bug#16133: 24.3; copy-file fails on chmod when copying to FAT filesystem

[Eli Zaretskii <eliz@gnu.org>]

> Date: Sun, 22 Dec 2013 10:54:48 -0800
> From: Paul Eggert <eggert@cs.ucla.edu>
> CC: 16133@debbugs.gnu.org, ran@lasgalen.net
> 
> Eli Zaretskii wrote:
> > A library function should
> > never decide on its own when to call it quits
> 
> Copy-file signals errors in many circumstances;
> this is just one of them.  If the goal is to have
> a library function that does not "decide on its
> own when to call it quits", then the function should
> return a value indicating the error, rather than
> throwing an exception.  That might be too much of
> a change to copy-file, but we could have a variant
> of copy-file that does that, suitable for library
> use.

It could be a workable alternative indeed, but I don't see why
copy-file couldn't do it itself: it currently returns no useful value,
so no code relies on its return value.

> I would favor changing the specification of copy-file,
> so that it doesn't copy the permissions
> unless its 6th argument is non-nil.

The case in point is when you want to copy permission, if possible,
but will settle for a copy without permission if not.  IOW, no need to
give up without trying.

Perhaps we should have a special value of the 6th argument which means
treat a failure to copy permissions as fatal.