QNetworkReply::readAll() causes app to crash

My app queues a series of files to download, and downloads each one in sequence. That is, it calls QNetworkAccessManager::get(), checks for errors in onFinished(), and if there are none, saves the data to disk. There are problems with this overall approach -- as opposed to writing the data as it arrives -- in that if a given file is too large, it will fill up the memory on a mobile device and cause the OS to kill the app.

That's not what I am seeing here.

And in fact, I am only seeing this for a particular file on a particular device, which happens to be a Nexus 7 running Android 5.1. Works fine on every iOS device I've tried, and a Nexus 7 running Android 6.0.1. The file downloads just fine, and there are no errors. But when I try to call readAll() on my non-null instance of QNetworkReply:

Code:

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F/libc    (23481): Fatal signal 6 (SIGABRT), code -6 in tid 23524 (QtThread)
I/DEBUG   (  182): pid: 23481, tid: 23524, name: QtThread  >>> my.org.appname <<<
E/lowmemorykiller(  176): Error writing /proc/23481/oom_score_adj; errno=22
I/Zygote  (  195): Process 23481 exited due to signal (6)
I/ActivityManager(  543): Process my.org.appname (pid 23481) has died

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Similar topic: http://www.qtcentre.org/threads/6684...-readAll-error

The author resolved it, but the solution was apparently so self-evident that no details were given.

My function is as follows:

Code:

---

bool DownloadManager::saveToDisk(const QString &fileName, QIODevice *data) {
    QFile file(fileName);
    bool ok = file.open(QIODevice::WriteOnly);
    if (ok) {
        QByteArray stuff = data->readAll(); // BOOM!
        file.write(stuff);
        file.close();
    }
    else
        qDebug() << QString("DownloadManager::saveToDisk(%1) - Failed: %2").arg(fileName).arg(file.errorString());
    return ok;
}

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And here is the calling code:

Code:

---

void DownloadManager::onFinished() {
    const QUrl urlObj = m_reply->url();
    const QString url = urlObj.toString();
    const QNetworkReply::NetworkError err = m_reply->error();
    if (err) {
        qDebug() << QString("DownloadManager::onFinished(%1) - Error: %2").arg(url).arg(m_reply->errorString());
        emit failure(url, (err == QNetworkReply::OperationCanceledError ? CanceledByUser : Network));
    }
    else {
        const QString fileName = absoluteSaveFileName(urlObj);
        if (saveToDisk(fileName, m_reply))
            emit success(url, fileName);
        else {
            qDebug() << QString("DownloadManager::onFinished(%1) - Failed to save to '%2'").arg(url).arg(fileName);
            emit failure(url, FileIO);
        }
    }
    m_reply->deleteLater();
    if (m_queue.isEmpty())
        setIsDownloading(false);
    else {
        m_reply = m_manager.get(m_queue.takeFirst());
        Q_ASSERT_X(m_reply, "onFinished", "Null pointer to QNetworkReply!");
#ifndef QT_NO_SSL
        connect(m_reply, SIGNAL(sslErrors(QList<QSslError>)), SLOT(sslErrors(QList<QSslError>)));
#endif
    }
}

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The cited topic and error message certainly make this look like a threading issue, but I need a hand with understanding and resolving it. I should add that I'm using Qt Creator 4.2.2 based on Qt 5.8.0.

Thanks in advance.

The working solution, for the moment (i.e., as opposed to writing the data as it arrives), involves appending the fully downloaded data to the output file a little at a time:

Code:

---

bool DownloadManager::saveToDisk(const QString &fileName, QIODevice *inputDevice) {
    QFile file(fileName);
    bool ok = file.open(QIODevice::Append);
    if (ok) {
        const qint64 MAX_LEN = 1024; // arbitrary...sort of
        while (ok && !inputDevice->atEnd())
            ok = file.write(inputDevice->read(MAX_LEN)) != -1;
        file.close();
    }
    return ok;
}

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Clearly the original problem was related to the file size, which at first glance doesn't seem to have much to do with threading. I'm frankly a bit annoyed at Qt, or Android, or both! :P