# 循環参照ガベージコレクションをサポートする

Python が循環参照を含むガベージの検出とコレクションをサポートするには、他のオブジェクトに対する "コンテナ" (他のオブジェクトには他のコンテナも含みます) となるオブジェクト型によるサポートが必要です。他のオブジェクトに対する参照を記憶しないオブジェクトや、(数値や文字列のような) アトム型 (atomic type) への参照だけを記憶するような型では、ガベージコレクションに際して特別これといったサポートを提供する必要はありません。

To create a container type, the `tp_flags` field of the type object must include the `Py_TPFLAGS_HAVE_GC` and provide an implementation of the `tp_traverse` handler. If instances of the type are mutable, a `tp_clear` implementation must also be provided.

`Py_TPFLAGS_HAVE_GC`

  このフラグをセットした型のオブジェクトは、この節に述べた規則に適合しなければなりません。簡単のため、このフラグをセットした型のオブジェクトをコンテナオブジェクトと呼びます。

コンテナ型のコンストラクタは以下の二つの規則に適合しなければなりません:

1. The memory for the object must be allocated using `PyObject_GC_New` or `PyObject_GC_NewVar`.

2. 他のコンテナへの参照が入るかもしれないフィールドが全て初期化されたら、すぐに `PyObject_GC_Track()` を呼び出さなければなりません。

同様に、オブジェクトのメモリ解放関数も以下の二つの規則に適合しなければなりません:

1. 他のコンテナを参照しているフィールドを無効化する前に、 `PyObject_GC_UnTrack()` を呼び出さなければなりません。

2. オブジェクトのメモリは `PyObject_GC_Del()` で解放しなければなりません。

   警告

   If a type adds the Py_TPFLAGS_HAVE_GC, then it must implement at least a `tp_traverse` handler or explicitly use one from its subclass or subclasses.

   When calling `PyType_Ready()` or some of the APIs that indirectly call it like `PyType_FromSpecWithBases()` or `PyType_FromSpec()` the interpreter will automatically populate the `tp_flags`, `tp_traverse` and `tp_clear` fields if the type inherits from a class that implements the garbage collector protocol and the child class does not include the `Py_TPFLAGS_HAVE_GC` flag.

PyObject_GC_New(TYPE, typeobj)

  Analogous to `PyObject_New` but for container objects with the `Py_TPFLAGS_HAVE_GC` flag set.

  Do not call this directly to allocate memory for an object; call the type's `tp_alloc` slot instead.

  When populating a type's `tp_alloc` slot, `PyType_GenericAlloc()` is preferred over a custom function that simply calls this macro.

  Memory allocated by this macro must be freed with `PyObject_GC_Del()` (usually called via the object's `tp_free` slot).

  参考

  - `PyObject_GC_Del()`

  - `PyObject_New`

  - `PyType_GenericAlloc()`

  - `tp_alloc`

PyObject_GC_NewVar(TYPE, typeobj, size)

  Analogous to `PyObject_NewVar` but for container objects with the `Py_TPFLAGS_HAVE_GC` flag set.

  Do not call this directly to allocate memory for an object; call the type's `tp_alloc` slot instead.

  When populating a type's `tp_alloc` slot, `PyType_GenericAlloc()` is preferred over a custom function that simply calls this macro.

  Memory allocated by this macro must be freed with `PyObject_GC_Del()` (usually called via the object's `tp_free` slot).

  参考

  - `PyObject_GC_Del()`

  - `PyObject_NewVar`

  - `PyType_GenericAlloc()`

  - `tp_alloc`

PyObject *PyUnstable_Object_GC_NewWithExtraData(PyTypeObject *type, size_t extra_size)
  これは Unstable APIです。マイナーリリースで予告なく変更されることがあります。

  Analogous to `PyObject_GC_New` but allocates extra_size bytes at the end of the object (at offset `tp_basicsize`). The allocated memory is initialized to zeros, except for the `Python object header`.

  The extra data will be deallocated with the object, but otherwise it is not managed by Python.

  Memory allocated by this function must be freed with `PyObject_GC_Del()` (usually called via the object's `tp_free` slot).

  警告

  The function is marked as unstable because the final mechanism for reserving extra data after an instance is not yet decided. For allocating a variable number of fields, prefer using `PyVarObject` and `tp_itemsize` instead.

  Added in version 3.12.

