Artem Chuprina wrote:
> As you appeal to GNU cp, see
> its default behavior: BY DEFAULT it TRIES to save permissions and
> owner/group

No, by default GNU cp does not try to copy either owner/group
or permissions to an existing destination.  It does not invoke
chmod or chown unless you use something like 'cp -p'.

> I can create files there, but I cannot change
> their metainfo, because they are not mine.

This is an unusual setup, at least for me.  If I create a file,
I should be able to change its metainformation.  I expect this
setup will cause problems with other applications, not just Emacs.
GNU tar would be one example.

That being said, we should be able to work around the problem
by having copy-file behave more like 'cp'.  That is, copy-file
should not invoke chmod by default; it should invoke chmod only
if it's told to preserve permissions (or preserve ownership,
since that often involves temporarily revoking permissions for
security reasons).  That way, plain copy-file should work with
your setup, although you'll still have trouble with
copy-file with the last arg t (which asks to copy permissions).

Attached is a proposed patch to do that, against trunk bzr 115721.
Does it solve your problem?