Fix crash in QMacPanGestureRecognizer

The reason it crashed was this:
1. Button was pressed => _panTimer started with the graphics view as
   destination.
2. Button was released => the graphicsview is destroyed
3. 300 ms later: Qt tries to deliver TimerEvent from _panTimer to the
   graphics view. Unfortunately, the graphics view is deleted, but Qt
   doesn't know that... (*crash*)

We therefore chose to start the timer with a destination we are controlling the
lifetime of: the QMacPanGestureRecognizer. Inside the timerEvent of that we can
check if the actual target is already destroyed.

Task-number: QTBUG-60404
Change-Id: Iff8f5b7217de42c4c5cf551ca507f0cff1c99a78
Reviewed-by: Frederik Gladhorn <frederik.gladhorn@qt.io>

src/widgets/kernel/qmacgesturerecognizer.cpp

@@ -43,6 +43,7 @@
 #include "qevent.h"
 #include "qwidget.h"
 #include "qdebug.h"
+#include <QtCore/qcoreapplication.h>
 
 #ifndef QT_NO_GESTURES
 
@@ -181,6 +182,16 @@ QGesture *QMacPanGestureRecognizer::create(QObject *target)
     return 0;
 }
 
+void QMacPanGestureRecognizer::timerEvent(QTimerEvent *ev)
+{
+    if (ev->timerId() == _panTimer.timerId()) {
+        if (_panTimer.isActive())
+           _panTimer.stop();
+        if (_target)
+            QCoreApplication::sendEvent(_target, ev);
+    }
+}
+
 QGestureRecognizer::Result
 QMacPanGestureRecognizer::recognize(QGesture *gesture, QObject *target, QEvent *event)
 {
@@ -195,7 +206,8 @@ QMacPanGestureRecognizer::recognize(QGesture *gesture, QObject *target, QEvent *
         if (ev->touchPoints().size() == 1) {
             reset(gesture);
             _startPos = QCursor::pos();
-            _panTimer.start(panBeginDelay, target);
+            _target = target;
+            _panTimer.start(panBeginDelay, this);
             _panCanceled = false;
             return QGestureRecognizer::MayBeGesture;
         }
@@ -242,7 +254,6 @@ QMacPanGestureRecognizer::recognize(QGesture *gesture, QObject *target, QEvent *
     case QEvent::Timer: {
         QTimerEvent *ev = static_cast<QTimerEvent *>(event);
         if (ev->timerId() == _panTimer.timerId()) {
-            _panTimer.stop();
             if (_panCanceled)
                 break;
             // Begin new pan session!

src/widgets/kernel/qmacgesturerecognizer_p.h

@@ -55,6 +55,7 @@
 #include "qtimer.h"
 #include "qpoint.h"
 #include "qgesturerecognizer.h"
+#include <QtCore/qpointer.h>
 
 #ifndef QT_NO_GESTURES
 
@@ -88,10 +89,13 @@ public:
     QGesture *create(QObject *target);
     QGestureRecognizer::Result recognize(QGesture *gesture, QObject *watched, QEvent *event);
     void reset(QGesture *gesture);
+protected:
+    void timerEvent(QTimerEvent *ev) override;
 private:
     QPointF _startPos;
     QBasicTimer _panTimer;
     bool _panCanceled;
+    QPointer<QObject> _target;
 };
 
 QT_END_NAMESPACE

tests/auto/other/gestures/tst_gestures.cpp

@@ -331,6 +331,9 @@ private slots:
     void graphicsViewParentPropagation();
     void panelPropagation();
     void panelStacksBehindParent();
+#ifdef Q_OS_MACOS
+    void deleteMacPanGestureRecognizerTargetWidget();
+#endif
     void deleteGestureTargetWidget();
     void deleteGestureTargetItem_data();
     void deleteGestureTargetItem();
@@ -1807,6 +1810,31 @@ void tst_Gestures::panelStacksBehindParent()
     QCOMPARE(panel->gestureOverrideEventsReceived, 0);
 }
 
+#ifdef Q_OS_MACOS
+void tst_Gestures::deleteMacPanGestureRecognizerTargetWidget()
+{
+    QWidget window;
+    window.resize(400,400);
+    QGraphicsScene scene;
+    QGraphicsView *view = new QGraphicsView(&scene, &window);
+    view->resize(400, 400);
+    window.show();
+
+    QVERIFY(QTest::qWaitForWindowExposed(&window));
+    QTouchDevice *device = QTest::createTouchDevice();
+    // QMacOSPenGestureRecognizer will start a timer on a touch press event
+    QTest::touchEvent(&window, device).press(1, QPoint(100, 100), &window);
+    delete view;
+
+    // wait until after that the QMacOSPenGestureRecognizer timer (300ms) is triggered.
+    // This is needed so that the whole test does not finish before the timer triggers
+    // and to make sure it crashes while executing *this* function. (otherwise it might give the
+    // impression that some of the subsequent test function caused the crash...)
+
+    QTest::qWait(400);  // DO NOT CRASH while waiting
+}
+#endif
+
 void tst_Gestures::deleteGestureTargetWidget()
 {
 }