bug#27986: 26.0.50; `rename-file' can rename files without confirmation

[Eli Zaretskii <eliz@gnu.org>]

> From: Philipp Stephani <p.stephani2@gmail.com>
> Date: Mon, 14 Aug 2017 17:09:35 +0000
> Cc: 27986@debbugs.gnu.org
> 
>  > Note how `rename-file' has silently overwritten `ß'. This is because on
>  > macOS, `ß' and `ẞ' are different file names, but Emacs treats them as
>  > equal. Probably the test for case-insensitive file names should be
>  > removed altogether
> 
>  Which one? there are two of them.
> 
> I guess all of them where correctness would depend on the outcome.

But then we lose a feature which just a few releases ago was deemed
valuable enough to have.

> As this example shows, there are cases where two case-insensitive filenames are considered equivalent by
> Emacs, but different by the actual filesystem. This is unavoidable, because the definition of "case-insensitive"
> changes all the time, both in Emacs and in the filesystems. Generally it's impossible to detect whether two
> filenames would refer to the same file without actually creating the file.

Wouldn't trying to rename it actually tell you that, by failing with EEXIST?
(On Windows, it simply succeeds and changes the letter-case silently,
but I understand that is not what happens on macOS.)

>  If the former,
>  then at least on MS-Windows we have a race anyway, because the
>  underlying system APIs are not atomic.
> 
> Wouldn't MoveFileExW with MOVE_FILE_REPLACE_EXISTING be atomic?

No, it isn't guaranteed to be atomic.  No documentation says that,
certainly not any official documentation.  If I had access to the
sources, I could have looked there, but I don't.  Failing that, my
assumption is that it isn't atomic, because meta-data of more than one
file needs to be touched in this case.

> Yes, I mean the functions described in section 2 of the man page. link(2) is a common markup for this.

My point was that GNU coding standards frown on use of such markup.

>  More to the point, how can this strategy work on a case-insensitive
>  filesystem? What am I missing?
> 
> IIUC link(2) + unlink(2) would, if successful, guarantee enough atomicity in the sense that the old file is now
> guaranteed to be the new file, and the call is guaranteed to fail if the new file already exists.

I actually think that if the old and the new name are equal but for
the letter-case, and the filesystem is case-insensitive, doing that
will delete the file, so you are left with nothing.

> I don't think anything can help with the case-changing problem; I
> think we just have to live with an occasional false positive signal
> in this case.

That'd be an unfortunate regression, IMO.  It isn't right for us to
back out changes we just introduced, which were considered important
when we did that.  We ought to find a solution that doesn't remove
this feature, not even on macOS.