Python renameat2 RENAME_EXCHANGE wrapper

rename-exchange.py

#!/usr/bin/env python3

import ctypes
import errno
import logging
import os
import tempfile

logger = logging.getLogger(os.path.basename(__file__))
libc = ctypes.CDLL('libc.so.6', use_errno=True)
AT_FDCWD = -100
RENAME_EXCHANGE = (1 << 1)


def rename_exchange(a, b):
    if hasattr(libc, 'renameat2'):
        if libc.renameat2(AT_FDCWD, a, AT_FDCWD, b, RENAME_EXCHANGE) == 0:
            logger.debug('renameat2 with RENAME_EXCHANGE succeeded')
            return

        err = ctypes.get_errno()
        if err == errno.EINVAL:
            logger.debug('RENAME_EXCHANGE flag not supported by kernel')
        elif err == errno.ENOSYS:
            logger.debug('renameat2 syscall not available in kernel')
        else:
            raise OSError(err, os.strerror(err), a, None, b)
    else:
        logger.debug('renameat2 not available in libc')

    # Fallback using a temporary name in the same directory as a. b is
    # renamed to the temporary name first so that if they're not on the
    # same filesystem no recovery is needed.
    logger.debug('Falling back to renames with temporary files')
    a_tmp = tempfile.mktemp(prefix=a, dir=os.path.dirname(a))
    logger.debug('Renaming "%s" to "%s"', b, a_tmp)
    os.rename(b, a_tmp)
    logger.debug('Renaming "%s" to "%s"', a, b)
    os.rename(a, b)
    logger.debug('Renaming "%s" to "%s"', a_tmp, a)
    os.rename(a_tmp, a)


if __name__ == '__main__':
    from argparse import ArgumentParser
    ap = ArgumentParser(description='Try out renameat2 with RENAME_EXCHANGE')
    ap.add_argument('A', help='first path')
    ap.add_argument('B', help='second path')
    ap.add_argument('--debug', dest='log_level', default=logging.WARNING,
                    action='store_const', const=logging.DEBUG,
                    help='enable debugging messages')
    args = ap.parse_args()
    logging.basicConfig(level=args.log_level)
    rename_exchange(args.A, args.B)