From mboxrd@z Thu Jan  1 00:00:00 1970
Message-ID: <14ec7b180701291558s18d500f0m56977475aa9c80d4@mail.gmail.com>
Date: Mon, 29 Jan 2007 16:58:03 -0700
From: "andrey mirtchovski" <mirtchovski@gmail.com>
To: "Fans of the OS Plan 9 from Bell Labs" <9fans@cse.psu.edu>
Subject: Re: [9fans] APE exit()
In-Reply-To: <93d3f4d8c7ad444db9626af772682441@proxima.alt.za>
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References: <e2523c524831b94df22562c68e59b147@terzarima.net>
	<93d3f4d8c7ad444db9626af772682441@proxima.alt.za>
Topicbox-Message-UUID: 0a358b5a-ead2-11e9-9d60-3106f5b1d025

I think I figured that one out.

/sys/src/ape/lib/ap/plan9/_buf.c:66 sets _killmuxsid() to be called
during exit, then forks the process nodefile. this is all fine until
exit time, when there is a race: _killmuxsid is called and tries to
kill the child via kill(-pidgroup) but the child has already exited.
kill() calls setpgid() on itself trying to become the same group as
the child, but since the child has exited setpgid() fails (there is no
such group anymore). kill() will then happily proceed and send SIGTERM
to the original note group with which it was born, which unfortunately
is the same notegroup of the shell, hence killing them both.

a proposed fix for /sys/src/ape/lib/ap/plan9/kill.c:

49,54c49,51
< 		if(setpgid(mpid, -pid) > 0) {
< 			r = note(mpid, msg, "/proc/%d/notepg");
< 			setpgid(mpid, sid);
< 		} else {
< 			r = -1;
< 		}
---
> 		setpgid(mpid, -pid);
> 		r = note(mpid, msg, "/proc/%d/notepg");
> 		setpgid(mpid, sid);