Michael,

Thanks a lot for this pointer. I had a look at the VW RecursionLock
class, and implemented something similar. I will probably need to
improve it by giving blocks unwind protection, which I think is done
in Tims exceptions package. Anyway, you will be pleased to know
that this technique is exactly what is needed to prevent nested
event triggering from blocking the PostOffice semaphore.

Thanks again!

Peter



>> I have found this to be a problem with OO and Semaphores
>> in general - there is so much cross-calling of messages it
>> is difficult to predict when a locked message will call itself.
>> It would be good to have a "smart semaphore" that forced
>> outside callers to wait, but let self invoke multiple invocations
>
>VW has a RecursionLock which is basicly a "smart Semaphore" that
>knows which Process holds the lock, thereby alowing recursive calls
>into the protected sections -- invaluable for thread-safe (I.E. Smalltalk 
>Process-safe) code.
>
>I've also found other similar smart mutex's to be usefull.  I had
>a TimeZone class which called-out to the OS (Unix) for doing timezone
>calculations.  Unfortunately, the Unix libraries have the timezone
>global to the OS's process. I had a lock that blocked all entrants who
>wanted to do a calculation in a different zone than the zone held by the 
>lock.
>
>Generally, I find that simple mutexes around small block's of code
>arise from cleaner designs.  Recursion locks are bigger armour-plating
>for putting around large pieces of functionality, usually in concert
>with general exception handling.
>
>-- Mike klein
>
>mklein at alumni.caltech.edu
>
>