bug#37892: 27.0.50; Crash when signaling a thread

[Eli Zaretskii <eliz@gnu.org>]

> From: Michał Krzywkowski <mkrzywkow@gmail.com>
> Date: Wed, 23 Oct 2019 19:06:42 +0200
> 
> When I evaluate this sexp, Emacs aborts:
> 
>   (let ((thread (make-thread (lambda () (sit-for 1.0)))))
>     (sit-for 0.5)
>     (thread-signal thread 'error nil))
> 
> Backtrace:
> 
>   Thread 5 "emacs" hit Breakpoint 1, terminate_due_to_signal (sig=sig@entry=6, backtrace_limit=backtrace_limit@entry=40) at emacs.c:371
>   371       signal (sig, SIG_DFL);
>   #0  0x000000000041f40b in terminate_due_to_signal (sig=sig@entry=6, backtrace_limit=backtrace_limit@entry=40) at emacs.c:371
>   #1  0x000000000041f866 in emacs_abort () at sysdep.c:2450
>   #2  0x000000000042154b in signal_or_quit (error_symbol=XIL(0x90), data=XIL(0), keyboard_quit=<optimized out>) at eval.c:1598
>   #3  0x0000000000421564 in Fsignal (error_symbol=<optimized out>, data=<optimized out>) at eval.c:1568
>   #4  0x0000000000423929 in post_acquire_global_lock (self=<optimized out>) at thread.c:115
>   #5  0x00000000005c5e62 in acquire_global_lock (self=0x1465e50) at thread.c:123

Emacs doesn't allow signals while it waits for input.

We could ignore thread-signal in such cases, or we could make it an
error (which will then signal an error in the thread which called
thread-signal).  What do people think?  Are there any other ideas for
handling this situation?

Michał, can you tell why you needed to call thread-signal while the
thread was in sit-for?

Thanks.