MXS-2490: Send error to client on unknown PS handle

If a client requests an unknown binary protocol prepared statement handle,
a custom error shows the actual ID used instead of the "empty" ID of 0
that the backend sends.

server/modules/routing/readwritesplit/rwsplit_route_stmt.cc

@@ -202,7 +202,12 @@ bool RWSplitSession::route_single_stmt(GWBUF* querybuf)
 
     SRWBackend target;
 
-    if (TARGET_IS_ALL(route_target))
+    if (command == MXS_COM_STMT_EXECUTE && stmt_id == 0)
+    {
+        // Unknown prepared statement ID
+        succp = send_unknown_ps_error(extract_binary_ps_id(querybuf));
+    }
+    else if (TARGET_IS_ALL(route_target))
     {
         succp = handle_target_is_all(route_target, querybuf, command, qtype);
     }

server/modules/routing/readwritesplit/rwsplitsession.cc

@@ -1221,3 +1221,11 @@ bool RWSplitSession::supports_hint(HINT_TYPE hint_type) const
 
     return rv;
 }
+
+bool RWSplitSession::send_unknown_ps_error(uint32_t stmt_id)
+{
+    std::stringstream ss;
+    ss << "Unknown prepared statement handler (" << stmt_id << ") given to MaxScale";
+    GWBUF* err = modutil_create_mysql_err_msg(1, 0, ER_UNKNOWN_STMT_HANDLER, "HY000", ss.str().c_str());
+    return m_client->func.write(m_client, err);
+}

server/modules/routing/readwritesplit/rwsplitsession.hh

@@ -214,6 +214,9 @@ private:
                                     mxs::SRWBackend& old_master,
                                     mxs::SRWBackend& curr_master);
 
+    // Send unknown prepared statement ID error to client
+    bool send_unknown_ps_error(uint32_t stmt_id);
+
     GWBUF* handle_causal_read_reply(GWBUF* writebuf, mxs::SRWBackend& backend);
     GWBUF* add_prefix_wait_gtid(SERVER* server, GWBUF* origin);
     void   correct_packet_sequence(GWBUF* buffer);