ExternalSemaphoreTable strange mutual exclusion

In ExternalSemaphoreTable I noticed two class variables set by class
initialization as:
   ProtectAdd := Semaphore forMutualExclusion.
   ProtectRemove := Semaphore forMutualExclusion

and I'm curious, what are the TWO shared resources that these TWO are
meant to protect?  it seems there is only one, that being
unprotectedExternalObjects.

I notice that "ExternalSemaphoreTable class>>safelyUnregisterExternalObject:"
stores unprotectedExternalObjects in its local variable "objects"
and then up removes an object from it.

But "ExternalSemaphoreTable class>>safelyRegisterExternalObject:"
also stores unprotectedExternalObjects in its local variable "objects"
which it then passes to "ExternalSemaphoreTable
class>>collectionBasedOn:withRoomFor:"
which takes a copy to grow a new collection to store into
unprotectedExternalObjects.

Even though its a rare case that the external semaphore might grow at
just the same time that an object is being unregistered, and also
practically #safelyUnregisterExternalObject: might not be preemptable,
this smells bad having two mutual exclusions covering one resource -
thus no mutual exclusion at all.  Is there something I'm missing?

cheers -ben

You can safely add and remove from the array concurrently, just concurrent adds or concurrent removes might lead to trouble.
Using two semaphores in this case is probably less useful in practice than as an example of how more finely grained locks are sometimes possible.
>
> I notice that "ExternalSemaphoreTable class>>safelyUnregisterExternalObject:"
> stores unprotectedExternalObjects in its local variable "objects"
> and then up removes an object from it.

The safely: should be moved to private category, probably renamed private* , and have a comment stating that they are the inner critical sections; calls must be protected by the appropriate locks. Not really.
The only valid caller of collectionBasedOn: is addExternalObject, so the method always executes inside ProtectAdd critical section.
There is, however, a ProtectRemove critical section missing, should be one around:

newObjects replaceFrom: 1 to:  externalObjects size with: externalObjects startingAt: 1.
self unprotectedExternalObjects: newObjects.

Cheers,
Henry