Add a way for auxiliary threads to handle events without CoreApp

Long-lived threads started by Qt itself can now receive events even if
QCoreApplication hasn't been created. This is required in all threads we
start that will handle events, unless we're sure that the thread will
exit before the global application object begins destruction.

Otherwise, those threads will have race conditions dealing with the
event delivery system trying to call the QCoreApplication::notify()
virtual while the object is being destroyed.

Change-Id: I27eaacb532114dd188c4ffff13d4ad2a4bb443e6
Reviewed-by: Olivier Goffart (Woboq GmbH) <ogoffart@woboq.com>

src/corelib/kernel/qcoreapplication.cpp

@@ -538,6 +538,14 @@ QThread *QCoreApplicationPrivate::mainThread()
     return theMainThread;
 }
 
+bool QCoreApplicationPrivate::threadRequiresCoreApplication()
+{
+    QThreadData *data = QThreadData::current(false);
+    if (!data)
+        return true;    // default setting
+    return data->requiresCoreApplication;
+}
+
 void QCoreApplicationPrivate::checkReceiverThread(QObject *receiver)
 {
     QThread *currentThread = QThread::currentThread();
@@ -926,6 +934,8 @@ bool QCoreApplication::isQuitLockEnabled()
     return quitLockRefEnabled;
 }
 
+static bool doNotify(QObject *, QEvent *);
+
 /*!
     Enables the ability of the QEventLoopLocker feature to quit
     the application.
@@ -960,7 +970,8 @@ bool QCoreApplication::notifyInternal(QObject *receiver, QEvent *event)
 */
 bool QCoreApplication::notifyInternal2(QObject *receiver, QEvent *event)
 {
-    if (!self)
+    bool selfRequired = QCoreApplicationPrivate::threadRequiresCoreApplication();
+    if (!self && selfRequired)
         return false;
 
     // Make it possible for Qt Script to hook into events even
@@ -978,6 +989,8 @@ bool QCoreApplication::notifyInternal2(QObject *receiver, QEvent *event)
     QObjectPrivate *d = receiver->d_func();
     QThreadData *threadData = d->threadData;
     QScopedLoopLevelCounter loopLevelCounter(threadData);
+    if (!selfRequired)
+        return doNotify(receiver, event);
     return self->notify(receiver, event);
 }
 
@@ -1039,7 +1052,11 @@ bool QCoreApplication::notify(QObject *receiver, QEvent *event)
     // no events are delivered after ~QCoreApplication() has started
     if (QCoreApplicationPrivate::is_app_closing)
         return true;
+    return doNotify(receiver, event);
+}
 
+static bool doNotify(QObject *receiver, QEvent *event)
+{
     if (receiver == 0) {                        // serious error
         qWarning("QCoreApplication::notify: Unexpected null receiver");
         return true;
@@ -1054,7 +1071,10 @@ bool QCoreApplication::notify(QObject *receiver, QEvent *event)
 
 bool QCoreApplicationPrivate::sendThroughApplicationEventFilters(QObject *receiver, QEvent *event)
 {
-    if (receiver->d_func()->threadData == this->threadData && extraData) {
+    // We can't access the application event filters outside of the main thread (race conditions)
+    Q_ASSERT(receiver->d_func()->threadData->thread == mainThread());
+
+    if (extraData) {
         // application event filters are only called for objects in the GUI thread
         for (int i = 0; i < extraData->eventFilters.size(); ++i) {
             QObject *obj = extraData->eventFilters.at(i);
@@ -1097,7 +1117,9 @@ bool QCoreApplicationPrivate::sendThroughObjectEventFilters(QObject *receiver, Q
 bool QCoreApplicationPrivate::notify_helper(QObject *receiver, QEvent * event)
 {
     // send to all application event filters (only does anything in the main thread)
-    if (QCoreApplication::self->d_func()->sendThroughApplicationEventFilters(receiver, event))
+    if (QCoreApplication::self
+            && receiver->d_func()->threadData->thread == mainThread()
+            && QCoreApplication::self->d_func()->sendThroughApplicationEventFilters(receiver, event))
         return true;
     // send to all receiver event filters
     if (sendThroughObjectEventFilters(receiver, event))

src/corelib/kernel/qcoreapplication_p.h

@@ -107,6 +107,8 @@ public:
 
     static QThread *theMainThread;
     static QThread *mainThread();
+    static bool threadRequiresCoreApplication();
+
     static void sendPostedEvents(QObject *receiver, int event_type, QThreadData *data);
 
     static void checkReceiverThread(QObject *receiver);

src/corelib/kernel/qeventloop.cpp

@@ -93,7 +93,7 @@ QEventLoop::QEventLoop(QObject *parent)
     : QObject(*new QEventLoopPrivate, parent)
 {
     Q_D(QEventLoop);
-    if (!QCoreApplication::instance()) {
+    if (!QCoreApplication::instance() && QCoreApplicationPrivate::threadRequiresCoreApplication()) {
         qWarning("QEventLoop: Cannot be used without QApplication");
     } else if (!d->threadData->eventDispatcher.load()) {
         QThreadPrivate::createEventDispatcher(d->threadData);

