QTextStream: Always use direct connection inside QDeviceClosedNotifier

Force direct connection when connecting aboutToClose() signal of device
to flushStream() slot of QDeviceClosedNotifier. This allows use of a
QTextStream from multiple threads when synchronization is handled by the
application.

Queued connections from aboutToClose() don't make much sense because the
device is closed immediately after emitting the signal. If a QTextStream
object is used by threads different from the one where it was created,
queued connection may result in attempting to flush the data after
the associated device is already closed, and accessing the QTextStream's
buffers from multiple threads.

Fixes: QTBUG-12055
Change-Id: If601d0f04f08b248b21ed1630b7dfd3546aee068
Reviewed-by: Oswald Buddenhagen <[email protected]>
Reviewed-by: Edward Welbourne <[email protected]>
Reviewed-by: Lars Knoll <[email protected]>

src/corelib/serialization/qtextstream_p.h

@@ -70,8 +70,13 @@ class QDeviceClosedNotifier : public QObject
     inline void setupDevice(QTextStream *stream, QIODevice *device)
     {
         disconnect();
-        if (device)
-            connect(device, SIGNAL(aboutToClose()), this, SLOT(flushStream()));
+        if (device) {
+            // Force direct connection here so that QTextStream can be used
+            // from multiple threads when the application code is handling
+            // synchronization (see also QTBUG-12055).
+            connect(device, SIGNAL(aboutToClose()), this, SLOT(flushStream()),
+                    Qt::DirectConnection);
+        }
         this->stream = stream;
     }