bug#66546: 30.0.50; save-buffer to write-protected file without backup fails

[Jens Schmidt]

Eli Zaretskii <eliz@gnu.org> writes:

>> From: Jens Schmidt <jschmidt4gnu@vodafonemail.de>
>> Cc: 66546@debbugs.gnu.org
>> Date: Sun, 15 Oct 2023 20:59:42 +0200
>>
>> Eli Zaretskii <eliz@gnu.org> writes:
>>
>> > In that case, does the change below fix the original problem?
>>
>> It does, thanks.
>
> I've now installed that on the master branch.

Thanks.  Now that's out of the way, should I then work on what I have
called issue B in the initial message and on ERT tests for both issues?
Or do you still think there is more discussion required on these
beforehand?

>> > +         ;; If set-file-extended-attributes fails to make the
>> > +         ;; file writable, fall back on set-file-modes.
>> > +         (with-demoted-errors "Error setting attributes: %s"
>> > +           (set-file-extended-attributes buffer-file-name
>> > +                                         (nth 1 setmodes)))
>>
>> How exactly could above call to `set-file-extended-attributes' *succeed*
>> to make the file writable?
>
> I don't know, and I don't think we should care.  Due to the
> above-mentioned system-dependencies, Emacs generally treats extended
> attributes as opaque objects, and only tries hard to preserve them
> where expected.  So the above is our best effort to preserve the
> attributes, which is why we call set-file-modes only if absolutely
> necessary, since doing that in general affects the extended
> attributes.

That explains (partially) what I did not understand in your solution.
But still, to me it seems that the two forms

  (setq setmodes
        (list (file-modes buffer-file-name)
              (with-demoted-errors
                  "Error getting extended attributes: %s"
                (file-extended-attributes buffer-file-name))
              buffer-file-name))
  (with-demoted-errors "Error setting attributes: %s"
    (set-file-extended-attributes buffer-file-name
                                  (nth 1 setmodes)))

, if limiting them to the extended attribute calls and if ignoring outer
context, could be simplified to

  (set-file-extended-attributes
   buffer-file-name
   (file-extended-attributes buffer-file-name))

(If not, why not?)

And wouldn't that be, in this context, just a no-op?

I fully understand that the extended attributes stored in `setmodes' are
required later to restore the attributes of the file after it has been
written to.  And in that context I understand why we call
`set-file-extended-attributes'.  But here not really, yet.