From 07a15916cc3c42616251b8ce3345bd77f248d9d8 Mon Sep 17 00:00:00 2001
From: Thiago Macieira <thiago.macieira@intel.com>
Date: Sat, 8 Feb 2025 15:31:55 -0800
Subject: [PATCH] QThreadStorage: add a warning suggesting use of thread_local
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thread_local can still be very broken for non-trivial types, 14 years
since C++11 was ratified, but it works plenty well for trivial types. So
suggest users use that (with a macro to opt-out).

[ChangeLog][QtCore][QThreadStorage] QThreadStorage will print a warning
suggesting the use of thread_local for use with primitive types that are
not pointers. In fact, switching to the C++11 functionality is advisable
for all types, but some runtimes have bugs concerning destruction order.
This warning can be suppressed by #define'ing the macro
Q_NO_THREAD_STORAGE_TRIVIAL_WARNING.

This code is actually very old (looks like it has pre-C++98
compatibility support for MSVC 6.0) and could use a refactor, but it's
not worth my time to do it or review it for such old content. Stop using
QThreadStorage if you can.

Task-number: QTBUG-117850
Change-Id: I906ba7872388fc578c78fffd92231118decfd67e
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>
Reviewed-by: Marc Mutz <marc.mutz@qt.io>
---
 src/corelib/thread/qthreadstorage.h               | 15 ++++++++++++++-
 .../qthreadstorage/crashonexit/crashOnExit.cpp    |  1 +
 .../thread/qthreadstorage/tst_qthreadstorage.cpp  |  2 ++
 3 files changed, 17 insertions(+), 1 deletion(-)

diff --git a/src/corelib/thread/qthreadstorage.h b/src/corelib/thread/qthreadstorage.h
index 35ba0700796..32dc29f0c26 100644
--- a/src/corelib/thread/qthreadstorage.h
+++ b/src/corelib/thread/qthreadstorage.h
@@ -14,6 +14,18 @@ QT_BEGIN_NAMESPACE
 
 #if QT_CONFIG(thread)
 
+template <bool ShouldWarn> struct QThreadStorageTraits
+{
+    static constexpr void warnAboutTrivial() {}
+};
+template <> struct QThreadStorageTraits<true>
+{
+#ifndef Q_NO_THREAD_STORAGE_TRIVIAL_WARNING
+    Q_DECL_DEPRECATED_X("QThreadStorage used with a trivial non-pointer type; consider using thread_local")
+#endif
+    static constexpr void warnAboutTrivial() noexcept {}
+};
+
 class Q_CORE_EXPORT QThreadStorageData
 {
 public:
@@ -87,6 +99,7 @@ template <class T>
 class QThreadStorage
 {
 private:
+    using Trait = QThreadStorageTraits<std::is_trivial_v<T> && !std::is_pointer_v<T>>;
     QThreadStorageData d;
 
     Q_DISABLE_COPY(QThreadStorage)
@@ -95,7 +108,7 @@ private:
     { qThreadStorage_deleteData(x, reinterpret_cast<T*>(0)); }
 
 public:
-    inline QThreadStorage() : d(deleteData) { }
+    inline QThreadStorage() : d(deleteData) { Trait::warnAboutTrivial(); }
     inline ~QThreadStorage() { }
 
     inline bool hasLocalData() const
diff --git a/tests/auto/corelib/thread/qthreadstorage/crashonexit/crashOnExit.cpp b/tests/auto/corelib/thread/qthreadstorage/crashonexit/crashOnExit.cpp
index 3b3a4d4813c..52dc5e22ac7 100644
--- a/tests/auto/corelib/thread/qthreadstorage/crashonexit/crashOnExit.cpp
+++ b/tests/auto/corelib/thread/qthreadstorage/crashonexit/crashOnExit.cpp
@@ -1,6 +1,7 @@
 // Copyright (C) 2016 The Qt Company Ltd.
 // SPDX-License-Identifier: LicenseRef-Qt-Commercial OR GPL-3.0-only
 
+#define Q_NO_THREAD_STORAGE_TRIVIAL_WARNING
 #include <QtCore/QCoreApplication>
 #include <QtCore/QThreadStorage>
 
diff --git a/tests/auto/corelib/thread/qthreadstorage/tst_qthreadstorage.cpp b/tests/auto/corelib/thread/qthreadstorage/tst_qthreadstorage.cpp
index 0449909c889..e3734d27f7f 100644
--- a/tests/auto/corelib/thread/qthreadstorage/tst_qthreadstorage.cpp
+++ b/tests/auto/corelib/thread/qthreadstorage/tst_qthreadstorage.cpp
@@ -1,6 +1,8 @@
 // Copyright (C) 2016 The Qt Company Ltd.
 // SPDX-License-Identifier: LicenseRef-Qt-Commercial OR GPL-3.0-only
 
+#define Q_NO_THREAD_STORAGE_TRIVIAL_WARNING
+
 #include <QTest>
 #if QT_CONFIG(process)
 #include <QProcess>