Fix potential race condition in QtConcurrent blocking methods

QtConcurrent::blocking*() methods are using the ExceptionStore directly,
which is not thread safe. In case if there's an exception thrown from
multiple threads there may be a race condition. Added a lock to avoid
that.

Change-Id: I5de9928f91f5f43951b9bf9c4594694dc0ca0328
Reviewed-by: Vitaly Fanaskov <vitaly.fanaskov@qt.io>
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>

src/concurrent/qtconcurrentthreadengine.cpp

@@ -322,10 +322,13 @@ void ThreadEngineBase::run() // implements QRunnable.
 
 void ThreadEngineBase::handleException(const QException &exception)
 {
-    if (futureInterface)
+    if (futureInterface) {
         futureInterface->reportException(exception);
-    else if (!exceptionStore.hasException())
-        exceptionStore.setException(exception);
+    } else {
+        QMutexLocker lock(&mutex);
+        if (!exceptionStore.hasException())
+            exceptionStore.setException(exception);
+    }
 }
 #endif
 

src/concurrent/qtconcurrentthreadengine.h

@@ -121,6 +121,7 @@ protected:
     QThreadPool *threadPool;
     ThreadEngineBarrier barrier;
     QtPrivate::ExceptionStore exceptionStore;
+    QBasicMutex mutex;
 };