bug#30364: 26.0.91; thread crash on macos

[Eli Zaretskii]

> From: Aaron Jensen <aaronjensen@gmail.com>
> Date: Sun, 18 Feb 2018 10:59:02 -0800
> Cc: Noam Postavsky <npostavs@gmail.com>, 30364@debbugs.gnu.org, 
>       Noam Postavsky <npostavs@users.sourceforge.net>, Alan Third 
> <alan@idiocy.org>
> 
> Out of curiosity, would it be possible to limit GCs to only happen on
> the main thread? They're stop the world any way, right, so does it
> matter which thread they happen on?

A thread that runs Lisp can continue running Lisp for a long time, and
will generate a lot of garbage.  If GC is prevented, we will risk
running out of memory.

IOW, leaving GC for the main thread only makes sense if we can be sure
the main thread will run shortly.  But that cannot be guaranteed with
the current thread model, where a thread must exit or yield before
another thread can run.