Fix mapping of QVideoFrame in write mode

After mapping of QVideoFrame with a write mode, we get this:
- the hw frame in QFFmpegVideoBuffer is not valid anymore
- the cached QImage is not valid anymore

The implemented approach might be revisited if we decide to
implement copy-on-modify idiom for QVideoFrame instead of
buffer sharing.

Fixes: QTBUG-123131
Pick-to: 6.7 6.6 6.5
Change-Id: I1c1662c5a4ac6f5bffa2cea8e36c4fbf60c20f64
Reviewed-by: Jøger Hansegård <joger.hansegard@qt.io>
Reviewed-by: Artem Dyomin <artem.dyomin@qt.io>

2 files changed, 14 insertions, 4 deletions

diff --git a/src/multimedia/video/qvideoframe.cpp b/src/multimedia/video/qvideoframe.cpp
index e58c7b2c4..8c57ce54b 100644
--- a/src/multimedia/video/qvideoframe.cpp
+++ b/src/multimedia/video/qvideoframe.cpp
@@ -427,6 +427,15 @@ bool QVideoFrame::map(QVideoFrame::MapMode mode)
     }
 
     d->mappedCount++;
+
+    // unlock mapMutex to avoid potential deadlock imageMutex <--> mapMutex
+    lock.unlock();
+
+    if ((mode & QVideoFrame::WriteOnly) != 0) {
+        QMutexLocker lock(&d->imageMutex);
+        d->image = {};
+    }
+
     return true;
 }
 
@@ -667,10 +676,12 @@ QImage QVideoFrame::toImage() const
     if (!isValid())
         return {};
 
-    std::call_once(d->imageOnceFlag, [this]() {
+    QMutexLocker lock(&d->imageMutex);
+
+    if (d->image.isNull()) {
         const bool mirrorY = surfaceFormat().scanLineDirection() != QVideoFrameFormat::TopToBottom;
         d->image = qImageFromVideoFrame(*this, rotation(), mirrored(), mirrorY);
-    });
+    }
 
     return d->image;
 }
diff --git a/src/multimedia/video/qvideoframe_p.h b/src/multimedia/video/qvideoframe_p.h
index f5037f7a5..1a43ae49d 100644
--- a/src/multimedia/video/qvideoframe_p.h
+++ b/src/multimedia/video/qvideoframe_p.h
@@ -19,7 +19,6 @@
 #include "qabstractvideobuffer_p.h"
 
 #include <qmutex.h>
-#include <mutex> // std::once
 
 QT_BEGIN_NAMESPACE
 
@@ -51,7 +50,7 @@ public:
     QtVideo::Rotation rotation = QtVideo::Rotation::None;
     bool mirrored = false;
     QImage image;
-    std::once_flag imageOnceFlag;
+    QMutex imageMutex;
 
 private:
     Q_DISABLE_COPY(QVideoFramePrivate)