EZ> You will have to provide a more detailed analysis, sorry.

There is no guarantee the static variable `threads_holding_glib_lock`
introduced in xgselect.c in commit 9c62ffb is synchronized across threads.  As
such, relying on it becoming zero in time for release_select_lock() is fraught.
(If you add print statements, this particular heisenbug disappears, at least
on Linux kernel 4.15.0-99-generic).

Four attachments:

1. Desired patch to master (reverts 9c62ffb, adds main-thread-p check).