bug#25172: 26.0.50; Concurrency feature, sit-for doesn't work (crashing and unexpected behaviour)

[Eli Zaretskii]

> From: Elias Mårtenson <lokedhs@gmail.com>
> Date: Tue, 13 Dec 2016 12:16:41 +0800
> Cc: Clément Pit--Claudel <clement.pit@gmail.com>, 
>       25172@debbugs.gnu.org
> 
> This is the output from the SIGABRT case. Again, there is a lot of 
> information before the actual "bt full" output:

This shows exactly the situation which I imagined could happen: 3
threads, including the main thread, all running the code in xg_select
which calls unblock_input, and aborting due to the block level being
negative.

Can you try removing the calls to block_input/unblock_input from
xgselect.c, and see if these crashes then stop?

Thanks.