QThread: Introduce isCurrentThread

This allows a more efficient way of checking whether a thread is the
currently executing one (without using private API).

Change-Id: I007edae6b258d7e42e901fa720d4f3cf9fe25a49
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>
Reviewed-by: Jarek Kobus <jaroslaw.kobus@qt.io>
Reviewed-by: David Faure <david.faure@kdab.com>

src/corelib/thread/qthread.cpp

@@ -913,6 +913,36 @@ int QThread::loopLevel() const
     return d->data->eventLoops.size();
 }
 
+/*!
+    \internal
+    Returns the thread handle of this thread.
+    It can be compared with the return value of currentThreadId().
+
+    This is used to implement isCurrentThread, and might be useful
+    for debugging (e.g. by comparing the value in gdb with info threads).
+
+    \note Thread handles of destroyed threads might be reused by the
+    operating system. Storing the return value of this function can
+    therefore give surprising results if it outlives the QThread object
+    (threads claimed to be the same even if they aren't).
+*/
+Qt::HANDLE QThreadPrivate::threadId() const
+{
+    return data->threadId.loadRelaxed();
+}
+
+/*!
+    \since 6.8
+    Returns true if this thread is QThread::currentThread.
+
+    \sa currentThreadId()
+*/
+bool QThread::isCurrentThread() const
+{
+    Q_D(const QThread);
+    return QThread::currentThreadId() == d->threadId();
+}
+
 #else // QT_CONFIG(thread)
 
 QThread::QThread(QObject *parent)
@@ -985,6 +1015,11 @@ QThread *QThread::currentThread()
     return QThreadData::current()->thread.loadAcquire();
 }
 
+bool QThread::isCurrentThread() const
+{
+    return true;
+}
+
 int QThread::idealThreadCount() noexcept
 {
     return 1;

src/corelib/thread/qthread.h

@@ -68,6 +68,8 @@ public:
     bool event(QEvent *event) override;
     int loopLevel() const;
 
+    bool isCurrentThread() const;
+
     template <typename Function, typename... Args>
     [[nodiscard]] static QThread *create(Function &&f, Args &&... args);
 

src/corelib/thread/qthread_p.h

@@ -179,6 +179,7 @@ public:
     ~QThreadPrivate();
 
     void setPriority(QThread::Priority prio);
+    Qt::HANDLE threadId() const;
 
     mutable QMutex mutex;
     QAtomicInt quitLockRef;

tests/auto/corelib/thread/qthread/tst_qthread.cpp

@@ -137,11 +137,13 @@ class Current_Thread : public QThread
 public:
     Qt::HANDLE id;
     QThread *thread;
+    bool runCalledInCurrentThread = false;
 
     void run() override
     {
         id = QThread::currentThreadId();
         thread = QThread::currentThread();
+        runCalledInCurrentThread = thread->isCurrentThread();
     }
 };
 
@@ -276,6 +278,11 @@ void tst_QThread::currentThreadId()
     QVERIFY(thread.wait(five_minutes));
     QVERIFY(thread.id != nullptr);
     QVERIFY(thread.id != QThread::currentThreadId());
+    QVERIFY(!thread.isCurrentThread());
+    QVERIFY(!thread.thread->isCurrentThread());
+    QVERIFY(thread.QThread::thread()->isCurrentThread());
+    QVERIFY(thread.runCalledInCurrentThread);
+    QVERIFY(qApp->thread()->isCurrentThread());
 }
 
 void tst_QThread::currentThread()