bug#55166: 28.1; tar-mode doesn't re-compress when saving remote files

[Michael Albinus <michael.albinus@gmx.de>]

Lars Ingebrigtsen <larsi@gnus.org> writes:

Hi,

>> dan@solstice:/tmp$ touch a b c
>> dan@solstice:/tmp$ tar cf test.tar a b c
>> dan@solstice:/tmp$ bzip2 -z test.tar
>> dan@solstice:/tmp$ file test.tar.bz2
>> test.tar.bz2: bzip2 compressed data, block size = 900k
>>
>> If you visit that remote file and save it, the result will have the .bz2
>> extension but won't actually be compressed:
>>
>> dan@solstice:/tmp$ file test.tar.bz2
>> test.tar.bz2: POSIX tar archive (GNU)
>>
>> This doesn't happen with compressed remote files which aren't tar archives.
>
> I can reproduce this problem with Emacs 29, too.  (And if I edit the
> file locally instead of via tramp, the tar file is saved compressed, so
> it seems to be tramp-related.)

Well, the problem seems to exist since Emacs 27, likely due to some
subtle changes in basic-save-buffer-1 or basic-save-buffer-2. I haven't
poked further there, because it looks like saving the buffer with this
constellation happened twice in Emacs 26, which looks not optimal.

The problem is an exotic feature of write-region, which first looks for
a file name handler in FILENAME, and if there isn't one, it checks
VISIT.

write-region of the compressed remote tar file in the scenario above is
called like

--8<---------------cut here---------------start------------->8---
  ...
  tramp-file-name-handler(write-region nil nil "/ssh:detlef:/tmp/tmpfSxlA9" nil "/ssh:detlef:/tmp/test.tar.bz2" "/ssh:detlef:/tmp/test.tar.bz2" nil)
  write-region(nil nil "/ssh:detlef:/tmp/tmpfSxlA9" nil "/ssh:detlef:/tmp/test.tar.bz2" "/ssh:detlef:/tmp/test.tar.bz2")
  basic-save-buffer-2()
  basic-save-buffer-1()
  basic-save-buffer(t)
  save-buffer(1)
  funcall-interactively(save-buffer 1)
  call-interactively(save-buffer nil nil)
  command-execute(save-buffer)
--8<---------------cut here---------------end--------------->8---

That is, a remote temporary file ("/ssh:detlef:/tmp/tmpfSxlA9") is
written, and the magic extension of the remote VISIT argument
"/ssh:detlef:/tmp/test.tar.bz2" is ignored.

When FILENAME is a local temporary file, the handler for VISIT
(jka-compr-handler) is triggered. That's the second test Lars has applied.

I've pushed a fix to master, which shall fix this. Dan, could you pls
check?

In parallel I'll try to write a test case for this problem. But that's
unrelated to closing this bug.

Best regards, Michael.