The NS port has an event loop that contains this code:

    if (++apploopnr != 1)
      {
        emacs_abort ();
      }

So if the loop is entered again it crashes Emacs instantly and gives
a stack trace. This shows up in a couple of bugs.

Bug#11049, for example, is where it receives a SIGHUP and should quit
cleanly and write out backups of open buffers, etc. but actually just
crashes because it re-enters the event loop.

What happens here is that the SIGHUP is received as an event, which
then calls a function that unblocks *all* input. Then Emacs tries to
clean up before quitting but that results with it handling input
events again, therefore re-entering the event loop and aborting.

I've sent a patch to this bug that simply re-blocks input, but the
alternative is to remove this check and let Emacs re-enter the event
loop. As far as I can tell this works fine for this particular case
(SIGHUP), but I don't know if it will break something else.

Is there any way, other than sending signal to kill it, that would
result in Emacs re-entering the event loop?

Thanks!
-- 
Alan Third