MDEV-27121 mariabackup incompatible with disabled dedicated undo log tablespaces

- mariabackup fails to assign srv_undo_space_id_start when the
dedicated undo tablespaces are disabled

extra/mariabackup/xtrabackup.cc

@@ -3578,10 +3578,6 @@ static dberr_t xb_assign_undo_space_start()
 	ulint		space;
 	int n_retries = 5;
 
-	if (srv_undo_tablespaces == 0) {
-		return error;
-	}
-
 	file = os_file_create(0, srv_sys_space.first_datafile()->filepath(),
 		OS_FILE_OPEN, OS_FILE_NORMAL, OS_DATA_FILE, true, &ret);
 

mysql-test/suite/mariabackup/full_backup.opt

+--innodb_undo_tablespaces=2

mysql-test/suite/mariabackup/full_backup.result

@@ -12,3 +12,17 @@ SELECT * FROM t;
 i
 1
 DROP TABLE t;
+#
+#  MDEV-27121 mariabackup incompatible with disabled dedicated
+#                  undo log tablespaces
+#
+call mtr.add_suppression("InnoDB: innodb_undo_tablespaces=0 disables dedicated undo log tablespaces");
+# xtrabackup backup
+# xtrabackup prepare
+# shutdown server
+# remove datadir
+# xtrabackup move back
+# restart server
+# Display undo log files from target directory
+undo001
+undo002

mysql-test/suite/mariabackup/full_backup.test

@@ -29,3 +29,27 @@ SELECT * FROM t;
 DROP TABLE t;
 rmdir $targetdir;
 
+--echo #
+--echo #  MDEV-27121 mariabackup incompatible with disabled dedicated
+--echo #                  undo log tablespaces
+--echo #
+call mtr.add_suppression("InnoDB: innodb_undo_tablespaces=0 disables dedicated undo log tablespaces");
+
+let $restart_parameters=--innodb_undo_tablespaces=0;
+--source include/restart_mysqld.inc
+
+echo # xtrabackup backup;
+--disable_result_log
+exec $XTRABACKUP --defaults-file=$MYSQLTEST_VARDIR/my.cnf --backup --target-dir=$targetdir;
+--enable_result_log
+
+echo # xtrabackup prepare;
+--disable_result_log
+exec $XTRABACKUP  --prepare --target-dir=$targetdir;
+-- source include/restart_and_restore.inc
+--enable_result_log
+
+--echo # Display undo log files from target directory
+list_files $targetdir undo*;
+
+rmdir $targetdir;