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


From: Ken Brown
Subject: bug#7225: 23.2.50; [PATCH] C-c C-c doesn't work in shell mode (Cygwin)
Date: Sun, 17 Oct 2010 19:39:59 -0400
User-agent: Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.9.2.9) Gecko/20100915 Thunderbird/3.1.4

On 10/17/2010 7:11 PM, Ken Brown wrote:
On 10/17/2010 6:08 PM, Eli Zaretskii wrote:
Date: Sun, 17 Oct 2010 17:40:39 -0400
From: Ken Brown<address@hidden>
CC: "address@hidden"<address@hidden>

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.

Am I misunderstanding the comment preceding the definition of
emacs_get_tty_pgrp?  Here's what it says:

/* Return the foreground process group for the tty/pty that
     the process P uses.  */

That's not the same as the process group of the shell, at least in
Cygwin.  See below.  You seem to be assuming that the process group of
the shell will include all of the shell's children.  Is that what
happens in GNU/Linux?

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.

It is the wrong process on Cygwin.  Here's an example.  I start a shell
and then run 'cat'.  In another shell I run 'ps'.  The relevant part of
the output is:

        PID    PPID    PGID     WINPID  TTY  UID    STIME COMMAND
        508     920     508       3552    1 1009 18:50:15 /usr/bin/sh
I     916     508     916       1832    1 1009 18:50:18 /usr/bin/cat

I've just checked that the same thing happens in GNU/Linux (or at least in the GNU/Linux system I have access to). Start a shell, start 'cat', and the PGID of cat will not be the same as that of the shell.





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