Replace grep with strings.Contains in lvdisplayGrep

[ashald]

We ran into a strange issue when using docker-lvm-plugin on a host with over a hundred lvm volumes. Once rebooted, if one would attempt to inspect a volume (or perform any operation that would internally call lvdisplayGrep) plugin will be rendered unusable. When looking at the process tree we saw some [lvdisplay] <defunct> zombies (and quite a few of them). Our theory is that if lvdisplay is called very "early" after host is rebooted it fails or does not produce the output lvdisplayGrep expects and this situation is not being handled properly. As a result plugin process becomes stuck in the named function that is being call within a lock and therefore no other operations will succeed after that.

@mortya has a script that shows how to reproduce this issue.

This probably could've been handled by revising error handling in lvdisplayGrep but I found it to be easier to get rid of the grep call along with the pipe and do "grepping" manually with strings.Contains. Code is simpler and we were not able to reproduce the issue with the given patch. Please note I used "dumb" strings.Split which appears to be good enough for the given task instead of a scanner from bufio as it's more succinct this way. If you prefer it the other way around please let me know.

[mortya]

To reproduce (at least on our VMs):

stem=test-$(hostname) count=600 size=100m set -o notify; for i in $(seq $count); do docker volume create -d lvm --opt size=0.1g ${stem}-${i}; done & function finished () { echo finished at: $(date); while true; do printf "\a"; sleep 1; done; } wait %1; finished

reboot; # wait for host to come back

set -o notify; docker volume ls & docker-volumes-prune -a "1 minute" & function finished () { echo finished at: $(date); while true; do printf "\a"; sleep 1; done; } wait %2; finished

lvs | grep -c -e $stem; # to see progress