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

O suporte do Python para detectar e coletar o lixo, que envolve
referencias circulares, requer suporte dos tipos de objetos que são
"contêineres" para outros objetos que também podem ser contêineres.
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 contêiner, o "tp_flags" campo do tipo do objeto
deve incluir o "Py_TPFLAGS_HAVE_GC" e fornecer uma implementação do
"tp_traverse" manipulador. Se as instâncias do tipo forem mutáveis,
uma "tp_clear" implementação também deverá ser fornecida.

Py_TPFLAGS_HAVE_GC

   Objetos com esse tipo de sinalizador definido devem estar em
   conformidade com 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
   contêineres foram inicializados, deve-se chamar
   "PyObject_GC_Track()".

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 contêineres sejam
   invalidados,  "PyObject_GC_UnTrack()" deve ser chamado.

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

   Aviso:

     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.

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)
    * Part of the Stable ABI.*

   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.

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)
    * Part of the Stable ABI since version 3.9.*

   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()".

   Novo na versão 3.9.

int PyObject_GC_IsFinalized(PyObject *op)
    * Part of the Stable ABI since version 3.9.*

   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()".

   Novo na versão 3.9.

void PyObject_GC_Del(void *op)
    * Part of the Stable ABI.*

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

void PyObject_GC_UnTrack(void *op)
    * Part of the Stable ABI.*

   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.

Alterado na versão 3.8: The "_PyObject_GC_TRACK()" and
"_PyObject_GC_UNTRACK()" macros have been removed from the public C
API.

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

typedef int (*visitproc)(PyObject *object, void *arg)
    * Part of the Stable ABI.*

   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:

typedef int (*traverseproc)(PyObject *self, visitproc visit, void *arg)
    * Part of the Stable ABI.*

   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.

typedef int (*inquiry)(PyObject *self)
    * Part of the Stable ABI.*

   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.


Controlando o estado do coletor de lixo
=======================================

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

Py_ssize_t PyGC_Collect(void)
    * Part of the 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)
    * Part of the Stable ABI since version 3.10.*

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

   Novo na versão 3.10.

int PyGC_Disable(void)
    * Part of the Stable ABI since version 3.10.*

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

   Novo na versão 3.10.

int PyGC_IsEnabled(void)
    * Part of the Stable ABI since version 3.10.*

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

   Novo na versão 3.10.
