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.

Manual conflict resolutions:
  - Removed Q_CONSTINIT, which doesn't exist in 6.2, yet.

Change-Id: I0305ab55be86a0e286016a3d1d97ee9bc0e28070
Reviewed-by: Ulf Hermann <ulf.hermann@qt.io>
(cherry picked from commit 0434e54b55d9a31c3b5331856bab639b7149058c)

1 files changed, 3 insertions, 1 deletions

diff --git a/src/qml/qml/qqmlengine_p.h b/src/qml/qml/qqmlengine_p.h
index 12d734574b..257b5371b6 100644
--- a/src/qml/qml/qqmlengine_p.h
+++ b/src/qml/qml/qqmlengine_p.h
@@ -84,6 +84,8 @@
 
 #include <qproperty.h>
 
+#include <atomic>
+
 QT_BEGIN_NAMESPACE
 
 class QQmlContext;
@@ -279,7 +281,7 @@ public:
     static bool designerMode();
     static void activateDesignerMode();
 
-    static bool qml_debugging_enabled;
+    static std::atomic<bool> qml_debugging_enabled;
 
     mutable QMutex networkAccessManagerMutex;