src/corelib/thread/qthread.cpp

@@ -50,7 +50,8 @@ QT_BEGIN_NAMESPACE
 
 QThreadData::QThreadData(int initialRefCount)
     : _ref(initialRefCount), loopLevel(0), thread(0), threadId(0),
-      eventDispatcher(0), quitNow(false), canWait(true), isAdopted(false)
+      eventDispatcher(0),
+      quitNow(false), canWait(true), isAdopted(false), requiresCoreApplication(true)
 {
     // fprintf(stderr, "QThreadData %p created\n", this);
 }
@@ -867,4 +868,28 @@ bool QThread::isInterruptionRequested() const
     return d->interruptionRequested;
 }
 
+/*!
+    \class QDaemonThread
+    \since 5.5
+    \brief The QDaemonThread provides a class to manage threads that outlive QCoreApplication
+    \internal
+
+    Note: don't try to deliver events from the started() signal.
+*/
+static void setThreadDoesNotRequireCoreApplication()
+{
+    QThreadData::current()->requiresCoreApplication = false;
+}
+
+QDaemonThread::QDaemonThread(QObject *parent)
+    : QThread(parent)
+{
+    // QThread::started() is emitted from the thread we start
+    connect(this, &QThread::started, setThreadDoesNotRequireCoreApplication);
+}
+
+QDaemonThread::~QDaemonThread()
+{
+}
+
 QT_END_NAMESPACE

src/corelib/thread/qthread_p.h

@@ -135,6 +135,13 @@ private:
 
 #ifndef QT_NO_THREAD
 
+class Q_CORE_EXPORT QDaemonThread : public QThread
+{
+public:
+    QDaemonThread(QObject *parent = 0);
+    ~QDaemonThread();
+};
+
 class QThreadPrivate : public QObjectPrivate
 {
     Q_DECLARE_PUBLIC(QThread)
@@ -224,7 +231,7 @@ public:
     QThreadData(int initialRefCount = 1);
     ~QThreadData();
 
-    static QThreadData *current(bool createIfNecessary = true);
+    static Q_AUTOTEST_EXPORT QThreadData *current(bool createIfNecessary = true);
     static void clearCurrentThreadData();
     static QThreadData *get2(QThread *thread)
     { Q_ASSERT_X(thread != 0, "QThread", "internal error"); return thread->d_func()->data; }
@@ -278,6 +285,7 @@ public:
     bool quitNow;
     bool canWait;
     bool isAdopted;
+    bool requiresCoreApplication;
 };
 
 class QScopedLoopLevelCounter

src/network/access/qnetworkreplyhttpimpl.cpp

@@ -49,6 +49,8 @@
 #include "qthread.h"
 #include "QtCore/qcoreapplication.h"
 
+#include <QtCore/private/qthread_p.h>
+
 #include "qnetworkcookiejar.h"
 
 #ifndef QT_NO_HTTP

src/network/bearer/qnetworkconfigmanager_p.cpp

@@ -41,6 +41,7 @@
 #include <QtCore/qstringlist.h>
 #include <QtCore/qthread.h>
 #include <QtCore/private/qcoreapplication_p.h>
+#include <QtCore/private/qthread_p.h>
 
 #ifndef QT_NO_BEARERMANAGEMENT
 
@@ -61,7 +62,7 @@ QNetworkConfigurationManagerPrivate::QNetworkConfigurationManagerPrivate()
 void QNetworkConfigurationManagerPrivate::initialize()
 {
     //Two stage construction, because we only want to do this heavyweight work for the winner of the Q_GLOBAL_STATIC race.
-    bearerThread = new QThread();
+    bearerThread = new QDaemonThread();
     bearerThread->setObjectName(QStringLiteral("Qt bearer thread"));
 
     bearerThread->moveToThread(QCoreApplicationPrivate::mainThread()); // because cleanup() is called in main thread context.

src/platformsupport/input/evdevtablet/qevdevtablet.cpp

@@ -292,7 +292,7 @@ void QEvdevTabletHandler::readData()
 
 
 QEvdevTabletHandlerThread::QEvdevTabletHandlerThread(const QString &spec, QObject *parent)
-    : QThread(parent), m_spec(spec), m_handler(0)
+    : QDaemonThread(parent), m_spec(spec), m_handler(0)
 {
     start();
 }

src/platformsupport/input/evdevtablet/qevdevtablet_p.h

@@ -48,6 +48,7 @@
 #include <QObject>
 #include <QString>
 #include <QThread>
+#include <QtCore/private/qthread_p.h>
 
 QT_BEGIN_NAMESPACE
 
@@ -68,7 +69,7 @@ private:
     QEvdevTabletData *d;
 };
 
-class QEvdevTabletHandlerThread : public QThread
+class QEvdevTabletHandlerThread : public QDaemonThread
 {
 public:
     explicit QEvdevTabletHandlerThread(const QString &spec, QObject *parent = 0);

src/platformsupport/input/evdevtouch/qevdevtouchhandler.cpp

@@ -642,7 +642,7 @@ void QEvdevTouchScreenData::reportPoints()
 
