linux-kernel - Re: openat(..., AT_UNLINKED) was Re: New copyfile system call

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Date:	Sun, 31 Mar 2013 19:28:40 -0400
From:	Ric Wheeler <rwheeler@...hat.com>
To:	Pavel Machek <pavel@....cz>
CC:	"Myklebust, Trond" <Trond.Myklebust@...app.com>,
	Andreas Dilger <adilger@...ger.ca>,
	Jörn Engel <joern@...fs.org>,
	Andy Lutomirski <luto@...capital.net>,
	Zach Brown <zab@...hat.com>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Linux FS Devel <linux-fsdevel@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"Chris L. Mason" <clmason@...ionio.com>,
	Christoph Hellwig <hch@...radead.org>,
	Alexander Viro <aviro@...hat.com>,
	"Martin K. Petersen" <mkp@....net>, Hannes Reinecke <hare@...e.de>,
	Joel Becker <jlbec@...lplan.org>
Subject: Re: openat(..., AT_UNLINKED) was Re: New copyfile system call - discuss
 before LSF?

On 03/31/2013 07:18 PM, Pavel Machek wrote:
> Hi!
>
>>>>>> Take a look at how many actively used filesystems out there that have
>>>>>> some variant of sillyrename(), and explain what you want to do in those
>>>>>> cases.
>>>>> Well. Yes, there are non-unix filesystems around. You have to deal
>>>>> with silly files on them, and this will not be different.
>>>> So this would be a local POSIX filesystem only solution to a problem
>>>> that has yet to be formulated?
>>> Problem is "clasical create temp file then delete it" is racy. See the
>>> archives. That is useful & common operation.
>> Which race are you concerned with exactly?
>>
>> User wants to test for a file with name "foo.txt"
>>
>> * create "foo.txt~" (or whatever)
>> * write contents into "foo.txt~"
>> * rename "foo.txt~" to "foo.txt"
>>
>> Until rename is done, the file does not exists and is not complete.
>> You will potentially have a garbage file to clean up if the program
>> (or system) crashes, but that is not racy in a classic sense, right?
> Well. If people rsync from you, they will start fetching incomplete
> foo.txt~. Plus the garbage issue.

That is not racy, just garbage (not trying to be pedantic, just trying to 
understand). I can see that the "~" file is annoying, but we have dealt with it 
for a *long* time :)

Until it has the right name (on either the source or target system for rsync), 
it is not the file you are looking for.
>
>> This is more of a garbage clean up issue?
> Also. Plus sometimes you want temporary "file" that is
> deleted. Terminals use it for history, etc...

There you would have a race, you can create a file and unlink it of course and 
still write to it, but you would have a potential empty file issue?

Ric


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