bundles/backups-server: read backup snapshot info from file instead of asking zfs every time

bundles/backup-server/files/check_backup_for_node

@@ -2,7 +2,6 @@
 
 from datetime import datetime
 from json import load
-from subprocess import check_output
 from sys import argv, exit
 from time import time
 
@@ -18,29 +17,17 @@ try:
     with open(f'/etc/backup-server/config.json', 'r') as f:
         server_settings = load(f)
 
-    # get all existing snapshots for NODE
-    for line in check_output('LC_ALL=C zfs list -H -t snapshot -o name', shell=True).splitlines():
-        line = line.decode('UTF-8')
+    with open(f'/etc/backup-server/backups.json', 'r') as f:
+        backups = load(f)
 
-        if line.startswith('{}/{}@'.format(server_settings['zfs-base'], NODE)):
-            _, snapname = line.split('@', 1)
-
-            if 'zfs-auto-snap' in snapname:
-                # migration from auto-snapshots, ignore
-                continue
-
-            ts, bucket = snapname.split('-', 1)
-            snaps.add(int(ts))
-
-    if not snaps:
+    if NODE not in backups:
         print('No backups found!')
         exit(2)
 
-    last_snap = sorted(snaps)[-1]
-    delta = NOW - last_snap
+    delta = NOW - backups[NODE]
 
     print('Last backup was on {} UTC'.format(
-        datetime.fromtimestamp(last_snap).strftime('%Y-%m-%d %H:%M:%S'),
+        datetime.fromtimestamp(backups[NODE]).strftime('%Y-%m-%d %H:%M:%S'),
     ))
 
     # One day without backups is still okay. There may be fluctuations