.. mode: -*- rst -*- Finalization ============ :Tag: design.mps.finalize :Author: David Jones :Date: 1997-02-14 :Status: incomplete design :Revision: $Id: //info.ravenbrook.com/project/mps/version/1.116/design/finalize.txt#1 $ :Copyright: See `Copyright and License`_. :Index terms: pair: finalization; design Overview -------- _`.overview`: Finalization is implemented internally using the Guardian pool class (design.mps.poolmrg_). Objects can be registered for finalization using ``mps_finalize()``. Notification of finalization is given to the client via the messaging interface. The Guardian pool class implements a ``Message`` subclass which implements the finalization messages. .. _design.mps.poolmrg: poolmrg 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. Architecture ------------ External interface .................. _`.if.register`: ``mps_finalize()`` increases the number of times that an object 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`: ``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 an object that has not been registered for finalization. _`.if.deregister.not`: At the moment (1997-08-20) ``mps_definalize()`` is not implemented. _`.if.get-ref`: ``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_). .. _design.mps.poolmrg: 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. ``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. .. note:: Instead of ``ArenaPoke()``, we could put in an ``ArenaWrite()`` that would be identical to ``ArenaPoke()``, except that it would ``AVER()`` the invariant (or it can just ``AVER()`` that 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 P. Pirinen, 1997-12-09. Document History ---------------- - 1997-02-14 David Jones. Incomplete design. - 2002-06-07 RB_ Converted from MMInfo database design document. - 2013-04-13 GDR_ Converted to reStructuredText. .. _RB: http://www.ravenbrook.com/consultants/rb/ .. _GDR: http://www.ravenbrook.com/consultants/gdr/ Copyright and License --------------------- Copyright © 2013-2014 Ravenbrook Limited . All rights reserved. This is an open source license. Contact Ravenbrook for commercial licensing options. Redistribution and use in source and binary forms, with or without modification, are permitted provided that the following conditions are met: #. Redistributions of source code must retain the above copyright notice, this list of conditions and the following disclaimer. #. Redistributions in binary form must reproduce the above copyright notice, this list of conditions and the following disclaimer in the documentation and/or other materials provided with the distribution. #. 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