Fix backup script to be able to handle nested btrfs mounts

Earlier it would only snapshot the top dir which resulted in some awkward moments when another btrfs drive that was mounted to /home was empty in the backups because snapshots only snapshot the topmost device in the tree.

roles/borg-client/templates/borg-backup.sh.j2

@@ -3,7 +3,8 @@
 set -e
 
 src="/"
-snapshotdir="/backup-snap"
+snapshotdir="backup-snapshot"
+backup_mountdir="/backup"
 
 ##
 #  usage : is_btrfs( $path )
@@ -11,19 +12,32 @@ snapshotdir="/backup-snap"
 # source: makechrootpkg from devtools
 ##
 is_btrfs() {
-	[[ -e "$1" && "$(stat -f -c %T "$1")" == btrfs ]]
+    [[ -e "$1" && "$(stat -f -c %T "$1")" == btrfs ]]
 }
 
+##
+#  usage : delete_snapshot( $path )
+##
 delete_snapshot() {
-	btrfs subvolume delete --commit-after "$snapshotdir"
+    btrfs subvolume delete --commit-after "$1"
 }
 
 if is_btrfs "$src"; then
-	if [[ -d "$snapshotdir" ]]; then
-		delete_snapshot
-	fi
-	btrfs subvolume snapshot -r "$src" "$snapshotdir"
-	src="$snapshotdir"
+    list_of_btrfs_submounts=$(findmnt -Rl -o target,fstype,options / | grep btrfs | grep -v "subvol=\/[[:alnum:]]" | cut -d ' ' -f1)
+
+    if [[ -d "$backup_mountdir" ]]; then
+        umount -R "$backup_mountdir"
+        rmdir "$backup_mountdir"
+    fi
+    mkdir "$backup_mountdir"
+    for vol in $list_of_btrfs_submounts; do
+        if [[ -d "$vol/$snapshotdir" ]]; then
+            delete_snapshot "$vol/$snapshotdir"
+        fi
+
+        btrfs subvolume snapshot -r "$vol" "$vol/$snapshotdir"
+        mount -o bind "$vol/$snapshotdir" "$backup_mountdir/$vol"
+    done
 fi
 
 systemctl is-active postgresql && /usr/local/bin/backup-postgres.sh || true
@@ -44,11 +58,13 @@ borg create -v --stats -C lz4 \
     -e "$snapshotdir/var/lib/archbuild" \
     -e "$snapshotdir/var/lib/archbuilddest" \
     -e "$snapshotdir/srv/archive" \
-    {{ backup_host }}:{{ backup_dir }}::$(date "+%Y%m%d-%H%M%S") "$src"
+    {{ backup_host }}:{{ backup_dir }}::$(date "+%Y%m%d-%H%M%S") "$backup_mountdir"
 borg prune -v {{ backup_host }}:{{ backup_dir }} --keep-daily=7 --keep-weekly=4 --keep-monthly=6
 
-if [[ -d "$snapshotdir" ]]; then
-	delete_snapshot
-fi
+umount -R "$backup_mountdir"
+for vol in $list_of_btrfs_submounts; do
+    delete_snapshot "$vol/$snapshotdir"
+done
+rmdir "$backup_mountdir"
 
 /usr/bin/vendor_perl/borg-restore.pl --update-cache