From: Jens Schmidt via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>
To: Eli Zaretskii <eliz@gnu.org>
Cc: 66546@debbugs.gnu.org
Subject: bug#66546: 30.0.50; save-buffer to write-protected file without backup fails
Date: Sun, 15 Oct 2023 11:34:17 +0200 [thread overview]
Message-ID: <87lec4cjqe.fsf@sappc2.fritz.box> (raw)
In-Reply-To: <83cyxgqwjm.fsf@gnu.org> (Eli Zaretskii's message of "Sun, 15 Oct 2023 08:33:49 +0300")
Eli Zaretskii <eliz@gnu.org> writes:
>> From: Jens Schmidt <jschmidt4gnu@vodafonemail.de>
>> Cc: 66546@debbugs.gnu.org
>> Date: Sat, 14 Oct 2023 22:31:36 +0200
>>
>> > Please explain what happens with "C-0 C-x C-s", and why. I don't
>> > think I understand that, given what you wrote.
>>
>> "C-0 C-x C-s" prompts me whether to write the modified buffer to the
>> write-protected file. When I reply "yes", I expect Emacs to do the
>> following:
>>
>> 1. Get the mode bits/extended attributes of "foo".
>> 2. Remove the write-potection on "foo".
>> 3. Write the buffer to "foo", not creating a backup file.
>> 4. Restore the mode bits/extended attributes on "foo" to their previous
>> state taken in step 1.
>
> So far, so good.
>
>> What happens instead:
>>
>> Emacs errors out that it cannot write to the file, even though it has
>> the permissions and means to temporarily remove the write-protection on
>> it.
>
> Why cannot Emacs write to the file, and what do the extended
> attributes have to do with that?
Let's check the working Emacs 23 code first, when there were no extended
attributes:
(setq setmodes (cons (file-modes buffer-file-name) buffer-file-name))
(set-file-modes buffer-file-name (logior (car setmodes) 128))
In procedural pseudo-code and without the cons, this does:
setmodes = file-modes (buffer-file-name);
set-file-modes (buffer-file-name, setmodes | 128);
which implements step 2 above: It takes the existing mode bits, adds bit
0b10000000 (u+w), and sets the resulting mode bits on the file. The
file gets writable. The following call to `write-region' succeeds.
Now the Emacs 29 code. We simplify that in the assumption, that the
extended attribute calls always succeed, and that we do not need the
regular calls:
(setq setmodes (list (file-extended-attributes buffer-file-name)
buffer-file-name))
(set-file-extended-attributes buffer-file-name
(nth 0 setmodes))
The pseudo-code here looks like this:
setmodes = file-extended-attributes (buffer-file-name);
set-file-extended-attributes (setmodes);
That is, the original extended attributes of the file get written back
to the file *unchanged*. No write permission is added to the file.
Accordingly, the following call to `write-region' fails.
next prev parent reply other threads:[~2023-10-15 9:34 UTC|newest]
Thread overview: 32+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-10-14 19:09 bug#66546: 30.0.50; save-buffer to write-protected file without backup fails Jens Schmidt via Bug reports for GNU Emacs, the Swiss army knife of text editors
2023-10-14 19:32 ` Eli Zaretskii
2023-10-14 20:31 ` Jens Schmidt via Bug reports for GNU Emacs, the Swiss army knife of text editors
2023-10-15 5:33 ` Eli Zaretskii
2023-10-15 9:34 ` Jens Schmidt via Bug reports for GNU Emacs, the Swiss army knife of text editors [this message]
2023-10-15 9:54 ` Eli Zaretskii
2023-10-15 11:39 ` Jens Schmidt via Bug reports for GNU Emacs, the Swiss army knife of text editors
2023-10-15 12:12 ` Eli Zaretskii
2023-10-15 18:59 ` Jens Schmidt via Bug reports for GNU Emacs, the Swiss army knife of text editors
2023-10-16 11:19 ` Eli Zaretskii
2023-10-16 20:04 ` Jens Schmidt via Bug reports for GNU Emacs, the Swiss army knife of text editors
2023-10-17 10:48 ` Eli Zaretskii
2023-10-17 20:12 ` Jens Schmidt via Bug reports for GNU Emacs, the Swiss army knife of text editors
2023-10-18 11:32 ` Eli Zaretskii
2023-10-18 20:36 ` Jens Schmidt via Bug reports for GNU Emacs, the Swiss army knife of text editors
2023-10-19 4:40 ` Eli Zaretskii
2023-10-19 21:12 ` Jens Schmidt via Bug reports for GNU Emacs, the Swiss army knife of text editors
2023-10-20 6:06 ` Eli Zaretskii
2023-10-21 17:56 ` Jens Schmidt via Bug reports for GNU Emacs, the Swiss army knife of text editors
2023-10-21 19:02 ` Eli Zaretskii
2023-10-21 20:17 ` Jens Schmidt via Bug reports for GNU Emacs, the Swiss army knife of text editors
2023-10-22 4:57 ` Eli Zaretskii
2023-10-22 9:45 ` Jens Schmidt via Bug reports for GNU Emacs, the Swiss army knife of text editors
2023-10-25 12:58 ` Eli Zaretskii
2023-10-29 14:23 ` Jens Schmidt via Bug reports for GNU Emacs, the Swiss army knife of text editors
2023-10-29 14:39 ` Eli Zaretskii
2023-11-01 19:06 ` Jens Schmidt via Bug reports for GNU Emacs, the Swiss army knife of text editors
2023-11-02 6:47 ` Eli Zaretskii
2023-11-03 21:02 ` Jens Schmidt via Bug reports for GNU Emacs, the Swiss army knife of text editors
2023-11-04 8:58 ` Eli Zaretskii
2023-11-04 12:30 ` Jens Schmidt via Bug reports for GNU Emacs, the Swiss army knife of text editors
2023-11-04 12:59 ` Eli Zaretskii
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