Fix race condition on QQmlEnginePrivate::qml_debugging_enabled

C++11 allows static dynamic initialization from different TUs to
happen concurrently, which means the QQmlDebuggingEnabler ctor must be
re-entrant and synchronized with other users of qml_debugging_enabled.

Thankfully, this is just a flag, so the fix is to simply make it
atomic<> and use relaxed loads and stores on it.

Pick-to: 6.3 6.2 5.15
Change-Id: I0305ab55be86a0e286016a3d1d97ee9bc0e28070
Reviewed-by: Ulf Hermann <ulf.hermann@qt.io>

1 files changed, 3 insertions, 1 deletions

diff --git a/src/qml/qml/qqmlengine_p.h b/src/qml/qml/qqmlengine_p.h
index b914afdaf1..e8211b1a38 100644
--- a/src/qml/qml/qqmlengine_p.h
+++ b/src/qml/qml/qqmlengine_p.h
@@ -80,6 +80,8 @@
 #include <QtCore/qstring.h>
 #include <QtCore/qthread.h>
 
+#include <atomic>
+
 QT_BEGIN_NAMESPACE
 
 class QNetworkAccessManager;
@@ -236,7 +238,7 @@ public:
     static bool designerMode();
     static void activateDesignerMode();
 
-    static bool qml_debugging_enabled;
+    static std::atomic<bool> qml_debugging_enabled;
 
     mutable QMutex networkAccessManagerMutex;