Hangups about vhangup()

[R.P.A.Collinson]

The vhangup problem discussed by Joe Kelsey and also the getty/dialup
problem are related.

One fundamental problem with the vhangup call is that it takes a file
descriptor and not a device name. This means that the terminal has to be
opened, vhangup'ed and then closed. This means that the act of opening
the device may foul up reference counts on inodes etc etc. I also suspect
that there is a fundamental problem in the fact that inodes for terminals
are never locked so that UNIX can scribble when you are inputting, there
are occasional race problems with the close code.

The terminals on our systems are connected via a network and we want all
terminals to detach cleanly from a machine on logout. The problem
is to do this in a benign way, not killing everything which a user has
carefully left running in the background. I have had various attempts
at this, and the current scheme is as follows:

1)	Invent a new system call (yes YET ANOTHER one) called
	hangtty(dev)
	char *dev;
	The dev argument is used to obtain an pointer to the incore inode
	for the terminal.

2)	Set a flag in this inode
	IDEAD	(UCB thoughtfully left a few spare bits)
	call the terminal close routine.

	We must now make sure that any further action on the inode
	does not get through to the device.

3)	A write/read to an IDEAD inode results in a SIGKILL being sent
	to the offending process.
	The user has no business leaving a background process running
	which writes or reads when the user has gone away.

4)	The call to get a new inode for the terminal (iget) ignores any
	inodes with IDEAD bits. So for short periods, there is more than
	one inode referring to the terminal. One is live, all the others
	have the IDEAD bit set.

This scheme appears to work, and is in use at mcvax. I believe that certain
ex-UCB people don't like it because it cuts across the layering in the
kernel. But still.....
(On the UCB/System V discussion, Bill Joy did us all a disservice in csh when
he allowed people to leave background jobs running without having said
nohup).

Peter Collinson
University of Kent, UK
vax135!ukc!pc
mcvax!ukc!pc