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

[Harald van Dijk <harald@gigawatt.nl>]

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.

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

> 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.

> 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.

> What is definitely required, though, is that the shell not read input or
> alter terminal settings if it is started in the background (possibly
> unless the script explicitly ignored SIGTTOU). This is what the loop
> with tcgetpgrp() does.

Definitely.

Cheers,
Harald van Dijk