bug#64325: ACL error

[Eli Zaretskii]

> Date: Wed, 28 Jun 2023 09:15:16 -0400
> From:  Charles Millar via "Bug reports for GNU Emacs,
>  the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>
> 
> Please see the attached debug on error which is generated when I attempt 
> to save a modified buffer/file, regardless of mode.

If, as I understand, you set debug-on-error non-nil when saving the
buffer, then this is normal: it means that the volume you have mounted
in /mnt/ doesn't support ACLs.  Emacs normally hides the ACL-related
errors and silently falls back on using set-file-modes, but setting
debug-on-error non-nil disables that.

So I don't think there's a bug here if you set debug-on-error.

If this happens even if you do not set debug-on-error, then I'd ask
you to step in Edebug through the code in backup-buffer-copy and tell
how come the with-demoted-errors form there doesn't do its job of
catching errors.

Thanks.