feat: add snapshot locking via chattr +i

the snapshot might get deleted from underneath us so lock it by setting
the immutable bit on the snapshot; note we don't need a recursive lock
because the snapshot already is read-only, we just need to prevent it
from getting pruned by 'btrfs subvolume delete'

README.md

@@ -1,6 +1,6 @@
 # btrfs-sync
 
-> **Note**: This fork simply adds in-transfer `zstd` compression support
+> **Note**: This fork simply adds in-transfer `zstd` compression support and snapshot locking capability (see [Features](#features))
 
 Smart and easy sync of BTRFS snapshots, locally or through SSH.
 
@@ -35,6 +35,7 @@ Delete subvolume (no-commit): '/media/USBdrive/home-snapshots/hourly_2018-03-08_
 - Simple syntax
 - Optional progress indication (using `pv`)
 - Support for `zstd`, `xz` or `pbzip2` compression in order to save bandwidth.
+- Snapshot locking via `chattr +i` while sync (`btrfs-send`) is in progress to prevent snapshot deletion, namely by `btrfs subvolume delete ...` invoked e.g as part of `btrfs-snp`
 - Retention policy
 - Automatic incremental synchronization
 - Cron friendly
@@ -114,4 +115,4 @@ chmod +x /etc/cron.daily/btrfs-sync
 
 Original work by *nachoparker* of [ownyourbits.com](https://ownyourbits.com/2018/03/09/easy-sync-of-btrfs-snapshots-with-btrfs-sync/)
 
-zstd compression support added by wanderer of [dotya.ml](https://git.dotya.ml/wanderer)
+zstd compression support and snapshot locking capability added by wanderer of [dotya.ml](https://git.dotya.ml/wanderer)

btrfs-sync

@@ -298,11 +298,24 @@ sync_snapshot() {
 echov "* Check destination"
 get_dst_snapshots "$DST" # sets DSTS DST_UUIDS
 for src in "${SRCS[@]}"; do
+  # the snapshot might get deleted from underneath us so lock it by setting
+  # the immutable bit on the snapshot; note we don't need a recursive lock
+  # because the snapshot already is read-only, we just need to prevent it from
+  # getting pruned by 'btrfs subvolume delete'
+  echov "* Locking '$src'"
+  chattr +i "$src"
   sync_snapshot "$src" && RET=0 || RET=1
+  # unlock after we're done
+  echov "* Freeing lock on '$src'"
+  chattr -i "$src"
   for i in $(seq 1 2); do
     [[ "$RET" != "1" ]] && break
     echo "* Retrying '$src'..."
+    echov "* Locking '$src'"
+    chattr +i "$src"
     sync_snapshot "$src" && RET=0 || RET=1
+    echov "* Freeing lock on '$src'"
+    chattr -i "$src"
   done
   [[ "$RET" == "1" ]] && { echo "Abort"; exit 1; }
 done