Fix crash when using Software Renderer on HiDPI QBackingStore

For some reason the QPaintDevice of a QBackingStore is not valid on
HiDPI QBackingStores until QBackingStore::begin() has been called.  I
suspect this is a bug in qtbase, but until this is fixed this change
will make sure that the paint device will always be valid, even in this
unusual situation.

Task-number: QTBUG-55875
Change-Id: I63137d0de9b68d29bfcf86d1807f576923875b5d
Reviewed-by: Laszlo Agocs <laszlo.agocs@qt.io>

1 files changed, 11 insertions, 2 deletions

diff --git a/src/quick/scenegraph/adaptations/software/qsgsoftwarerenderer.cpp b/src/quick/scenegraph/adaptations/software/qsgsoftwarerenderer.cpp
index ae89ed7d8a..cad826fb27 100644
--- a/src/quick/scenegraph/adaptations/software/qsgsoftwarerenderer.cpp
+++ b/src/quick/scenegraph/adaptations/software/qsgsoftwarerenderer.cpp
@@ -101,8 +101,13 @@ void QSGSoftwareRenderer::render()
         return;
 
     // If there is a backingstore, set the current paint device
-    if (m_backingStore)
+    if (m_backingStore) {
+        // For HiDPI QBackingStores, the paint device is not valid
+        // until begin() has been called. See: QTBUG-55875
+        m_backingStore->beginPaint(QRegion());
         m_paintDevice = m_backingStore->paintDevice();
+        m_backingStore->endPaint();
+    }
 
     QElapsedTimer renderTimer;
 
@@ -133,8 +138,12 @@ void QSGSoftwareRenderer::render()
     qint64 optimizeRenderListTime = renderTimer.restart();
 
     // If Rendering to a backingstore, prepare it to be updated
-    if (m_backingStore != nullptr)
+    if (m_backingStore != nullptr) {
         m_backingStore->beginPaint(updateRegion);
+        // It is possible that a QBackingStore's paintDevice() will change
+        // when begin() is called.
+        m_paintDevice = m_backingStore->paintDevice();
+    }
 
     QPainter painter(m_paintDevice);
     painter.setRenderHint(QPainter::Antialiasing);