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| PEP: 442 | |
| Title: Safe object finalization | |
| Version: $Revision$ | |
| Last-Modified: $Date$ | |
| Author: Antoine Pitrou <solipsis@pitrou.net> | |
| BDFL-Delegate: Benjamin Peterson <benjamin@python.org> | |
| Status: Final | |
| Type: Standards Track | |
| Content-Type: text/x-rst | |
| Created: 18-May-2013 | |
| Python-Version: 3.4 | |
| Post-History: 18-May-2013 | |
| Resolution: https://mail.python.org/pipermail/python-dev/2013-June/126746.html | |
| Abstract | |
| ======== | |
| This PEP proposes to deal with the current limitations of object | |
| finalization. The goal is to be able to define and run finalizers | |
| for any object, regardless of their position in the object graph. | |
| This PEP doesn't call for any change in Python code. Objects | |
| with existing finalizers will benefit automatically. | |
| Definitions | |
| =========== | |
| Reference | |
| A directional link from an object to another. The target of the | |
| reference is kept alive by the reference, as long as the source is | |
| itself alive and the reference isn't cleared. | |
| Weak reference | |
| A directional link from an object to another, which doesn't keep | |
| alive its target. This PEP focuses on non-weak references. | |
| Reference cycle | |
| A cyclic subgraph of directional links between objects, which keeps | |
| those objects from being collected in a pure reference-counting | |
| scheme. | |
| Cyclic isolate (CI) | |
| A standalone subgraph of objects in which no object is referenced | |
| from the outside, containing one or several reference cycles, *and* | |
| whose objects are still in a usable, non-broken state: they can | |
| access each other from their respective finalizers. | |
| Cyclic garbage collector (GC) | |
| A device able to detect cyclic isolates and turn them into cyclic | |
| trash. Objects in cyclic trash are eventually disposed of by | |
| the natural effect of the references being cleared and their | |
| reference counts dropping to zero. | |
| Cyclic trash (CT) | |
| A former cyclic isolate whose objects have started being cleared | |
| by the GC. Objects in cyclic trash are potential zombies; if they | |
| are accessed by Python code, the symptoms can vary from weird | |
| AttributeErrors to crashes. | |
| Zombie / broken object | |
| An object part of cyclic trash. The term stresses that the object | |
| is not safe: its outgoing references may have been cleared, or one | |
| of the objects it references may be zombie. Therefore, | |
| it should not be accessed by arbitrary code (such as finalizers). | |
| Finalizer | |
| A function or method called when an object is intended to be | |
| disposed of. The finalizer can access the object and release any | |
| resource held by the object (for example mutexes or file | |
| descriptors). An example is a ``__del__`` method. | |
| Resurrection | |
| The process by which a finalizer creates a new reference to an | |
| object in a CI. This can happen as a quirky but supported | |
| side-effect of ``__del__`` methods. | |
| Impact | |
| ====== | |
| While this PEP discusses CPython-specific implementation details, the | |
| change in finalization semantics is expected to affect the Python | |
| ecosystem as a whole. In particular, this PEP obsoletes the current | |
| guideline that "objects with a ``__del__`` method should not be part of a | |
| reference cycle". | |
| Benefits | |
| ======== | |
| The primary benefits of this PEP regard objects with finalizers, such | |
| as objects with a ``__del__`` method and generators with a ``finally`` | |
| block. Those objects can now be reclaimed when they are part of a | |
| reference cycle. | |
| The PEP also paves the way for further benefits: | |
| * The module shutdown procedure may not need to set global variables to | |
| None anymore. This could solve a well-known class of irritating issues. | |
| The PEP doesn't change the semantics of: | |
| * Weak references caught in reference cycles. | |
| * C extension types with a custom ``tp_dealloc`` function. | |
| Description | |
| =========== | |
| Reference-counted disposal | |
| -------------------------- | |
| In normal reference-counted disposal, an object's finalizer is called | |
| just before the object is deallocated. If the finalizer resurrects | |
| the object, deallocation is aborted. | |
| *However*, if the object was already finalized, then the finalizer isn't | |
| called. This prevents us from finalizing zombies (see below). | |
| Disposal of cyclic isolates | |
| --------------------------- | |
| Cyclic isolates are first detected by the garbage collector, and then | |
| disposed of. The detection phase doesn't change and won't be described | |
| here. Disposal of a CI traditionally works in the following order: | |
| 1. Weakrefs to CI objects are cleared, and their callbacks called. At | |
| this point, the objects are still safe to use. | |
| 2. The CI becomes a CT as the GC systematically breaks all | |
| known references inside it (using the ``tp_clear`` function). | |
| 3. Nothing. All CT objects should have been disposed of in step 2 | |
| (as a side-effect of clearing references); this collection is | |
| finished. | |
| This PEP proposes to turn CI disposal into the following sequence (new | |
| steps are in bold): | |
| 1. Weakrefs to CI objects are cleared, and their callbacks called. At | |
| this point, the objects are still safe to use. | |
| 2. **The finalizers of all CI objects are called.** | |
| 3. **The CI is traversed again to determine if it is still isolated. | |
