Suporte a Coleta de Lixo Cíclica
********************************

O suporte do Python para detectar e coletar o lixo, que envolve
referencias circulares, requer suporte dos tipos de objetos que são
"contêiners" para outros objetos que também podem ser contêiners.
Tipos que não armazenam referências a outros tipos de objetos, ou que
apenas armazenam referências a tipos atômicos (como números ou
strings), não precisam fornecer nenhum suporte explicito para coleta
de lixo.

Para criar um tipo container, o "tp_flags" campo do tipo do objeto
deve incluir o "Py_TPFLAGS_HAVE_GC`e fornecer uma implementação do
:c:member:`~PyTypeObject.tp_traverse" manipulador. Se as instâncias do
tipo forem mutáveis, uma:c:member:*~PyTypeObject.tp_clear*
implementação também deverá ser fornecida.

Py_TPFLAGS_HAVE_GC

   Objetos com esse tipo de sinalizador definido devem seguir as
   regras documentadas aqui. Por conveniência esses objetos serão
   referenciados como objetos de contêiner.

Construtores para tipos de contêiner devem obedecer a duas regras:

1. A memória para o objeto deve ser alocada usando "PyObject_GC_New()"
   ou "PyObject_GC_NewVar()".

2. Uma vez que todos os campos que podem conter referências a outros
   containers foram inicializados, deve-se chamar
   "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)

   Resize an object allocated by "PyObject_NewVar()".  Returns the
   resized object or "NULL" on failure.  *op* must not be tracked by
   the collector yet.

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.

void _PyObject_GC_TRACK(PyObject *op)

   A macro version of "PyObject_GC_Track()".  It should not be used
   for extension modules.

   Obsoleto desde a versão 3.6: This macro is removed from Python 3.8.

Da mesma forma, o desalocador para o objeto deve estar em conformidade
com regras semelhantes:

1. Antes que os campos que fazer referência a outros containers sejam
   invalidados,  "PyObject_GC_UnTrack()" deve ser chamado.

2. A memória destinada ao objeto deve ser desalocada usando
   "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.

void _PyObject_GC_UNTRACK(PyObject *op)

   A macro version of "PyObject_GC_UnTrack()".  It should not be used
   for extension modules.

   Obsoleto desde a versão 3.6: This macro is removed from Python 3.8.

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)

   Traversal function for a container object.  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.

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)

   If *o* is not "NULL", call the *visit* callback, with arguments *o*
   and *arg*.  If *visit* returns a non-zero value, then return it.
   Using this macro, "tp_traverse" handlers look like:

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

The "tp_clear" handler must be of the "inquiry" type, or "NULL" if the
object is immutable.

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.
