Re: [PATCH] mv: add --swap (-s) option to atomically swap 2 paths

[Pádraig Brady]

On 02/03/2024 12:46, Dominique Martinet wrote:
> Thanks for remembering me; didn't try the patch yet but overall looks
> good to me.
>
> Just one nitpick on the not supported message check:
> Pádraig Brady wrote on Sat, Mar 02, 2024 at 11:01:42AM +0000:
> > +      if (renameatu (AT_FDCWD, file[0], AT_FDCWD, file[1],
> > +                                   RENAME_EXCHANGE))
> > +        {
> > +          if (errno == EINVAL || is_ENOTSUP (errno))
>
> checking the man page, renameeat with RENAME_EXCHANGE on a dir and
> something inside it also fails with EINVAL, so that will print the
> operation isn't supported when it can probably be considered a user
> error (I did't take the time to try this patch, using another tool to
> illustrate but it should be the same):
> $ mkdir a
> $ touch a/b
> $ ./renameat2 --exchange a a/b
> renameat2: Invalid argument

For comparison, mv now does:

  $ mv -x a a/b
  mv: atomic swap of 'a' and 'a/b' is not supported

> I guess there's not much we can do about this though, EINVAL could also
> really mean not supported in this case and checking if one path is a
> prefix of another seems overkill to me...

Right. Since the syscall doesn't distinguish errors in this case,
our error is no better or worse I think.  Having mv distinguish
based on path processing would be overkill I agree.
Also it's a bit of an edge case anyway.

cheers,
Pádraig