QObject: Q_ASSERT the object type before calling a PMF

The old-syle signal-slot syntax had the advantage of not delivering
signals to slots in derived classes after that derived class's
destructor had finished running (because we called via the virtual
qt_metacall). The new syntax made no checks, so a conversion from the
old to the new syntax may introduce crashes or other data corruptions at
runtime if the destructor had completed.

This commit introduces a Q_ASSERT to print the class name that the
object is not any more. Since this is in inline code, this should get
enabled for users' debug modes and does not therefore depend on Qt being
built in debug mode.

It required some Private classes to be adapted to the new form, by
exposing the public q_func() in the public: part.

Pick-to: 6.2
Fixes: QTBUG-33908
Change-Id: Iccb47e5527544b6fbd75fffd16b874cdc08c1f3e
Reviewed-by: Marc Mutz <marc.mutz@qt.io>
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: Giuseppe D'Angelo <giuseppe.dangelo@kdab.com>

src/corelib/kernel/qobject_p.h

@@ -98,9 +98,9 @@ public:
 
 class Q_CORE_EXPORT QObjectPrivate : public QObjectData
 {
+public:
     Q_DECLARE_PUBLIC(QObject)
 
-public:
     struct ExtraData
     {
         ExtraData(QObjectPrivate *ptr) : parent(ptr) { }
@@ -492,6 +492,12 @@ using FunctionStorage = typename std::conditional_t<
         FunctionStorageByValue<Func>
     >;
 
+template <typename ObjPrivate> inline void assertObjectType(QObjectPrivate *d)
+{
+    using Obj = std::remove_pointer_t<decltype(std::declval<ObjPrivate *>()->q_func())>;
+    assertObjectType<Obj>(d->q_ptr);
+}
+
 template<typename Func, typename Args, typename R>
 class QPrivateSlotObject : public QSlotObjectBase, private FunctionStorage<Func>
 {

src/corelib/kernel/qobjectdefs_impl.h

@@ -50,6 +50,7 @@
 
 QT_BEGIN_NAMESPACE
 class QObject;
+class QObjectPrivate;
 
 namespace QtPrivate {
     template <typename T> struct RemoveRef { typedef T Type; };
@@ -139,6 +140,22 @@ namespace QtPrivate {
 
     template<typename Func> struct FunctionPointer { enum {ArgumentCount = -1, IsPointerToMemberFunction = false}; };
 
+    template<typename ObjPrivate> inline void assertObjectType(QObjectPrivate *d);
+    template<typename Obj> inline void assertObjectType(QObject *o)
+    {
+        // ensure all three compile
+        [[maybe_unused]] auto staticcast = [](QObject *obj) { return static_cast<Obj *>(obj); };
+        [[maybe_unused]] auto qobjcast = [](QObject *obj) { return Obj::staticMetaObject.cast(obj); };
+#ifdef __cpp_rtti
+        [[maybe_unused]] auto dyncast = [](QObject *obj) { return dynamic_cast<Obj *>(obj); };
+        auto cast = dyncast;
+#else
+        auto cast = qobjcast;
+#endif
+        Q_ASSERT_X(cast(o), Obj::staticMetaObject.className(),
+                   "Called object is not of the correct type (class destructor may have already run)");
+    }
+
     template <typename, typename, typename, typename> struct FunctorCall;
     template <int... II, typename... SignalArgs, typename R, typename Function>
     struct FunctorCall<IndexesList<II...>, List<SignalArgs...>, R, Function> {
@@ -148,25 +165,33 @@ namespace QtPrivate {
     };
     template <int... II, typename... SignalArgs, typename R, typename... SlotArgs, typename SlotRet, class Obj>
     struct FunctorCall<IndexesList<II...>, List<SignalArgs...>, R, SlotRet (Obj::*)(SlotArgs...)> {
-        static void call(SlotRet (Obj::*f)(SlotArgs...), Obj *o, void **arg) {
+        static void call(SlotRet (Obj::*f)(SlotArgs...), Obj *o, void **arg)
+        {
+            assertObjectType<Obj>(o);
             (o->*f)((*reinterpret_cast<typename RemoveRef<SignalArgs>::Type *>(arg[II+1]))...), ApplyReturnValue<R>(arg[0]);
         }
     };
     template <int... II, typename... SignalArgs, typename R, typename... SlotArgs, typename SlotRet, class Obj>
     struct FunctorCall<IndexesList<II...>, List<SignalArgs...>, R, SlotRet (Obj::*)(SlotArgs...) const> {
-        static void call(SlotRet (Obj::*f)(SlotArgs...) const, Obj *o, void **arg) {
+        static void call(SlotRet (Obj::*f)(SlotArgs...) const, Obj *o, void **arg)
+        {
+            assertObjectType<Obj>(o);
             (o->*f)((*reinterpret_cast<typename RemoveRef<SignalArgs>::Type *>(arg[II+1]))...), ApplyReturnValue<R>(arg[0]);
         }
     };
     template <int... II, typename... SignalArgs, typename R, typename... SlotArgs, typename SlotRet, class Obj>
     struct FunctorCall<IndexesList<II...>, List<SignalArgs...>, R, SlotRet (Obj::*)(SlotArgs...) noexcept> {
-        static void call(SlotRet (Obj::*f)(SlotArgs...) noexcept, Obj *o, void **arg) {
+        static void call(SlotRet (Obj::*f)(SlotArgs...) noexcept, Obj *o, void **arg)
+        {
+            assertObjectType<Obj>(o);
             (o->*f)((*reinterpret_cast<typename RemoveRef<SignalArgs>::Type *>(arg[II+1]))...), ApplyReturnValue<R>(arg[0]);
         }
     };
     template <int... II, typename... SignalArgs, typename R, typename... SlotArgs, typename SlotRet, class Obj>
     struct FunctorCall<IndexesList<II...>, List<SignalArgs...>, R, SlotRet (Obj::*)(SlotArgs...) const noexcept> {
-        static void call(SlotRet (Obj::*f)(SlotArgs...) const noexcept, Obj *o, void **arg) {
+        static void call(SlotRet (Obj::*f)(SlotArgs...) const noexcept, Obj *o, void **arg)
+        {
+            assertObjectType<Obj>(o);
             (o->*f)((*reinterpret_cast<typename RemoveRef<SignalArgs>::Type *>(arg[II+1]))...), ApplyReturnValue<R>(arg[0]);
         }
     };

