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

[Stefan Monnier via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

> 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.

Hmm... the nested runs happens from within an outer `sit-for`, so the
time taken by the nested runs could/should count as part of the wait
time of the outer `sit-for`, thus sort of "running in parallel".

Apparently you're right, tho:

    (progn (run-with-timer 0 nil #'sit-for 2)
           (benchmark-call (lambda () (sit-for 2))))
    ==> (4.020784627 0 0.0)

but this looks like a bug in `sit-for` to me.  🙁

> 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.

Indeed, the upside is that it means my patch doesn't make things worse
in this respect.  🙂


        Stefan