QTlsBackend: Fix living QObjects after QCoreApplication shutdown

Since switching to the plugin bases system for the tls backends,
Qt again retains some QObjects after QCoreApplication shutdown.
This was previously fixed in QTBUG-84234, so make sure we destroy the
newly introduced QObjects as well.

Task-number: QTBUG-84234
Pick-to: 6.3
Change-Id: I1aaea2c90f7d55793c19259be4f9173b4befb246
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>

src/network/ssl/qsslsocket.cpp

@@ -3118,7 +3118,12 @@ QTlsBackend *QSslSocketPrivate::tlsBackendInUse()
         return nullptr;
     }
 
-    return tlsBackend = QTlsBackend::findBackend(activeBackendName);
+    tlsBackend = QTlsBackend::findBackend(activeBackendName);
+    QObject::connect(tlsBackend, &QObject::destroyed, [] {
+        const QMutexLocker locker(&backendMutex);
+        tlsBackend = nullptr;
+    });
+    return tlsBackend;
 }
 
 /*!

src/network/ssl/qtlsbackend.cpp

@@ -52,6 +52,7 @@
 
 #include <QtCore/private/qfactoryloader_p.h>
 
+#include "QtCore/qapplicationstatic.h"
 #include <QtCore/qbytearray.h>
 #include <QtCore/qmutex.h>
 
@@ -60,8 +61,8 @@
 
 QT_BEGIN_NAMESPACE
 
-Q_GLOBAL_STATIC_WITH_ARGS(QFactoryLoader, loader,
-                          (QTlsBackend_iid, QStringLiteral("/tls")))
+Q_APPLICATION_STATIC(QFactoryLoader, loader, QTlsBackend_iid,
+                     QStringLiteral("/tls"))
 
 namespace {
 
@@ -92,7 +93,7 @@ public:
 
         static QBasicMutex mutex;
         const QMutexLocker locker(&mutex);
-        if (loaded)
+        if (backends.size())
             return true;
 
 #if QT_CONFIG(library)
@@ -102,7 +103,7 @@ public:
         while (loader->instance(index))
             ++index;
 
-        return loaded = true;
+        return true;
     }
 
     QList<QString> backendNames()
@@ -139,7 +140,6 @@ public:
 private:
     std::vector<QTlsBackend *> backends;
     QMutex collectionMutex;
-    bool loaded = false;
 };
 
 } // Unnamed namespace
@@ -202,6 +202,10 @@ QTlsBackend::QTlsBackend()
 {
     if (backends())
         backends->addBackend(this);
+
+    connect(QCoreApplication::instance(), &QCoreApplication::destroyed, this, [this] {
+        delete this;
+    });
 }
 
 /*!