| If it is determined that at least one object in CI is now reachable | |
| from outside the CI, this collection is aborted and the whole CI | |
| is resurrected. Otherwise, proceed.** | |
| 4. The CI becomes a CT as the GC systematically breaks all | |
| known references inside it (using the ``tp_clear`` function). | |
| 5. Nothing. All CT objects should have been disposed of in step 4 | |
| (as a side-effect of clearing references); this collection is | |
| finished. | |
| .. note:: | |
| The GC doesn't recalculate the CI after step 2 above, hence the need | |
| for step 3 to check that the whole subgraph is still isolated. | |
| C-level changes | |
| =============== | |
| Type objects get a new ``tp_finalize`` slot to which ``__del__`` methods | |
| are mapped (and reciprocally). Generators are modified to use this slot, | |
| rather than ``tp_del``. A ``tp_finalize`` function is a normal C | |
| function which will be called with a valid and alive ``PyObject`` as its | |
| only argument. It doesn't need to manipulate the object's reference count, | |
| as this will be done by the caller. However, it must ensure that the | |
| original exception state is restored before returning to the caller. | |
| For compatibility, ``tp_del`` is kept in the type structure. Handling | |
| of objects with a non-NULL ``tp_del`` is unchanged: when part of a CI, | |
| they are not finalized and end up in ``gc.garbage``. However, a non-NULL | |
| ``tp_del`` is not encountered anymore in the CPython source tree (except | |
| for testing purposes). | |
| Two new C API functions are provided to ease calling of ``tp_finalize``, | |
| especially from custom deallocators. | |
| On the internal side, a bit is reserved in the GC header for GC-managed | |
| objects to signal that they were finalized. This helps avoid finalizing | |
| an object twice (and, especially, finalizing a CT object after it was | |
| broken by the GC). | |
| .. note:: | |
| Objects which are not GC-enabled can also have a ``tp_finalize`` slot. | |
| They don't need the additional bit since their ``tp_finalize`` function | |
| can only be called from the deallocator: it therefore cannot be called | |
| twice, except when resurrected. | |
| Discussion | |
| ========== | |
| Predictability | |
| -------------- | |
| Following this scheme, an object's finalizer is always called exactly | |
| once, even if it was resurrected afterwards. | |
| For CI objects, the order in which finalizers are called (step 2 above) | |
| is undefined. | |
| Safety | |
| ------ | |
| It is important to explain why the proposed change is safe. There | |
| are two aspects to be discussed: | |
| * Can a finalizer access zombie objects (including the object being | |
| finalized)? | |
| * What happens if a finalizer mutates the object graph so as to impact | |
| the CI? | |
| Let's discuss the first issue. We will divide possible cases in two | |
| categories: | |
| * If the object being finalized is part of the CI: by construction, no | |
| objects in CI are zombies yet, since CI finalizers are called before | |
| any reference breaking is done. Therefore, the finalizer cannot | |
| access zombie objects, which don't exist. | |
| * If the object being finalized is not part of the CI/CT: by definition, | |
| objects in the CI/CT don't have any references pointing to them from | |
| outside the CI/CT. Therefore, the finalizer cannot reach any zombie | |
| object (that is, even if the object being finalized was itself | |
| referenced from a zombie object). | |
| Now for the second issue. There are three potential cases: | |
| * The finalizer clears an existing reference to a CI object. The CI | |
| object may be disposed of before the GC tries to break it, which | |
| is fine (the GC simply has to be aware of this possibility). | |
| * The finalizer creates a new reference to a CI object. This can only | |
| happen from a CI object's finalizer (see above why). Therefore, the | |
| new reference will be detected by the GC after all CI finalizers are | |
| called (step 3 above), and collection will be aborted without any | |
| objects being broken. | |
| * The finalizer clears or creates a reference to a non-CI object. By | |
| construction, this is not a problem. | |
| Implementation | |
| ============== | |
| An implementation is available in branch ``finalize`` of the repository | |
| at http://hg.python.org/features/finalize/. | |
| Validation | |
| ========== | |
| Besides running the normal Python test suite, the implementation adds | |
| test cases for various finalization possibilities including reference cycles, | |
| object resurrection and legacy ``tp_del`` slots. | |
| The implementation has also been checked to not produce any regressions on | |
| the following test suites: | |
| * `Tulip <http://code.google.com/p/tulip/>`_, which makes an extensive | |
| use of generators | |
| * `Tornado <http://www.tornadoweb.org>`_ | |
| * `SQLAlchemy <http://www.sqlalchemy.org/>`_ | |
| * `Django <https://www.djangoproject.com/>`_ | |
| * `zope.interface <http://pypi.python.org/pypi/zope.interface>`_ | |
| References | |
| ========== | |
| Notes about reference cycle collection and weak reference callbacks: | |
| http://hg.python.org/cpython/file/4e687d53b645/Modules/gc_weakref.txt | |
| Generator memory leak: http://bugs.python.org/issue17468 | |
| Allow objects to decide if they can be collected by GC: | |
| http://bugs.python.org/issue9141 | |
| Module shutdown procedure based on GC | |
| http://bugs.python.org/issue812369 | |
| Copyright | |
| ========= | |
| This document has been placed in the public domain. | |
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