--destroy-only keeps one snapshot less than specified

[emz00]

When I run zfs-auto-snapshot --syslog --label=daily --keep=3 --destroy-only myzpool it keeps the last 2 snapshots, not 3.
Similarly zfs-auto-snapshot --syslog --label=daily --keep=2 --destroy-only myzpool it keeps the last 1 snapshot, not 2.

Ubuntu 18.04.5, installed zfs-auto-snapshot using apt install. I don't know what version of it I have - it doesn't report that and there is no --version.

[mschout]

Same issue on Ubuntu 20.04.1 LTS

[mschout]

Basically from a quick skim, I think it is because the "destroy" loop assumes that it just created a snapshot. One workaround is after reading the args, just decrement increment $opt_keep by one if --destroy-only was given.

[mschout]

Something like this around line 350:

if [ -z "$opt_do_snapshots" -a ! -z "$opt_keep" ]
then
        opt_keep=$(( $opt_keep + 1 ))
fi