innobackupex 1.6.4 need empty datadir when copy-back

When restore by xtrabackup '--copy-back' with version 1.6.4, you may get error:

IMPORTANT: Please check that the copy-back run completes successfully.
           At the end of a successful copy-back run innobackupex- 1.6.4
           prints  " completed OK! " .

Original data directory is not empty! at innobackupex- 1.6.4  line  544 .

when read those lines we found that:

    # check that original data directories exist and they are empty
    if_directory_exists_and_empty($orig_datadir ,   " Original data " ) ;
    if_directory_exists_and_empty($orig_ibdata_dir ,   " Original InnoDB data " ) ;
    if_directory_exists_and_empty($orig_iblog_dir ,   " Original InnoDB log " ) ;

Google for reason for this check and find it mentioned in:

http://www.mysqlperformanceblog.com/2011/12/19/percona-xtrabackup-1-6-4/

innobackupex did not check that MySQL datadir was empty before –copy-back was run. With this bug fix ,
innobackupex will now error out of the –copy-back operation if the destination is not empty ,  avoiding
potential data loss or a strang combination of a restored backup and previous data. Bug Fixed: # 737569  (Valentine Gostev)