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

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Message-ID: <20130331231854.GA32174@amd.pavel.ucw.cz>
Date:	Mon, 1 Apr 2013 01:18:54 +0200
From:	Pavel Machek <pavel@....cz>
To:	Ric Wheeler <rwheeler@...hat.com>
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?

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.

> 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...
									Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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