MDEV-27902 Spider check trx and get conn in rnd_next()

This allows creation of SPIDER_CONN on demand, if the previous one was
freed.

Also, the first_link_idx's are reset during
spider_check_trx_and_get_conn(), which in the case of remote HANDLER
commands, might not match the link to use correct first_link_idx for
remote HANDLER statement that was later set in
ha_spider::rnd_handler_init() (causing testing regressions in the
spider/handler suite). Therefore we fix the first_link_idx there.

storage/spider/ha_spider.cc

@@ -7182,6 +7182,8 @@ int ha_spider::rnd_next(
       DBUG_RETURN(error_num);
     use_pre_call = FALSE;
   }
+  if ((error_num= spider_check_trx_and_get_conn(ha_thd(), this, FALSE)))
+    DBUG_RETURN(error_num);
   DBUG_RETURN(rnd_next_internal(buf));
 }
 
@@ -12553,6 +12555,9 @@ int ha_spider::rnd_handler_init()
           DBUG_RETURN(error_num);
         }
         set_handler_opened(roop_count);
+        spider_db_handler *dbton_hdl=
+          dbton_handler[share->sql_dbton_ids[conn_link_idx[roop_count]]];
+        dbton_hdl->first_link_idx= roop_count;
       }
     }
     if (sql_kinds & SPIDER_SQL_KIND_HANDLER)

storage/spider/mysql-test/spider/bugfix/r/mdev_27902.result

+#
+# MDEV-27902 Crashes, asserts, hangs and corruptions in Spider when using HANDLER
+#
+for master_1
+for child2
+for child3
+CREATE SERVER srv FOREIGN DATA WRAPPER mysql
+OPTIONS (SOCKET "$MASTER_1_MYSOCK", DATABASE 'test',user 'root');
+CREATE TABLE t (c INT) ENGINE=Spider;
+HANDLER t OPEN;
+Warnings:
+Error	1429	Unable to connect to foreign data source: localhost
+Error	1429	Unable to connect to foreign data source: localhost
+Error	1429	Unable to connect to foreign data source: localhost
+Error	1429	Unable to connect to foreign data source: localhost
+HANDLER t READ next;
+ERROR HY000: Unable to connect to foreign data source: localhost
+dummy;
+ERROR 42000: You have an error in your SQL syntax; check the manual that corresponds to your MariaDB server version for the right syntax to use near 'dummy' at line 1
+HANDLER t READ next;
+ERROR HY000: Unable to connect to foreign data source: localhost
+drop table t;
+CREATE TABLE t (c INT) ENGINE=Spider;
+HANDLER t OPEN;
+Warnings:
+Error	1429	Unable to connect to foreign data source: localhost
+Error	1429	Unable to connect to foreign data source: localhost
+Error	1429	Unable to connect to foreign data source: localhost
+Error	1429	Unable to connect to foreign data source: localhost
+HANDLER t READ FIRST;
+ERROR HY000: Unable to connect to foreign data source: localhost
+HANDLER t READ NEXT;
+ERROR HY000: Unable to connect to foreign data source: localhost
+drop table t;
+CREATE TABLE t (c INT) ENGINE=Spider;
+HANDLER t OPEN;
+Warnings:
+Error	1429	Unable to connect to foreign data source: localhost
+Error	1429	Unable to connect to foreign data source: localhost
+Error	1429	Unable to connect to foreign data source: localhost
+Error	1429	Unable to connect to foreign data source: localhost
+HANDLER t READ NEXT;
+ERROR HY000: Unable to connect to foreign data source: localhost
+SELECT * FROM t;
+ERROR HY000: Unable to connect to foreign data source: localhost
+HANDLER t READ NEXT;
+ERROR HY000: Unable to connect to foreign data source: localhost
+drop table t;
+drop server srv;
+for master_1
+for child2
+for child3
+#
+# end of test mdev_27902
+#

storage/spider/mysql-test/spider/bugfix/t/mdev_27902.test

+--echo #
+--echo # MDEV-27902 Crashes, asserts, hangs and corruptions in Spider when using HANDLER
+--echo #
+--disable_query_log
+--disable_result_log
+--source ../../t/test_init.inc
+--enable_result_log
+--enable_query_log
+
+evalp CREATE SERVER srv FOREIGN DATA WRAPPER mysql
+OPTIONS (SOCKET "$MASTER_1_MYSOCK", DATABASE 'test',user 'root');
+
+# original case
+CREATE TABLE t (c INT) ENGINE=Spider;
+HANDLER t OPEN;
+--error ER_CONNECT_TO_FOREIGN_DATA_SOURCE
+HANDLER t READ next;
+--error ER_PARSE_ERROR
+dummy;
+--error ER_CONNECT_TO_FOREIGN_DATA_SOURCE
+HANDLER t READ next;
+
+drop table t;
+
+# case by nayuta
+CREATE TABLE t (c INT) ENGINE=Spider;
+HANDLER t OPEN;
+--error ER_CONNECT_TO_FOREIGN_DATA_SOURCE
+HANDLER t READ FIRST;
+--error ER_CONNECT_TO_FOREIGN_DATA_SOURCE
+HANDLER t READ NEXT;
+
+drop table t;
+
+# Another case by Roel
+CREATE TABLE t (c INT) ENGINE=Spider;
+HANDLER t OPEN;
+--error ER_CONNECT_TO_FOREIGN_DATA_SOURCE
+HANDLER t READ NEXT;
+--error ER_CONNECT_TO_FOREIGN_DATA_SOURCE
+SELECT * FROM t;
+--error ER_CONNECT_TO_FOREIGN_DATA_SOURCE
+HANDLER t READ NEXT;
+
+drop table t;
+
+drop server srv;
+
+--disable_query_log
+--disable_result_log
+--source ../../t/test_deinit.inc
+--enable_result_log
+--enable_query_log
+--echo #
+--echo # end of test mdev_27902
+--echo #