After this commit xtrabackup doesn't create --target-dir anymore when --stream option is used: https://github.com/MariaDB/server/commit/f37161070db5e800136c5610f4ddb2226396ce52

This results in error and aborted backup when --debug-sync option is used with --stream, because it tries to create a file in --target-dir (for synchronization).

xtrabackup: Generating a list of tablespaces

xtrabackup: Error: cannot open /backup/xtrabackup_debug_sync

The fix should ensure that --target-dir is created when --debug-sync option is used