bug#7225: 23.2.50; [PATCH] C-c C-c doesn't work in shell mode (Cygwin)

[Ken Brown <kbrown@cornell.edu>]

On 10/16/2010 9:26 AM, Eli Zaretskii wrote:
>> Date: Sat, 16 Oct 2010 08:44:28 -0400
>> From: Ken Brown<kbrown@cornell.edu>
>> CC: "7225@debbugs.gnu.org"<7225@debbugs.gnu.org>
>>
>>>> C-c C-c
>>>> (The cat process doesn't get killed.)
>>>
>>> Is the reason for this known?  It looks like the `kill' syscall isn't
>>> doing its job, but if that's so, there should be a good reason for
>>> that.
>>
>> The problem is the group id in the call to 'kill'.  Cygwin doesn't
>> define TIOCGPGRP, and the group id as set in line 6233 of process.c
>> (emacs-23 branch) is the wrong group id.
>
> Does Cygwin have another way of getting the process group ID?  If so,
> can it be used in process_send_signal instead of emacs_get_tty_pgrp?
[...]
> So I suggest to look into tweaking process_send_signal so that it
> works with Cygwin.  It looks like it makes all kinds of assumptions
> regarding systems that don't have TIOCGPGRP, and they are probably
> wrong for Cyugwin.  Or maybe you need something special in
> EMACS_KILLPG.
>
> When you have fixed process_send_signal, you can then introduce
> SIGNALS_VIA_CHARACTERS for Cygwin.

I haven't yet found an alternative to TIOCGPGRP that works for Cygwin, 
and it may well be that there isn't one.  But I think there's a separate 
issue that is not specific to Cygwin.  It seems to me that line 6233 of 
process.c is simply wrong, and not just for Cygwin.

When we reach that line, we want the process group ID of the foreground 
process group of the terminal associated with p (which is a shell in the 
most common use case).  We don't have TIOCGPGRP, so we don't know how to 
do this.  We therefore give up and set gid = p->pid.  Is there any 
situation in which this is the right thing to do?  It means that (in the 
common use case) we'll send the signal to the shell instead of to the 
process running in the shell.  Wouldn't it be better to just return at 
that point and issue a warning message saying that we can't send the signal?

What this would mean for Cygwin, once I make the change I proposed (and 
assuming I don't find a better solution), is that the only signals we'll 
be able to send to the foreground process of a shell are SIGINT, 
SIGQUIT, and SIGTSTP, and we'll see a failure message if we try to send 
a different signal.  That's better than sending a signal to the wrong 
process.