#!/bin/bash
# backup-zfs: use zfs send/recv to push/pull snapshots

usage() {
	echo "$(basename "$0") [-hvq] [-t tag] [-k keep] [-d dateopts] [srchost:]srcfs [desthost:]destfs" >&2
	echo "$(basename "$0"): use zfs send/recv to push/pull snapshots" >&2
	printf "%15s: %s\n" >&2 "-h" "this help statement" \
	                        "-v" "verbose" \
				"-q" "quiet" \
	                    "-t tag" "tag to use for naming snapshots and in syslog" \
	                   "-k keep" "number of snapshots to keep on src" \
	               "-d dateopts" "options string for date(1), used in naming snapshots"
	exit $1
}

# log to syslog; if verbose or on a tty, also to stdout
# usage: log msg
log() {
	logger -t $tag -- "$@"
	if ! $quiet && [[ -t 1 ]] || $verbose ; then
		echo "$@" >&2
	fi
}

# exit with a code & message
# usage: die $exitcode msg
die() {
	code="$1"
	shift
	if [[ $code -ne 0 ]] ; then
		verbose=true log "FATAL: $@"
	else
		log "$@"
	fi
	exit $code
}

# run zfs(1) command either locally or via ssh
# usage: ZFS "$host" command args...
ZFS() {
	host="$1"
	shift
	if [[ -n $host ]] ; then
		log "remote ($host): zfs $@"
		ssh "$host" sudo zfs "$@"
	else
		log "local: zfs $@"
		sudo zfs "$@"
	fi
}

###
### defaults
###
tag=backup-zfs
dateopts="+%F_%T"
keep=5
verbose=false
quiet=false

###
### parse options
###
while getopts "hvqk:t:d:" opt ; do
	case $opt in
		h) usage 0 ;;
		v) verbose=true ;;
		q) quiet=true ;;
		k) keep=$OPTARG ;;
		t) tag=$OPTARG ;;
		d) dateopts=$OPTARG ;;
		*) usage 1 ;;
	esac
done
shift $((OPTIND-1))
date="$(date $dateopts)"

###
### parse src & dest host/fs info
###
# fail if there's ever >1 colon
if [[ $1 =~ :.*: || $2 =~ :.*: ]] ; then
	die 1 "invalid fsspec: '$1' or '$2'"
fi

# fail if src or dest isn't specified
if [[ -z $1 || -z $2 ]] ; then
	usage 1
fi
src="$1"
dest="$2"

# discard anything before a colon to get the fs
srcfs="${src#*:}"
destfs="${dest#*:}"

# iff there is a colon, discard everything after it to get the host
[[ $src =~ : ]] && srchost="${src%:*}"
[[ $dest =~ : ]] && desthost="${dest%:*}"

# get the last src component
srcbase="${srcfs##*/}"

###
### create new snapshot on src
###
cur="$srcfs@${tag}_$date"
ZFS "$srchost" snapshot -r "$cur"

###
### find newest snapshot matching the tag on dest
###
last="$(ZFS "$desthost" list -d 1 -t snapshot -H -S creation -o name $destfs/$srcbase 2>/dev/null \
	| grep -F "@${tag}_" | head -n1 | cut -f2 -d@)"

###
### send & receive
###
# 1st time: send full snapshot
if [[ -z $last ]] ; then
	log "sending full recursive snapshot from $src to $dest"
	ZFS "$srchost" send -R "$cur" | ZFS "$desthost" receive -Fud "$destfs"
# special case: tagged snapshots exist on dest, but src has rotated through all
elif ! ZFS "$srchost" list $srcfs@$last &>/dev/null ; then
	die 1 "no incremental path from from $src to $dest"
# normal case: send incremental
else
	log "sending incremental snapshot from $src to $dest (${last#${tag}_}..${cur#*@${tag}_})"
	ZFS "$srchost" send -R -I "$last" "$cur" | ZFS "$desthost" receive -Fud "$destfs"
fi

###
### clean up old snapshots
###
for snap in $(ZFS "$srchost" list -d 1 -t snapshot -H -S creation -o name $srcfs \
	      | grep -F "@${tag}_" | cut -f2 -d@ | tail -n+$((keep+1)) ) ;
do
	ZFS "$srchost" destroy -r $srcfs@$snap
done