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:
  - dropped the Q_CONSTINIT (doesn't exist in 5.15)
  - actual line conflicts in src/qml/qml/qqmlengine_p.h

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

4 files changed, 6 insertions, 4 deletions

diff --git a/src/qml/debugger/qqmldebug.cpp b/src/qml/debugger/qqmldebug.cpp
index 6bf49b9671..4af711171a 100644
--- a/src/qml/debugger/qqmldebug.cpp
+++ b/src/qml/debugger/qqmldebug.cpp
@@ -63,7 +63,7 @@ QQmlDebuggingEnabler::QQmlDebuggingEnabler(bool printWarning)
 {
     if (printWarning && !s_printedWarning.test_and_set(std::memory_order_relaxed))
         fprintf(stderr, "QML debugging is enabled. Only use this in a safe environment.\n");
-    QQmlEnginePrivate::qml_debugging_enabled = true;
+    QQmlEnginePrivate::qml_debugging_enabled.store(true, std::memory_order_relaxed);
 }
 
 /*!
diff --git a/src/qml/debugger/qqmldebugconnector.cpp b/src/qml/debugger/qqmldebugconnector.cpp
index 690d98c17f..72043ecd1f 100644
--- a/src/qml/debugger/qqmldebugconnector.cpp
+++ b/src/qml/debugger/qqmldebugconnector.cpp
@@ -111,7 +111,7 @@ QQmlDebugConnector *QQmlDebugConnector::instance()
     if (!params)
         return nullptr;
 
-    if (!QQmlEnginePrivate::qml_debugging_enabled) {
+    if (!QQmlEnginePrivate::qml_debugging_enabled.load(std::memory_order_relaxed)) {
         if (!params->arguments.isEmpty()) {
             qWarning().noquote() << QString::fromLatin1(
                                         "QML Debugger: Ignoring \"-qmljsdebugger=%1\". Debugging "
diff --git a/src/qml/qml/qqmlengine.cpp b/src/qml/qml/qqmlengine.cpp
index ba87c9810f..5c6a7b1f37 100644
--- a/src/qml/qml/qqmlengine.cpp
+++ b/src/qml/qml/qqmlengine.cpp
@@ -193,7 +193,7 @@ int qmlRegisterUncreatableMetaObject(const QMetaObject &staticMetaObject,
     \endcode
 */
 
-bool QQmlEnginePrivate::qml_debugging_enabled = false;
+std::atomic<bool> QQmlEnginePrivate::qml_debugging_enabled{false};
 bool QQmlEnginePrivate::s_designerMode = false;
 
 #if QT_VERSION < QT_VERSION_CHECK(6, 0, 0)
diff --git a/src/qml/qml/qqmlengine_p.h b/src/qml/qml/qqmlengine_p.h
index 5e78471a5b..147024044c 100644
--- a/src/qml/qml/qqmlengine_p.h
+++ b/src/qml/qml/qqmlengine_p.h
@@ -80,6 +80,8 @@
 #include <private/qjsengine_p.h>
 #include <private/qqmldirparser_p.h>
 
+#include <atomic>
+
 QT_BEGIN_NAMESPACE
 
 class QQmlContext;
@@ -267,7 +269,7 @@ public:
     static bool designerMode();
     static void activateDesignerMode();
 
-    static bool qml_debugging_enabled;
+    static std::atomic<bool> qml_debugging_enabled;
 
     mutable QMutex networkAccessManagerMutex;