MXS-3054: Prevent pooling of DCBs with inbound data

If a DCB is expecting a response from the server, it must not be placed
into the connection pool. This prevents the case where unexpected
responses would be delivered to DCBs taken out of the connection pool.

include/maxscale/dcb.hh

@@ -189,6 +189,7 @@ struct DCB : public MXB_POLL_DATA
                                                  *      -1: Evicted from the persistent pool and being closed.
                                                  *   non-0: Time when placed in the persistent pool.
                                                  */
+    bool           valid_for_pool = true;       // TODO: Make this private in 2.5
     SERVICE*       service = nullptr;           /**< The related service */
     void*          data = nullptr;              /**< Client protocol data, owned by client DCB */
     void*          authenticator_data = nullptr;/**< The authenticator data for this DCB */

server/core/backend.cc

@@ -61,6 +61,7 @@ void Backend::close(close_type type)
             /** Clean operation counter in bref and in SERVER */
             if (is_waiting_result())
             {
+                m_dcb->valid_for_pool = false;
                 clear_state(WAITING_RESULT);
             }
             clear_state(IN_USE);

server/core/dcb.cc

@@ -1260,6 +1260,7 @@ static bool dcb_maybe_add_persistent(DCB* dcb)
         && strlen(dcb->user)
         && server
         && dcb->session
+        && dcb->valid_for_pool
         && session_valid_for_pool(dcb->session)
         && server->persistpoolmax()
         && (server->status & SERVER_RUNNING)