Synchronize QEventDispatcherWin32::processEvents() with QCoreApplication

QCoreApplication has a special internal mechanism to control whether
the event dispatcher should block after delivering the posted events.

To handle queued connections in nested loops properly, we should use
that functionality.

Pick-to: 5.15
Fixes: QTBUG-85981
Change-Id: I124179a23b26a995cf95ed379e97bfa62c95f42a
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@gmx.de>
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>

2 files changed, 29 insertions, 1 deletions

diff --git a/src/corelib/kernel/qeventdispatcher_win.cpp b/src/corelib/kernel/qeventdispatcher_win.cpp
index 7e6d478473..b3ae226e58 100644
--- a/src/corelib/kernel/qeventdispatcher_win.cpp
+++ b/src/corelib/kernel/qeventdispatcher_win.cpp
@@ -529,6 +529,7 @@ bool QEventDispatcherWin32::processEvents(QEventLoop::ProcessEventsFlags flags)
     // QCoreApplication::sendPostedEvents() takes care about recursions.
     sendPostedEvents();
 
+    auto threadData = d->threadData.loadRelaxed();
     bool canWait;
     bool retVal = false;
     do {
@@ -599,7 +600,8 @@ bool QEventDispatcherWin32::processEvents(QEventLoop::ProcessEventsFlags flags)
         // wait for message
         canWait = (!retVal
                    && !d->interrupt.loadRelaxed()
-                   && (flags & QEventLoop::WaitForMoreEvents));
+                   && flags.testFlag(QEventLoop::WaitForMoreEvents)
+                   && threadData->canWaitLocked());
         if (canWait) {
             emit aboutToBlock();
             MsgWaitForMultipleObjectsEx(0, NULL, INFINITE, QS_ALLINPUT, MWMO_ALERTABLE | MWMO_INPUTAVAILABLE);
diff --git a/tests/auto/corelib/kernel/qeventdispatcher/tst_qeventdispatcher.cpp b/tests/auto/corelib/kernel/qeventdispatcher/tst_qeventdispatcher.cpp
index 85a2dae3b6..7aadd14466 100644
--- a/tests/auto/corelib/kernel/qeventdispatcher/tst_qeventdispatcher.cpp
+++ b/tests/auto/corelib/kernel/qeventdispatcher/tst_qeventdispatcher.cpp
@@ -67,6 +67,7 @@ private slots:
     void sendPostedEvents_data();
     void sendPostedEvents();
     void processEventsOnlySendsQueuedEvents();
+    void postedEventsPingPong();
     void eventLoopExit();
 };
 
@@ -349,6 +350,31 @@ void tst_QEventDispatcher::processEventsOnlySendsQueuedEvents()
     QCOMPARE(object.eventsReceived, 4);
 }
 
+void tst_QEventDispatcher::postedEventsPingPong()
+{
+    QEventLoop mainLoop;
+
+    // We need to have at least two levels of nested loops
+    // for the posted event to get stuck (QTBUG-85981).
+    QMetaObject::invokeMethod(this, [this, &mainLoop]() {
+        QMetaObject::invokeMethod(this, [&mainLoop]() {
+            // QEventLoop::quit() should be invoked on the next
+            // iteration of mainLoop.exec().
+            QMetaObject::invokeMethod(&mainLoop, &QEventLoop::quit,
+                                      Qt::QueuedConnection);
+        }, Qt::QueuedConnection);
+        mainLoop.processEvents();
+    }, Qt::QueuedConnection);
+
+    // We should use Qt::CoarseTimer on Windows, to prevent event
+    // dispatcher from sending a posted event.
+    QTimer::singleShot(500, Qt::CoarseTimer, [&mainLoop]() {
+        mainLoop.exit(1);
+    });
+
+    QCOMPARE(mainLoop.exec(), 0);
+}
+
 void tst_QEventDispatcher::eventLoopExit()
 {
     // This test was inspired by QTBUG-79477. A particular