// bug in Qt 5.15.2 QPixmap::convertFromImage() and QPixmap::fromImage() <<<<<<<

#include <QImage>
#include <QPixmap>
#include <QWidget>

#include <QTimer>
#include <QFuture>
#include <QMessageBox>

class PIXMAPBUG : public QWidget
{ private:
    // image dimensions
    static constexpr int W = 960, H = 540;

  private:
    QImage  image;
    QPixmap pixmap;

    QTimer  timer;
    int     left;

  private:
    void UpdateFrame();

  public:
    PIXMAPBUG();

    bool  Finished() const { return left==0; }
};


PIXMAPBUG::PIXMAPBUG()
  : image(W,H,QImage::Format_ARGB32/* NOT pre-multiplied */),
    pixmap(W,H)
{ image.fill(Qt::transparent);

  pixmap.convertFromImage(image);

  left = 10*50;
  connect(&timer,&QTimer::timeout,this,&PIXMAPBUG::UpdateFrame);
  timer.start(20);
}

void PIXMAPBUG::UpdateFrame()
{ // finshed?
  if(left==0)
    return;
  left -= 1;

  // fill entire image with 0xAA
  memset(image.bits(),0xAA,image.sizeInBytes());

  // call below will corrupt memory
  // this will write (at least) 4 pixels out-of-bounds (in pre-multiplied format RRGGBBAA).
  // note: pixmap = QPixmap::fromImage(image); will also cause memory corruption.
  pixmap.convertFromImage(image);
}


static bool BufferOverwriteTest()
{ static constexpr size_t BUFSIZE = 11749294080; // in bytes
  //static constexpr size_t BUFSIZE = size_t(1)<<28; // in bytes

  // an all-zero dynamically allocated buffer
  unsigned char *const buffer = new unsigned char[BUFSIZE];
  (void)memset(buffer,0x00,BUFSIZE);

  // detect any change to buffer
  for(size_t t=0; t<40; ++t)
    { for(size_t i=0; i<BUFSIZE; ++i)
        { if(buffer[i]!=0x00)
            { // memory corruption
              delete[] buffer;
              return false;
            }
        }
    }

  // no memory corruption detected
  delete[] buffer;
  return true;
}



int main(int argc, char *argv[])
{ QApplication app(argc,argv);

  PIXMAPBUG pixmapbug;
  pixmapbug.show();

  const QFuture<bool> future(QtConcurrent::run(BufferOverwriteTest));
  while(!( future.isFinished() && pixmapbug.Finished() ))
    { QThread::yieldCurrentThread();
      QApplication::processEvents();
    }

  const bool memokay = future.result();
  if(memokay)
    QMessageBox::information(nullptr,"Okay","No error detected.");
  else
    QMessageBox::critical(nullptr,"Memory Corruption","Out-of-bounds error detected.");
}