Move prev deletion below and guarantee data flush before marking backup as finished

btrfs/subvolume-delta-file

@@ -2,6 +2,9 @@
 #
 # BTRFS delta snapshots with incomplete send handling
 # CC0 ~caskd
+#
+# Note that this script currently has no locking mechanism
+# This is left as a requirement to be guaranteed from the outside
 
 : ${SRC:?'Source not defined'}
 : ${SNAPDIR:?'Snapdir not defined'}
@@ -37,6 +40,7 @@ delta() {
 	[ -e "$DESTPATH.tmp" ] && rm -v "$DESTPATH.tmp"
 
 	if btrfs -v send --compressed-data -p "$CUR" "$TMP" | compress > "$DESTPATH.tmp"; then
+		sync "$DESTPATH.tmp"
 		mv -v "$DESTPATH.tmp" "$DESTPATH"
 		ln -sv "../$TS_PARENT" "$DEST/$TS_CURRENT/$PARENTNAME"
 	fi
@@ -51,6 +55,7 @@ full() {
 	[ -e "$DESTPATH.tmp" ] && rm -v "$DESTPATH.tmp"
 
 	if btrfs -v send --compressed-data "$TMP" | compress > "$DESTPATH.tmp"; then
+		sync "$DESTPATH.tmp"
 		mv -v "$DESTPATH.tmp" "$DESTPATH"
 	fi
 }
@@ -65,14 +70,13 @@ symdel() {
 mkdir -vp "$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
+symdel "$PREV"
 [ -L "$CUR" ] && \
 	mv -v "$CUR" "$PREV" >&2
 mv -v "$TMP" "$CUR" >&2