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

bundles/backup-server/files/check_backup_for_node-cron

@@ -0,0 +1,39 @@
+#!/usr/bin/env python3
+
+from json import load, dump
+from subprocess import check_output
+from shutil import move
+from os import remove
+from collections import defaultdict
+
+with open('/etc/backup-server/config.json', 'r') as f:
+    server_settings = load(f)
+
+snapshots = defaultdict(set)
+
+for line in check_output('LC_ALL=C zfs list -H -t snapshot -o name', shell=True).splitlines():
+    line = line.decode('UTF-8')
+
+    if line.startswith('{}/'.format(server_settings['zfs-base'])):
+        dataset, snapname = line.split('@', 1)
+
+        dataset = dataset.split('/')[-1]
+        ts, bucket = snapname.split('-', 1)
+
+        if not ts.isdigit():
+            # garbage, ignore
+            continue
+
+        snapshots[dataset].add(int(ts))
+
+backups = {}
+for dataset, snaps in snapshots.items():
+    backups[dataset] = sorted(snaps)[-1]
+
+with open('/etc/backup-server/backups.tmp.json', 'w') as f:
+    dump(backups, f)
+
+move(
+    '/etc/backup-server/backups.tmp.json',
+    '/etc/backup-server/backups.json',
+)