Re: [Chicken-users] Threads and dynamic-wind still problematic

[felix winkelmann]

On Mon, Mar 2, 2009 at 9:48 PM, Peter Bex <address@hidden> wrote:
>
> However, it turns out that an exception thrown in a thread simply
> terminates the thread instead of unwinding the stack.  The same does not
> happen in the primordial thread:
>

Ok, here is what Marc Feeley, the author of SRFI-18 has to say
about this:

>>    I have a question about SRFI-18: the default exception handler
>>   for a thread (the one that stores the thrown object in the
>>  thread to be later accessed via thread-join!), should it invoke
>> dynamic-wind after thunks?
>
>No the default exception handler shouldn't invoke the after thunks of the 
>current continuation.  That's because the >exception handler doesn't 
>"continue" at the initial continuation of that thread.  Here are the relevant 
>words of SRFI >18:
>
>  Moreover, in this dynamic environment the exception handler
>  is bound to the "initial exception handler" which is a unary
>  procedure which causes the (then) current thread to store in
>  its end-exception field an "uncaught exception" object whose
>  "reason" is the argument of the handler, abandon all mutexes
>  it owns, and finally terminate.
>
>The rationale is that, when an uncaught exception occurs in a thread the 
>thread is in bad shape and things have >gone sufficiently wrong that there is 
>no universally acceptable way to continue execution.  Executing after thunks 
>>could require a whole lot of processing that the thread is not in a shape to 
>do.  So the safe thing is to terminate >the thread.  If the programmer knows 
>how to recover from an exception, then he can capture the continuation >early 
>on, and install an exception handler which invokes the continuation.  When the 
>continuation is invoked the >after thunks will execute.
>
>Hope that clarifies things.

Hope that clarifies things.


cheers,
felix