link(2) to rename files in sftp

[Dan Yefimov]

On Tue, 6 Apr 2004, Darren Tucker wrote:

> Carson Gaspar wrote:
> 
> > --On Tuesday, April 06, 2004 09:24:31 +1000 Darren Tucker 
> > 
> >> According to the CVS log, the link shuffle is used to "fix races in
> >> rename/symlink" (revs 1.46 and 1.44).
> > 
> > If rename() has a race condition, the OS is broken. Plain and simple.
> 
> The original code for rename looked like the following:
>      if (stat(newpath, &st) == -1) {
>          ret = rename(oldpath, newpath);
>          status = (ret == -1) ? errno_to_portable(errno) : SSH2_FX_OK;
>      }
> 
> The idea is obviously to not clobber existing files, but the 
> implementation is racy (hence the change, I guess).
> 
> I have no idea how to implement that portably for filesystems without 
> Unix semantics without the race.
> 
May be the following code could be used:

	if ((ret = open(newpath, O_WRONLY|O_CREAT|O_EXCL, S_IRUSR)) != -1) {
		close(ret);
		ret = rename(oldpath, newpath);
		status = (ret == -1) ? errno_to_portable(errno) : SSH2_FX_OK;
	}

Of course, someone could modify temporary file mode and write something into it 
between calls to open() and rename() are made, but does somebody really care 
about that case?
-- 

    Sincerely Your, Dan.