widgets: Don't create winId when the widget is being destroyed

When QWidget is being destroyed, its winId is cleared, and
a QEvent::WinIdChange is sent. If a listener of this event
reacted by calling winId() again, we might crash.

A crash can be observed when this child widget is destroyed in dtor of its parent.
E.g. here is a hierarchy of widgets:
1:QWidget
 2:QObject
 3:QWidget
  4:QWidget

If a listener subscribed for WinIdChange events from (4),
and there is a connection to destroy (4) when (2) is destroyed.

This will lead to infinite loop:

1. QWidget::~QWidget
2. QWidget::destroy
3. QWidgetPrivate::setWinId(0)
4. QCoreApplication::sendEvent(q, QEvent::WinIdChange);
5. eventFilter
6. QWidget::winId
7. QWidgetPrivate::createWinId (this=0x555555957600) at kernel/qwidget.cpp:2380
8. QWidgetPrivate::createWinId (this=0x55555596b040) at kernel/qwidget.cpp:2387
9. QWidget::create (this=0x5555558f2010, window=0, initializeWindow=true, destroyOldWindow=true) at kernel/qwidget.cpp:1163
10. QWidgetPrivate::createWinId (this=0x55555596b040) at kernel/qwidget.cpp:2387
11. QWidget::create (this=0x5555558f2010, window=0, initializeWindow=true, destroyOldWindow=true) at kernel/qwidget.cpp:1163
12. QWidgetPrivate::createWinId (this=0x55555596b040) at kernel/qwidget.cpp:2387

Fixes: QTBUG-81849
Change-Id: Ib4c33ac97d9a79c701431ae107bddfb22720ba0d
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>

1 files changed, 51 insertions, 0 deletions

diff --git a/tests/auto/widgets/kernel/qwidget/tst_qwidget.cpp b/tests/auto/widgets/kernel/qwidget/tst_qwidget.cpp
index b5d174f340..2dd9e13b80 100644
--- a/tests/auto/widgets/kernel/qwidget/tst_qwidget.cpp
+++ b/tests/auto/widgets/kernel/qwidget/tst_qwidget.cpp
@@ -409,6 +409,8 @@ private slots:
     void closeEvent();
     void closeWithChildWindow();
 
+    void winIdAfterClose();
+
 private:
     bool ensureScreenSize(int width, int height);
 
@@ -11261,5 +11263,54 @@ void tst_QWidget::closeWithChildWindow()
     QVERIFY(!childWidget->isVisible());
 }
 
+class WinIdChangeSpy : public QObject
+{
+    Q_OBJECT
+public:
+    QWidget *widget = nullptr;
+    WId winId = 0;
+    explicit WinIdChangeSpy(QWidget *w, QObject *parent = nullptr)
+        : QObject(parent)
+        , widget(w)
+        , winId(widget->winId())
+    {
+    }
+
+public slots:
+    bool eventFilter(QObject *obj, QEvent *event) override
+    {
+        if (obj == widget) {
+            if (event->type() == QEvent::WinIdChange) {
+                winId = widget->winId();
+                return true;
+            }
+        }
+        return false;
+    }
+};
+
+void tst_QWidget::winIdAfterClose()
+{
+    auto widget = new QWidget;
+    auto notifier = new QObject(widget);
+    auto deleteWidget = new QWidget(new QWidget(widget));
+    auto spy = new WinIdChangeSpy(deleteWidget);
+    deleteWidget->installEventFilter(spy);
+    connect(notifier, &QObject::destroyed, [&] { delete deleteWidget; });
+
+    widget->setAttribute(Qt::WA_NativeWindow);
+    widget->windowHandle()->create();
+    widget->show();
+
+    QVERIFY(QTest::qWaitForWindowExposed(widget));
+    QVERIFY(spy->winId);
+
+    widget->windowHandle()->close();
+    delete widget;
+
+    QCOMPARE(spy->winId, WId(0));
+    delete spy;
+}
+
 QTEST_MAIN(tst_QWidget)
 #include "tst_qwidget.moc"