bug#43723: 27.1; Errors in file-extended-attributes prevent from saving buffer

[Eli Zaretskii <eliz@gnu.org>]

> From: Lars Ingebrigtsen <larsi@gnus.org>
> Cc: 43723@debbugs.gnu.org
> Date: Mon, 06 Jun 2022 18:22:13 +0200
> 
> Eli Zaretskii <eliz@gnu.org> writes:
> 
> > When some volume is mounted such that file-extended-attributes fails
> > for files there (because the agent used to mount doesn't support ACLs
> > or SELinux), this prevents users from saving their edits to files on
> > that volume, because file-extended-attributes signals an error.  This
> > appears as a regression to users because Emacs 26 silently ignored
> > such errors.
> >
> > To allow users to save the files in these cases, but still keep them
> > informed about the loss of potentially important attributes, Emacs
> > should probably warn about this, allow the user to decide he/she wants
> > to ignore the problem, and record this fact somewhere, to avoid asking
> > the same question again for the same volume.
> 
> Do you have a test case to reproduce this problem?  I don't really use
> SELinux myself...

No, I don't have a recipe.

This bug report was the result of this discussion on emacs-devel:

  https://lists.gnu.org/archive/html/emacs-devel/2020-09/msg02248.html

The basic concern is that Emacs 26 silently ignored errors in file-acl
and file-selinux-context, whereas Emacs 27 and later doesn't ignore
them.  My point was that preventing the user from saving the edits
just because we cannot preserve the ACLs is too radical, since most
users don't care about ACLs, and because support for ACLs on volumes
mounted by all kinds of network disk drivers that have trouble mapping
extended attributes between different systems.

You can easily simulate this situation by writing a replacement for
file-acl that always signals a file-error, or advising it to that
effect.