MDEV-29962 Spider: creates connections if needed before lock_tables()

Same cause as MDEV-31996. This prevents reuse of conns freed
previously, e.g. during the commit of previous statement.

It does not occur in 10.4 because of the commit for MDEV-19002 removed
the call to spider_check_trx_and_get_conn().

storage/spider/ha_spider.cc

@@ -1130,6 +1130,8 @@ int ha_spider::external_lock(
     }
   }
 
+  if ((error_num= spider_check_trx_and_get_conn(thd, this, FALSE)))
+    DBUG_RETURN(error_num);
   if (!partition_handler || !partition_handler->handlers)
   {
     DBUG_RETURN(lock_tables()); /* Non-partitioned table */

storage/spider/mysql-test/spider/bugfix/r/mdev_29962.result

+#
+# MDEV-29962 SIGSEGV in ha_spider::lock_tables on BEGIN after table lock
+#
+for master_1
+for child2
+for child3
+set spider_same_server_link= 1;
+CREATE SERVER srv FOREIGN DATA WRAPPER mysql
+OPTIONS (SOCKET "$MASTER_1_MYSOCK", DATABASE 'test',user 'root');
+CREATE TABLE t (c INT) ENGINE=InnoDB;
+CREATE TABLE t1 (c INT) ENGINE=Spider COMMENT='WRAPPER "mysql",srv "srv",TABLE "t"';
+SELECT * FROM t1;
+c
+LOCK TABLES t1 WRITE CONCURRENT,t1 AS t2 READ;
+BEGIN;
+drop table t, t1;
+drop server srv;
+for master_1
+for child2
+for child3
+#
+# end of test mdev_29962
+#

storage/spider/mysql-test/spider/bugfix/t/mdev_29962.test

+--echo #
+--echo # MDEV-29962 SIGSEGV in ha_spider::lock_tables on BEGIN after table lock
+--echo #
+--disable_query_log
+--disable_result_log
+--source ../../t/test_init.inc
+--enable_result_log
+--enable_query_log
+
+set spider_same_server_link= 1;
+evalp CREATE SERVER srv FOREIGN DATA WRAPPER mysql
+OPTIONS (SOCKET "$MASTER_1_MYSOCK", DATABASE 'test',user 'root');
+
+CREATE TABLE t (c INT) ENGINE=InnoDB;
+CREATE TABLE t1 (c INT) ENGINE=Spider COMMENT='WRAPPER "mysql",srv "srv",TABLE "t"';
+SELECT * FROM t1;
+LOCK TABLES t1 WRITE CONCURRENT,t1 AS t2 READ;
+BEGIN;
+
+drop table t, t1;
+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_29962
+--echo #