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.
