Fix a crash on the focus chain when removing items from the scene.

The crash was because of the dangling pointer set on the tabFocusFirst
attribute in QGraphicsScene. A child which was the tabFocusFirst was
removed from the scene and fixFocusChainBeforeReparenting was setting the
new tabFocusFirst pointer to the parent (since in that example it was
the focusNext) but later on the parent was removed also from the scene
(due to the recursion of removeItem if you call it with the parent :
first children, then the parent itself) and
fixFocusChainBeforeReparenting was not called again so if you delete
the parent then the scene has the dangling pointer set.

Task-number:QTBUG-6544
Reviewed-by:janarve

4 files changed, 10 insertions, 9 deletions

diff --git a/src/gui/graphicsview/qgraphicsscene.cpp b/src/gui/graphicsview/qgraphicsscene.cpp
index 27ebb79..2af90b8 100644
--- a/src/gui/graphicsview/qgraphicsscene.cpp
+++ b/src/gui/graphicsview/qgraphicsscene.cpp
@@ -569,14 +569,10 @@ void QGraphicsScenePrivate::removeItemHelper(QGraphicsItem *item)
 
     item->d_ptr->clearSubFocus();
 
-    if (!item->d_ptr->inDestructor && item == tabFocusFirst) {
-        QGraphicsWidget *widget = static_cast<QGraphicsWidget *>(item);
-        widget->d_func()->fixFocusChainBeforeReparenting(0, 0);
-    }
-
     if (item->flags() & QGraphicsItem::ItemSendsScenePositionChanges)
         unregisterScenePosItem(item);
 
+    QGraphicsScene *oldScene = item->d_func()->scene;
     item->d_func()->scene = 0;
 
     //We need to remove all children first because they might use their parent
@@ -587,6 +583,11 @@ void QGraphicsScenePrivate::removeItemHelper(QGraphicsItem *item)
             q->removeItem(item->d_ptr->children.at(i));
     }
 
+    if (!item->d_ptr->inDestructor && item == tabFocusFirst) {
+        QGraphicsWidget *widget = static_cast<QGraphicsWidget *>(item);
+        widget->d_func()->fixFocusChainBeforeReparenting(0, oldScene, 0);
+    }
+
     // Unregister focus proxy.
     item->d_ptr->resetFocusProxy();
 
diff --git a/src/gui/graphicsview/qgraphicswidget.cpp b/src/gui/graphicsview/qgraphicswidget.cpp
index fe569f4..8de81c2 100644
--- a/src/gui/graphicsview/qgraphicswidget.cpp
+++ b/src/gui/graphicsview/qgraphicswidget.cpp
@@ -1054,7 +1054,7 @@ QVariant QGraphicsWidget::itemChange(GraphicsItemChange change, const QVariant &
         break;
     case ItemParentChange: {
         QGraphicsItem *parent = qVariantValue<QGraphicsItem *>(value);
-        d->fixFocusChainBeforeReparenting((parent && parent->isWidget()) ? static_cast<QGraphicsWidget *>(parent) : 0);
+        d->fixFocusChainBeforeReparenting((parent && parent->isWidget()) ? static_cast<QGraphicsWidget *>(parent) : 0, scene());
 
         // Deliver ParentAboutToChange.
         QEvent event(QEvent::ParentAboutToChange);
diff --git a/src/gui/graphicsview/qgraphicswidget_p.cpp b/src/gui/graphicsview/qgraphicswidget_p.cpp
index b747a30..5b6490f 100644
--- a/src/gui/graphicsview/qgraphicswidget_p.cpp
+++ b/src/gui/graphicsview/qgraphicswidget_p.cpp
@@ -743,7 +743,7 @@ bool QGraphicsWidgetPrivate::hasDecoration() const
 /**
  * is called after a reparent has taken place to fix up the focus chain(s)
  */
-void QGraphicsWidgetPrivate::fixFocusChainBeforeReparenting(QGraphicsWidget *newParent, QGraphicsScene *newScene)
+void QGraphicsWidgetPrivate::fixFocusChainBeforeReparenting(QGraphicsWidget *newParent, QGraphicsScene *oldScene, QGraphicsScene *newScene)
 {
     Q_Q(QGraphicsWidget);
 
@@ -789,7 +789,7 @@ void QGraphicsWidgetPrivate::fixFocusChainBeforeReparenting(QGraphicsWidget *new
     // update tabFocusFirst for oldScene if the item is going to be removed from oldScene
     if (newParent)
         newScene = newParent->scene();
-    QGraphicsScene *oldScene = q->scene();
+
     if (oldScene && newScene != oldScene)
         oldScene->d_func()->tabFocusFirst = firstOld;
 
diff --git a/src/gui/graphicsview/qgraphicswidget_p.h b/src/gui/graphicsview/qgraphicswidget_p.h
index eb53649..65e6962 100644
--- a/src/gui/graphicsview/qgraphicswidget_p.h
+++ b/src/gui/graphicsview/qgraphicswidget_p.h
@@ -98,7 +98,7 @@ public:
     mutable qreal *margins;
     void ensureMargins() const;
 
-    void fixFocusChainBeforeReparenting(QGraphicsWidget *newParent, QGraphicsScene *newScene = 0);
+    void fixFocusChainBeforeReparenting(QGraphicsWidget *newParent, QGraphicsScene *oldScene, QGraphicsScene *newScene = 0);
     void setLayout_helper(QGraphicsLayout *l);
 
     // Layouts