GNU bug report logs - #51820
29.0.50; process_pending_signals is almost useless without SIGIO

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From: help-debbugs <at> gnu.org (GNU bug Tracking System)
To: Ken Brown <kbrown <at> cornell.edu>
Subject: bug#51820: closed (Re: bug#51820: 29.0.50; process_pending_signals
 is almost useless without SIGIO)
Date: Wed, 17 Nov 2021 18:12:01 +0000

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Your bug report

#51820: 29.0.50; process_pending_signals is almost useless without SIGIO

which was filed against the emacs package, has been closed.

The explanation is attached below, along with your original report.
If you require more details, please reply to 51820 <at> debbugs.gnu.org.

-- 
51820: http://debbugs.gnu.org/cgi/bugreport.cgi?bug=51820
GNU Bug Tracking System
Contact help-debbugs <at> gnu.org with problems

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From: Ken Brown <kbrown <at> cornell.edu>
To: Eli Zaretskii <eliz <at> gnu.org>
Cc: eggert <at> cs.ucla.edu, 51820-done <at> debbugs.gnu.org
Subject: Re: bug#51820: 29.0.50; process_pending_signals is almost useless
 without SIGIO
Date: Wed, 17 Nov 2021 13:11:06 -0500

On 11/14/2021 1:48 AM, Eli Zaretskii wrote:
>> Date: Sat, 13 Nov 2021 18:25:55 -0500
>> Cc: 51820 <at> debbugs.gnu.org, Eli Zaretskii <eliz <at> gnu.org>
>> From: Ken Brown <kbrown <at> cornell.edu>
>>
>>> In that case, perhaps someone using Cygwin (i.e., not me) could look into this.
>>> Evidently it has not been much of a practical problem all these years.
>>
>> It's hard to know whether it's been a problem.  Maybe emacs would be more
>> responsive to C-g on Cygwin if we removed the #ifdef from handle_async_input.
>> My preference would be to try that (on master of course) and see if anything breaks.
>>
>> Eli, WDYT?
> 
> Sure, feel free to install such a change on master.

Done.

> I don't know how
> many Cygwin users track the Emacs master branch, but if there are more
> than a couple, perhaps even provide a variable exposed to Lisp that
> users could tweak to see the result without rebuilding and without
> leaving the session.

Good idea.  I think I'll wait a couple weeks so that I can see how the change 
works myself, as well as to see if anyone notices.  But then I might do that. 
For now I'm closing the bug.

Ken

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From: Ken Brown <kbrown <at> cornell.edu>
To: bug-gnu-emacs <at> gnu.org
Subject: 29.0.50; process_pending_signals is almost useless without SIGIO
Date: Sat, 13 Nov 2021 17:49:59 -0500

I might be missing something, but it seems to me that the main purpose of 
process_pending_signals is to read input in case the user typed C-g.  This is in 
fact done on systems with SIGIO, via a call to handle_async_input.  But 
handle_async_input is a NOOP on systems without SIGIO.  This has been the case 
since the following commit:

commit 4d7e6e51dd4acecff466a28d958c50f34fc130b8
Author: Paul Eggert <eggert <at> cs.ucla.edu>
Date:   Sun Sep 23 01:44:20 2012 -0700

    Simplify and avoid signal-handling races.
[...]
    Fixes: debbugs:12471

I can't find anything in the commit message or in the discussion of Bug#12471 
that explains this.  Paul, I realize that the commit was 9 years ago, but do you 
remember why you made this change?

On systems without SIGIO, an atimer "poll_timer" is created that fires every 2 
seconds.  This causes pending_signals to be set, which has practically no effect 
AFAICT.  The only thing I see that poll_timer accomplishes is that if emacs 
happens to be in the select call in wait_reading_process_output when the timer 
fires, then select will return.

Ken

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