How to stop a long-running function prematurely?

Question

I am working on a plotting algorithm. To do this I get the data from a DAQ board in my main GUI thread and then I send the data over to a worker thread to be processed, the worker thread emits a signal with the new QImage which I display as a plot in my GUI. The problem is the function, let's call it generateImage(), to calculate and generate the QImage takes a long time (~50-70 milliseconds, depending on data length) and in between this time another set of data might arrive which will require the worker thread to recalculate the plot from the beginning. I want the generateImage() to abandon the calculation and restart from the beginning if the new data arrives while it is still calculating. My approach is to set a member boolean variable, let's call it b_abort_, and check if it is set to true inside generateImage() and return if it's true, outside generateImage() it always remains true and I only set it to false before generateImage() is called.

All this happens in a worker thread, I subclass QObject and use moveToThread() to move it to a worker thread.

The function which starts calculation:

void WorkerThread::startCalc()
{
    b_abort_ = false;
    generateImage();
    // if b_abort_ is set to true, generateImage() will return prematurely 
    if(b_abort_)
        emit calcFinished();
    else
        b_abort_ = true;
}

Function which does all calculations and generates image:

void WorkerThread::generateImage()
{
    /* Calculation of some parts */
    for(int ii = 0; ii < Rawdata.length(); ++ii) // Starting main time consuming loop
    { 
        if(b_abort_)
            return;
        /* Perform calculation for one data point from Rawdata */
    }

    // data calculation complete, now it's time to generate QImage
    // before that I check if b_abort_ is set to true
    if(b_abort_)
        return;

    for(int ii = 0; ii < CalculatedData.length(); ++ii) // plotting calculated data on QImage
    {
        if(b_abort_)
            return;
        /* plot one data point from CalculatedData vector */
    }

    // generation of QImage finished, time to send the signal
    emit renderedPlot(image); // image is a QImage object
}

In my worker thread, I have a slot to receive data from the main GUI Thread, it is configured with Qt::QueuedConnection (the default) as the connection type:

void WorkerThread::receiveData(QVector<double> data)
{
    if(!b_abort_) // check if calculation is still running
    { 
        QEventLoop loop;
        connect(this, &WorkerThread::calcFinished, &loop, &QEventLoop::quit);
        b_abort_ = true; // set it to true and wait for calculation to stop
        loop.exec();
        // start new calculation
        RawData = data;
        startClac();
    }
    else
    {
        RawData = data;
        startClac();
    }
}

When I use this approach in my main GUI Thread, the generateImage() function blocks all event loops, and my GUI freezes, which makes me think that inside a single thread (main GUI thread or a worker thread) only one function can run at a time and so any change in b_abort_ is not applied until the thread's event loop returns to process other functions. When using WorkerThread it is difficult to verify if this is working, some times it works fine while other times it generates bad allocation error which seems like it is not working (although it might be because of a different reason entirely, I am not sure). I would like to ask your opinion, is this the right approach to stop a long-running calculation prematurely? Are there any other methods that I can use which will be more robust than my current approach?

Answer 1

How to stop a long-running function in another thread prematurely?

You're correct that the only sane way to do this is to have the long-running thread check, at regular intervals, whether it should stop early.

Note that the flag you're checking must be atomic, or protected by a mutex, or otherwise somehow synchronized. Otherwise it's entirely legitimate for the worker thread to check the variable and never see the value change (no, you can't use volatile for this).

... which makes me think that inside a single thread (main GUI thread or a worker thread) only one function can run at a time ...

Yes, that's exactly what a thread is! It is a single, linear thread of execution. It can't do two things at once. Doing two things at once is the whole reason for having more than one thread.

The approach should be to have a worker thread waiting for work to do, and a main thread that only ever sends it asynchronous messages (start generating an image with this data, or interrupt processing and start again with this data instead, or whatever).

If the main thread calls a function that should happen in the worker thread instead, well, you've deliberately started executing it in the main thread, and the main thread won't do anything until it returns. Just like every other function.

---

As an aside, your design has a problem: it's possible to never finish generating a single image if it keeps being interrupted by new data.

The usual solution is double-buffering: you let the worker thread finish generating the current image while the main thread accumulates data for the next one. When the worker has finished one image, it can be passed back to the main thread for display. Then the worker can start processing the next, so it takes the buffer of "dirty" updates that the main thread has prepared for it. Subsequent updates are again added to the (now empty) buffer for the next image.