Fix dead lock in the Qt event handling

The deadlock is caused because the QEvent is destroyed while holding the
event list mutex. And the QEvent may have a custom destructor that will
re-enter the event handlng code.

The QScopedPointer that should destroy the event must be created after
the MutexUnlocker.

Regression introduced by commit f9035587b9

Task-number: QTBUG-31606
Change-Id: I6b2cbc2656eacdec61b641886953f00bf5b3ff36
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/kernel/qcoreapplication.cpp

@@ -1451,7 +1451,7 @@ void QCoreApplicationPrivate::sendPostedEvents(QObject *receiver, int event_type
         // first, we diddle the event so that we can deliver
         // it, and that no one will try to touch it later.
         pe.event->posted = false;
-        QScopedPointer<QEvent> e(pe.event);
+        QEvent *e = pe.event;
         QObject * r = pe.receiver;
 
         --r->d_func()->postedEvents;
@@ -1469,8 +1469,10 @@ void QCoreApplicationPrivate::sendPostedEvents(QObject *receiver, int event_type
         };
         MutexUnlocker unlocker(locker);
 
+        QScopedPointer<QEvent> event_deleter(e); // will delete the event (with the mutex unlocked)
+
         // after all that work, it's time to deliver the event.
-        QCoreApplication::sendEvent(r, e.data());
+        QCoreApplication::sendEvent(r, e);
 
         // careful when adding anything below this point - the
         // sendEvent() call might invalidate any invariants this

tests/auto/corelib/kernel/qcoreapplication/tst_qcoreapplication.cpp

@@ -69,6 +69,7 @@ private slots:
     void eventLoopExecAfterExit();
     void customEventDispatcher();
     void testQuitLock();
+    void QTBUG31606_QEventDestructorDeadLock();
 };
 
 class EventSpy : public QObject
@@ -769,6 +770,33 @@ void tst_QCoreApplication::testQuitLock()
     app.exec();
 }
 
+
+void tst_QCoreApplication::QTBUG31606_QEventDestructorDeadLock()
+{
+    class MyEvent : public QEvent
+    { public:
+        MyEvent() : QEvent(QEvent::Type(QEvent::User + 1)) {}
+        ~MyEvent() {
+            QCoreApplication::postEvent(qApp, new QEvent(QEvent::Type(QEvent::User+2)));
+        }
+    };
+
+    int argc = 1;
+    char *argv[] = { const_cast<char*>("tst_qcoreapplication") };
+    QCoreApplication app(argc, argv);
+
+    EventSpy spy;
+    app.installEventFilter(&spy);
+
+    QCoreApplication::postEvent(&app, new MyEvent);
+    QCoreApplication::processEvents();
+    QVERIFY(spy.recordedEvents.contains(QEvent::User + 1));
+    QVERIFY(!spy.recordedEvents.contains(QEvent::User + 2));
+    QCoreApplication::processEvents();
+    QVERIFY(spy.recordedEvents.contains(QEvent::User + 2));
+}
+
+
 static void createQObjectOnDestruction()
 {
     // Make sure that we can create a QObject after the last QObject has been