Mini Shell

#!/usr/bin/python
# coding=utf-8
#
# Copyright © Cloud Linux GmbH & Cloud Linux Software, Inc 2010-2019 All Rights Reserved
#
# Licensed under CLOUD LINUX LICENSE AGREEMENT
# http://cloudlinux.com/docs/LICENSE.TXT

import fcntl
import os
from contextlib import contextmanager

CONFIG_DIR = '/etc/container'
LOCK_FILE = os.path.join(CONFIG_DIR, '.ve.cfg.lock')

# TODO: probably should be encapsulated
LOCK_FD = None
LOCK_WRITE = False


class LockFailedException(Exception):
    pass


def setup_global_lock(write=False):
    global LOCK_FD
    global LOCK_WRITE

    if LOCK_FD is not None:
        return False

    if write:
        LOCK_WRITE = True
    try:
        LOCK_FD = open(LOCK_FILE, 'a+', encoding='utf-8')  # pylint: disable=consider-using-with
        fcntl.flock(LOCK_FD.fileno(), fcntl.LOCK_EX)
    except IOError as e:
        raise LockFailedException("IO error happened while getting lock") from e
    LOCK_FD.seek(0, 0)
    LOCK_FD.truncate()
    LOCK_FD.write(str(os.getpid()))
    LOCK_FD.flush()
    return True


def release_lock():
    global LOCK_FD

    # write locks released only on process exit
    if LOCK_WRITE:
        return False

    if LOCK_FD is not None:
        LOCK_FD.close()
        LOCK_FD = None
        return True


@contextmanager
def acquire_lock(write=False):
    """
    Usage:
    with acquire_lock():
       ... do something with ve.cfg ...
    """
    setup_global_lock(write)
    yield
    release_lock()