#!/bin/bash -l
set -eE -o pipefail
trap 'echo "$0:$LINENO:error: \"$BASH_COMMAND\" returned $?" >&2' ERR

sv() { echo "$@"; "$@"; }

date=unison-$(date "+%Y:%m:%d")
for mountp in /mnt/root/q /mnt/iroot/i; do
    [[ -e $mountp ]] || continue
    date_path=${mountp%/*}/$date
    todays=( $date_path-* )
    IFS=$'\n' todays=($(sort -Vr <<<"${todays[*]}")); unset IFS
    if [[ -e ${todays[0]} ]]; then
        snapshot=$date_path-$(( ${todays[0]##*-} + 1 ))
    else
        snapshot=$date_path-1
    fi
    sv s btrfs subvolume snapshot -r $mountp $snapshot
    existing=( ${mountp%/*}/unison-* )
    IFS=$'\n' existing=($(sort -Vr <<<"${existing[*]}")); unset IFS
    # keep 10 unison snapshots around
    if (( ${#existing[@]} > 10 )); then
        sv s btrfs subvolume delete ${existing[@]:10}
    fi
done