QAbstractSocket: optimize skipping

Avoid an unnecessary call to the slow base implementation on
buffered TCP socket.

Change-Id: Icc823b416b267aa8e0c1106b20872df9ef0e22d7
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>

src/network/socket/qabstractsocket.cpp

@@ -1371,6 +1371,21 @@ void QAbstractSocketPrivate::fetchConnectionParameters()
 #endif
 }
 
+/*! \internal
+*/
+qint64 QAbstractSocketPrivate::skip(qint64 maxSize)
+{
+    // if we're not connected, return -1 indicating EOF
+    if (!socketEngine || !socketEngine->isValid() || state != QAbstractSocket::ConnectedState)
+        return -1;
+
+    // Caller, QIODevice::skip(), has ensured buffer is empty. So, wait
+    // for more data in buffered mode.
+    if (isBuffered)
+        return 0;
+
+    return QIODevicePrivate::skip(maxSize);
+}
 
 void QAbstractSocketPrivate::pauseSocketNotifiers(QAbstractSocket *socket)
 {

src/network/socket/qabstractsocket_p.h

@@ -71,6 +71,9 @@ public:
     QAbstractSocketPrivate();
     virtual ~QAbstractSocketPrivate();
 
+    // from QIODevicePrivate
+    qint64 skip(qint64 maxSize) override;
+
     // from QAbstractSocketEngineReceiver
     inline void readNotification() override { canReadNotification(); }
     inline void writeNotification() override { canWriteNotification(); }