.. Sources:
`<https://info.ravenbrook.com/project/mps/master/design/finalize/>`_
.. index::
single: finalization
.. _topic-finalization:
Finalization
============
It is sometimes necessary to perform actions when a block of memory
:term:`dies <dead>`. For example, a block may represent the
acquisition of an external resource such as a file handle or a network
connection. When the block dies, the corresponding resource must be
released. This procedure is known as :term:`finalization`.
A block requiring finalization must be registered by calling :c:func:`mps_finalize`::
mps_addr_t ref = block_requiring_finalization;
mps_finalize(arena, &ref);
A block that been registered for finalization becomes *finalizable* as
soon as the :term:`garbage collector` observes that it would otherwise
be :term:`reclaimed` (that is, the only thing keeping it alive is the
fact that it needs to be finalized). If a block is finalizable the MPS
may choose to finalize it (by posting a finalization message: see
below) at *any* future time.
.. note::
This means that a block that was determined to be finalizable, but
then became unconditionally :term:`live` by the creation of a new
:term:`strong reference` to it, may still be finalized.
:term:`Weak references (1)` do not prevent blocks
from being finalized. At the point that a block is finalized, weak
references will still validly refer to the block. The fact that a
block is registered for finalization prevents weak references to that
block from being :term:`splatted <splat>`. See :ref:`topic-weak`.
The Memory Pool System finalizes a block by posting a *finalization
message* to the :term:`message queue` of the :term:`arena` in which
the block was allocated.
.. note::
This design avoids the problems that can result from the
:term:`garbage collector` calling a function in the client program
to do the finalization. In such an implementation, the client
program's finalization code may end up running concurrently with
other code that accesses the underlying resource, and so access to
the resource need to be guarded with a lock, but then an unlucky
scheduling of finalization can result in deadlock. See :ref:`Boehm
(2002) <BOEHM02>` for a detailed discussion of this issue.
The :term:`message type` of finalization messages is
:c:func:`mps_message_type_finalization`, and the client program must
enable the posting of these messages by calling
:c:func:`mps_message_type_enable` before any block becomes
finalizable::
mps_message_type_enable(arena, mps_message_type_finalization());
When a finalization message has been retrieved from the message queue
by calling :c:func:`mps_message_get`, the finalization reference may
be accessed by calling :c:func:`mps_message_finalization_ref`. The
finalization message keeps the block alive until it is discarded by
calling :c:func:`mps_message_discard`.
.. note::
The client program may choose to keep the finalized block alive by
keeping a strong reference to the finalized object after
discarding the finalization message.
This process is known as :term:`resurrection` and in some
finalization systems requires special handling, but in the MPS
this just is just the usual result of the rule that strong
references keep objects alive.
It is fine to re-register a block for finalization after
retrieving its finalization message from the message queue. This
will cause it to be finalized again should all strong references
disappear again.
See :ref:`topic-message` for details of the message mechanism.
.. index::
single: finalization; multiple
Multiple finalizations
----------------------
A block may be registered for finalization multiple times. A block
that has been registered for finalization *n* times will be finalized
at most *n* times.
This may mean that there are multiple finalization messages on the
queue at the same time, or it may not (it may be necessary for the
client program to discard previous finalization messages for a block
before a new finalization messages for that block are posted to the
message queue). The MPS provides no guarantees either way: a client
program that registers the same block multiple times must cope with
either behaviour.
.. index::
pair: finalization; cautions
.. _topic-finalization-cautions:
Cautions
--------
1. Don't rely on finalization for your program to work. Treat it as an
optimization that enables the freeing of resources that the
garbage collector can prove are unreachable.
2. The MPS provides no guarantees about the promptness of
finalization. The MPS does not finalize a block until it
determines that the block is finalizable, which may require a full
garbage collection in the worst case, and such a collection may
not :ref:`scheduled <topic-collection-schedule>` for some time. Or
the block may never become finalizable because it is incorrectly
determined to be reachable due to an :term:`ambiguous reference`
pointing to it. Or the block may never become finalizable because
it remains reachable through a reference, even if that reference
might never be used.
3. Even when blocks are finalized in a reasonably timely fashion, the
client needs to process the finalization messages in time to avoid
the resource running out. For example, in the Scheme interpreter,
finalization messages are only processed at the end of the
read–eval–print loop, so a program that opens many files may run
out of handles even though the associated objects are all
finalizable, as shown here:
.. code-block:: none
MPS Toy Scheme Example
9960, 0> (define (repeat n f _) (if (eqv? n 0) '() (repeat (- n 1) f (f))))
repeat
10840, 0> (repeat 300 (lambda () (open-input-file "scheme.c")) 0)
open-input-file: cannot open input file
A less naïve interpreter might process finalization messages on a
more regular schedule, or might take emergency action in the event
of running out of open file handles by carrying out a full garbage
collection and processing any finalization messages that are
posted as a result.
