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)

   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.

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.

Changed in version 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:

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.
