MXS-1776: Handle recursive COM_STMT_EXECUTE commands

Readwritesplit would not handle multiple overlapping COM_STMT_EXECUTE
commands properly if they opened cursors. This was due to the fact that
the result would not be marked as complete and COM_STMT_FETCH commands
were executed as if they did not return results.

The correct implementation is to consider a COM_STMT_EXECUTE that opens a
cursor complete only when the first EOF packet is read (that is, when the
resultset header is read). This allows subsequent COM_STMT_FETCH commands
to be handled separately.

The separate COM_STMT_FETCH handling must count the number of packets that
are being fetched. This allows correct tracking of the state of a
COM_STMT_FETCH by checking that the number of packets is correct or the
second EOF/ERR packet is read.

server/modules/routing/readwritesplit/rwsplitsession.hh

@@ -59,6 +59,9 @@ public:
     bool execute_session_command();
     bool write(GWBUF* buffer, response_type type = EXPECT_RESPONSE);
 
+    // For COM_STMT_FETCH processing
+    bool consume_fetched_rows(GWBUF* buffer);
+
     inline void set_large_packet(bool value)
     {
         m_large_packet = value;
@@ -74,6 +77,11 @@ public:
         return m_command;
     }
 
+    inline bool cursor_is_open() const
+    {
+        return m_open_cursor;
+    }
+
 private:
     reply_state_t    m_reply_state;
     BackendHandleMap m_ps_handles; /**< Internal ID to backend PS handle mapping */
@@ -81,6 +89,8 @@ private:
                                       *calculation for result sets when the result
                                       * contains very large rows */
     uint8_t          m_command;
+    bool             m_open_cursor; /**< Whether we have an open cursor */
+    uint32_t         m_expected_rows; /**< Number of rows a COM_STMT_FETCH is retrieving */
 };
 
 typedef std::tr1::shared_ptr<RWBackend> SRWBackend;