FFI, signals and exceptions

[Simon Marlow]

On 06/08/2010 21:16, Edward Z. Yang wrote:
> Excerpts from Corey O'Connor's message of Fri Aug 06 16:15:21 -0400 2010:
>> In your test cases that fail are your C computations foreign unsafe imports?
>
> First thing I checked. :-) They were safe imports, and the Haskell code
> did get called--just the C code kept marching on.

Right, the RTS won't try to interrupt a foreign call even when there's a 
pending throwTo for the thread making the call.  The reason is that, 
well, there's no way to interrupt C calls.  You could try pthread_cancel 
I suppose, but only if the thread making the call is not a bound thread 
(because pthread_cancel kills the thread, it's not an exception 
mechanism).  That might be quite interesting to try, actually.  You'll 
need to modify the RTS: the place where we decide what to do when a 
throwTo is received for a thread involved in a foreign call is around 
line 396 of rts/RaiseAsync.c (in the HEAD):

     case BlockedOnCCall:
     case BlockedOnCCall_NoUnblockExc:
	blockedThrowTo(cap,target,msg);
	return THROWTO_BLOCKED;

this is where you would call pthread_cancel (after checking for a bound 
thread).  You should look into pthread_setcancelstate and 
pthread_setcanceltype, and call these appropriately for worker threads.

Cheers,
	Simon