QThread: Introduce static isMainThread() helper

Useful for QtCreator, as a replacement for
Utils::isMainThread() inside threadutils.h,
may serve for other projects, too.

Introduce static QCoreApplicationPrivate::theMainThreadId
atomic helper field holding the id of the main thread.

Change-Id: Iccc0302f423f47b5ecad86c4cd3de4d1ee36155f
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/kernel/qcoreapplication.cpp

@@ -525,6 +525,7 @@ void QCoreApplicationPrivate::eventDispatcherReady()
 }
 
 Q_CONSTINIT QBasicAtomicPointer<QThread> QCoreApplicationPrivate::theMainThread = Q_BASIC_ATOMIC_INITIALIZER(nullptr);
+Q_CONSTINIT QBasicAtomicPointer<void> QCoreApplicationPrivate::theMainThreadId = Q_BASIC_ATOMIC_INITIALIZER(nullptr);
 QThread *QCoreApplicationPrivate::mainThread()
 {
     Q_ASSERT(theMainThread.loadRelaxed() != nullptr);

src/corelib/kernel/qcoreapplication_p.h

@@ -108,6 +108,7 @@ public:
     virtual void quit();
 
     static QBasicAtomicPointer<QThread> theMainThread;
+    static QBasicAtomicPointer<void> theMainThreadId;
     static QThread *mainThread();
     static bool threadRequiresCoreApplication();
 

src/corelib/thread/qthread.cpp

@@ -66,8 +66,9 @@ QThreadData::~QThreadData()
     // safeguard the main thread here.. This fix is a bit crude, but it solves
     // the problem...
     if (this->thread.loadAcquire() == QCoreApplicationPrivate::theMainThread.loadAcquire()) {
-       QCoreApplicationPrivate::theMainThread.storeRelease(nullptr);
+        QCoreApplicationPrivate::theMainThread.storeRelease(nullptr);
-       QThreadData::clearCurrentThreadData();
+        QCoreApplicationPrivate::theMainThreadId.storeRelaxed(nullptr);
+        QThreadData::clearCurrentThreadData();
     }
 
     // ~QThread() sets thread to nullptr, so if it isn't null here, it's
@@ -425,6 +426,23 @@ QThread *QThread::currentThread()
     return data->thread.loadAcquire();
 }
 
+/*!
+    \since 6.8
+
+    Returns whether the currently executing thread is the main thread.
+
+    The main thread is the thread in which QCoreApplication was created.
+    This is usually the thread that called the \c{main()} function, but not necessarily so.
+    It is the thread that is processing the GUI events and in which graphical objects
+    (QWindow, QWidget) can be created.
+
+    \sa currentThread(), QCoreApplication::instance()
+*/
+bool QThread::isMainThread()
+{
+    return currentThreadId() == QCoreApplicationPrivate::theMainThreadId.loadRelaxed();
+}
+
 /*!
     Constructs a new QThread to manage a new thread. The \a parent
     takes ownership of the QThread. The thread does not begin
@@ -1066,8 +1084,10 @@ QThreadData *QThreadData::current(bool createIfNecessary)
         data->threadId.storeRelaxed(Qt::HANDLE(data->thread.loadAcquire()));
         data->deref();
         data->isAdopted = true;
-        if (!QCoreApplicationPrivate::theMainThread.loadAcquire())
+        if (!QCoreApplicationPrivate::theMainThread.loadAcquire()) {
             QCoreApplicationPrivate::theMainThread.storeRelease(data->thread.loadRelaxed());
+            QCoreApplicationPrivate::theMainThreadId.storeRelaxed(data->threadId.loadRelaxed());
+        }
     }
     return data;
 }

src/corelib/thread/qthread.h

@@ -30,6 +30,7 @@ class Q_CORE_EXPORT QThread : public QObject
 public:
     static Qt::HANDLE currentThreadId() noexcept Q_DECL_PURE_FUNCTION;
     static QThread *currentThread();
+    static bool isMainThread();
     static int idealThreadCount() noexcept;
     static void yieldCurrentThread();
 

src/corelib/thread/qthread_unix.cpp

@@ -191,8 +191,10 @@ QThreadData *QThreadData::current(bool createIfNecessary)
         data->deref();
         data->isAdopted = true;
         data->threadId.storeRelaxed(to_HANDLE(pthread_self()));
-        if (!QCoreApplicationPrivate::theMainThread.loadAcquire())
+        if (!QCoreApplicationPrivate::theMainThread.loadAcquire()) {
             QCoreApplicationPrivate::theMainThread.storeRelease(data->thread.loadRelaxed());
+            QCoreApplicationPrivate::theMainThreadId.storeRelaxed(data->threadId.loadRelaxed());
+        }
     }
     return data;
 }

src/corelib/thread/qthread_win.cpp

@@ -89,6 +89,7 @@ QThreadData *QThreadData::current(bool createIfNecessary)
 
         if (!QCoreApplicationPrivate::theMainThread) {
             QCoreApplicationPrivate::theMainThread = threadData->thread.loadRelaxed();
+            QCoreApplicationPrivate::theMainThreadId.storeRelaxed(threadData->threadId.loadRelaxed());
         } else {
             HANDLE realHandle = INVALID_HANDLE_VALUE;
             DuplicateHandle(GetCurrentProcess(),