[TUHS] Sockets and the true UNIX

[Chet Ramey]

On 9/22/17 4:57 PM, Chris Torek wrote:
>> In that sense, job control and process groups *are* independent of
>> the terminal device.
>>
>> The complication arises when you have to multiplex one terminal
>> between several different jobs in an interactive shell. Setting
>> the terminal process group correctly -- the only way to accomplish
>> that -- is fragile and the consequences of setting it to the wrong
>> value are inconvenient, to say the least.
> 
> Sure, but here again you can have the shell hang on to the
> (kernel allocated) pgroup, and any control operation amounts
> to, e.g., "set tty pgroup to the one for my pgroupid X".
> Since the shell is waiting for everything in the pipeline
> to exit, the shell knows when to close its "pgid descriptor".

That's not the problem this solves. The problem with allocating
process groups in the shell (and making them equal to the process id
of the pgrp leader) is that you end up with a race condition when
trying to add subsequent processes in a pipeline to that pgrp.
It forces you to artifically prevent the process group leader from
running or exiting until you're finished setting up the entire
pipeline.

The shell has to set the terminal process group to arbitrary process
groups, not just its own: that's how `fg' has to work. For that purpose,
tcsetpgrp is fine. You just have to make sure that you only call it
when the shell is in the foreground.

As for other processes, I'm not sure exactly what you mean by `control
operation', but you surely would want a background process to get stopped
if it tries to set the terminal attributes.


> (If you abandon the pipeline entirely, you also abandon the
> ability to attach it to a tty.  There should be some way to
> "rejoin" pgroups, which would let you attach them to new tty
> sessions, which is something that is missing from the current
> system.  But that's another thing entirely.)

This is something that would be nice to have, but there are security
considerations that make it tricky. Not unsolvable, but tricky to get
right.

Chet
-- 
``The lyf so short, the craft so long to lerne.'' - Chaucer
		 ``Ars longa, vita brevis'' - Hippocrates
Chet Ramey, UTech, CWRU    chet at case.edu    http://cnswww.cns.cwru.edu/~chet/