Re: SIGINT handling

[Jilles Tjoelker]

On Mon, Sep 21, 2015 at 10:48:07AM +0100, Stephane Chazelas wrote:
> 2015-09-19 21:36:28 +0100, Stephane Chazelas:
> > 2015-09-18 16:14:39 +0100, Stephane Chazelas:
> > [...]
> > > In:
> > > 
> > > bash -c 'sh -c "trap exit INT; sleep 10; :"; echo hi'
> > > 
> > > If I press Ctrl-C, I still see "hi".
> > [...]
> > 
> > Jilles provided with the explanation at
> > http://unix.stackexchange.com/a/230731

> > with a link to:
> > http://www.cons.org/cracauer/sigint.html
> [...]

> Note that bash (and ksh, contrary to FreeBSD sh) is not
> consistent in its handling of that "WCE" (for "wait and
> cooperative exit") approach in that pressing ^C in:

> bash -c '
>   var=$(sh -c "trap \"\" INT; sleep 3; echo result)
>   echo "$var"
> '

> kills bash, leaving the "sh" and "sleep" running unattended in
> background.

> Same for:

> bash -O lastpipe -c '
>   sh -c "trap \"\" INT; sleep 3; echo test" | read var; echo done'

> One could also argue, that to be consistent, SIGTSTP and SIGQUIT
> should be treated similarly (strangely enough
> http://www.cons.org/cracauer/sigint.html doesn't mention SIGTSTP).

Agreed for SIGQUIT, but not for SIGTSTP. For SIGTSTP, either the shell
has job control enabled or it does not. If it does, SIGTSTP stops the
job and continues the shell; if it does not, SIGTSTP stops the whole job
including the shell.

> I'm not sure I prefer that WCE approach over WUE. Wouldn't it be
> preferable that applications that intercept SIGINT/QUIT/TSTP for
> anything other than clean-up before exit/suspend implement job
> control themselves instead (like vi's :! should create a process
> group and make that the foreground process group of the
> terminal so pressing ^C in sh -c vi, :!sleep 10, only sends the
> SIGINT to sleep)?

This kind of job control manipulation is very hard to get right in the
general case. FreeBSD's su does it, and it needed various iterations to
fix hanging processes or unexpected logouts, some of which only occur
when the application is started from certain shells.

Also, it is not possible to fix generally cases like
  su SOMEUSER -c 'while sleep 0.1; do echo @@@; done' | less
where there are other processes in the same process group as the one
doing job control manipulations. If su changes the tty's foreground
process group, it will prevent less from reconfiguring terminal modes.

-- 
Jilles Tjoelker