QAbstractEventDispatcher: prevent too late unregistering of timers

Timers can't unregister when the dispatcher object is no longer of a
dispatcher type. It's too late even at ~QAbstractEventDispatcher,
because unregisterTimer() is a pure virtual. To prevent their attempting
to unregister, we set the thread's dispatcher to nullptr if it is this
object.

This has been a latent bug, so it's worth fixing. This started happening
for me with an un-pushed change that changed the order of how
QCoreApplication and QGuiApplication destroy the main thread event
dispatcher (namely, in their destructors, not waiting for ~QObject to
deleteChildren()).

Drive-by relax the store in QThread::setEventDispatcher().

Fixes: QTBUG-137130
Pick-to: 6.9 6.8
Change-Id: I8845736c38a931af62e3fffdfd3554874df89e8e
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>

src/corelib/kernel/qabstracteventdispatcher.cpp

@@ -175,7 +175,11 @@ QAbstractEventDispatcher::QAbstractEventDispatcher(QAbstractEventDispatcherPriva
     Destroys the event dispatcher.
 */
 QAbstractEventDispatcher::~QAbstractEventDispatcher()
-{ }
+{
+    QThreadData *data = QThreadData::current();
+    if (data->eventDispatcher.loadRelaxed() == this)
+        data->eventDispatcher.storeRelaxed(nullptr);
+}
 
 /*!
     Returns a pointer to the event dispatcher object for the specified

src/corelib/thread/qthread.cpp

@@ -1236,7 +1236,7 @@ void QThread::setEventDispatcher(QAbstractEventDispatcher *eventDispatcher)
     } else {
         eventDispatcher->moveToThread(this);
         if (eventDispatcher->thread() == this) // was the move successful?
-            d->data->eventDispatcher = eventDispatcher;
+            d->data->eventDispatcher.storeRelaxed(eventDispatcher);
         else
             qWarning("QThread::setEventDispatcher: Could not move event dispatcher to target thread");
     }

tests/auto/corelib/kernel/qtimer/tst_qtimer.cpp

@@ -83,6 +83,7 @@ private slots:
     void singleShotToFunctors();
     void singleShot_chrono();
     void singleShot_static();
+    void singleShotDestructionBeforeEventDispatcher();
     void crossThreadSingleShotToFunctor_data();
     void crossThreadSingleShotToFunctor();
 #ifdef QT_BUILD_INTERNAL
@@ -1193,6 +1194,22 @@ void tst_QTimer::postedEventsShouldNotStarveTimers()
     QTRY_VERIFY_WITH_TIMEOUT(timeoutSpy.size() > 5, 100);
 }
 
+void tst_QTimer::singleShotDestructionBeforeEventDispatcher()
+{
+    // This makes sure the QSingleShotTimer doesn't cause a crash when the
+    // event dispatcher is deleted. As of the time of this test's writing, the
+    // QSST was parented to the dispatcher, so if it hadn't yet expired, it
+    // would be deleted by the QObject destructor, which is too late to
+    // unregister the timer.
+
+    auto thr = QThread::create([] {
+        QObject o;
+        QTimer::singleShot(300s, &o, [] {});
+    });
+    thr->start();
+    thr->wait();
+}
+
 struct DummyFunctor {
     static QThread *callThread;
     void operator()() {
@@ -1673,6 +1690,10 @@ void tst_QTimer::initMain()
 void tst_QTimer::cleanupTestCase()
 {
     delete s_staticSingleShotUser;
+
+    // Same as singleShotDestructionBeforeEventDispatcher() above, but for the
+    // main thread.
+    QTimer::singleShot(300s, this, [] {});
 }
 
 void tst_QTimer::singleShot_static()