gistfile1.sc

# vim:set ts=4 sw=4 et ai:
from __future__ import with_statement

"""
:Author: Toby DiPasquale <toby@cbcg.net>
:Copyright: Copyright (c) 2008 Toby DiPasquale. Free for use for any purpose 
            by anyone forever.

Introduction
============

Built from the recommendations found in the "Eat My Data" talk by Stewart 
Smith of MySQL AB (online here: http://www.flamingspork.com/talks/), this 
module will help you avoid data corruption issues on multiple platforms.

This module requires Python 2.5 or higher.

"""

__docformat__ = "restructuredtext"

import sys
import os

#
# Mac OS X's fsync() doesn't really sync to disk; instead we have
# to issue this fcntl() to make sure the data is on disk on that
# platform.
#
if sys.platform == 'darwin':
    # 51 == F_FULLSYNC on Mac OS X (/usr/include/sys/fcntl.h)
    import fcntl
    _sync = lambda fd : fcntl.fcntl(fd, 51)
else:
    _sync = os.fsync

def save(path, data, mode='w', offset=None, whence=os.SEEK_SET):
    """
    Write `data` into `path` in the safest way possible vis a vis avoiding 
    data corruption.
    
    :Parameters:
        path : str
            path to file into which to write `data`
        data : str
            contents to write to `path`
        mode : str
            mode with which to open `path` (e.g. ``'w'`` for writing, ``'a'``
            for appending, etc.; see `open` docs for details)
        offset : long
            byte offset into `path` into which to write `data`
        whence : int
            how to interpret `offset` in terms of positioning in the file;
            see `os.seek` for details on valid arguments

    :raise `OSError`: raised by underlying OS ops in case of error
    :raise `IOError`: raised by underlying file ops in case of error
    """
    tmp = _tempfile(path)
    with open(tmp, mode) as f:
        if offset:
            f.seek(offset, whence)
        f.write(data)
        f.flush()
        _sync(f.fileno())
    os.rename(tmp, path)

def _tempfile(path):
    """
    Create a tempfile name for the given `path`.
    
    :Parameters:
        path : str
            original file path

    :rtype: str
    :return: path to tempfile for `path`
    """
    return '%s.tmp' % path