Short live QCoreApplication::instanceExists()

This is a thread-safe version of

    QCoreApplication::instance() != nullptr

for Qt 6.x, because QCoreApplication::self is not atomic and thus
reading it from outside the main thread could be a data race.

That's not to say it always is: if by construction the code can only run
in the main thread or while QCoreApplication definitely exists, that's
safe. Therefore, this commit focuses on places that are meant to be used
in multi-threaded environment (ruling out most of QtGui and QtWidgets)
or where the code was going to dereference the returned pointer anyway.

Change-Id: I6fc556c5fe5cbe0b5902fffdfb6b3bb345b0ee50
Reviewed-by: Marc Mutz <marc.mutz@qt.io>

src/corelib/global/qlibraryinfo.cpp

@@ -62,14 +62,14 @@ QLibrarySettings::QLibrarySettings() : paths(false), reloadOnQAppAvailable(false
 
 QSettings *QLibrarySettings::configuration()
 {
-    if (reloadOnQAppAvailable && QCoreApplication::instance() != nullptr)
+    if (reloadOnQAppAvailable && QCoreApplication::instanceExists())
         load();
     return settings.get();
 }
 
 bool QLibrarySettings::havePaths()
 {
-    if (reloadOnQAppAvailable && QCoreApplication::instance() != nullptr)
+    if (reloadOnQAppAvailable && QCoreApplication::instanceExists())
         load();
     return paths;
 }
@@ -78,7 +78,7 @@ void QLibrarySettings::load()
 {
     // If we get any settings here, those won't change when the application shows up.
     settings = findConfiguration();
-    reloadOnQAppAvailable = !settings && !QCoreApplication::instance();
+    reloadOnQAppAvailable = !settings && !QCoreApplication::instanceExists();
 
     if (settings) {
         // This code needs to be in the regular library, as otherwise a qt.conf that
@@ -121,7 +121,7 @@ static std::unique_ptr<QSettings> findConfiguration()
         }
     }
 #endif
-    if (QCoreApplication::instance()) {
+    if (QCoreApplication::instanceExists()) {
         QString pwd = QCoreApplication::applicationDirPath();
         qtconfig = pwd + u"/qt" QT_STRINGIFY(QT_VERSION_MAJOR) ".conf"_s;
         if (QFile::exists(qtconfig))
@@ -270,7 +270,7 @@ QVersionNumber QLibraryInfo::version() noexcept
 
 static QString prefixFromAppDirHelper()
 {
-    if (QCoreApplication::instance()) {
+    if (QCoreApplication::instanceExists()) {
 #ifdef Q_OS_DARWIN
         CFBundleRef bundleRef = CFBundleGetMainBundle();
         if (bundleRef) {

src/corelib/kernel/qcoreapplication.cpp

@@ -144,6 +144,21 @@ Q_CONSTINIT QCoreApplication *QCoreApplication::self = nullptr;
 Q_CONSTINIT static QBasicAtomicPointer<QCoreApplication> g_self = nullptr;
 #  undef qApp
 #  define qApp g_self.loadRelaxed()
+
+/*!
+    \internal
+
+    This function is a Qt 6 thread-safe (no data races) version of:
+    \code
+        QCoreApplication::instance() != nullptr
+    \endcode
+
+    We may remove it in Qt 7.0 because the above will be thread-safe.
+*/
+bool QCoreApplication::instanceExists() noexcept
+{
+    return qApp != nullptr;
+}
 #endif
 
 #if !defined(Q_OS_WIN) || defined(QT_BOOTSTRAPPED)
@@ -276,7 +291,7 @@ void qAddPreRoutine(QtStartUpFunction p)
         return;
 
     if (preRoutinesCalled) {
-        Q_ASSERT(QCoreApplication::instance());
+        Q_ASSERT(qApp);
         p();
     }
 
@@ -467,11 +482,6 @@ QCoreApplicationPrivate::~QCoreApplicationPrivate()
 #endif
 }
 
-bool QCoreApplicationPrivate::isAlive() noexcept
-{
-    return qApp != nullptr;
-}
-
 #ifndef QT_NO_QOBJECT
 
 void QCoreApplicationPrivate::cleanupThreadData()

src/corelib/kernel/qcoreapplication.h

@@ -96,8 +96,10 @@ public:
 
 #if QT_VERSION >= QT_VERSION_CHECK(7, 0, 0)
     static QCoreApplication *instance() noexcept { return self.loadRelaxed(); }
+    static bool instanceExists() noexcept { return instance() != nullptr; }
 #else
     static QCoreApplication *instance() noexcept { return self; }
+    static bool instanceExists() noexcept;
 #endif
 
 #ifndef QT_NO_QOBJECT

src/corelib/kernel/qcoreapplication_p.h

@@ -61,7 +61,6 @@ public:
 #endif
     ~QCoreApplicationPrivate();
 
-    static bool isAlive() noexcept;
     void init();
 
