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.

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"
   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. The memory for the object must be allocated using "PyObject_GC_New"
   or "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.

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.

   Ao preencher o slot "tp_alloc" de um tipo, "PyType_GenericAlloc()"
   é preferível a uma função personalizada que simplesmente chama esta
   macro.

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

   Ver também:

     * "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.

   Ao preencher o slot "tp_alloc" de um tipo, "PyType_GenericAlloc()"
   é preferível a uma função personalizada que simplesmente chama esta
   macro.

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

   Ver também:

     * "PyObject_GC_Del()"

     * "PyObject_NewVar"

     * "PyType_GenericAlloc()"

     * "tp_alloc"

PyObject *PyUnstable_Object_GC_NewWithExtraData(PyTypeObject *type, size_t extra_size)

   *Esta é uma API Instável. Isso pode se alterado sem aviso em
   lançamentos menores.*

   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).

   Aviso:

     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.

   Adicionado na versão 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)
    * Parte da ABI Estável.*

   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)
    * Parte da ABI Estável desde a versão 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()".

   Adicionado na versão 3.9.

int PyObject_GC_IsFinalized(PyObject *op)
    * Parte da ABI Estável desde a versão 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()".

   Adicionado na versão 3.9.

void PyObject_GC_Del(void *op)
    * Parte da ABI Estável.*

   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.

   Ver também:

     * "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)
    * Parte da ABI Estável.*

   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)
    * Parte da ABI Estável.*

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

typedef int (*inquiry)(PyObject *self)
    * Parte da ABI Estável.*

   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.


Traversal
=========

The "tp_traverse" handler must have the following type:

typedef int (*traverseproc)(PyObject *self, visitproc visit, void *arg)
    * Parte da ABI Estável.*

   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;
      }

   Nota:

     "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:

   Aviso:

     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.

   Nota:

     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.On the other hand,
     even if you know a member can never be part of a cycle, as a
     debugging aid you may want to visit it anyway just so the "gc"
     module's "get_referents()" function will include it.

   Nota:

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

     **Detalhes da implementação do CPython:** Garbage collection is a
     "stop-the-world" operation: even in *free threading* builds, only
     one thread state is *attached* when "tp_traverse" handlers run.

   Alterado na versão 3.9: 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)
    * Parte da ABI Estável desde a versão 3.15.*

   See "DuringGC" functions for common information.

   Adicionado na versão 3.15.

   Ver também:

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

int PyType_GetBaseByToken_DuringGC(PyTypeObject *type, void *tp_token, PyTypeObject **result)
    * Parte da ABI Estável desde a versão 3.15.*

   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.

   Adicionado na versão 3.15.

   Ver também: "PyType_GetBaseByToken()"

PyObject *PyType_GetModule_DuringGC(PyTypeObject *type)
PyObject *PyType_GetModuleByToken_DuringGC(PyTypeObject *type, const void *mod_token)
    *Retorna valor: Referência emprestada.** Parte da ABI Estável
   desde a versão 3.15.*

   See "DuringGC" functions for common information.

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

   Adicionado na versão 3.15.

   Ver também: "PyType_GetModule()", "PyType_GetModuleByToken()"


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)
    * Parte da ABI Estável.*

   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)
    * Parte da ABI Estável desde a versão 3.10.*

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

   Adicionado na versão 3.10.

int PyGC_Disable(void)
    * Parte da ABI Estável desde a versão 3.10.*

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

   Adicionado na versão 3.10.

int PyGC_IsEnabled(void)
    * Parte da ABI Estável desde a versão 3.10.*

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

   Adicionado na versão 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)

   *Esta é uma API Instável. Isso pode se alterado sem aviso em
   lançamentos menores.*

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

   Aviso:

     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.

   Adicionado na versão 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.

   Adicionado na versão 3.12.
