LP #1644694: pt-stalk get wrong mysqld pid when the host installed multi mysql instance

**Reported in Launchpad by zhechen last update 25-11-2016 14:13:03

host env:
Centos 6.8
percona-toolkit-2.2.19

two mysql instance:
$ pidof mysqld
33530 17881

my host have two mysql instance, 3304 port(pid: 17881) and 3306 port(pid: 33530), when I use pt-stalk to monitor 3304 port, the mysql info logs in /var/lib/pt-stalk/ is about 3306 port, this is wrong, I just want log the mysql 3304 port.

the following code in pt-stalk mabye get wrong mysql pid:

792 collect() {
793 local d="$1" # directory to save results in
794 local p="$2" # prefix for each result file
795
796 local mysqld_pid=$(_pidof mysqld | awk '

')
797

the subsequent mysql info will be wrong when mysqld_pid is wrong, pt-stalk should read mysql pid_file to get right pid number.