An Xtrabackup SST may fail because of a ./ibdata1 race condition

[Bogdan Dobrelya]

Repro steps:
- Run a galera cluster of 5 nodes with a random-halves network partitions on/off (via an iptables reject rule), unless the issue's reproduced an a node or nodes. For each partition, kill -9 the nodes in a minority halve, then start them back once the partition ended.

An example logs snippet:
WSREP_SST: [INFO] Proceeding with SST (20160426 08:41:30.576)
WSREP_SST: [INFO] Cleaning the existing datadir (20160426 08:41:30.578)
removed '/var/lib/mysql/ib_logfile0'
removed '/var/lib/mysql/ib_logfile1'
removed '/var/lib/mysql/mysql/slave_worker_info.ibd'
removed '/var/lib/mysql/mysql/slave_master_info.ibd'
removed directory: '/var/lib/mysql/mysql'
removed '/var/lib/mysql/ibdata1'
removed '/var/lib/mysql/backup-my.cnf'
removed '/var/lib/mysql/auto.cnf'
WSREP_SST: [INFO] Cleaning the binlog directory /var/log/mysql as well (20160426 08:41:30.609)
WSREP_SST: [INFO] Evaluating socat -u TCP-LISTEN:4444,reuseaddr stdio | xbstream -x; RC=( ${PIPESTATUS[@]} ) (20160426 08:41:30.613)
xbstream: Can't create/write to file './ibdata1' (Errcode: 17 - File exists)

Similar topic
https://groups.google.com/forum/#!topic/codership-team/UEuWLDtCqh8

Packages used: percona-xtradb-cluster-server-5.6 5.6.21-25.8-0ubuntu2,
percona-xtrabackup 2.2.3-2.1build1

To ensure the issue will always happen, add to the config file:
[sst]
cpat = '.*galera\.cache$\|.*ibdata.*$\|.*sst_in_progress$\|.*gvwstate\.dat$\|.*grastate\.dat$\|.*\.err$\|.*\.log$\|.*RPM_UPGRADE_MARKER$\|.*RPM_UPGRADE_HISTORY$'

this will make the ibdata file to be preserved and the File exists error to pop up.