Populates a "snapshots" directory in each user's home directory with links at separate versions

zfs_link_snapshots.py

#!/usr/bin/env python

################################################################################
# zfs_link_snapshots.py                                                        #
# Creates a "snapshots" directory in users home directories                    #
# (c) 2019 Andreas Stöckel, licensed under the GPLv3                           #
################################################################################


################################################################################
# COMMAND LINE ARGUMENT PARSING                                                #
################################################################################

# See https://stackoverflow.com/a/43357954
def str2bool(v):
	if v.lower() in ('yes', 'true', 't', 'y', '1'):
		return True
	elif v.lower() in ('no', 'false', 'f', 'n', '0'):
		return False
	else:
		raise argparse.ArgumentTypeError('Boolean value expected.')

import argparse
parser = argparse.ArgumentParser()
parser.add_argument('volume', help='The ZFS volume name')
parser.add_argument('--dry', help='Perform a dry run, only print actions', type=str2bool)
args = parser.parse_args()

volume = args.volume


################################################################################
# STEP 1: List snapshots                                                       #
################################################################################

def exec_process(args):
	import subprocess
	"""
	Executes a child process and exits if the child process fails
	"""
	# Open the child process with the given arguments
	child = subprocess.Popen(args, stdout=subprocess.PIPE, stderr=subprocess.PIPE)

	# Fetch stderr and stdout
	stdout, stderr = child.communicate()

	# Raise an exception in case the return code is not zero
	if child.returncode != 0:
		raise Exception("Error while executing subprocess\n" + stderr)
	return str(stdout, "ascii")

# Fetch all snapshots
snapshots = []
for snapshot in exec_process(["zfs", "get", "-Hro", "name,value", "-p", "creation", volume]).split('\n'):
	snapshot = snapshot.split('\t')
	if len(snapshot) == 2:
		if snapshot[0].startswith(volume + "@"):
			snapshot_name = snapshot[0][len(volume + "@"):]
			snapshot_time = int(float(snapshot[1]))
			snapshots.append((snapshot_name, snapshot_time))

# Get the directory in which the volume is mounted
mnt = exec_process(["zfs", "list", "-Ho", "mountpoint", volume]).strip()

# List all subdirectories and check whether a "snapshots" and a "backup"
# directory exists within that directory
import os, datetime
for path in os.listdir(mnt):
	home_dir = os.path.join(mnt, path)
	snap_dir = os.path.join(home_dir, "snapshots")
	back_dir = os.path.join(home_dir, "backup")
	if not (os.path.isdir(snap_dir) or os.path.isdir(back_dir)):
		continue

	# Remember the set of all existing symlinks
	links_old = set()
	for path in os.listdir(snap_dir):
		path = os.path.join(snap_dir, path)
		if os.path.islink(path):
			if args.dry:
				print("Found existing symlink \"{}\"".format(path))
			links_old.add(path)

	# For each snapshot, generate a link that points at the backup directory
	# in the corresponding snapshot directory
	links_new = set()
	for snapshot_name, snapshot_time in snapshots:
		# Find the target ZFS snapshot directory
		zfs_snap_dir = os.path.join(mnt, ".zfs", "snapshot", snapshot_name)
		rel_back_dir = os.path.relpath(back_dir, mnt)
		zfs_snap_back_dir = os.path.join(zfs_snap_dir, rel_back_dir)
		if not os.path.isdir(zfs_snap_back_dir):
			if args.dry:
				print("Link target", zfs_snap_back_dir, "does not exist for snapshot", snapshot_name)
			continue

		# Construct the symlink name
		dt = datetime.datetime.fromtimestamp(snapshot_time)
		link = os.path.join(snap_dir, "snapshot_" + dt.strftime("%Y-%m-%d_%H%M"))
		if link in links_new:
			cnt = 1
			while "{}_{}".format(link, cnt) in links_new:
				cnt = cnt + 1
			link = "{}_{}".format(link, cnt)
		links_new.add(link)

		# If the target symlink file already exists, make sure it points at the
		# right target, otherwise remove it
		if link in links_old:
			links_old.remove(link)
			zfs_snap_back_dir_old = os.readlink(link)
			if zfs_snap_back_dir_old == zfs_snap_back_dir:
				if args.dry:
					print("Checked existing symlink \"{}\" with target \"{}\"".format(link, zfs_snap_back_dir))
				continue
			print("Removing wrong link",  link)
			if not args.dry:
				os.unlink(link)

		print("Creating new link", link, "->", zfs_snap_back_dir, "for", snapshot_name)
		if not args.dry:
			os.symlink(zfs_snap_back_dir, link)

	# Remove all old links that were not removed from the set above
	for link in links_old:
		print("Removing outdated link",  link)
		if not args.dry:
			os.unlink(link)