src/network/socket/qlocalsocket_p.h

@@ -116,9 +116,9 @@ public:
 
 class QLocalSocketPrivate : public QIODevicePrivate
 {
+public:
     Q_DECLARE_PUBLIC(QLocalSocket)
 
-public:
     QLocalSocketPrivate();
     void init();
 

src/widgets/widgets/qwidgettextcontrol_p_p.h

@@ -73,8 +73,9 @@ class QAbstractScrollArea;
 
 class QWidgetTextControlPrivate : public QObjectPrivate
 {
-    Q_DECLARE_PUBLIC(QWidgetTextControl)
 public:
+    Q_DECLARE_PUBLIC(QWidgetTextControl)
+
     QWidgetTextControlPrivate();
 
     bool cursorMoveKeyEvent(QKeyEvent *e);

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

@@ -1,7 +1,8 @@
 /****************************************************************************
 **
 ** Copyright (C) 2021 The Qt Company Ltd.
-** Copyright (C) 2015 Olivier Goffart <ogoffart@woboq.com>
+** Copyright (C) 2020 Olivier Goffart <ogoffart@woboq.com>
+** Copyright (C) 2021 Intel Corporation.
 ** Contact: https://www.qt.io/licensing/
 **
 ** This file is part of the test suite of the Qt Toolkit.
@@ -169,6 +170,7 @@ private slots:
     void disconnectDisconnects();
     void singleShotConnection();
     void objectNameBinding();
+    void emitToDestroyedClass();
 };
 
 struct QObjectCreatedOnShutdown
@@ -5946,8 +5948,8 @@ public:
 };
 
 class ConnectToPrivateSlotPrivate : public QObjectPrivate {
-    Q_DECLARE_PUBLIC(ConnectToPrivateSlot)
 public:
+    Q_DECLARE_PUBLIC(ConnectToPrivateSlot)
     int receivedCount;
     QVariant receivedValue;
 
@@ -8140,6 +8142,71 @@ void tst_QObject::objectNameBinding()
                                                                 "objectName");
 }
 
+namespace EmitToDestroyedClass {
+static int assertionCallCount = 0;
+static int wouldHaveAssertedCount = 0;
+struct WouldAssert : std::exception {};
+class Base : public QObject
+{
+    Q_OBJECT
+public:
+    ~Base()
+    {
+        try {
+            emit theSignal();
+        } catch (const WouldAssert &) {
+            ++wouldHaveAssertedCount;
+        }
+    }
+
+signals:
+    void theSignal();
+};
+
+class Derived : public Base
+{
+    Q_OBJECT
+public:
+    ~Derived() { }
+
+public slots:
+    void doNothing() {}
+};
+} // namespace EmitToDestroyedClass
+
+QT_BEGIN_NAMESPACE
+namespace QtPrivate {
+template<> void assertObjectType<EmitToDestroyedClass::Derived>(QObject *o)
+{
+    // override the assertion so we don't assert and so something does happen
+    // when assertions are disabled. By throwing, we also prevent the UB from
+    // happening.
+    using namespace EmitToDestroyedClass;
+    ++assertionCallCount;
+    if (!qobject_cast<Derived *>(o))
+        throw WouldAssert();
+}
+}
+QT_END_NAMESPACE
+
+void tst_QObject::emitToDestroyedClass()
+{
+    using namespace EmitToDestroyedClass;
+    std::unique_ptr ptr = std::make_unique<Derived>();
+    QObject::connect(ptr.get(), &Base::theSignal, ptr.get(), &Derived::doNothing);
+    QCOMPARE(assertionCallCount, 0);
+    QCOMPARE(wouldHaveAssertedCount, 0);
+
+    // confirm our replacement function did get called
+    emit ptr->theSignal();
+    QCOMPARE(assertionCallCount, 1);
+    QCOMPARE(wouldHaveAssertedCount, 0);
+
+    ptr.reset();
+    QCOMPARE(assertionCallCount, 2);
+    QCOMPARE(wouldHaveAssertedCount, 1);
+}
+
 // Test for QtPrivate::HasQ_OBJECT_Macro
 static_assert(QtPrivate::HasQ_OBJECT_Macro<tst_QObject>::Value);
 static_assert(!QtPrivate::HasQ_OBJECT_Macro<SiblingDeleter>::Value);