Crash when rendering a scene using DeviceCoordinateCache

The execution flow would allow the widget the scene would be rendered on to be
null. However this very widget was being used when using DeviceCoordinateCache
to get the desktop size. We now skip this optimisation when no widget is
given. Auto-test included.

Reviewed-by: bnilsen
Task-number: QTBUG-5904

2 files changed, 22 insertions, 3 deletions

diff --git a/src/gui/graphicsview/qgraphicsscene.cpp b/src/gui/graphicsview/qgraphicsscene.cpp
index a463c01..10d251d 100644
--- a/src/gui/graphicsview/qgraphicsscene.cpp
+++ b/src/gui/graphicsview/qgraphicsscene.cpp
@@ -4420,9 +4420,12 @@ void QGraphicsScenePrivate::drawItemHelper(QGraphicsItem *item, QPainter *painte
         bool allowPartialCacheExposure = !viewRect.contains(deviceRect);
 #else
         // Only if deviceRect is 20% taller or wider than the desktop.
-        QRect desktopRect = QApplication::desktop()->availableGeometry(widget);
-        bool allowPartialCacheExposure = (desktopRect.width() * 1.2 < deviceRect.width()
-                                          || desktopRect.height() * 1.2 < deviceRect.height());
+        bool allowPartialCacheExposure = false;
+        if (widget) {
+            QRect desktopRect = QApplication::desktop()->availableGeometry(widget);
+            allowPartialCacheExposure = (desktopRect.width() * 1.2 < deviceRect.width()
+                                         || desktopRect.height() * 1.2 < deviceRect.height());
+        }
 #endif
         QRegion scrollExposure;
         if (deviceData->cacheIndent != QPoint() || allowPartialCacheExposure) {
diff --git a/tests/auto/qgraphicsscene/tst_qgraphicsscene.cpp b/tests/auto/qgraphicsscene/tst_qgraphicsscene.cpp
index a49c2c1..20d9eb8 100644
--- a/tests/auto/qgraphicsscene/tst_qgraphicsscene.cpp
+++ b/tests/auto/qgraphicsscene/tst_qgraphicsscene.cpp
@@ -276,6 +276,7 @@ private slots:
     void task176178_itemIndexMethodBreaksSceneRect();
     void task160653_selectionChanged();
     void task250680_childClip();
+    void taskQTBUG_5904_crashWithDeviceCoordinateCache();
 };
 
 void tst_QGraphicsScene::initTestCase()
@@ -4180,5 +4181,20 @@ void tst_QGraphicsScene::isActive()
 
 }
 
+void tst_QGraphicsScene::taskQTBUG_5904_crashWithDeviceCoordinateCache()
+{
+    QGraphicsScene scene;
+    QGraphicsRectItem *rectItem = scene.addRect(QRectF(0, 0, 100, 200), QPen(Qt::black), QBrush(Qt::green));
+
+    rectItem->setCacheMode(QGraphicsItem::DeviceCoordinateCache);
+
+    QPixmap pixmap(100,200);
+    QPainter painter(&pixmap);
+    painter.setRenderHint(QPainter::Antialiasing);
+    scene.render(&painter);
+    painter.end();
+    // No crash, then it passed!
+}
+
 QTEST_MAIN(tst_QGraphicsScene)
 #include "tst_qgraphicsscene.moc"