bug#69606: [PATCH] Ensure default-directory exists when generating diff

[Philip Kaludercic <philipk@posteo.net>]

Michael Albinus <michael.albinus@gmx.de> writes:

> Philip Kaludercic <philipk@posteo.net> writes:
>
> Hi Philip,
>
>> diff --git a/lisp/vc/diff.el b/lisp/vc/diff.el
>> index a64fbc47853..47566dbc40b 100644
>> --- a/lisp/vc/diff.el
>> +++ b/lisp/vc/diff.el
>> @@ -188,7 +188,9 @@ diff-no-select
>>                                    (list (or old-alt old)
>>                                          (or new-alt new)))))
>>  		     " "))
>> -	 (thisdir default-directory))
>> +	 (thisdir (if (file-exists-p default-directory)
>> +                      default-directory
>> +                    (expand-file-name "~"))))
>>      (with-current-buffer buf
>>        (setq buffer-read-only t)
>>        (buffer-disable-undo (current-buffer))
>
> I would use temporary-file-directory (or even
> small-temporary-file-directory). Spamming the home directory with
> (temporary) diff files doesn't sound like a good idea.

If I am not mistaken, the command does not create any files, the output
of diff is written directly into the *diff* buffer.

>> Any other ideas?  Perhaps we should always set the default directory to
>> that
>
> Perhaps, but I don't know whether there are undesired side effects, for
> example when using relative file names. OTOH, if default-directory is
> remote or unwritable, we might have problems anyway.

I don't quite understand how, but it seems that some specific sequence
of commands can start diff in a directory I just deleted, without this
being necessary.  The reason I was thinking about a more general
solution, is that something like

--8<---------------cut here---------------start------------->8---
(let ((default-directory "/this/does/not/exists"))
  (make-process :command '("true")))
--8<---------------cut here---------------end--------------->8---

will always fail, even though "true" doesn't use the current working
directory.  I think it would be useful to have some :fallback option for
these situations, to ensure that if `default-directory' doesn't exist,
any other directory should be used instead.

> Best regards, Michael.

-- 
	Philip Kaludercic on peregrine