 
 QEvdevTouchScreenHandlerThread::QEvdevTouchScreenHandlerThread(const QString &device, const QString &spec, QObject *parent)
-    : QThread(parent), m_device(device), m_spec(spec), m_handler(0)
+    : QDaemonThread(parent), m_device(device), m_spec(spec), m_handler(0)
 {
     start();
 }

src/platformsupport/input/evdevtouch/qevdevtouchhandler_p.h

@@ -49,6 +49,7 @@
 #include <QString>
 #include <QList>
 #include <QThread>
+#include <QtCore/private/qthread_p.h>
 #include <qpa/qwindowsysteminterface.h>
 
 #if !defined(QT_NO_MTDEV)
@@ -80,7 +81,7 @@ private:
 #endif
 };
 
-class QEvdevTouchScreenHandlerThread : public QThread
+class QEvdevTouchScreenHandlerThread : public QDaemonThread
 {
 public:
     explicit QEvdevTouchScreenHandlerThread(const QString &device, const QString &spec, QObject *parent = 0);

src/widgets/kernel/qapplication.cpp

@@ -3688,7 +3688,9 @@ bool QApplication::notify(QObject *receiver, QEvent *e)
 bool QApplicationPrivate::notify_helper(QObject *receiver, QEvent * e)
 {
     // send to all application event filters
-    if (sendThroughApplicationEventFilters(receiver, e))
+    if (threadRequiresCoreApplication()
+        && receiver->d_func()->threadData->thread == mainThread()
+        && sendThroughApplicationEventFilters(receiver, e))
         return true;
 
     if (receiver->isWidgetType()) {

tests/auto/corelib/kernel/qcoreapplication/qcoreapplication.pro

@@ -3,3 +3,4 @@ TARGET = tst_qcoreapplication
 QT = core testlib core-private
 SOURCES = tst_qcoreapplication.cpp
 HEADERS = tst_qcoreapplication.h
+requires(contains(QT_CONFIG,private_tests))

tests/auto/corelib/kernel/qcoreapplication/tst_qcoreapplication.cpp

@@ -78,6 +78,21 @@ public:
     }
 };
 
+class Thread : public QDaemonThread
+{
+    void run() Q_DECL_OVERRIDE
+    {
+        QThreadData *data = QThreadData::current();
+        QVERIFY(!data->requiresCoreApplication);        // daemon thread
+        data->requiresCoreApplication = requiresCoreApplication;
+        QThread::run();
+    }
+
+public:
+    Thread() : requiresCoreApplication(true) {}
+    bool requiresCoreApplication;
+};
+
 void tst_QCoreApplication::sendEventsOnProcessEvents()
 {
     int argc = 1;
@@ -853,6 +868,41 @@ void tst_QCoreApplication::applicationEventFilters_auxThread()
     QVERIFY(!spy.recordedEvents.contains(QEvent::User + 1));
 }
 
+void tst_QCoreApplication::threadedEventDelivery_data()
+{
+    QTest::addColumn<bool>("requiresCoreApplication");
+    QTest::addColumn<bool>("createCoreApplication");
+    QTest::addColumn<bool>("eventsReceived");
+
+    // invalid combination:
+    //QTest::newRow("default-without-coreapp") << true << false << false;
+    QTest::newRow("default") << true << true << true;
+    QTest::newRow("independent-without-coreapp") << false << false << true;
+    QTest::newRow("independent-with-coreapp") << false << true << true;
+}
+
+// posts the event before the QCoreApplication is destroyed, starts thread after
+void tst_QCoreApplication::threadedEventDelivery()
+{
+    QFETCH(bool, requiresCoreApplication);
+    QFETCH(bool, createCoreApplication);
+    QFETCH(bool, eventsReceived);
+
+    int argc = 1;
+    char *argv[] = { const_cast<char*>(QTest::currentAppName()) };
+    QScopedPointer<TestApplication> app(createCoreApplication ? new TestApplication(argc, argv) : 0);
+
+    Thread thread;
+    thread.requiresCoreApplication = requiresCoreApplication;
+    ThreadedEventReceiver receiver;
+    receiver.moveToThread(&thread);
+    QCoreApplication::postEvent(&receiver, new QEvent(QEvent::Type(QEvent::User + 1)));
+
+    thread.start();
+    QVERIFY(thread.wait(1000));
+    QCOMPARE(receiver.recordedEvents.contains(QEvent::User + 1), eventsReceived);
+}
+
 static void createQObjectOnDestruction()
 {
     // Make sure that we can create a QObject after the last QObject has been

tests/auto/corelib/kernel/qcoreapplication/tst_qcoreapplication.h

@@ -61,6 +61,8 @@ private slots:
     void QTBUG31606_QEventDestructorDeadLock();
     void applicationEventFilters_mainThread();
     void applicationEventFilters_auxThread();
+    void threadedEventDelivery_data();
+    void threadedEventDelivery();
 };
 
 #endif // TST_QCOREAPPLICATION_H