On Tue, 9 Apr 2019 at 10:33, Eli Zaretskii <eliz@gnu.org> wrote:

> > but I can't see a way to trigger
> > this for call_process: it searches for PROGRAM and signals an error
> > early, regardless of whether the filename is absolute or not.
>
> One way is to delete the program between the time Emacs searches for
> it and the time it actually invokes it.  Another way is to make the
> program be a file whose name includes non-ASCII characters outside of
> the current system codepage (I'm assuming the search for the program
> uses file-oriented primitives which support any Unicode characters).
>
> Having said that, this isn't worth too much of your time, if those
> ideas cannot be easily implemented, or turn out wrong, and no others
> present themselves.

I was inspired by your suggestions to think of a simpler idea: use "C:/nul.exe".

There is unfortunately one additional wrinkle: each of the test passes
on its own, but when running both together the second one fails due to
this check in maybe_call_debugger:

      /* RMS: What's this for?  */
      && when_entered_debugger < num_nonmacro_input_events)

RMS' question is (now) answered in the commentary for when_entered_debugger:

/* The value of num_nonmacro_input_events as of the last time we
   started to enter the debugger.  If we decide to enter the debugger
   again when this is still equal to num_nonmacro_input_events, then we
   know that the debugger itself has an error, and we should just
   signal the error instead of entering an infinite loop of debugger
   invocations.  */

static intmax_t when_entered_debugger;

So I guess we'd need some way of resetting it from Lisp? (which does
open the tiny danger of a debugger inf-looping by resetting
when_entered_debugger and then hitting an error).
As far as I can tell, the normal debugger resets it by calling
recursive-edit, but there's no way to return from that without human
intervention (I think?).