MDEV-36325 MDEV-36357 Call limit_mode on spider_db_conn instead of spider_db_result in spider_db_store_result

This prevents segv on NULL result_list->current->result on handler
calls that are not the first. Reasons supporting this change:

- spider_db_result::limit_mode is not called anywhere else. The only
  calls to limit_mode method are on spider_db_conn
- it is very unlikely (impossible?) for the connection to change from
  one backend to another between execution of sql statements and
  storing result:

/* in spider_bg_conn_action: */
                if (dbton_handler->execute_sql(
                  sql_type,
                  conn,
                  result_list->quick_mode,
                  &spider->need_mons[conn->link_idx])
// [... 9 lines elided]
                    if (!(result_list->bgs_error =
                      spider_db_store_result(spider, conn->link_idx,
                        result_list->table)))

  this also means it is very unlikely (impossible?) for the backend
  type (dbton_id) to differ between conn->db_conn and
  result_list->current->result, also considering that
  spider_db_result::dbton_id comes from spider_db_conn::dbton_id:

spider_db_result::spider_db_result(
  SPIDER_DB_CONN *in_db_conn
) : db_conn(in_db_conn), dbton_id(in_db_conn->dbton_id)

Since this was the only call to spider_db_result::limit_mode, we also
remove the method altogether.

Author: mariadb-YuchenPei

storage/spider/mysql-test/spider/bg/t/basic_sql.test

@@ -476,11 +476,8 @@ SELECT count(*) FROM ta_l ORDER BY a;
     --enable_query_log
     --enable_result_log
 --connection master_1
-# MDEV-36357
---disable_view_protocol
 SELECT a.a, a.b, date_format(a.c, '%Y-%m-%d %H:%i:%s') FROM tb_l a WHERE
 EXISTS (SELECT * FROM ta_l b WHERE b.b = a.b) ORDER BY a.a;
---enable_view_protocol
     --disable_query_log
     --disable_result_log
   --connection child2_1

storage/spider/spd_db_conn.cc

@@ -3104,7 +3104,7 @@ int spider_db_store_result(
           result_list->quick_phase == 2
         ) {
           if (result_list->low_mem_read &&
-            result_list->current->result->limit_mode() == 0)
+            conn->db_conn->limit_mode() == 0)
           {
             do {
               spider_db_free_one_result(result_list,

storage/spider/spd_db_include.cc

@@ -47,13 +47,6 @@ int spider_db_result::fetch_table_checksum(
   DBUG_RETURN(0);
 }
 
-uint spider_db_result::limit_mode()
-{
-  DBUG_ENTER("spider_db_result::limit_mode");
-  DBUG_PRINT("info",("spider this=%p", this));
-  DBUG_RETURN(spider_dbton[dbton_id].db_util->limit_mode());
-}
-
 spider_db_conn::spider_db_conn(
   SPIDER_CONN *in_conn
 ) : conn(in_conn), dbton_id(in_conn->dbton_id)

storage/spider/spd_db_include.h

@@ -883,7 +883,6 @@ class spider_db_result
     SPIDER_SHARE *spider_share,
     CHARSET_INFO *access_charset
   ) = 0;
-  virtual uint limit_mode();
 };
 
 class spider_db_conn