From: Brian Candler <b.candler@...>
Date: 2009-07-17T17:28:55+09:00
Subject: Re: making sure a file isn't being written/copied before moving

Paul Archer wrote:
>> Paul Archer wrote:
>>> Can someone suggest an easy (or at least reliable)  way to make sure
>>> that
>>> any file I'm about to modify isn't being touched by another program?
>>
>> As you said yourself: rely on the atomic semantics of the filesystem.
>> Rename it to an extension which the other program will not recognise, or
>> into another directory which the other program won't be looking in.
>>
> 
> Perhaps you missed it in my original post (or perhaps I simply wasn't
> clear), but I may be operating on files that are added arbitrarily. My
> concern is that the script starts acting on a file that is still being
> copied. Renaming it won't help there.

The program which drops files into the directory has to work the same 
way:

- open temporary file
- write to it
- close it
- fsync if you want to be sure it's on disk even if power is pulled
- rename it to final location

That's why I said to look at Maildir semantics - adding new E-mails to a 
maildir works like this. (They are written into the tmp/ directory, and 
then renamed into the new/ directory)
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