Re: [PATCH 05/11] vfs: add RENAME_NOREPLACE flag

["J. Bruce Fields" <bfields@fieldses.org>]

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.