#!/usr/bin/env python3


import re
from datetime import datetime
from json import loads
from subprocess import check_call, check_output
from sys import argv


def list_datasets():
    datasets = set()
    for line in check_output(['zfs', 'list', '-H', '-o', 'name']).splitlines():
        line = line.decode('UTF-8')

        for prefix in metadata.get('snapshot_never', set()):
            if line.startswith(prefix):
                break
        else:
            datasets.add(line)

    return datasets


def get_filtered_snapshots_for_dataset(ds):
    all_snapshots = check_output(['zfs', 'list', '-H', '-o', 'name', '-t', 'snapshot']).decode('UTF-8').splitlines()

    prefix = '{}@zfs-auto-snap_{}-'.format(ds, label)
    snapshots = set()

    for i in sorted(all_snapshots):
        if i.startswith(prefix):
            snapshots.add(i)

    return snapshots


def delete_snapshot(snap):
    assert '@' in snap, 'BUG! Dataset "{}" has no @!'.format(snap)
    print('deleting snapshot {}'.format(snap))
    check_call(['zfs', 'destroy', snap])


def create_snapshot(ds, label, now):
    check_call(['zfs', 'snapshot', '{}@zfs-auto-snap_{}-{}'.format(ds, label, now)])
    print('created snapshot {}@zfs-auto-snap_{}-{}'.format(ds, label, now))


label = argv[1]

with open('/etc/zfs-snapshot-config.json', 'r') as fp:
    metadata = loads(fp.read())

default_retain = metadata['retain_defaults'][label]
now = datetime.now().strftime('%F-%H%M')
snapshots_created = False

for ds in list_datasets():
    retain = int(metadata.get('retain_per_dataset', {}).get(ds, {}).get(label, default_retain))

    if retain > 0:
        create_snapshot(ds, label, now)
        snapshots_created = True

    existing_snapshots = get_filtered_snapshots_for_dataset(ds)

    if retain > 0:
        # Why +1 here? Because we're specifying the amount of hours we want
        # to go back in time, not the amount of snapshots we want to keep.
        # Stating '1 month' does mean 'i want to be able to go back atleast
        # one monthly snapshot', not 'keep one monthly snapshot'. If we only
        # kept one snapshot, that wouldn't be possible for most of the time,
        # because the monthly snapshot is actually less than a month old.
        snapshots_to_keep = retain+1

        for snapshot in sorted(existing_snapshots)[:-snapshots_to_keep]:
            delete_snapshot(snapshot)
    else:
        for snapshot in existing_snapshots:
            delete_snapshot(snapshot)

with open('/var/tmp/zfs-auto-snapshot.status', 'w') as fp:
    fp.write('{}\n'.format(datetime.now().strftime('%s') if snapshots_created else 0))
