Re: How to safely maintain a status file

Path	csiph.com!usenet.pasdenom.info!gegeweb.org!de-l.enfer-du-nord.net!feeder1.enfer-du-nord.net!feeds.phibee-telecom.net!newsfeed.xs4all.nl!newsfeed5.news.xs4all.nl!xs4all!post.news.xs4all.nl!not-for-mail
Return-Path	<python-python-list@m.gmane.org>
X-Original-To	python-list@python.org
Delivered-To	python-list@mail.python.org
X-Spam-Status	OK 0.000
X-Spam-Evidence	'H': 1.00; 'S': 0.00; 'rename': 0.07; 'subject:file': 0.07; 'terminated': 0.07; 'subject:How': 0.09; '(although': 0.09; 'flush': 0.09; 'overwrite': 0.09; 'received:80.91': 0.09; 'received:80.91.229': 0.09; 'received:gmane.org': 0.09; 'received:list': 0.09; 'atomically': 0.16; 'deleted,': 0.16; 'fancy': 0.16; 'from:addr:cheimes.de': 0.16; 'from:addr:lists': 0.16; 'from:name:christian heimes': 0.16; 'maildir': 0.16; 'message-id:@dough.gmane.org': 0.16; 'posix': 0.16; 'received:80.91.229.3': 0.16; 'received:plane.gmane.org': 0.16; 'renames': 0.16; 'side.': 0.16; 'stuff.': 0.16; 'temp': 0.16; 'documented': 0.17; 'platforms': 0.18; 'windows': 0.19; 'file.': 0.20; 'exists.': 0.22; 'work,': 0.22; '(you': 0.23; 'header:In-Reply-To:1': 0.25; 'header:User-Agent:1': 0.26; 'disk': 0.27; "doesn't": 0.28; 'header:X-Complaints-To:1': 0.28; 'locks': 0.29; 'case,': 0.29; 'e.g.': 0.30; 'compatible': 0.30; 'file': 0.32; 'could': 0.32; 'to:addr:python-list': 0.33; "can't": 0.34; 'christian': 0.34; 'too.': 0.35; 'something': 0.35; 'there': 0.35; 'received:org': 0.36; 'but': 0.36; 'correctly': 0.37; 'two': 0.37; 'data': 0.37; 'subject:: ': 0.38; 'some': 0.38; 'sure': 0.38; 'to:addr:python.org': 0.39; 'header:Received:5': 0.40; 'your': 0.60; 'matter': 0.61; 'side': 0.61; 'provide': 0.62; 'race': 0.71; 'integrity': 0.75; 'guaranteed': 0.76; 'subject:status': 0.84; 'reasons:': 0.91
X-Injected-Via-Gmane	http://gmane.org/
To	python-list@python.org
From	Christian Heimes <lists@cheimes.de>
Subject	Re: How to safely maintain a status file
Date	Sun, 08 Jul 2012 23:52:40 +0200
References	<CAOV1wRVtm27yWez1HZuN8=ia-TyM2aXp9QCUbSZ5aZExP_ZChA@mail.gmail.com> <sanjv7lo0vb3rlhip4ov1gpgp4gs51bvfr@invalid.netcom.com> <4FF9F454.40207@shopzeus.com>
Mime-Version	1.0
Content-Type	text/plain; charset=ISO-8859-1
Content-Transfer-Encoding	7bit
X-Gmane-NNTP-Posting-Host	f048046007.adsl.alicedsl.de
User-Agent	Mozilla/5.0 (X11; Linux x86_64; rv:12.0) Gecko/20120430 Thunderbird/12.0.1
In-Reply-To	<4FF9F454.40207@shopzeus.com>
X-BeenThere	python-list@python.org
X-Mailman-Version	2.1.12
Precedence	list
List-Id	General discussion list for the Python programming language <python-list.python.org>
List-Unsubscribe	<http://mail.python.org/mailman/options/python-list>, <mailto:python-list-request@python.org?subject=unsubscribe>
List-Archive	<http://mail.python.org/pipermail/python-list>
List-Post	<mailto:python-list@python.org>
List-Help	<mailto:python-list-request@python.org?subject=help>
List-Subscribe	<http://mail.python.org/mailman/listinfo/python-list>, <mailto:python-list-request@python.org?subject=subscribe>
Newsgroups	comp.lang.python
Message-ID	<mailman.1929.1341784379.4697.python-list@python.org> (permalink)
Lines	28
NNTP-Posting-Host	2001:888:2000:d::a6
X-Trace	1341784379 news.xs4all.nl 6988 [2001:888:2000:d::a6]:47043
X-Complaints-To	abuse@xs4all.nl
Xref	csiph.com comp.lang.python:25055

Show key headers only | View raw

Am 08.07.2012 22:57, schrieb Laszlo Nagy:
> But even if the rename operation is atomic, there is still a race
> condition. Your program can be terminated after the original status file
> has been deleted, and before the temp file was renamed. In this case,
> you will be missing the status file (although your program already did
> something just it could not write out the new status).

You are contradicting yourself. Either the OS is providing a fully
atomic rename or it doesn't. All POSIX compatible OS provide an atomic
rename functionality that renames the file atomically or fails without
loosing the target side. On POSIX OS it doesn't matter if the target exists.

You don't need locks or any other fancy stuff. You just need to make
sure that you flush the data and metadata correctly to the disk and
force a re-write of the directory inode, too. It's a standard pattern on
POSIX platforms and well documented in e.g. the maildir RFC.

You can use the same pattern on Windows but it doesn't work as good and
doesn't guaranteed file integrity for two reasons:

1) Windows's rename isn't atomic if the right side exists.

2) Windows locks file when a program opens a file. Other programs can't
rename or overwrite the file. (You can get around the issue with some
extra work, though.)

Christian

Back to comp.lang.python | Previous | Next — Next in thread | Find similar | Unroll thread

Thread

Re: How to safely maintain a status file Christian Heimes <lists@cheimes.de> - 2012-07-08 23:52 +0200
  Re: How to safely maintain a status file John Nagle <nagle@animats.com> - 2012-07-09 13:24 -0700
    Re: How to safely maintain a status file Christian Heimes <lists@cheimes.de> - 2012-07-10 01:41 +0200
    Re: How to safely maintain a status file alex23 <wuwei23@gmail.com> - 2012-07-09 19:04 -0700
    Re: How to safely maintain a status file Laszlo Nagy <gandalf@shopzeus.com> - 2012-07-12 14:31 +0200

csiph-web