Backup-Scripts/btrfs/subvolume-delta-file

#!/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="$SNAPPATH/prev"
CUR="$SNAPPATH/cur"
TMP="$SNAPPATH/tmp"
SNAPDEST="$SNAPPATH/$CDATE"
PARENTNAME="parent"
DESTNAME="data"

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

compress() {
	zstd -T0 -z -9
}

delta() {
	local TS_PARENT="$(basename "$(realpath "$CUR")")"
	local TS_CURRENT="$(basename "$(realpath "$TMP")")"
	[ -d "$(realpath "$CUR")" ] || return 1
	[ -e "$DEST/$TS_PARENT/$DATA" ] || return
	msg "Attempting to send delta"

	mkdir -vp "$DEST/$TS_CURRENT"
	ln -sv "../$TS_PARENT" "$DEST/$TS_CURRENT/$PARENTNAME"
	btrfs -v send --compressed-data -p "$CUR" "$TMP" | compress > "$DEST/$TS_CURRENT/$DESTNAME.tmp"
	mv -v "$DEST/$TS_CURRENT/$DESTNAME.tmp" "$DEST/$TS_CURRENT/$DESTNAME"
}

full() {
	local TS_CURRENT="$(basename "$(realpath "$TMP")")"
	msg "Attempting to send full"

	mkdir -vp "$DEST/$TS_CURRENT"
	btrfs -v send --compressed-data "$TMP" | compress > "$DEST/$TS_CURRENT/$DESTNAME.tmp"
	mv -v "$DEST/$TS_CURRENT/$DESTNAME.tmp" "$DEST/$TS_CURRENT/$DESTNAME"
}

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 || return 1

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