     QString appName() const;

src/corelib/kernel/qcoreevent.cpp

@@ -329,7 +329,7 @@ QEvent::QEvent(Type type)
 
 QEvent::~QEvent()
 {
-    if (m_posted && QCoreApplication::instance())
+    if (m_posted && QCoreApplication::instanceExists())
         QCoreApplicationPrivate::removePostedEvent(this);
 }
 

src/corelib/kernel/qeventloop.cpp

@@ -68,7 +68,7 @@ QEventLoop::QEventLoop(QObject *parent)
 {
     Q_D(QEventLoop);
     QThreadData *threadData = d->threadData.loadRelaxed();
-    if (!QCoreApplication::instance() && threadData->requiresCoreApplication) {
+    if (!QCoreApplication::instanceExists() && threadData->requiresCoreApplication) {
         qWarning("QEventLoop: Cannot be used without QCoreApplication");
     } else {
         threadData->ensureEventDispatcher();

src/corelib/kernel/qtranslator.cpp

@@ -397,7 +397,7 @@ QTranslator::QTranslator(QObject * parent)
 
 QTranslator::~QTranslator()
 {
-    if (QCoreApplication::instance())
+    if (QCoreApplication::instanceExists())
         QCoreApplication::removeTranslator(this);
     Q_D(QTranslator);
     d->clear();

src/corelib/thread/qthreadstorage.cpp

@@ -157,7 +157,7 @@ void QThreadStorageData::finish(void **p)
         locker.unlock();
 
         if (!destructor) {
-            if (QCoreApplicationPrivate::isAlive())
+            if (QCoreApplication::instanceExists())
                 qWarning("QThreadStorage: entry %d destroyed before end of thread %p",
                          i, QThread::currentThread());
             continue;

src/dbus/qdbusintegrator.cpp

@@ -559,7 +559,7 @@ bool QDBusConnectionPrivate::handleMessage(const QDBusMessage &amsg)
         // run it through the spy filters (if any) before the regular processing:
         // a) if it's a local message, we're in the caller's thread, so invoke the filter directly
         // b) if it's an external message, post to the main thread
-        if (Q_UNLIKELY(qDBusSpyHookList.exists()) && qApp) {
+        if (Q_UNLIKELY(qDBusSpyHookList.exists()) && QCoreApplication::instanceExists()) {
             if (isLocal) {
                 Q_ASSERT(QThread::currentThread() != thread());
                 qDBusDebug() << this << "invoking message spies directly";

src/gui/image/qpixmap.cpp

@@ -48,7 +48,7 @@ QT_WARNING_DISABLE_MSVC(4723)
 
 static bool qt_pixmap_thread_test()
 {
-    if (Q_UNLIKELY(!QCoreApplication::instance())) {
+    if (!QCoreApplication::instanceExists()) {
         qFatal("QPixmap: Must construct a QGuiApplication before a QPixmap");
         return false;
     }

src/network/kernel/qdnslookup.cpp

@@ -776,7 +776,7 @@ void QDnsLookup::lookup()
     Q_D(QDnsLookup);
     d->isFinished = false;
     d->reply = QDnsLookupReply();
-    if (!QCoreApplication::instance()) {
+    if (!QCoreApplication::instanceExists()) {
         // NOT qCWarning because this isn't a result of the lookup
         qWarning("QDnsLookup requires a QCoreApplication");
         return;

src/sql/kernel/qsqldatabase.cpp

@@ -23,12 +23,12 @@ Q_STATIC_LOGGING_CATEGORY(lcSqlDb, "qt.sql.qsqldatabase")
 using namespace Qt::StringLiterals;
 
 #define CHECK_QCOREAPPLICATION \
-    if (Q_UNLIKELY(!QCoreApplication::instance())) { \
+    if (Q_UNLIKELY(!QCoreApplication::instanceExists())) { \
         qCWarning(lcSqlDb, "QSqlDatabase requires a QCoreApplication"); \
         return; \
     }
 #define CHECK_QCOREAPPLICATION_RETVAL \
-    if (Q_UNLIKELY(!QCoreApplication::instance())) { \
+    if (Q_UNLIKELY(!QCoreApplication::instanceExists())) { \
         qCWarning(lcSqlDb, "QSqlDatabase requires a QCoreApplication"); \
         return {}; \
     }
@@ -661,7 +661,7 @@ void QSqlDatabasePrivate::init(const QString &type)
         qCWarning(lcSqlDb, "QSqlDatabase: %ls driver not loaded", qUtf16Printable(type));
         qCWarning(lcSqlDb, "QSqlDatabase: available drivers: %ls",
                   qUtf16Printable(QSqlDatabase::drivers().join(u' ')));
-        if (QCoreApplication::instance() == nullptr)
+        if (!QCoreApplication::instanceExists())
             qCWarning(lcSqlDb, "QSqlDatabase: an instance of QCoreApplication is required for loading driver plugins");
         driver = shared_null()->driver;
     }