From: Eric Wong <normalperson@...>
Date: 2012-05-22T10:39:49+09:00
Subject: Re: [C ext and GVL] Why UBF() is called even if Ruby traps signals ??

I単aki Baz Castillo <ibc@aliax.net> wrote:
> But that's not my problem. In fact, I have a mechanism to communicate
> with UV loop at any time safely from any thread:
> 
>   uv_async_send()   (thread safe)

OK, perfect.  You can probably just call in your ubf(), nothing else.

> So from the ubf() function I have no problem in telling UV "please do
> this now or in your next instant iteration". My problem is: what to
> say?
> 
> 1) If ubf() has been called from Thread#kill then I need to run a
> function that closes all the active UV handlers so uv_run() exits.

Can you run those cleanup handlers in the killer and not the killee?
You should Thread#join the killee, and then cleanup in the killer.

On the other hand, with uv_async_send(), you can probably avoid
Thread#kill entirely.  Thread#kill can be dangerous/unreliable,
as ensure clauses will still fire, and ensure clauses can be
broken, too.

> 2) If ubf() has been called due to a signal receipt, I don't want
> uv_run() to exit. Instead the signal wakeups the UV loop and generates
> an iteration in which I call to rb_check_interrupts() for handling the
> signal in Ruby land and I'm happy. But the ubf() has been called !!!
> 
> So the problem is that when a signal is received, it makes the ubf()
> function to be called and, at that point, I have no way to know,
> within the ubf() function, whether it has been called by Thread#kill
> or by a received signal.

You're right, there's no way for the ubf() to know why it's called.

The purpose of the ubf() is only to wake up a thread and have it
reacquire the GVL.  That's it.

Once a thread reacquires the GVL, it can:
- run Ruby signal handlers (automatically done by VM)
- die (respond to Thread#kill, again automatically done by VM)
- give up the GVL again and resume blocking (your choice)