Comment 3 for bug 1551706

As follow-up, Xtrabackup should ignore user under [mysqld] because this user is not a MySQL server's user, it is a system user, which will own the MySQL process while starting, i.e MySQL server will start under that user.
http://dev.mysql.com/doc/refman/5.7/en/changing-mysql-user.html

In contrast the user under [client] is for client connection.

[client]
user = xtrbackup
[mysqld]
user = mysql

It is especially affected while doing some test in Jenkins. There will be no 'root' in jenkins, the default is 'jenkins' system user.

[client]
port=10000
socket=/tmp/mysql.sock
user=root
[mysqld]
basedir=/mnt/workspace/PTB-run-percona-server-5.6-pxb-2.3.5-2.4.4/Host/centos7-64/Percona-Server-5.6.33-rel79.0-Linux.x86_64.ssl101
datadir=/mnt/workspace/PTB-run-percona-server-5.6-pxb-2.3.5-2.4.4/Host/centos7-64/mnt/var/ps_5.6.33_sysbench/test-0/mysql.1/data
tmpdir=/mnt/workspace/PTB-run-percona-server-5.6-pxb-2.3.5-2.4.4/Host/centos7-64/mnt/var/ps_5.6.33_sysbench/test-0/mysql.1/tmp
port=10000
socket=/tmp/mysql.sock
pid-file=/mnt/workspace/PTB-run-percona-server-5.6-pxb-2.3.5-2.4.4/Host/centos7-64/mnt/var/ps_5.6.33_sysbench/test-0/mysql.1/mysql.pid
console
user=jenkins
server-id=1
general-log-file=/mnt/workspace/PTB-run-percona-server-5.6-pxb-2.3.5-2.4.4/Host/centos7-64/mnt/var/ps_5.6.33_sysbench/test-0/mysql.1/query.log

Temporary workaround was to create 'jenkins' user for MySQL.