QObject: Let moveToThread return succcess state

Add bool QObjectPrivate::moveToThread() which returns the success state
of the attempt to move the object to the new thread. For Qt7 the public
signature of QObject::moveToThread() should be changed to directly
return this value.

[ChangeLog][QtCore][QObject] QObject::moveToThread() now returns a
boolean success state.

Change-Id: I8441c8155a76b804b473ab9c45c2be6840ef1677
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Marc Mutz <marc.mutz@qt.io>
(cherry picked from commit e752c77b35185fa51cdc3684329a9733abe2be3b)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/corelib/compat/removed_api.cpp

@@ -852,6 +852,11 @@ QObject *qt_qFindChild_helper(const QObject *parent, const QString &name, const
     return qt_qFindChild_helper(parent, QAnyStringView{name}, mo, options);
 }
 
+void QObject::moveToThread(QThread *targetThread)
+{
+    moveToThread(targetThread, QT6_CALL_NEW_OVERLOAD);
+}
+
 #include "qobjectdefs.h"
 
 int QMetaObject::indexOfEnumerator(const char *name) const

src/corelib/kernel/qobject.cpp

@@ -1628,9 +1628,9 @@ QThread *QObject::thread() const
 }
 
 /*!
-    Changes the thread affinity for this object and its children. The
-    object cannot be moved if it has a parent. Event processing will
-    continue in the \a targetThread.
+    Changes the thread affinity for this object and its children and
+    returns \c true on success. The object cannot be moved if it has a
+    parent. Event processing will continue in the \a targetThread.
 
     To move an object to the main thread, use QApplication::instance()
     to retrieve a pointer to the current application, and then use
@@ -1667,26 +1667,26 @@ QThread *QObject::thread() const
 
     \sa thread()
  */
-void QObject::moveToThread(QThread *targetThread)
+bool QObject::moveToThread(QThread *targetThread QT6_IMPL_NEW_OVERLOAD_TAIL)
 {
     Q_D(QObject);
 
     if (d->threadData.loadRelaxed()->thread.loadAcquire() == targetThread) {
         // object is already in this thread
-        return;
+        return true;
     }
 
     if (d->parent != nullptr) {
         qWarning("QObject::moveToThread: Cannot move objects with a parent");
-        return;
+        return false;
     }
     if (d->isWidget) {
         qWarning("QObject::moveToThread: Widgets cannot be moved to a new thread");
-        return;
+        return false;
     }
     if (!d->bindingStorage.isEmpty()) {
         qWarning("QObject::moveToThread: Can not move objects that contain bindings or are used in bindings to a new thread.");
-        return;
+        return false;
     }
 
     QThreadData *currentData = QThreadData::current();
@@ -1706,7 +1706,7 @@ void QObject::moveToThread(QThread *targetThread)
                  "DYLD_PRINT_LIBRARIES=1 and check that only one set of binaries are being loaded.");
 #endif
 
-        return;
+        return false;
     }
 
     // prepare to move
@@ -1740,6 +1740,7 @@ void QObject::moveToThread(QThread *targetThread)
 
     // now currentData can commit suicide if it wants to
     currentData->deref();
+    return true;
 }
 
 void QObjectPrivate::moveToThread_helper()

src/corelib/kernel/qobject.h

@@ -135,7 +135,10 @@ public:
     bool blockSignals(bool b) noexcept;
 
     QThread *thread() const;
+#if QT_CORE_REMOVED_SINCE(6, 7)
     void moveToThread(QThread *thread);
+#endif
+    bool moveToThread(QThread *thread QT6_DECL_NEW_OVERLOAD_TAIL);
 
     int startTimer(int interval, Qt::TimerType timerType = Qt::CoarseTimer);
     int startTimer(std::chrono::milliseconds time, Qt::TimerType timerType = Qt::CoarseTimer);

tests/auto/corelib/kernel/qobject/tst_qobject.cpp

@@ -1870,13 +1870,15 @@ void tst_QObject::moveToThread()
         QObject *child = new QObject(object);
         QCOMPARE(object->thread(), currentThread);
         QCOMPARE(child->thread(), currentThread);
-        object->moveToThread(0);
+        QVERIFY(object->moveToThread(nullptr));
         QCOMPARE(object->thread(), (QThread *)0);
         QCOMPARE(child->thread(), (QThread *)0);
-        object->moveToThread(currentThread);
+        QVERIFY(object->moveToThread(currentThread));
         QCOMPARE(object->thread(), currentThread);
         QCOMPARE(child->thread(), currentThread);
-        object->moveToThread(0);
+        QTest::ignoreMessage(QtWarningMsg, "QObject::moveToThread: Cannot move objects with a parent");
+        QVERIFY(!child->moveToThread(nullptr));
+        QVERIFY(object->moveToThread(nullptr));
         QCOMPARE(object->thread(), (QThread *)0);
         QCOMPARE(child->thread(), (QThread *)0);
         // can delete an object with no thread anywhere