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

[Eli Zaretskii <eliz@gnu.org>]

> Date: Sun, 17 Oct 2010 17:40:39 -0400
> From: Ken Brown <kbrown@cornell.edu>
> CC: "7225@debbugs.gnu.org" <7225@debbugs.gnu.org>
> 
> 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?

The problem that code is trying to solve is how to send a signal to
the whole process group starting at the shell (or whatever process is
the group leader).  Failure to do so could mean that the immediate
subprocess of Emacs will get the signal, but its children will not.
If the signal kills the subprocess, its children may remain behind as
orphans.

> 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.

It's not the wrong process.  The problem is that its children might
not get the signal.

So the question is: how to send a signal to the whole process group
with Cygwin?  Does Cygwin support the "minus pgid" method of calling
`kill'?  Or maybe there's some other method?