QLocalSocket/Win: prevent writing to broken pipe

When a peer closes the connection, the device remains opened for reading
purposes. However, we should disable writing on disconnected socket.
Otherwise, if the user issues a write() call, a new pipe writer object
will be created and the write call occurs with invalid handle value.

Pick-to: 5.15
Change-Id: Id136798c7663df1fce7ed0aa4e3c6f5c65218a11
Reviewed-by: Timur Pocheptsov <timur.pocheptsov@qt.io>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@gmx.de>

src/network/socket/qlocalsocket_win.cpp

@@ -212,6 +212,12 @@ qint64 QLocalSocket::skipData(qint64 maxSize)
 qint64 QLocalSocket::writeData(const char *data, qint64 len)
 {
     Q_D(QLocalSocket);
+    if (!isValid()) {
+        d->error = OperationError;
+        d->errorString = tr("Socket is not connected");
+        return -1;
+    }
+
     if (len == 0)
         return 0;
     d->write(data, len);

tests/auto/network/socket/qlocalsocket/tst_qlocalsocket.cpp

@@ -1267,6 +1267,7 @@ void tst_QLocalSocket::syncDisconnectNotify()
     QVERIFY(serverSocket);
     delete serverSocket;
     QCOMPARE(client.waitForReadyRead(), false);
+    QVERIFY(!client.putChar(0));
 }
 
 void tst_QLocalSocket::asyncDisconnectNotify()