Fix living QObject after shutdown of QCoreApplication

QThreadPool is a QObject and must be deleted if the QCoreApplication
is being destroyed to release the underlying ThreadData.
A Q_GLOBAL_STATIC won't release any memory is not able to
manually release it.

Pick-to: 5.15
Task-number: QTBUG-84234
Change-Id: Ia82bcff2b564b753ed687f025ff86fa1bed1e64c
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/kernel/qcoreapplication.cpp

@@ -892,8 +892,10 @@ QCoreApplication::~QCoreApplication()
     } QT_CATCH (...) {
         // swallow the exception, since destructors shouldn't throw
     }
-    if (globalThreadPool)
+    if (globalThreadPool) {
         globalThreadPool->waitForDone();
+        delete globalThreadPool;
+    }
 #endif
 
 #ifndef QT_NO_QOBJECT

src/corelib/thread/qthreadpool.cpp

@@ -40,13 +40,12 @@
 #include "qthreadpool.h"
 #include "qthreadpool_p.h"
 #include "qdeadlinetimer.h"
+#include "qcoreapplication.h"
 
 #include <algorithm>
 
 QT_BEGIN_NAMESPACE
 
-Q_GLOBAL_STATIC(QThreadPool, theInstance)
-
 /*
     QThread wrapper, provides synchronization against a ThreadPool
 */
@@ -478,7 +477,13 @@ QThreadPool::~QThreadPool()
 */
 QThreadPool *QThreadPool::globalInstance()
 {
-    return theInstance();
+    static QPointer<QThreadPool> theInstance;
+    static QBasicMutex theMutex;
+
+    const QMutexLocker locker(&theMutex);
+    if (theInstance.isNull() && !QCoreApplication::closingDown())
+        theInstance = new QThreadPool();
+    return theInstance;
 }
 
 /*!