Interruptible GHC

[Edward Z. Yang]

Excerpts from Simon Marlow's message of Mon Sep 13 05:10:13 -0400 2010:
> The idea of having user-definable cancellation mechanisms seems quite 
> sensible, given that we have so many ways to do this.  However it seems 
> quite hard in practice: for pthread_cancel, the RTS has to behave quite 
> differently from pthread_kill.  The API for defining the cancellation 
> mechanism could get quite complicated.
> 
> For now I would go with 'interruptible' (meaning either pthread_kill() 
> or CancelSynchronousIO()).  It's not nearly as dangerous as 
> pthread_cancel(), but it covers a lot of the cases we're interested in, 
> and it doesn't have problems with bound threads.

Sounds good.  I'll roll this change tomorrow.  Maybe we should emit a warning
pre-Vista that "interruptible" is not being honored?  Also, it occurs
to me that a lot of this functionality could be implemented in user
space.

Cheers,
Edward