this prevents a race condition where an event is sent as a port is being destroyed.
when the server gets around to handling the event, it now refers to a different port
that was created in the mean-time.
address-space is a non-recursive data structure, which contains a flat list of vm_areas representing
mapped vm-objects.
userspace programs can no longer create sub-address-spaces. instead, they can reserve portions of
the address space, and use that reserved space to create mappings.
wash