循環垃圾回收的支援
******************

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()".

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()".

   警告:

     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.

   When populating a type's "tp_alloc" slot, "PyType_GenericAlloc()"
   is preferred over a custom function that simply calls this macro.

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

   也參考:

     * "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.

   When populating a type's "tp_alloc" slot, "PyType_GenericAlloc()"
   is preferred over a custom function that simply calls this macro.

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

   也參考:

     * "PyObject_GC_Del()"

     * "PyObject_NewVar"

     * "PyType_GenericAlloc()"

     * "tp_alloc"

PyObject *PyUnstable_Object_GC_NewWithExtraData(PyTypeObject *type, size_t extra_size)

   *這是 不穩定 API，它可能在小版本發布中沒有任何警告地被變更。*

   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).

   警告:

     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.

   在 3.12 版被加入.

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)
    * 為 穩定 ABI 的一部分.*

   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)
    * 為 穩定 ABI 的一部分 自 3.9 版本開始.*

   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()".

   在 3.9 版被加入.

int PyObject_GC_IsFinalized(PyObject *op)
    * 為 穩定 ABI 的一部分 自 3.9 版本開始.*

   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()".

   在 3.9 版被加入.

void PyObject_GC_Del(void *op)
    * 為 穩定 ABI 的一部分.*

   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.

   也參考:

     * "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)
    * 為 穩定 ABI 的一部分.*

   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.

在 3.8 版的變更: 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)
    * 為 穩定 ABI 的一部分.*

   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)
    * 為 穩定 ABI 的一部分.*

   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)
    * 為 穩定 ABI 的一部分.*

   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;
      }

   備註:

     "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:

   警告:

     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.

   備註:

     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.

   備註:

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

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

   在 3.9 版的變更: 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)
    * 為 穩定 ABI 的一部分 自 3.15 版本開始.*

   See "DuringGC" functions for common information.

   在 3.15 版被加入.

   也參考:

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

int PyType_GetBaseByToken_DuringGC(PyTypeObject *type, void *tp_token, PyTypeObject **result)
    * 為 穩定 ABI 的一部分 自 3.15 版本開始.*

   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.

   在 3.15 版被加入.

   也參考: "PyType_GetBaseByToken()"

PyObject *PyType_GetModule_DuringGC(PyTypeObject *type)
PyObject *PyType_GetModuleByToken_DuringGC(PyTypeObject *type, const void *mod_token)
    *回傳值：借用參照。** 為 穩定 ABI 的一部分 自 3.15 版本開始.*

   See "DuringGC" functions for common information.

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

   在 3.15 版被加入.

   也參考: "PyType_GetModule()", "PyType_GetModuleByToken()"


Controlling the Garbage Collector State
=======================================

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

Py_ssize_t PyGC_Collect(void)
    * 為 穩定 ABI 的一部分.*

   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)
    * 為 穩定 ABI 的一部分 自 3.10 版本開始.*

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

   在 3.10 版被加入.

int PyGC_Disable(void)
    * 為 穩定 ABI 的一部分 自 3.10 版本開始.*

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

   在 3.10 版被加入.

int PyGC_IsEnabled(void)
    * 為 穩定 ABI 的一部分 自 3.10 版本開始.*

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

   在 3.10 版被加入.


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)

   *這是 不穩定 API，它可能在小版本發布中沒有任何警告地被變更。*

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

   警告:

     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.

   在 3.12 版被加入.

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.

   在 3.12 版被加入.
