FINALIZATION
                          design.mps.finalize
                           incomplete design
                             drj 1997-02-14


OVERVIEW:

Finalization is implemented internally using the Guardian Pool Class 
(design.mps.poolmrg).  Objects can be registered for finalization using an 
interface function (called mps_finalize).  Notification of finalization is 
given to the client via the messaging interface.  PoolClassMRG 
(design.mps.poolmrg) implements a Message Class which implements the 
finalization messages.


REQUIREMENTS:

.req: Currently only Dylan has requirements for finalization, see 
req.dylan.fun.final.


ARCHITECTURE:

External Interface


.if.register:
mps_res_t mps_finalize(mps_arena_t arena, mps_addr_t obj);

increases the number of times that the object located at obj has been 
registered for finalization by one.  The object must have been allocated from 
the arena (space).  Any finalization messages that are created for this object 
will appear on the arena's message queue.  The MPS will attempt to finalize the 
object that number of times.

.if.deregister:
void mps_definalize(mps_arena_t arena, mps_addr_t obj);

mps_definalize reduces the number of times that the object located at obj has 
been registered for finalization by one.  It is an error to definalize that has 
not been registered for finalization.

.if.deregister.not: At the moment (1997-08-20) mps_definalize is not implemented

.if.get-ref:
void mps_message_finalization_ref(mps_addr_t *mps_addr_return,
                                  mps_arena_t mps_arena,
                                  mps_message_t mps_message)


mps_message_finalization_ref returns the reference to the finalized object 
stored in the finalization message.


IMPLEMENTATION:

.int.over: Registering an object for finalization corresponds to allocating a 
reference of rank FINAL to that object.  This reference is allocated in a 
guardian object in a pool of PoolClassMRG (see design.mps.poolmrg).

.int.arena.struct: The MRG pool used for managing final references is kept in 
the Arena (Space), referred to as the "final pool".  .int.arena.lazy: The pool 
is lazily created, it will not be created until the first object is registered 
for finalization.  .int.arena.flag: There is a flag in the Arena that indicates 
whether the final pool has been created yet or not.

.int.finalize:

Res ArenaFinalize(Arena arena, Ref addr)

.int.finalize.create: Creates the final pool if it has not been created yet.  
.int.finalize.alloc: Allocates a guardian in the final pool.  
.int.finalize.write: Writes a reference to the object into the guardian 
object.  .int.finalize.all: That's all.  .int.finalize.error: if either the 
creation of the pool or the allocation of the object fails then the error will 
be reported back to the caller.  .int.finalize.error.no-unwind: This function 
does not need to do any unwinding in the error cases because the creation of 
the pool is not something that needs to be undone.

.int.arena-destroy.empty: ArenaDestroy empties the message queue by calling 
MessageEmpty.

.int.arena-destroy.final-pool: If the final pool has been created then 
ArenaDestroy destroys the final pool.

.access: mps_message_finalization_ref needs to access the finalization message 
to retrieve the reference and then write it to where the client asks.  This 
must be done carefully, in order to avoid breaking the invariants or creating a 
hidden root.

.access.invariants: We protect the invariants by using special routines 
ArenaRead and ArenaPoke to read and write the reference.  This works as long as 
there's no write-barrier collection.  [Instead of ArenaPoke, we could put in an 
ArenaWrite that would be identical to ArenaPoke, except for AVERring the 
invariant (or it can just AVER there are no busy traces unflipped).  When we 
get write-barrier collection, we could change it to do the real thing, but in 
the absence of a write-barrier, it's functionally identical to ArenaPoke.  
Pekka 1997-12-09]