Fix invalid memory read when shutting down QML applications

As the last line in the QObject destructor, we call setParentHelper(0) to
remove ourselves from the parent. In the process of that we also initiate the
QML parentChanged callback. The first thing that parentChanged callback used to
do (but now does it too late, after 26350b5ceafa0ade1328037f6234a7d288eb8f48 in
qtdeclarative) is to check if the object was deleted and then return. We could
re-introduce the check there, but I think it's cleaner to not bother calling
the callback on a dead object in the first place.

Change-Id: Ia4d43b65a9b3744a451b4c312a2d6f9c0e3b67dc
Reviewed-by: Lars Knoll <lars.knoll@digia.com>

src/corelib/kernel/qobject.cpp

@@ -1876,7 +1876,7 @@ void QObjectPrivate::setParent_helper(QObject *o)
             }
         }
     }
-    if (!isDeletingChildren && declarativeData && QAbstractDeclarativeData::parentChanged)
+    if (!wasDeleted && !isDeletingChildren && declarativeData && QAbstractDeclarativeData::parentChanged)
         QAbstractDeclarativeData::parentChanged(declarativeData, q, o);
 }
 

tests/auto/corelib/kernel/qobject/tst_qobject.cpp

@@ -152,6 +152,7 @@ private slots:
     void connectBase();
     void qmlConnect();
     void exceptions();
+    void noDeclarativeParentChangedOnDestruction();
 };
 
 struct QObjectCreatedOnShutdown
@@ -6233,6 +6234,43 @@ void tst_QObject::exceptions()
 #endif
 }
 
+#ifdef QT_BUILD_INTERNAL
+static bool parentChangeCalled = false;
+
+static void testParentChanged(QAbstractDeclarativeData *, QObject *, QObject *)
+{
+    parentChangeCalled = true;
+}
+#endif
+
+void tst_QObject::noDeclarativeParentChangedOnDestruction()
+{
+#ifdef QT_BUILD_INTERNAL
+    typedef void (*ParentChangedCallback)(QAbstractDeclarativeData *, QObject *, QObject *);
+    QScopedValueRollback<ParentChangedCallback> rollback(QAbstractDeclarativeData::parentChanged);
+    QAbstractDeclarativeData::parentChanged = testParentChanged;
+
+    QObject *parent = new QObject;
+    QObject *child = new QObject;
+
+    QAbstractDeclarativeData dummy;
+    QObjectPrivate::get(child)->declarativeData = &dummy;
+
+    parentChangeCalled = false;
+    child->setParent(parent);
+
+    QVERIFY(parentChangeCalled);
+    parentChangeCalled = false;
+
+    delete child;
+    QVERIFY(!parentChangeCalled);
+
+    delete parent;
+#else
+    QSKIP("Needs QT_BUILD_INTERNAL");
+#endif
+}
+
 // Test for QtPrivate::HasQ_OBJECT_Macro
 Q_STATIC_ASSERT(QtPrivate::HasQ_OBJECT_Macro<tst_QObject>::Value);
 Q_STATIC_ASSERT(!QtPrivate::HasQ_OBJECT_Macro<SiblingDeleter>::Value);