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

[Paul Eggert <eggert@cs.ucla.edu>]

On 08/16/2017 03:31 PM, Stefan Monnier wrote:
>>> This should be quite rare.  The only scenario I see matching your
>>> concern is if the source is a directory, the destination is not
>>> a directory name but is an empty directory and is not a symlink, and
>>> the destination is not a descendant of  the source.  Although not
>>> impossible, this will happen so rarely that it doesn't invalidate
>>> the proposed change.
> Paul's suggestion makes a lot of sense to me, but I don't quite
> understand the above: why does the emptiness of the destination
> directory matter?

It's because the system call rename(A,B) always fails when B is a 
nonempty directory. Hence the proposed (rename-file A B) will fail if B 
is not a directory name but happens to name a nonempty directory, and 
therefore rename-file noisily fails instead of silently behaving 
differently in this case (which was Eli's concern). Conversely, if B is 
an empty directory then rename(A,B) can succeed (and thereby remove the 
old B) if all the other conditions are met.