PyObject_GC_Resize(TYPE, op, newsize)

  Resize an object allocated by `PyObject_NewVar`. Returns the resized object of type `TYPE*` (refers to any C type) or `NULL` on failure.

  op must be of type PyVarObject* and must not be tracked by the collector yet. newsize must be of type `Py_ssize_t`.

void PyObject_GC_Track(PyObject *op)
  次に属します: Stable ABI.

  オブジェクト op を、コレクタによって追跡されるオブジェクトの集合に追加します。コレクタは何回動くのかは予想できないので、追跡されている間はオブジェクトは正しい状態でいなければなりません。 `tp_traverse` の対象となる全てのフィールドが正しい状態になってすぐに、たいていはコンストラクタの末尾付近で、呼び出すべきです。

int PyObject_IS_GC(PyObject *obj)

  Returns non-zero if the object implements the garbage collector protocol, otherwise returns 0.

  The object cannot be tracked by the garbage collector if this function returns 0.

int PyObject_GC_IsTracked(PyObject *op)
  次に属します: Stable ABI (バージョン 3.9 より).

  Returns 1 if the object type of op implements the GC protocol and op is being currently tracked by the garbage collector and 0 otherwise.

  This is analogous to the Python function `gc.is_tracked()`.

  Added in version 3.9.

int PyObject_GC_IsFinalized(PyObject *op)
  次に属します: Stable ABI (バージョン 3.9 より).

  Returns 1 if the object type of op implements the GC protocol and op has been already finalized by the garbage collector and 0 otherwise.

  This is analogous to the Python function `gc.is_finalized()`.

  Added in version 3.9.

void PyObject_GC_Del(void *op)
  次に属します: Stable ABI.

  Releases memory allocated to an object using `PyObject_GC_New` or `PyObject_GC_NewVar`.

  Do not call this directly to free an object's memory; call the type's `tp_free` slot instead.

  Do not use this for memory allocated by `PyObject_New`, `PyObject_NewVar`, or related allocation functions; use `PyObject_Free()` instead.

  参考

  - `PyObject_Free()` is the non-GC equivalent of this function.

  - `PyObject_GC_New`

  - `PyObject_GC_NewVar`

  - `PyType_GenericAlloc()`

  - `tp_free`

void PyObject_GC_UnTrack(void *op)
  次に属します: Stable ABI.

  オブジェクト op を、コレクタによって追跡されるオブジェクトの集合から除去します。このオブジェクトに対して `PyObject_GC_Track()` を再度呼び出して、追跡されるオブジェクトの集合に戻すことも可能です。 `tp_traverse` ハンドラの対象となるフィールドが正しくない状態になる前に、デアロケータ (`tp_dealloc` ハンドラ) はオブジェクトに対して、この関数を呼び出すべきです。

バージョン 3.8 で変更: The `_PyObject_GC_TRACK()` and `_PyObject_GC_UNTRACK()` macros have been removed from the public C API.

`tp_traverse` ハンドラはこの型の関数パラメータを受け取ります:

typedef int (*visitproc)(PyObject *object, void *arg)
  次に属します: Stable ABI.

  `tp_traverse` ハンドラに渡されるビジター関数 (visitor function) の型です。この関数は、探索するオブジェクトを object として、 `tp_traverse` ハンドラの第 3 引数を arg として呼び出します。 Python のコアはいくつかのビジター関数を使って、ゴミとなった循環参照を検出する仕組みを実装します; ユーザが自身のためにビジター関数を書く必要が出てくることはないでしょう。

`tp_clear` ハンドラは `inquiry` 型であるか、オブジェクトが不変 (immutable) な場合は `NULL` でなければなりません。

typedef int (*inquiry)(PyObject *self)
  次に属します: Stable ABI.

  循環参照を形成しているとおぼしき参照群を放棄します。変更不可能なオブジェクトは循環参照を直接形成することが決してないので、この関数を定義する必要はありません。このメソッドを呼び出した後でもオブジェクトは有効なままでなければならないので注意してください (参照に対して `Py_DECREF()` を呼ぶだけにしないでください)。ガベージコレクタは、オブジェクトが循環参照を形成していることを検出した際にこのメソッドを呼び出します。

## Traversal

`tp_traverse` ハンドラは次の型を持っていなければなりません:

typedef int (*traverseproc)(PyObject *self, visitproc visit, void *arg)
  次に属します: Stable ABI.

