bug#51177: 29.0.50; stop-process on pipes

[Eli Zaretskii <eliz@gnu.org>]

> From: Helmut Eller <eller.helmut@gmail.com>
> Date: Sat, 16 Oct 2021 18:24:02 +0200
> Cc: 51177@debbugs.gnu.org, jakanakaevangeli@chiru.no
> 
> On Thu, Oct 14 2021, Lars Ingebrigtsen wrote:
> 
> > Helmut Eller <eller.helmut@gmail.com> writes:
> >
> >> I did not read the docstring but I did read the manual.  This feature is
> >> not described in the manual.  At least not near set-process-filter.
> >
> > Yup.  I've now documented the t value in the manual in emacs-28.
> 
> The t value also doesn't seem be handled correctly by make-process:
> 
> (ert-deftest test-filter=t ()
>   (let ((p (make-process :command '("dd" "if=/dev/zero" "count=0")
> 			 :name "foo"
> 			 :filter t)))
>     ;;(set-process-filter p t)
>     (while (eq (process-status p) 'run)
>       (accept-process-output p))))
> 
> when executed with
> 
>   emacs -Q --batch -l test.el -f ert-run-tests-batch-and-exit
> 
> produces:
> 
> Running 1 tests (2021-10-16 18:21:53+0200, selector ‘t’)
> Test test-filter=t backtrace:
>   t(#<process foo> "0+0 records in\n0+0 records out\n")
>   accept-process-output(#<process foo>)

What do we expect to happen when a Lisp program calls
accept-process-output on a process that is stopped?