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SubjectRe: [PATCH 05/11] vfs: add RENAME_NOREPLACE flag
On Wed, Jan 15, 2014 at 10:26:23AM -0800, Andy Lutomirski wrote:
> On Wed, Jan 15, 2014 at 10:19 AM, J. Bruce Fields <bfields@fieldses.org> wrote:
> > On Wed, Jan 08, 2014 at 11:10:09PM +0100, Miklos Szeredi wrote:
> >> From: Miklos Szeredi <mszeredi@suse.cz>
> >>
> >> If this flag is specified and the target of the rename exists then the
> >> rename syscall fails with EEXIST.
> >
> > Why is this useful?
>
> The trivial answer: to eliminate the race condition from 'mv -i'.
>
> Another answer: there's a common pattern to atomically create a file
> with contents: open a temporary file, write to it, optionally fsync
> it, close it, then link(2) it to the final name, then unlink the
> temporary file.
>
> The reason to use link(2) is because it won't silently clobber the destination.
>
> This is annoying:
> - It requires an extra system call that shouldn't be necessary.
> - It doesn't work on (IMO sensible) filesystems that don't support
> hard links (e.g. vfat).
> - It's not atomic -- there's an intermediate state where both files exist.
> - It's ugly.
>
> The new rename flag will make this totally sensible.

Makes sense, thanks! Could that just get cut-n-pasted into the
changelog? (Unless there's some obvious other place to put it.)

--b.

>
> To be fair, on new enough kernels, you can also use O_TMPFILE and
> linkat to achieve the same thing even more cleanly.


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