bug#71223: 30.0.50; stack overflow after very fast opening and closing of frames

[Eli Zaretskii <eliz@gnu.org>]

> Cc: 71223@debbugs.gnu.org
> Date: Wed, 29 May 2024 15:56:17 -0400
> From:  Stefan Monnier via "Bug reports for GNU Emacs,
>  the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>
> 
> IIUC this problem is not really new with my code (tho, I guess when the
> process-filters are run in a nested fashion, the various `sit-for` can
> be done in parallel, whereas with my patch they are done sequentially,
> significantly increasing the visible delay when you have many such
> requests).

No, the nested sit-for calls are NOT running in parallel, because
there's just one thread running them all.  Only the innermost sit-for
is actually "running", in the sense that its pselect loop in waiting
for input.

So what your change does is replace a depth-first sequence of sit-for
with a kind-of breadth-first sequence.  But it's a sequence in either
case.