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Re: How to safely maintain a status file

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Date Thu, 12 Jul 2012 14:30:41 +0200
From Laszlo Nagy <gandalf@shopzeus.com>
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To Christian Heimes <lists@cheimes.de>, python-list@python.org
Subject Re: How to safely maintain a status file
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> 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.
This is not a contradiction. Although the rename operation is atomic, 
the whole "change status" process is not. It is because there are two 
operations: #1 delete old status file and #2. rename the new status 
file. And because there are two operations, there is still a race 
condition. I see no contradiction here.
>
> 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.
It is not entirely true. We are talking about two processes. One is 
reading a file, another one is writting it. They can run at the same 
time, so flushing disk cache forcedly won't help.

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Thread

Re: How to safely maintain a status file Laszlo Nagy <gandalf@shopzeus.com> - 2012-07-12 14:30 +0200
  Re: How to safely maintain a status file Hans Mulder <hansmu@xs4all.nl> - 2012-07-12 15:19 +0200
    Re: How to safely maintain a status file Laszlo Nagy <gandalf@shopzeus.com> - 2012-07-12 19:43 +0200
    Re: How to safely maintain a status file Christian Heimes <lists@cheimes.de> - 2012-07-12 20:39 +0200
      Re: How to safely maintain a status file Rick Johnson <rantingrickjohnson@gmail.com> - 2012-07-12 18:20 -0700
        Re: How to safely maintain a status file Chris Angelico <rosuav@gmail.com> - 2012-07-13 12:12 +1000
          Re: How to safely maintain a status file Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-07-13 03:13 +0000
            Re: How to safely maintain a status file Gene Heskett <gheskett@wdtv.com> - 2012-07-12 23:49 -0400
              Re: How to safely maintain a status file Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-07-13 04:21 +0000
            Re: How to safely maintain a status file rantingrickjohnson@gmail.com - 2012-07-12 21:26 -0700
              Re: How to safely maintain a status file Chris Angelico <rosuav@gmail.com> - 2012-07-13 16:02 +1000
              Re: How to safely maintain a status file Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-07-13 07:14 +0000
                Re: How to safely maintain a status file Dennis Lee Bieber <wlfraed@ix.netcom.com> - 2012-07-13 13:29 -0400
              RE: How to safely maintain a status file "Prasad, Ramit" <ramit.prasad@jpmorgan.com> - 2012-07-13 16:00 +0000
              Re: [Python] RE: How to safely maintain a status file Chris Gonnerman <chris@gonnerman.org> - 2012-07-13 12:27 -0500
              RE: [Python] RE: How to safely maintain a status file "Prasad, Ramit" <ramit.prasad@jpmorgan.com> - 2012-07-13 17:59 +0000
                Re: [Python] RE: How to safely maintain a status file Hans Mulder <hansmu@xs4all.nl> - 2012-07-13 20:28 +0200
                Re: [Python] RE: How to safely maintain a status file MRAB <python@mrabarnett.plus.com> - 2012-07-13 20:57 +0100
                Re: [Python] RE: How to safely maintain a status file Christian Heimes <lists@cheimes.de> - 2012-07-13 22:21 +0200
              Re: [Python] RE: How to safely maintain a status file Chris Angelico <rosuav@gmail.com> - 2012-07-14 04:19 +1000
              RE: How to safely maintain a status file Chris Gonnerman <chris@gonnerman.org> - 2012-07-13 15:15 -0500
                Re: How to safely maintain a status file Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-07-14 01:53 +0000

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