bug#37892: 27.0.50; Crash when signaling a thread

[Eli Zaretskii <eliz@gnu.org>]

> From: Michał Krzywkowski <mkrzywkow@gmail.com>
> Cc: Michał Krzywkowski <mkrzywkow@gmail.com>,
>  37892@debbugs.gnu.org
> Date: Sat, 26 Oct 2019 14:29:43 +0200
> 
>     Eli> Emacs doesn't allow signals while it waits for input.
> 
>     Eli> We could ignore thread-signal in such cases, or we could make it an
>     Eli> error (which will then signal an error in the thread which called
>     Eli> thread-signal).  What do people think?  Are there any other ideas for
>     Eli> handling this situation?
> 
> Maybe the signal should be raised in the thread right after it returns
> from sit-for?

We don't have any mechanism for doing that.

> Ignoring signals sounds like a bad idea.

It is better than crashing.  It is also not much worse than
interrupting the thread at some arbitrary random point.

>     Eli> Michał, can you tell why you needed to call thread-signal while the
>     Eli> thread was in sit-for?
> 
> I wanted to signal some background thread which was doing some work and
> then Emacs crashed, because it just so happened that it was in sit-for.
> The thread function was doing _more_ than just sit-for.
> 
> I actually don't care (and can't know) what another thread is doing, I
> just want to send a signal to it.

But thread-signal is not for causing an error in a thread, it is for
unblocking a thread that waits on a mutex or a condvar.  So why would
you use it when the thread is not blocked?