  Traversal function for a garbage-collected object, used by the garbage collector to detect reference cycles. Implementations must call the visit function for each object directly contained by self, with the parameters to visit being the contained object and the arg value passed to the handler. The visit function must not be called with a `NULL` object argument. If visit returns a non-zero value, that value should be returned immediately.

  A typical `tp_traverse` function calls the `Py_VISIT()` convenience macro on each of the instance's members that are Python objects that the instance owns. For example, this is a (slightly outdated) traversal function for the `threading.local` class:

      static int
      local_traverse(PyObject *op, visitproc visit, void *arg)
      {
          localobject *self = (localobject *) op;
          Py_VISIT(Py_TYPE(self));
          Py_VISIT(self->args);
          Py_VISIT(self->kw);
          Py_VISIT(self->dict);
          return 0;
      }

  注釈

  `Py_VISIT()` requires the visit and arg parameters to `local_traverse()` to have these specific names; don't name them just anything.

  Instances of heap-allocated types hold a reference to their type. Their traversal function must therefore visit the type:

      Py_VISIT(Py_TYPE(self));

  Alternately, the type may delegate this responsibility by calling `tp_traverse` of a heap-allocated superclass (or another heap-allocated type, if applicable). If they do not, the type object may not be garbage-collected.

  If the `Py_TPFLAGS_MANAGED_DICT` bit is set in the `tp_flags` field, the traverse function must call `PyObject_VisitManagedDict()` like this:

      int err = PyObject_VisitManagedDict((PyObject*)self, visit, arg);
      if (err) {
          return err;
      }

  Only the members that the instance owns (by having strong references to them) must be visited. For instance, if an object supports weak references via the `tp_weaklist` slot, the pointer supporting the linked list (what tp_weaklist points to) must not be visited as the instance does not directly own the weak references to itself.

  The traversal function has a limitation:

  警告

  The traversal function must not have any side effects. Implementations may not modify the reference counts of any Python objects nor create or destroy any Python objects, directly or indirectly.

  This means that most Python C API functions may not be used, since they can raise a new exception, return a new reference to a result object, have internal logic that uses side effects. Also, unless documented otherwise, functions that happen to not have side effects may start having them in future versions, without warning.

  For a list of safe functions, see a separate section below.

  注釈

  The `Py_VISIT()` call may be skipped for those members that provably cannot participate in reference cycles. In the `local_traverse` example above, there is also a `self->key` member, but it can only be `NULL` or a Python string and therefore cannot be part of a reference cycle.

  一方、メンバが循環参照の一部になり得ないと判っていても、デバッグ目的で巡回したい場合があるかもしれないので、 `gc` モジュールの `get_referents()` 関数は循環参照になり得ないメンバも返します。

  注釈

  The `tp_traverse` function can be called from any thread.

  CPython 実装の詳細: Garbage collection is a "stop-the-world" operation: even in free threading builds, only one thread state is attached when `tp_traverse` handlers run.

  バージョン 3.9 で変更: Heap-allocated types are expected to visit `Py_TYPE(self)` in `tp_traverse`. In earlier versions of Python, due to bug 40217, doing this may lead to crashes in subclasses.

To simplify writing `tp_traverse` handlers, a `Py_VISIT()` macro is provided. In order to use this macro, the `tp_traverse` implementation must name its arguments exactly visit and arg:

Py_VISIT(o)

  If the PyObject* o is not `NULL`, call the visit callback, with arguments o and arg. If visit returns a non-zero value, then return it.

  This corresponds roughly to:

      #define Py_VISIT(o)                             \
         if (op) {                                    \
            int visit_result = visit(o, arg);         \
            if (visit_result != 0) {                  \
               return visit_result;                   \
            }                                         \
         }

### Traversal-safe functions

The following functions and macros are safe to use in a `tp_traverse` handler:

- the visit function passed to `tp_traverse`

- `Py_VISIT()`

- `Py_SIZE()`

- `Py_TYPE()`: if called from a `tp_traverse` handler, `Py_TYPE()`'s result will be valid for the duration of the handler call

- `PyObject_VisitManagedDict()`

- `PyObject_TypeCheck()`, `PyType_IsSubtype()`, `PyType_HasFeature()`

- `Py<type>_Check` and `Py<type>_CheckExact` -- for example, `PyTuple_Check()`

- "DuringGC" functions

### "DuringGC" functions

The following functions should only be used in a `tp_traverse` handler; calling them in other contexts may have unintended consequences.

