Supporting Cyclic Garbage Collection
************************************

Python's support for detecting and collecting garbage which involves
circular references requires support from object types which are
"containers" for other objects which may also be containers.  Types
which do not store references to other objects, or which only store
references to atomic types (such as numbers or strings), do not need
to provide any explicit support for garbage collection.

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

   Objects with a type with this flag set must conform with the rules
   documented here.  For convenience these objects will be referred to
   as container objects.

Constructors for container types must conform to two rules:

1. The memory for the object must be allocated using
   "PyObject_GC_New()" or "PyObject_GC_NewVar()".

2. Once all the fields which may contain references to other
   containers are initialized, it must call "PyObject_GC_Track()".

TYPE* PyObject_GC_New(TYPE, PyTypeObject *type)

   Analogous to "PyObject_New()" but for container objects with the
   "Py_TPFLAGS_HAVE_GC" flag set.

TYPE* PyObject_GC_NewVar(TYPE, PyTypeObject *type, Py_ssize_t size)

   Analogous to "PyObject_NewVar()" but for container objects with the
   "Py_TPFLAGS_HAVE_GC" flag set.

TYPE* PyObject_GC_Resize(TYPE, PyVarObject *op, Py_ssize_t newsize)

   为 "PyObject_NewVar()" 所分配对象重新调整大小。 返回调整大小后的对
   象或在失败时返回 "NULL"。 *op* 必须尚未被垃圾回收器追踪。

void PyObject_GC_Track(PyObject *op)

   Adds the object *op* to the set of container objects tracked by the
   collector.  The collector can run at unexpected times so objects
   must be valid while being tracked.  This should be called once all
   the fields followed by the "tp_traverse" handler become valid,
   usually near the end of the constructor.

Similarly, the deallocator for the object must conform to a similar
pair of rules:

1. Before fields which refer to other containers are invalidated,
   "PyObject_GC_UnTrack()" must be called.

2. The object's memory must be deallocated using
   "PyObject_GC_Del()".

void PyObject_GC_Del(void *op)

   Releases memory allocated to an object using "PyObject_GC_New()" or
   "PyObject_GC_NewVar()".

void PyObject_GC_UnTrack(void *op)

   Remove the object *op* from the set of container objects tracked by
   the collector.  Note that "PyObject_GC_Track()" can be called again
   on this object to add it back to the set of tracked objects.  The
   deallocator ("tp_dealloc" handler) should call this for the object
   before any of the fields used by the "tp_traverse" handler become
   invalid.

3.8 版更變: "_PyObject_GC_TRACK()" 和 "_PyObject_GC_UNTRACK()" 宏已从
公有 C API 中移除。

The "tp_traverse" handler accepts a function parameter of this type:

int (*visitproc)(PyObject *object, void *arg)

   Type of the visitor function passed to the "tp_traverse" handler.
   The function should be called with an object to traverse as
   *object* and the third parameter to the "tp_traverse" handler as
   *arg*.  The Python core uses several visitor functions to implement
   cyclic garbage detection; it's not expected that users will need to
   write their own visitor functions.

The "tp_traverse" handler must have the following type:

int (*traverseproc)(PyObject *self, visitproc visit, void *arg)

   用于容器对象的遍历函数。 它的实现必须对 *self* 所直接包含的每个对象
   调用 *visit* 函数，*visit* 的形参为所包含对象和传给处理程序的 *arg*
   值。 *visit* 函数调用不可附带 "NULL" 对象作为参数。 如果 *visit* 返
   回非零值，则该值应当被立即返回。

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*:

void Py_VISIT(PyObject *o)

   如果 *o* 不为 "NULL"，则调用 *visit* 回调函数，附带参数 *o* 和
   *arg*。 如果 *visit* 返回一个非零值，则返回该值。 使用此宏之后，
   "tp_traverse" 处理程序的形式如下:

      static int
      my_traverse(Noddy *self, visitproc visit, void *arg)
      {
          Py_VISIT(self->foo);
          Py_VISIT(self->bar);
          return 0;
      }

"tp_clear" 处理程序必须为 "inquiry" 类型，如果对象不可变则为 "NULL"。

int (*inquiry)(PyObject *self)

   Drop references that may have created reference cycles.  Immutable
   objects do not have to define this method since they can never
   directly create reference cycles.  Note that the object must still
   be valid after calling this method (don't just call "Py_DECREF()"
   on a reference).  The collector will call this method if it detects
   that this object is involved in a reference cycle.
