Subject: Re: bug#28542: Temporary failure in name resolution while quitting emacs

From: Lars Ingebrigtsen <[email-masked]>
Date: Wed, 02 Dec 2020 10:47:31 +0100

Eli Zaretskii <[email-masked]> writes:

> The ELisp manual says ab out kill-emacs-hook:
>
>      Because ‘kill-emacs’ can be called in situations where user
>      interaction is impossible (e.g., when the terminal is
>      disconnected), functions on this hook should not attempt to
>      interact with the user.  If you want to interact with the user when
>      Emacs is shutting down, use ‘kill-emacs-query-functions’, described
>      below.
>
> So I don't think we can safely ask whether to continue.

I don't quite interpret it that way -- this only says that if your
intention is to communicate with the user, then use
`kill-emacs-query-functions'.

The current situation is that if you have an error in `kill-emacs-hook'
(and you're running an interactive Emacs), then you can't kill Emacs at
all.  Adding a query to the user about what to do is an improvement, and
won't regress anything.

> We could either use safe_run_hooks, as we do in the noninteractive
> case (thus silently ignoring errors in this hook even if we do have a
> means to communicate with the user), or maybe move the offending
> function to kill-emacs-query-functions.  Or try a more limited
> solution of ensuring this particular function doesn't signal an error,
> or catches it and returns.

I'm a bit leery at doing anything automatically (either ignoring the
error or re-running it from a different context).  The hook may be, for
instance, writing stuff to a data file and then clearing out some
structures, and re-writing the stuff may clear the data, for instance.

Only the user should be making a judgement call in the error situation
here.

>> The flow control here is a bit odd, though.  `save-buffers-kill-emacs'
>> calls Fkill_emacs, which starts:
>> 
>> DEFUN ("kill-emacs", Fkill_emacs, Skill_emacs, 0, 1, "P",
>> [...]
>>   /* Fsignal calls emacs_abort () if it sees that waiting_for_input is
>>      set.  */
>>   waiting_for_input = 0;
>>   if (noninteractive)
>>     safe_run_hooks (Qkill_emacs_hook);
>>   else
>>     run_hook (Qkill_emacs_hook);
>> 
>> Is this bit done from the C level because of that waiting_for_input
>> setting?  And...  I don't understand the comment -- the `error' (which
>> calls signal?) doesn't abort Emacs?  Anybody?
>
> I think the comment has this exact scenario in mind: if we don't make
> sure waiting_for_input is zero, and the hook just happens to signal an
> error, Emacs will dump core.

Oh, I interpreted it the opposite way -- that "waiting_for_input = 0" is
setting it.  To zero.  But it means "isn't cleared"?

-- 
(domestic pets only, the antidote for overdose, milk.)
   bloggy blog: http://lars.ingebrigtsen.no