'Try to acquire the lock file without blocking.
:param int fd: file descriptor of the opened file to lock'

def _try_lock(self, fd):

    try:
        fcntl.lockf(fd, (fcntl.LOCK_EX | fcntl.LOCK_NB))
    except IOError as err:
        if (err.errno in (errno.EACCES, errno.EAGAIN)):
            logger.debug('A    lock    on    %s    is    held    by    another    process.', self._path)
            raise errors.LockError('Another    instance    of    Certbot    is    already    running.')
        raise
