MDEV-16519 : mariabackup should fail if MDL could not be acquired with lock-ddl-per-table

There is a tiny chance for race condition during MDL acquisition.

If table is renamed just prior to
SELECT 1 FROM <table_name> LIMIT 0

then this query would  fail, yet mariabackup --backup does not handle
it as fatal error and continues, only to fail later during file copy.

The fix is to die on error, of MDL lock query fails.

extra/mariabackup/backup_mysql.cc

@@ -186,6 +186,7 @@ xb_mysql_query(MYSQL *connection, const char *query, bool use_result,
 
 		if (!use_result) {
 			mysql_free_result(mysql_result);
+			mysql_result = NULL;
 		}
 	}
 
@@ -1783,12 +1784,17 @@ mdl_lock_table(ulint space_id)
   MYSQL_RES *mysql_result = xb_mysql_query(mdl_con, oss.str().c_str(), true, true);
 
   while (MYSQL_ROW row = mysql_fetch_row(mysql_result)) {
+
+    DBUG_EXECUTE_IF("rename_during_mdl_lock_table",
+      if (strcmp(row[0], "test/t1") == 0)
+        xb_mysql_query(mysql_connection, "RENAME TABLE test.t1 to test.t2", false, true
+    ););
+
     std::string full_table_name =  ut_get_name(0,row[0]);
     std::ostringstream lock_query;
     lock_query << "SELECT 1 FROM " << full_table_name  << " LIMIT 0";
-
     msg_ts("Locking MDL for %s\n", full_table_name.c_str());
-    xb_mysql_query(mdl_con, lock_query.str().c_str(), false, false);
+    xb_mysql_query(mdl_con, lock_query.str().c_str(), false, true);
   }
 
   pthread_mutex_unlock(&mdl_lock_con_mutex);

mysql-test/suite/mariabackup/rename_during_mdl_lock.result

+CREATE TABLE t1(i int) ENGINE INNODB;
+FOUND 1 /failed to execute query SELECT 1 FROM/ in backup.log
+DROP TABLE t2;

mysql-test/suite/mariabackup/rename_during_mdl_lock.test

+--source include/have_debug.inc
+let $targetdir=$MYSQLTEST_VARDIR/backup;
+mkdir $targetdir;
+CREATE TABLE t1(i int) ENGINE INNODB;
+--error 1
+exec  $XTRABACKUP --defaults-file=$MYSQLTEST_VARDIR/my.cnf  --backup  --target-dir=$targetdir  --lock-ddl-per-table --dbug=+d,rename_during_mdl_lock_table 2>$targetdir/backup.log;
+
+let SEARCH_FILE=$targetdir/backup.log;
+let SEARCH_PATTERN=failed to execute query SELECT 1 FROM;
+source include/search_pattern_in_file.inc;
+
+DROP TABLE t2;
+rmdir $targetdir;