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>

6 files changed, 24 insertions, 7 deletions

diff --git a/src/corelib/kernel/qcoreapplication.cpp b/src/corelib/kernel/qcoreapplication.cpp
index 1696aeb77b..46de52ec9f 100644
--- a/src/corelib/kernel/qcoreapplication.cpp
+++ b/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
diff --git a/src/corelib/thread/qthread.cpp b/src/corelib/thread/qthread.cpp
index bd8c6341c1..79199c97e0 100644
--- a/src/corelib/thread/qthread.cpp
+++ b/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;
diff --git a/src/corelib/thread/qthread_p.h b/src/corelib/thread/qthread_p.h
index 526633cafa..2cf988260f 100644
--- a/src/corelib/thread/qthread_p.h
+++ b/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; }
 
diff --git a/src/corelib/thread/qthread_unix.cpp b/src/corelib/thread/qthread_unix.cpp
index 8104cc8938..44ad8d3ac2 100644
--- a/src/corelib/thread/qthread_unix.cpp
+++ b/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();
diff --git a/src/corelib/thread/qthread_win.cpp b/src/corelib/thread/qthread_win.cpp
index 8614330d6f..0cf903bb3a 100644
--- a/src/corelib/thread/qthread_win.cpp
+++ b/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();
diff --git a/tests/auto/corelib/kernel/qcoreapplication/tst_qcoreapplication.cpp b/tests/auto/corelib/kernel/qcoreapplication/tst_qcoreapplication.cpp
index 42bf9eeca3..10c00e2a67 100644
--- a/tests/auto/corelib/kernel/qcoreapplication/tst_qcoreapplication.cpp
+++ b/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"