#!/usr/bin/env python3

import os
import tempfile

def test_atomic_file_replace():
    # an example of SAFE atomic file replacement in modern UNIX-like FS
    # This sample is incomplete since does not call fsync() on a directory.
    # so, in case of power outage, old file (with old contents) may be
    # visible in dir
    with tempfile.TemporaryDirectory() as testdir:

        # just create sample file with initial contents
        with open(os.path.join(testdir, 'xxx.dat'), 'wb') as sample:
            sample.write(b'foo')

        target = sample.name
        # if testdir is symlink, we should create temporary file in dir
        # where pointed file placed, instead of dir where symlink itself
        # is placed
        realdir = os.path.dirname(os.path.realpath(target))
        with tempfile.NamedTemporaryFile(dir=realdir, mode='wb') as replacement:
            replacement.write(b'bar')

            # To prevent data loss in case of power outage
            # we should first flush data to disk.
            replacement.flush()
            os.fdatasync(replacement.fileno())

            # rename atomically. If exception happen here, temporary file
            # will be removed and original file will be left intact
            os.rename(replacement.name, target)
            replacement.delete = False

        assert not os.path.exists(replacement.name)

        # open replaced file and check that it has been actually replaced
        with open(target, 'rb') as two:
            assert two.read() == b'bar'

test_atomic_file_replace()