(0) at reply finish. It's also incorrect here if you use more then one call of makeRequest. Remember that QThread is interface to a single thread, not thread pool. So you can't call start() second time on thread instance when it is still running. Also if you're creating network access manager in entry point run() you should delete it in same place after exec(). Remember that exec() is blocking, so your objects won't be deleted before your thread exits.