Add delta snapshot restoration script

btrfs/merge-delta-file

@@ -0,0 +1,67 @@
+#!/bin/sh
+#
+# BTRFS delta snapshots restoration
+# CC0 ~caskd
+#
+# This recursively goes to the initial full backup and restores every step to the requested source
+
+set -e
+
+: ${SRC:?'Source not defined'}
+: ${SIZE:=1G}
+
+DATANAME="data"
+PARENTNAME="parent"
+LOOPBACKNAME="restore"
+TMPPATTERN="btrfs-restore.XXXXXXXXXX"
+
+msg() {
+	echo "$1" >&2
+}
+
+cleanup_err() {
+	cleanup
+	rm -v "$LOOPBACKNAME"
+}
+
+cleanup() {
+	# Disable traps and cleanup
+	trap '' INT TERM EXIT HUP
+	umount -v "$TMPDIR"
+	rmdir "$TMPDIR"
+}
+
+decompress() {
+	zstd -d
+}
+
+delta_recieve() {
+	local CURRENT="$1"
+	local DEST="$2"
+	if [ -L "$CURRENT/$PARENTNAME" ]; then
+		local PARENT="$(realpath "$CURRENT/$PARENTNAME")"
+		if ! delta_recieve "$PARENT" "$DEST"; then
+			msg "Cannot receive $(basename "$PARENT") > $(basename "$CURRENT")"
+			return
+		fi
+	fi
+	if ! < "$CURRENT"/"$DATANAME" decompress | btrfs -v receive "$DEST"; then
+		msg "Failed to receive $(basename "$CURRENT")"
+		return
+	fi
+}
+
+trap cleanup_err INT TERM EXIT HUP
+
+fallocate -l "$SIZE" "$LOOPBACKNAME" || return
+LOOPBACK="$(losetup --show -f "$LOOPBACKNAME")" || return
+echo "$LOOPBACK"
+
+mkfs.btrfs "$LOOPBACK" || return
+TMPDIR="$(mktemp -d "$TMPPATTERN")" || return
+echo "$TMPDIR"
+
+mount -t btrfs "$LOOPBACK" "$TMPDIR" || return
+delta_recieve "$SRC" "$TMPDIR" || return
+
+cleanup