bug#40576: call-process-region does not accept nil as first argument

[Eli Zaretskii <eliz@gnu.org>]

> From: Philipp Stephani <p.stephani2@gmail.com>
> Date: Sun, 12 Apr 2020 18:44:58 +0200
> Cc: Pietro Giorgianni <giorgian@gmail.com>, 40576@debbugs.gnu.org
> 
> > Right.  But there's more here than meets the eye, because the change
> > after which we started advertising the special meaning of nil for
> > START exposed a problem: write_region, called from create_temp_file,
> > has special meaning for START = nil: it widens the buffer and writes
> > the entire buffer contents to the temp file.  Which isn't right when
> > write_region is called from call-process-region, as it allows access
> > to inaccessible portion of the buffer, something we shouldn't do.
> 
> I think that's pretty much intentional. The documentation says
> 
> "If START is nil, that means to use the entire buffer contents"

That sentence is an addition made in 2016, AFAICT.

> It specifically doesn't say to only use the accessible portion of the
> buffer. Given that this behavior probably has been in place since
> commit 561cb8e159e7eff7a6487a45a1cfab47ba456030 from 1994, it would be
> rather unwise to introduce such a breaking change.

But then START = nil would work, whereas START = 2 will signal an
error if the buffer is narrowed.  Does that make sense?