These functions act like their counterparts without the `_DuringGC` suffix, but they are guaranteed to not have side effects, they do not set an exception on failure, and they return/set borrowed references as detailed in the individual documentation.

Note that these functions may fail (return `NULL` or `-1`), but as they do not set an exception, no error information is available. In some cases, failure is not distinguishable from a successful `NULL` result.

void *PyObject_GetTypeData_DuringGC(PyObject *o, PyTypeObject *cls)
void *PyObject_GetItemData_DuringGC(PyObject *o)
void *PyType_GetModuleState_DuringGC(PyTypeObject *type)
void *PyModule_GetState_DuringGC(PyObject *module)
int PyModule_GetToken_DuringGC(PyObject *module, void **result)
  次に属します: Stable ABI (バージョン 3.15 より).

  See "DuringGC" functions for common information.

  Added in version 3.15.

  参考

  `PyObject_GetTypeData()`, `PyObject_GetItemData()`, `PyType_GetModuleState()`, `PyModule_GetState()`, `PyModule_GetToken()`, `PyType_GetBaseByToken()`

int PyType_GetBaseByToken_DuringGC(PyTypeObject *type, void *tp_token, PyTypeObject **result)
  次に属します: Stable ABI (バージョン 3.15 より).

  See "DuringGC" functions for common information.

  Sets *result to a borrowed reference rather than a strong one. The reference is valid for the duration of the `tp_traverse` handler call.

  Added in version 3.15.

  参考

  `PyType_GetBaseByToken()`

PyObject *PyType_GetModule_DuringGC(PyTypeObject *type)
PyObject *PyType_GetModuleByToken_DuringGC(PyTypeObject *type, const void *mod_token)
  戻り値: 借用参照。 次に属します: Stable ABI (バージョン 3.15 より).

  See "DuringGC" functions for common information.

  These functions return a borrowed reference, which is valid for the duration of the `tp_traverse` handler call.

  Added in version 3.15.

  参考

  `PyType_GetModule()`, `PyType_GetModuleByToken()`

## Controlling the Garbage Collector State

The C-API provides the following functions for controlling garbage collection runs.

Py_ssize_t PyGC_Collect(void)
  次に属します: Stable ABI.

  Perform a full garbage collection, if the garbage collector is enabled. (Note that `gc.collect()` runs it unconditionally.)

  Returns the number of collected + unreachable objects which cannot be collected. If the garbage collector is disabled or already collecting, returns `0` immediately. Errors during garbage collection are passed to `sys.unraisablehook`. This function does not raise exceptions.

int PyGC_Enable(void)
  次に属します: Stable ABI (バージョン 3.10 より).

  Enable the garbage collector: similar to `gc.enable()`. Returns the previous state, 0 for disabled and 1 for enabled.

  Added in version 3.10.

int PyGC_Disable(void)
  次に属します: Stable ABI (バージョン 3.10 より).

  Disable the garbage collector: similar to `gc.disable()`. Returns the previous state, 0 for disabled and 1 for enabled.

  Added in version 3.10.

int PyGC_IsEnabled(void)
  次に属します: Stable ABI (バージョン 3.10 より).

  Query the state of the garbage collector: similar to `gc.isenabled()`. Returns the current state, 0 for disabled and 1 for enabled.

  Added in version 3.10.

## Querying Garbage Collector State

The C-API provides the following interface for querying information about the garbage collector.

void PyUnstable_GC_VisitObjects(gcvisitobjects_t callback, void *arg)
  これは Unstable APIです。マイナーリリースで予告なく変更されることがあります。

  Run supplied callback on all live GC-capable objects. arg is passed through to all invocations of callback.

  警告

  If new objects are (de)allocated by the callback it is undefined if they will be visited.

  Garbage collection is disabled during operation. Explicitly running a collection in the callback may lead to undefined behaviour e.g. visiting the same objects multiple times or not at all.

  Added in version 3.12.

typedef int (*gcvisitobjects_t)(PyObject *object, void *arg)

  Type of the visitor function to be passed to `PyUnstable_GC_VisitObjects()`. arg is the same as the arg passed to `PyUnstable_GC_VisitObjects`. Return `1` to continue iteration, return `0` to stop iteration. Other return values are reserved for now so behavior on returning anything else is undefined.

  Added in version 3.12.