QZXingFilter: Fix crash when destructing QZXingFilter

QZXingFilterRunnable runs QFuture to perform
decoder::decodeImage() and stores it in a
class var of its owner (QZXingFilter), but
when QZXingFilter is destructed it was not canceling
that QFuture, making possible to the QFuture to try to
access filter->decoder->decodeImage() after filter's
are already destructed (dangling pointer).

Issue detected when using QZXingFilter QML Element in
an Page pushed in a StackView. When the page containing the
QZXingFilter is poped from the StackView, (by default) the
filter is unloaded/destructed, what makes the app crash
without this patch.

Signed-off-by: Nick Diego Yamane <nick.diego@gmail.com>

src/QZXingFilter.cpp

@@ -39,6 +39,10 @@ QZXingFilter::QZXingFilter(QObject *parent)
 
 QZXingFilter::~QZXingFilter()
 {
+    if(!processThread.isFinished()) {
+      processThread.cancel();
+      processThread.waitForFinished();
+    }
     if(decoder_p)
         delete decoder_p;
 }
@@ -82,6 +86,10 @@ QZXingFilterRunnable::QZXingFilterRunnable(QZXingFilter * filter)
 {
 
 }
+QZXingFilterRunnable::~QZXingFilterRunnable()
+{
+    filter = nullptr;
+}
 
 QVideoFrame QZXingFilterRunnable::run(QVideoFrame * input, const QVideoSurfaceFormat &surfaceFormat, RunFlags flags)
 {
@@ -261,7 +269,7 @@ void QZXingFilterRunnable::processVideoFrameProbed(SimpleVideoFrame & videoFrame
 //    const QString path = QStandardPaths::writableLocation(QStandardPaths::PicturesLocation) + "/qrtest/test_" + QString::number(i % 100) + ".png";
 //    qDebug() << "saving image" << i << "at:" << path << image.save(path);
 
-    QString tag = filter->decoder_p->decodeImage(image, image.width(), image.height());
+    QString tag = decode(image);
 
     const bool tryHarder = filter->decoder_p->getTryHarder();
     /// The frames we get from the camera may be reflected horizontally or vertically
@@ -269,14 +277,20 @@ void QZXingFilterRunnable::processVideoFrameProbed(SimpleVideoFrame & videoFrame
     /// TODO: Maybe there is a better way to know this orientation beforehand? Or should we try decoding all of them?
     if (tag.isEmpty() && tryHarder) {
         image = image.mirrored(true, false);
-        tag = filter->decoder_p->decodeImage(image, image.width(), image.height());
+        tag = decode(image);
     }
     if (tag.isEmpty() && tryHarder) {
         image = image.mirrored(false, true);
-        tag = filter->decoder_p->decodeImage(image, image.width(), image.height());
+        tag = decode(image);
     }
     if (tag.isEmpty() && tryHarder) {
         image = image.mirrored(true, true);
-        tag = filter->decoder_p->decodeImage(image, image.width(), image.height());
+        tag = decode(image);
     }
 }
+
+QString QZXingFilterRunnable::decode(const QImage &image)
+{
+    return (filter != nullptr) ?
+      filter->decoder_p->decodeImage(image, image.width(), image.height()) : QString();
+}

src/QZXingFilter.h

@@ -103,10 +103,14 @@ class QZXingFilterRunnable : public QObject, public QVideoFilterRunnable
 
     public:
         explicit QZXingFilterRunnable(QZXingFilter * filter);
+        virtual ~QZXingFilterRunnable();
         /// This method is called whenever we get a new frame. It runs in the UI thread.
         QVideoFrame run(QVideoFrame * input, const QVideoSurfaceFormat &surfaceFormat, RunFlags flags);
         void processVideoFrameProbed(SimpleVideoFrame & videoFrame, const QRect& captureRect);
 
+    private:
+        QString decode(const QImage &image);
+
     private:
         QZXingFilter * filter;
 };