QSessionManager cancel() function does not work

I want my application to prevent a systen shutdown in some cases.
QSessionManager should do the trick, when calling it's cancel() function.

So I have a code like this from the documentation:

MyMainWidget::MyMainWidget(QWidget *parent)
    :QWidget(parent)
{
    connect(qApp, SIGNAL(commitDataRequest(QSessionManager&)), this, SLOT(commitData(QSessionManager&)));
}

void MyMainWidget::commitData(QSessionManager& manager)
{
    if (m_prevent_shutdown) {
        qDebug() << Q_FUNC_INFO << "cancel the shutdown";
        manager.cancel();
    }
}

This neither works in Windows 7, nor in Ubuntu 14.04.
But from the debug output I can see that the cancel() is called right in front of the Windows/Ubuntu shutdown, after pressing the systems shutdown button.

In Windows, handling WM_QUERYENDSESSION "manually" in a custom QAbstractNativeEventFilter does work.