Clear the current thread data for the main thread

This avoids crashes accessing deleted memory when creating a QObject
after the last QObject had been deleted, like a qDebug() in global
destructors.

==41000== Invalid read of size 4
==41000==    at 0x5F01ED5: bool QBasicAtomicOps<4>::ref<int>(int&) (qatomic_x86.h:208)
==41000==    by 0x5F01309: QBasicAtomicInteger<int>::ref() (qbasicatomic.h:147)
==41000==    by 0x5F24051: QThreadData::ref() (qthread.cpp:100)
==41000==    by 0x614A984: QObject::QObject(QObject*) (qobject.cpp:681)
==41000==  Address 0x6ee73f0 is 0 bytes inside a block of size 152 free'd
==41000==    at 0x4A0736C: operator delete(void*) (vg_replace_malloc.c:480)
==41000==    by 0x5F240BF: QThreadData::deref() (qthread.cpp:109)
==41000==    by 0x6113F6B: QCoreApplicationData::~QCoreApplicationData() (qcoreapplication.cpp:268)

The comment right above the change in qthread.cpp looks eerily similar
to the problem I'm trying to fix. However, the actual change that
introduced the change is not in the Qt public history, so we can't
know for sure what the problem was then.

Change-Id: I0dba895b041fe6cf81e6f8939ca85035cd00aad1
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>

src/corelib/kernel/qcoreapplication.cpp

@@ -300,13 +300,6 @@ struct QCoreApplicationData {
 #ifndef QT_NO_LIBRARY
         delete app_libpaths;
 #endif
-
-        // cleanup the QAdoptedThread created for the main() thread
-       if (QCoreApplicationPrivate::theMainThread) {
-           QThreadData *data = QThreadData::get2(QCoreApplicationPrivate::theMainThread);
-           QCoreApplicationPrivate::theMainThread = 0;
-           data->deref(); // deletes the data and the adopted thread
-       }
     }
 
 #ifdef Q_OS_BLACKBERRY

src/corelib/thread/qthread.cpp

@@ -76,6 +76,7 @@ QThreadData::~QThreadData()
     // the problem...
     if (this->thread == QCoreApplicationPrivate::theMainThread) {
        QCoreApplicationPrivate::theMainThread = 0;
+       QThreadData::clearCurrentThreadData();
     }
 
     QThread *t = thread;

src/corelib/thread/qthread_p.h

@@ -223,6 +223,7 @@ public:
     ~QThreadData();
 
     static QThreadData *current();
+    static void clearCurrentThreadData();
     static QThreadData *get2(QThread *thread)
     { Q_ASSERT_X(thread != 0, "QThread", "internal error"); return thread->d_func()->data; }
 

src/corelib/thread/qthread_unix.cpp

@@ -204,6 +204,11 @@ static void clear_thread_data()
     pthread_setspecific(current_thread_data_key, 0);
 }
 
+void QThreadData::clearCurrentThreadData()
+{
+    clear_thread_data();
+}
+
 QThreadData *QThreadData::current()
 {
     QThreadData *data = get_thread_data();

src/corelib/thread/qthread_win.cpp

@@ -96,6 +96,11 @@ Q_DESTRUCTOR_FUNCTION(qt_free_tls)
 /*
     QThreadData
 */
+void QThreadData::clearCurrentThreadData()
+{
+    TlsSetValue(qt_current_thread_data_tls_index, 0);
+}
+
 QThreadData *QThreadData::current()
 {
     qt_create_tls();

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

@@ -769,6 +769,18 @@ void tst_QCoreApplication::testQuitLock()
     app.exec();
 }
 
+static void createQObjectOnDestruction()
+{
+    // Make sure that we can create a QObject after the last QObject has been
+    // destroyed (especially after QCoreApplication has).
+    //
+    // Before the fixes, this would cause a dangling pointer dereference. If
+    // the problem comes back, it's possible that the following causes no
+    // effect.
+    QObject obj;
+    obj.thread()->setProperty("testing", 1);
+}
+Q_DESTRUCTOR_FUNCTION(createQObjectOnDestruction)
 
 QTEST_APPLESS_MAIN(tst_QCoreApplication)
 #include "tst_qcoreapplication.moc"