FIXUP: Fix swap condition in DisplayGLOutputSurface::updatePaintNode

Amends commit 99a668c324. Task 'swapBuffersOnVizThread' is supposed to
be scheduled only when m_middleBuffer is ready after swap on gpu thread.
Checking for m_taskRunner is not enough to verify this. Call to
updatePaintNode may come from SG backend on expose event, thus breaking
expected sequence of buffers swaps.

Task-number: QTBUG-85817
Fixes: QTBUG-86599
Change-Id: Ia97a7a24b5672a2979b514d3cb4879e7630941eb
Reviewed-by: Jüri Valdmann <juri.valdmann@qt.io>

3 files changed, 4 insertions, 1 deletions

diff --git a/src/core/compositor/display_gl_output_surface.cpp b/src/core/compositor/display_gl_output_surface.cpp
index a75daf222..370eb07e6 100644
--- a/src/core/compositor/display_gl_output_surface.cpp
+++ b/src/core/compositor/display_gl_output_surface.cpp
@@ -210,6 +210,7 @@ void DisplayGLOutputSurface::swapBuffersOnGpuThread(unsigned int id, std::unique
         QMutexLocker locker(&m_mutex);
         m_middleBuffer->serviceId = id;
         m_middleBuffer->fence = CompositorResourceFence::create(std::move(fence));
+        m_readyToUpdate = true;
     }
 
     m_sink->scheduleUpdate();
diff --git a/src/core/compositor/display_gl_output_surface.h b/src/core/compositor/display_gl_output_surface.h
index c9c9343ae..d2e203b4e 100644
--- a/src/core/compositor/display_gl_output_surface.h
+++ b/src/core/compositor/display_gl_output_surface.h
@@ -141,6 +141,7 @@ private:
     std::unique_ptr<Buffer> m_backBuffer;
     std::unique_ptr<Buffer> m_middleBuffer;
     std::unique_ptr<Buffer> m_frontBuffer;
+    bool m_readyToUpdate = false;
     scoped_refptr<base::SingleThreadTaskRunner> m_taskRunner;
     scoped_refptr<viz::VizProcessContextProvider> m_vizContextProvider;
 };
diff --git a/src/core/compositor/display_gl_output_surface_qsg.cpp b/src/core/compositor/display_gl_output_surface_qsg.cpp
index d4b9154d1..0e9fff6e3 100644
--- a/src/core/compositor/display_gl_output_surface_qsg.cpp
+++ b/src/core/compositor/display_gl_output_surface_qsg.cpp
@@ -88,12 +88,13 @@ QSGNode *DisplayGLOutputSurface::updatePaintNode(QSGNode *oldNode, RenderWidgetH
 {
     {
         QMutexLocker locker(&m_mutex);
-        if (m_taskRunner) {
+        if (m_readyToUpdate) {
             std::swap(m_middleBuffer, m_frontBuffer);
             m_taskRunner->PostTask(
                     FROM_HERE,
                     base::BindOnce(&DisplayGLOutputSurface::swapBuffersOnVizThread, base::Unretained(this)));
             m_taskRunner.reset();
+            m_readyToUpdate = false;
         }
     }