#!/bin/sh
#
# BTRFS delta snapshots with incomplete send handling
# CC0 ~caskd

: ${SRC:?'Source not defined'}
: ${SNAPDIR:?'Snapdir not defined'}
: ${DEST:?'Destination not defined'}

CDATE="$(date '+%s')"
SNAPSRC="$SRC"
SNAPPATH="$SNAPSRC/$SNAPDIR"
PREV="$SRC/prev"
CUR="$SRC/cur"
TMP="$SRC/tmp"
SNAPDEST="$SNAPPATH/$CDATE"

msg() {
	echo "$1" >&2
}

delta() {
	[ -d "$(realpath "$CUR")" ] || return 1
	msg "Attempting to send delta"
	btrfs -v send --compressed-data -p "$CUR" "$TMP" | btrfs -v receive "$DEST"
}

full() {
	msg "Attempting to send full"
	btrfs -v send --compressed-data "$TMP" | btrfs -v receive "$DEST"
}

symdel() {
	if [ -L "$1" ]; then
		btrfs subvolume delete "$1" >&2 || true
		rm -v "$1" >&2
	fi
}

mkdir -vp "$SNAPPATH" >&2

symdel "$TMP"
symdel "$PREV"

ln -sv "$CDATE" "$TMP" >&2
btrfs subvolume snapshot -r "$SNAPSRC" "$SNAPDEST" >&2

delta || full

# Mark snapshot transmission as complete
[ -L "$CUR" ] && \
	mv -v "$CUR" "$PREV" >&2
mv -v "$TMP" "$CUR" >&2