Re: Core dump when throwing an exception from a resumed partial continuation

[Andy Wingo <wingo@pobox.com>]

On Fri 15 Mar 2013 22:01, Brent Pinkney <brp@4dst.com> writes:

> When I resume the continuation in another thread, all works perfectly
> UNLESS the continued execution throws and exception.
> Then guile exits with a core dump.
>
> By contrast if I resume the continuation in the same thread and then
> throw and exception all works as expected.

I think I know what this is.

So, a delimited continuation should capture that part of the dynamic
environment made in its extent.  (See Oleg Kiselyov and Chung-Chieh
Shan's "Delimited Dynamic Binding" paper.)  That is what Guile does, for
fluids, prompts, and dynamic-wind blocks.

Our implementation of exception handling uses a fluid,
%exception-handler (boot-9.scm:86).  However that fluid references a
stack of exception handlers on the heap.  There is the problem: an
exception in a reinstated delimited continuation continuation will walk
the captured exception handler stack from the heap, not from its own
dynamic environment.  Therefore it could abort to a continuation that is
not present on the new thread.

The solution is to have the exception handler find the next handler from
the dynamic environment.  This will need a new primitive to walk the
dynamic stack, I think.

I can't look at this atm as I broke my arm (!) and so typing is tough.
For now as a workaround I suggest you put a catch #t in each of your
delimited continuations.  This way all throws will be handled by catches
established by the continuation.

Regards,

Andy
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