If you are designing a programming language then it is generally a
good idea to provide the programmer with a mechanism for ensuring
prompt release of scarce resources. For example, Scheme provides
the ``(with-input-from-file)`` procedure which specifies that the
created port has :term:`dynamic extent` (and so can be closed as
soon as the procedure exits).
4. The MPS does not finalize objects in the context of
:c:func:`mps_arena_destroy` or :c:func:`mps_pool_destroy`.
:c:func:`mps_pool_destroy` should therefore not be invoked on pools
containing objects registered for finalization.
.. note::
Under normal circumstances, finalization code can assume that
objects referenced by the object being finalized ("object F")
have themselves not yet been finalized. (Because object F is
keeping them alive.) If finalization code is run at program
exit, this assumption is no longer true. It is much more
difficult to write correct code if it has to run under both
circumstances.
This is why Java's ``System.runFinalizersOnExit`` is
deprecated. See Appendix A of :ref:`Boehm (2002) <BOEHM02>`
for a discussion of this problem.
4. Not all :term:`pool classes` support finalization. In general, only
pools that manage objects whose liveness is determined by garbage
collection do so. See the :ref:`pool`.
.. index::
single: finalization; interface
Finalization interface
----------------------
.. c:function:: mps_res_t mps_finalize(mps_arena_t arena, mps_addr_t *ref_p)
Register a :term:`block` for :term:`finalization`.
``arena`` is the arena in which the block lives.
``ref_p`` points to a :term:`reference` to the block to be
registered for finalization.
Returns :c:macro:`MPS_RES_OK` if successful, or another
:term:`result code` if not.
This function registers the block pointed to by ``*ref_p`` for
finalization. This block must have been allocated from a
:term:`pool` in ``arena``. Violations of this constraint may not
be checked by the MPS, and may be unsafe, causing the MPS to crash
in undefined ways.
.. note::
This function receives a pointer to a reference. This is to
avoid placing the restriction on the :term:`client program`
that the C call stack be a :term:`root`.
.. c:function:: mps_res_t mps_definalize(mps_arena_t arena, mps_addr_t *ref_p)
Deregister a :term:`block` for :term:`finalization`.
``arena`` is the arena in which the block lives.
``ref_p`` points to a :term:`reference` to the block to be
deregistered for finalization.
Returns :c:macro:`MPS_RES_OK` if successful, or
:c:macro:`MPS_RES_FAIL` if the block was not previously registered
for finalization.
.. note::
This function receives a pointer to a reference. This is to
avoid placing the restriction on the :term:`client program`
that the C call stack be a :term:`root`.
.. index::
pair: finalization; message
Finalization messages
---------------------
.. c:function:: mps_message_type_t mps_message_type_finalization(void)
Return the :term:`message type` of finalization messages.
Finalization messages are used by the MPS to implement
:term:`finalization`. When the MPS detects that a block that has
been registered for finalization (by calling
:c:func:`mps_finalize`) is finalizable, it finalizes it by posting
a :term:`message` of this type.
Note that there might be delays between the block becoming
finalizable, the MPS detecting that, and the message being
posted.
In addition to the usual methods applicable to messages,
finalization messages support the
:c:func:`mps_message_finalization_ref` method which returns a
reference to the block that was registered for finalization.
.. seealso::
:ref:`topic-message`.
.. c:function:: void mps_message_finalization_ref(mps_addr_t *ref_o, mps_arena_t arena, mps_message_t message)
Returns the finalization reference for a finalization message.
``ref_o`` points to a location that will hold the finalization
reference.
``arena`` is the :term:`arena` which posted the message.
``message`` is a message retrieved by :c:func:`mps_message_get` and
not yet discarded. It must be a finalization message: see
:c:func:`mps_message_type_finalization`.
The reference returned by this method is a reference to the block
that was originally registered for :term:`finalization` by a call
to :c:func:`mps_finalize`.
.. note::
The reference returned is subject to the normal constraints,
such as might be imposed by a :term:`moving <moving garbage
collector>` collection, if appropriate. For this reason, it is
stored into the location pointed to by ``ref_o`` in order to
enable the :term:`client program` to place it directly into
scanned memory, without imposing the restriction that the C
stack be a :term:`root`.
The message itself is not affected by invoking this method.
Until the client program calls :c:func:`mps_message_discard`
to discard the message, it will refer to the object and
prevent its reclamation.
.. seealso::
:ref:`topic-message`.