Fix crash when calling QWidget::grab() on a QOpenGLWidget

By avoiding unneeded nested QPainters.

Crash was:
ASSERT: "s" in file /data/sources/qt/qt5/qtbase/src/gui/painting/qpaintengine_raster.cpp, line 2239

s was nullptr because the inner QPainter had called updateState(0), which is then dereferenced by the outer QPainter.

Task-number: QTBUG-61036
Change-Id: I7aad648f805f1abac4d38dfbefa2292da8b52af4
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>
Reviewed-by: Laszlo Agocs <laszlo.agocs@qt.io>
(cherry picked from commit 0265a23bb02b68534bb3c86514cc93bc45a7444f)

2 files changed, 6 insertions, 2 deletions

diff --git a/src/widgets/kernel/qwidget.cpp b/src/widgets/kernel/qwidget.cpp
index 53b39e0394..5dd43390ee 100644
--- a/src/widgets/kernel/qwidget.cpp
+++ b/src/widgets/kernel/qwidget.cpp
@@ -5592,13 +5592,15 @@ void QWidgetPrivate::drawWidget(QPaintDevice *pdev, const QRegion &rgn, const QP
                 // punch a hole in the backingstore, so the texture will be visible.
                 if (!q->testAttribute(Qt::WA_AlwaysStackOnTop)) {
                     beginBackingStorePainting();
-                    QPainter p(q);
                     if (backingStore) {
+                        QPainter p(q);
                         p.setCompositionMode(QPainter::CompositionMode_Source);
                         p.fillRect(q->rect(), Qt::transparent);
                     } else {
+                        QImage img = grabFramebuffer();
+                        QPainter p(q);
                         // We are not drawing to a backingstore: fall back to QImage
-                        p.drawImage(q->rect(), grabFramebuffer());
+                        p.drawImage(q->rect(), img);
                         skipPaintEvent = true;
                     }
                     endBackingStorePainting();
diff --git a/tests/auto/widgets/widgets/qopenglwidget/tst_qopenglwidget.cpp b/tests/auto/widgets/widgets/qopenglwidget/tst_qopenglwidget.cpp
index f5c127a063..ca4488744f 100644
--- a/tests/auto/widgets/widgets/qopenglwidget/tst_qopenglwidget.cpp
+++ b/tests/auto/widgets/widgets/qopenglwidget/tst_qopenglwidget.cpp
@@ -228,6 +228,8 @@ void tst_QOpenGLWidget::painter()
     glw->m_clear = true;
     image = glw->grabFramebuffer();
     QVERIFY(image.pixel(20, 10) == qRgb(0, 255, 0));
+
+    QPixmap pix = glw->grab(); // QTBUG-61036
 }
 
 void tst_QOpenGLWidget::reparentToAlreadyCreated()