Use QThread::isMainThread() in a few places

It's thread-safe, whereas trying to load qApp isn't in Qt 6.x (will be
in 7.0) and dereferencing it to call QObject::thread() will probably
never be. It's also faster, being a single function call instead of two
or three.

This is not an exhaustive search, it's just a few places I found while
searching for QThread::instance().

Pick-to: 6.8
Change-Id: I3b4e1c75bb3966e2cd2dfffd79bfc8a40f6cf40b
Reviewed-by: Ivan Solovev <ivan.solovev@qt.io>
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>
(cherry picked from commit 6fb39ca2cbb111469ce09c8342997eb5375e3239)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/assets/downloader/tasking/qprocesstask.cpp

@@ -217,7 +217,7 @@ ProcessReaper::ProcessReaper()
 
 ProcessReaper::~ProcessReaper()
 {
-    if (QThread::currentThread() != qApp->thread())
+    if (!QThread::isMainThread())
         qWarning() << "Destructing process reaper from non-main thread.";
 
     instance()->m_private->waitForFinished();

src/dbus/qdbusconnectionmanager.cpp

@@ -33,7 +33,7 @@ QDBusConnectionPrivate *QDBusConnectionManager::busConnection(QDBusConnection::B
 
     // we'll start in suspended delivery mode if we're in the main thread
     // (the event loop will resume delivery)
-    bool suspendedDelivery = qApp && qApp->thread() == QThread::currentThread();
+    bool suspendedDelivery = QThread::isMainThread();
 
     const auto locker = qt_scoped_lock(defaultBusMutex);
     if (defaultBuses[type])

src/dbus/qdbusintegrator.cpp

@@ -2028,7 +2028,7 @@ public:
         // if this call is running on the main thread, we have a much lower
         // tolerance for delay because any long-term delay will wreck user
         // interactivity.
-        if (qApp && qApp->thread() == QThread::currentThread())
+        if (QThread::isMainThread())
             m_maxCallTimeoutMs = mainThreadWarningAmount;
         else
             m_maxCallTimeoutMs = otherThreadWarningAmount;

src/gui/image/qpixmap.cpp

@@ -53,7 +53,7 @@ static bool qt_pixmap_thread_test()
         return false;
     }
     if (QGuiApplicationPrivate::instance()
-        && qApp->thread() != QThread::currentThread()
+        && !QThread::isMainThread()
         && !QGuiApplicationPrivate::platformIntegration()->hasCapability(QPlatformIntegration::ThreadedPixmaps)) {
         qWarning("QPixmap: It is not safe to use pixmaps outside the GUI thread on this platform");
         return false;

src/gui/image/qpixmapcache.cpp

@@ -71,7 +71,7 @@ static inline qsizetype cost(const QPixmap &pixmap)
 
 static inline bool qt_pixmapcache_thread_test()
 {
-    if (Q_LIKELY(QCoreApplication::instance() && QThread::currentThread() == QCoreApplication::instance()->thread()))
+    if (Q_LIKELY(QThread::isMainThread()))
         return true;
 
     return false;

src/gui/kernel/qoffscreensurface.cpp

@@ -121,7 +121,7 @@ void QOffscreenSurface::create()
         d->platformOffscreenSurface = QGuiApplicationPrivate::platformIntegration()->createPlatformOffscreenSurface(this);
         // No platform offscreen surface, fallback to an invisible window
         if (!d->platformOffscreenSurface) {
-            if (QThread::currentThread() != qGuiApp->thread())
+            if (!QThread::isMainThread())
                 qWarning("Attempting to create QWindow-based QOffscreenSurface outside the gui thread. Expect failures.");
             d->offscreenWindow = new QWindow(d->screen);
             // Make the window frameless to prevent Windows from enlarging it, should it

src/gui/kernel/qwindow.cpp

@@ -2840,7 +2840,7 @@ void QWindowPrivate::maybeSynthesizeContextMenuEvent(QMouseEvent *event)
 */
 void QWindow::requestUpdate()
 {
-    Q_ASSERT_X(QThread::currentThread() == QCoreApplication::instance()->thread(),
+    Q_ASSERT_X(QThread::isMainThread(),
         "QWindow", "Updates can only be scheduled from the GUI (main) thread");
 
     Q_D(QWindow);

src/gui/kernel/qwindowsysteminterface.cpp

@@ -95,7 +95,7 @@ template<>
 template<typename EventType, typename ...Args>
 bool QWindowSystemHelper<QWindowSystemInterface::SynchronousDelivery>::handleEvent(Args ...args)
 {
-    if (QThread::currentThread() == QGuiApplication::instance()->thread()) {
+    if (QThread::isMainThread()) {
         EventType event(args...);
         // Process the event immediately on the Gui thread and return the accepted state
         if (QWindowSystemInterfacePrivate::eventHandler) {

src/gui/text/qtextdocument.cpp

@@ -2331,7 +2331,7 @@ QVariant QTextDocument::loadResource(int type, const QUrl &name)
 
     if (!r.isNull()) {
         if (type == ImageResource && r.userType() == QMetaType::QByteArray) {
-            if (qApp->thread() != QThread::currentThread()) {
+            if (!QThread::isMainThread()) {
                 // must use images in non-GUI threads
                 QImage image;
                 image.loadFromData(r.toByteArray());

src/gui/vulkan/qvulkanwindow.cpp

@@ -2152,7 +2152,7 @@ void QVulkanWindowPrivate::endFrame()
  */
 void QVulkanWindow::frameReady()
 {
-    Q_ASSERT_X(QThread::currentThread() == QCoreApplication::instance()->thread(),
+    Q_ASSERT_X(QThread::isMainThread(),
         "QVulkanWindow", "frameReady() can only be called from the GUI (main) thread");
 
     Q_D(QVulkanWindow);

src/widgets/dialogs/qerrormessage.cpp

@@ -210,7 +210,7 @@ static void jump(QtMsgType t, const QMessageLogContext &context, const QString &
         rich.chop(4);
 
     if (!metFatal) {
-        if (QThread::currentThread() == qApp->thread()) {
+        if (QThread::isMainThread()) {
             qtMessageHandler->showMessage(rich);
         } else {
             QMetaObject::invokeMethod(qtMessageHandler,

src/widgets/kernel/qwidget.cpp

@@ -955,7 +955,7 @@ void QWidgetPrivate::init(QWidget *parentWidget, Qt::WindowFlags f)
 
 #if QT_CONFIG(thread)
     if (!parent) {
-        Q_ASSERT_X(q->thread() == qApp->thread(), "QWidget",
+        Q_ASSERT_X(QThread::isMainThread(), "QWidget",
                    "Widgets must be created in the GUI thread.");
     }
 #endif