Add file-based delta snapshots

btrfs/subvolume-delta-file

@@ -0,0 +1,70 @@
+#!/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 "../$TS_PARENT/$DESTNAME" "$DEST/$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