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

[Achim Gratz <Stromeko@nexgo.de>]

Glenn Morris writes:
> Someone may be relying on the fact that copy-file copies permissions,
> and want it it throw an error if it fails:
>
> http://debbugs.gnu.org/cgi/bugreport.cgi?bug=8306
>
> Others, like you, may not care.

If the contract of copy-file is that it always copies permissions, then
it should throw an error and not silently corrupt the permissions.  That
makes it useless on filesystems without permissions and where/when the
user has insufficient rights to install permissions which is possible
when ACL are in effect, so the function would need to have an extra
argument to ignore the error or not try to change permissions at all.

> Since copy-file has only relatively recently starting giving an error,
> maybe the least bad thing would be for it to give a message instead of
> an error?

In this case an alternative would be that copying permissions is done on
a best effort basis and may fail across file systems or in certain other
situations without raising an error (although I still think that it
should be possible to check for that).

> I don't know how we tidy this up for the future though.
> Add yet another optional argument "error-if-permissions-fail"? Yuck.

How about copy-file-with-permissions (which fails when it can't do what
the name says) and copy-file which does best-effort?


Regards,
Achim.
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