bug#25247: 26.0.50; Concurrency crashes with XLib

[Eli Zaretskii]

> From: Elias Mårtenson <lokedhs@gmail.com>
> Date: Fri, 30 Dec 2016 19:21:08 +0800
> Cc: Tino Calancha <tino.calancha@gmail.com>, raeburn@raeburn.org, 
> 25247@debbugs.gnu.org
> 
> One interesting fact is that if I replace ‘sleep-for’ with ‘sit-for’, then 
> the updates come at exactly the expected
> time.

But only as long as blink-cursor-mode is turned on, right?  If you
turn it off before running the experiment, sit-for behaves the same as
sleep-for, right?

When blink-cursor-mode is ON, it supplies 2 events each second, and
that allows the threads that finished waiting to acquire the global
lock and insert the string.  Otherwise, the threads wait for the
global lock and do the insertions at the end.