11. Finalization

11.1. Overview

.overview: Finalization is implemented internally using the MRG pool class (design.mps.poolmrg). Objects can be registered for finalization by calling mps_finalize(). Notification of finalization is given to the client via the messaging interface (design.mps.message). The MRG pool class implements a Message subclass which implements the finalization messages.

11.2. Requirements

.req: Historically only Dylan had requirements for finalization, see req.dylan.fun.final. Now (2003-02-19) Configura have requirements for finalization. Happily they are very similar.

11.3. Implementation

.impl.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 belonging to the MRG pool class (see design.mps.poolmrg).

.impl.arena.struct: A single pool belonging to the MRG pool class and used for managing final references is kept in the arena and referred to as the “final pool”.

.impl.arena.lazy: The final pool is lazily created. It is not created until the first object is registered for finalization.

.impl.arena.flag: There is a flag in the Arena that indicates whether the final pool has been created yet or not.

.impl.scan: An object is determined to be finalizable if it is fixed at rank FINAL for a trace, and was not fixed at any lower rank for that trace. See design.mps.poolmrg.scan.wasold.

.impl.message: When an object is determined to be finalizable, a message for that object is posted to the arena’s message queue.

.impl.arena-destroy.empty: ArenaDestroy() empties the message queue by calling MessageEmpty().

.impl.arena-destroy.final-pool: If the final pool has been created then ArenaDestroy() destroys the final pool.

.impl.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 invalidating collection invariants such as the segment summary.

.impl.invariants: We protect the invariants by using ArenaRead() and ArenaWrite() to read and write the reference via the software barrier.

11.4. External interface

.if.register: mps_finalize() registers an object for finalization.

.if.deregister: mps_definalize() deregisters an object for finalization. It is an error to definalize an object that has not been registered for finalization.

.if.get-ref: mps_message_finalization_ref() returns the reference to the finalized object stored in the finalization message.

.if.multiple: The external interface allows an object to be registered multiple times, but does not specify the number of finalization messages that will be posted for that object.

11.5. Internal interface

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.alloc.multiple: A consequence of this implementation is that if an object is finalized multiple times, then multiple guardians are created in the final pool, and so multiple messages will be posted to the message queue when the object is determined to be finalizable. But this behaviour is not guaranteed by the documentation, leaving us free to change the iplementation.

.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 is returned 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.

Res ArenaDefinalize(Arena arena, Ref obj)

.int.definalize.fail: If the final pool has not been created, return ResFAIL immediately.

.int.definalize.search: Otherwise, search for a guardian in the final pool that refers to the object and which has not yet been finalized. If one is found, delete it and return ResOK. Otherwise no guardians in the final pool refer to the object, so return ResFAIL.