MXS-1752: Add prototype concatenation router

The router concatenates multiple result sets.

server/modules/routing/cat/catsession.hh

@@ -0,0 +1,82 @@
+#pragma once
+/*
+ * Copyright (c) 2016 MariaDB Corporation Ab
+ *
+ * Use of this software is governed by the Business Source License included
+ * in the LICENSE.TXT file and at www.mariadb.com/bsl11.
+ *
+ * Change Date: 2020-01-01
+ *
+ * On the date above, in accordance with the Business Source License, use
+ * of this software will be governed by version 2 or later of the General
+ * Public License.
+ */
+
+#include "cat.hh"
+
+#include "../readwritesplit/rwbackend.hh"
+
+class Cat;
+
+/**
+ * The client session structure used within this router.
+ */
+class CatSession: public mxs::RouterSession
+{
+    CatSession(const CatSession&) = delete;
+    CatSession& operator =(const CatSession&) = delete;
+public:
+
+    CatSession(MXS_SESSION* session, Cat* router, mxs::SRWBackendList& backends);
+
+    /**
+     * The RouterSession instance will be deleted when a client session
+     * has terminated. Will be called only after @c close() has been called.
+     */
+    ~CatSession();
+
+    /**
+     * Called when a client session has been closed.
+     */
+    void close();
+
+    /**
+     * Called when a packet being is routed to the backend. The router should
+     * forward the packet to the appropriate server(s).
+     *
+     * @param pPacket A client packet.
+     */
+    int32_t routeQuery(GWBUF* pPacket);
+
+    /**
+     * Called when a packet is routed to the client. The router should
+     * forward the packet to the client using `MXS_SESSION_ROUTE_REPLY`.
+     *
+     * @param pPacket  A client packet.
+     * @param pBackend The backend the packet is coming from.
+     */
+    void clientReply(GWBUF* pPacket, DCB* pBackend);
+
+    /**
+     *
+     * @param pMessage  The error message.
+     * @param pProblem  The DCB on which the error occurred.
+     * @param action    The context.
+     * @param pSuccess  On output, if false, the session will be terminated.
+     */
+    void handleError(GWBUF*             pMessage,
+                     DCB*               pProblem,
+                     mxs_error_action_t action,
+                     bool*              pSuccess);
+private:
+
+    MXS_SESSION*                  m_session;
+    mxs::SRWBackendList           m_backends;
+    uint64_t                      m_completed;
+    uint8_t                       m_packet_num;
+    mxs::SRWBackendList::iterator m_current;
+    GWBUF*                        m_query;
+
+    // Skip over unused backend servers
+    void skip_unused();
+};