QThreadDataPrivate: fix data race on canWait boolean.

postEvent() accesses it with the postEventList mutex locked, but
processEvent() was checking it without any mutex locked.

Change-Id: I31bbb50f7a1c337067b8e3de16ee7cd11400b517
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/kernel/qeventdispatcher_glib.cpp

@@ -259,10 +259,11 @@ static gboolean postEventSourcePrepare(GSource *s, gint *timeout)
     gint dummy;
     if (!timeout)
         timeout = &dummy;
-    *timeout = data->canWait ? -1 : 0;
+    const bool canWait = data->canWaitLocked();
+    *timeout = canWait ? -1 : 0;
 
     GPostEventSource *source = reinterpret_cast<GPostEventSource *>(s);
-    return (!data->canWait
+    return (!canWait
             || (source->serialNumber.load() != source->lastSerialNumber));
 }
 

src/corelib/kernel/qeventdispatcher_unix.cpp

@@ -589,7 +589,7 @@ bool QEventDispatcherUNIX::processEvents(QEventLoop::ProcessEventsFlags flags)
     QCoreApplicationPrivate::sendPostedEvents(0, 0, d->threadData);
 
     int nevents = 0;
-    const bool canWait = (d->threadData->canWait
+    const bool canWait = (d->threadData->canWaitLocked()
                           && !d->interrupt.load()
                           && (flags & QEventLoop::WaitForMoreEvents));
 

src/corelib/thread/qthread_p.h

@@ -231,6 +231,12 @@ public:
     void ref();
     void deref();
 
+    bool canWaitLocked()
+    {
+        QMutexLocker locker(&postEventList.mutex);
+        return canWait;
+    }
+
     QThread *thread;
     Qt::HANDLE threadId;
     bool quitNow;