Re: dash 0.5.11.2, busybox sh 1.32.0, FreeBSD 12.2 sh: spring TTOU but should not i think

[Jilles Tjoelker <jilles@stack.nl>]

On Wed, Dec 23, 2020 at 08:18:24PM +0000, Harald van Dijk wrote:
> On 21/12/2020 16:24, Jilles Tjoelker wrote:
> > Also, simply entering the command
> >      trap "echo TTOU" TTOU
> > in an interactive shell makes it behave strangely. Created jobs
> > immediately stop, "fg" works once but after that the shell tends to get
> > stuck quickly.

> Good catch, there is some work to be done there too.

> > This seems a good approach, but certain writes to the terminal may need
> > the same treatment, for example the error message when fork fails for
> > the second and further elements of a pipeline. This also makes me wonder
> > why SIGTTOU is ignored at all by default.

> True. This is hard to create a reliable test case for, but there is only
> limited code that can get executed while a job-control-enabled shell may not
> be in the foreground process group.

An rlimit (ulimit -u) will cause fork to fail after a number of
processes, but will not work reliably if other code is executing
concurrently with the same UID.

> If fork fails halfway through a pipeline, it may also be a problem that some
> of the commands in the pipeline may still run.

This can be handled much like the case where an element in the pipeline
fails immediately such as because a utility cannot be found. I am not
sure how well this currently works.

> > In mksh, the issue is resolved slightly differently: setting a trap on
> > TTOU pretends to work but the signal disposition remains set to ignored.

> zsh also rejects traps on TTOU, but does so explicitly:

>   zsh: can't trap SIGTTOU in interactive shells

> Amusingly, it prints this in any shell where job control is enabled,
> regardless of whether the shell is interactive.

The rejection makes sense for any shell instance that has job control
and uses tcsetpgrp(), whether interactive or not.

I am not entirely happy with the rejection idea, though, since the check
can be bypassed by temporarily disabling job control:
    set +m; trap 'echo TTOU' TTOU; set -m
and running external utilities then fails with:
    zsh: can't set tty pgrp: interrupt

> > Tradition is for job control shells to be a process group leader, but I
> > don't really know why. Changing this will not fix the issue entirely
> > anyway since the shell must perform tcsetpgrp() from the background when
> > a foreground job has terminated.

> I've been thinking more about this, and I suspect it's a another conflation
> between interactive mode and job control. In an interactive shell that's not
> executing any external program, you want any Ctrl-C to only send SIGINT to
> that shell, not to any other process. In order for that to work, it needs to
> be its own process group.

> This should, in my opinion, *only* happen for interactive shells, not for
> job-control-enabled non-interactive shells. Consider

>   sh -c 'sh -mc "while :; do :; done"; echo bye'

> The behaviour I would want is that Ctrl-C kills the parent shell, so that
> this does not print "bye". Different shells behave differently.

I think the main effect of the -m option is that it places jobs in
separate process groups, which may also be useful in scripts. After
something like:

    set -m
    foo &
    foopid=$!
    set +m

it is possible to  kill -- "-$foopid"  .

Although non-portable kernel features such as cgroups (Linux) or reaper
(FreeBSD) might be more useful for tracking processes like this, the
work to define how to use them in a shell has not been done.

In FreeBSD sh, I extended the possibilities here by allowing job control
without an accessible tty in non-interactive mode. This applies both if
there is no controlling terminal at all and if the shell is in the
background.

In the example 
    sh -c 'sh -mc "while :; do :; done"; echo bye'
the -m flag seems to have no effect since that shell only runs builtins.

Changing it to
    sh -mc 'sh -c "while :; do :; done"; echo bye'
the desired Ctrl+C behaviour can still work because the outer shell
exits when it notices the inner shell has terminated because of SIGINT.

-- 
Jilles Tjoelker