Check if socketEngine is set before accessing it.

QTcpServer::addPendingConnection() is protected, so it is possible to
add pending connections from a class derived from QTcpServer. This
derived class can get a QTcpSocket from somewhere else, in which case
d->socketEngine will not be set (or used). Given that it is possible to
add pending connections in this scenario, it would make sense to be able
to retrieve them as well.

[ChangeLog][QtNetwork][QTcpServer] Permit using a QTcpServer with
externally created QTcpSocket.

Task-number: QTBUG-51288
Change-Id: I830c10f1a881e2bca4e4ad716d8be865e1c27a9f
Reviewed-by: Lars Knoll <lars.knoll@theqtcompany.com>

src/network/socket/qtcpserver.cpp

@@ -543,8 +543,11 @@ QTcpSocket *QTcpServer::nextPendingConnection()
     if (d->pendingConnections.isEmpty())
         return 0;
 
-    if (!d->socketEngine->isReadNotificationEnabled())
+    if (!d->socketEngine) {
+        qWarning("QTcpServer::nextPendingConnection() called while not listening");
+    } else if (!d->socketEngine->isReadNotificationEnabled()) {
         d->socketEngine->setReadNotificationEnabled(true);
+    }
 
     return d->pendingConnections.takeFirst();
 }

tests/auto/network/socket/qtcpserver/tst_qtcpserver.cpp

@@ -106,6 +106,8 @@ private slots:
 
     void eagainBlockingAccept();
 
+    void qtbug51288();
+
 private:
 #ifndef QT_NO_BEARERMANAGEMENT
     QNetworkSession *networkSession;
@@ -990,5 +992,22 @@ void tst_QTcpServer::eagainBlockingAccept()
     server.close();
 }
 
+class NonListeningTcpServer : public QTcpServer
+{
+public:
+    void addSocketFromOutside(QTcpSocket* s)
+    {
+        addPendingConnection(s);
+    }
+};
+
+void tst_QTcpServer::qtbug51288()
+{
+    NonListeningTcpServer server;
+    QTcpSocket socket;
+    server.addSocketFromOutside(&socket);
+    QCOMPARE(&socket, server.nextPendingConnection());
+}
+
 QTEST_MAIN(tst_QTcpServer)
 #include "tst_qtcpserver.moc"