bpo-35081: Make some _PyGC macros internal (GH-10507)

* Move "GC" macros together:

  * PyObject_IS_GC()
  * _Py_AS_GC()
  * _PyObject_GC_IS_TRACKED()
  * _PyObject_GC_MAY_BE_TRACKED()

* Mark other GC macros as internal (#ifdef Py_BUILD_CORE):

  * _PyGCHead_NEXT(g), _PyGCHead_SET_NEXT(g, p)
  * _PyGCHead_PREV(g), _PyGCHead_SET_PREV(g, p)
  * _PyGCHead_FINALIZED(g), _PyGCHead_SET_FINALIZED(g)
  * _PyGC_FINALIZED(o), _PyGC_SET_FINALIZED(o)
  * _PyObject_GC_TRACK(o), _PyObject_GC_UNTRACK(o)
  * _PyGC_PREV_MASK_FINALIZED
  * _PyGC_PREV_MASK_COLLECTING
  * _PyGC_PREV_SHIFT
  * _PyGC_PREV_MASK

* Replace _PyGC_generation0 with _PyRuntime.gc.generation0
* _queuemodule.c: replace _PyObject_GC_UNTRACK()
  with with PyObject_GC_UnTrack()
* Document that  _PyObject_GC_TRACK() _PyObject_GC_UNTRACK() macros
  have been removed from the public C API.

Doc/c-api/gcsupport.rst

@@ -61,11 +61,6 @@ Constructors for container types must conform to two rules:
    end of the constructor.
 
 
-.. c:function:: void _PyObject_GC_TRACK(PyObject *op)
-
-   A macro version of :c:func:`PyObject_GC_Track`.  It should not be used for
-   extension modules.
-
 Similarly, the deallocator for the object must conform to a similar pair of
 rules:
 
@@ -90,10 +85,10 @@ rules:
    the fields used by the :c:member:`~PyTypeObject.tp_traverse` handler become invalid.
 
 
-.. c:function:: void _PyObject_GC_UNTRACK(PyObject *op)
+.. versionchanged:: 3.8
 
-   A macro version of :c:func:`PyObject_GC_UnTrack`.  It should not be used for
-   extension modules.
+   The :c:func:`_PyObject_GC_TRACK` and :c:func:`_PyObject_GC_UNTRACK` macros
+   have been removed from the public C API.
 
 The :c:member:`~PyTypeObject.tp_traverse` handler accepts a function parameter of this type:
 

Include/internal/pycore_pymem.h

@@ -147,8 +147,6 @@ struct _gc_runtime_state {
 
 PyAPI_FUNC(void) _PyGC_Initialize(struct _gc_runtime_state *);
 
-#define _PyGC_generation0 _PyRuntime.gc.generation0
-
 
 /* Set the memory allocator of the specified domain to the default.
    Save the old allocator into *old_alloc if it's non-NULL.

Include/objimpl.h

@@ -256,18 +256,18 @@ PyAPI_FUNC(Py_ssize_t) _PyGC_CollectIfEnabled(void);
 /* Test if a type has a GC head */
 #define PyType_IS_GC(t) PyType_HasFeature((t), Py_TPFLAGS_HAVE_GC)
 
-/* Test if an object has a GC head */
-#ifndef Py_LIMITED_API
-#define PyObject_IS_GC(o) (PyType_IS_GC(Py_TYPE(o)) && \
-    (Py_TYPE(o)->tp_is_gc == NULL || Py_TYPE(o)->tp_is_gc(o)))
-#endif
-
 PyAPI_FUNC(PyVarObject *) _PyObject_GC_Resize(PyVarObject *, Py_ssize_t);
 #define PyObject_GC_Resize(type, op, n) \
                 ( (type *) _PyObject_GC_Resize((PyVarObject *)(op), (n)) )
 
-/* GC information is stored BEFORE the object structure. */
+
 #ifndef Py_LIMITED_API
+/* Test if an object has a GC head */
+#define PyObject_IS_GC(o) \
+    (PyType_IS_GC(Py_TYPE(o)) \
+     && (Py_TYPE(o)->tp_is_gc == NULL || Py_TYPE(o)->tp_is_gc(o)))
+
+/* GC information is stored BEFORE the object structure. */
 typedef struct {
     // Pointer to next object in the list.
     // 0 means the object is not tracked
@@ -278,10 +278,21 @@ typedef struct {
     uintptr_t _gc_prev;
 } PyGC_Head;
 
-extern PyGC_Head *_PyGC_generation0;
-
 #define _Py_AS_GC(o) ((PyGC_Head *)(o)-1)
 
+/* True if the object is currently tracked by the GC. */
+#define _PyObject_GC_IS_TRACKED(o) (_Py_AS_GC(o)->_gc_next != 0)
+
+/* True if the object may be tracked by the GC in the future, or already is.
+   This can be useful to implement some optimizations. */
+#define _PyObject_GC_MAY_BE_TRACKED(obj) \
+    (PyObject_IS_GC(obj) && \
+        (!PyTuple_CheckExact(obj) || _PyObject_GC_IS_TRACKED(obj)))
+#endif
+
+
+#if defined(Py_BUILD_CORE) || defined(Py_BUILD_CORE_BUILTIN)
+
 /* Bit flags for _gc_prev */
 /* Bit 0 is set when tp_finalize is called */
 #define _PyGC_PREV_MASK_FINALIZED  (1)
@@ -304,20 +315,26 @@ extern PyGC_Head *_PyGC_generation0;
         | ((uintptr_t)(p)); \
     } while (0)
 
-#define _PyGCHead_FINALIZED(g) (((g)->_gc_prev & _PyGC_PREV_MASK_FINALIZED) != 0)
-#define _PyGCHead_SET_FINALIZED(g) ((g)->_gc_prev |= _PyGC_PREV_MASK_FINALIZED)
+#define _PyGCHead_FINALIZED(g) \
+    (((g)->_gc_prev & _PyGC_PREV_MASK_FINALIZED) != 0)
+#define _PyGCHead_SET_FINALIZED(g) \
+    ((g)->_gc_prev |= _PyGC_PREV_MASK_FINALIZED)
 
-#define _PyGC_FINALIZED(o) _PyGCHead_FINALIZED(_Py_AS_GC(o))
-#define _PyGC_SET_FINALIZED(o) _PyGCHead_SET_FINALIZED(_Py_AS_GC(o))
+#define _PyGC_FINALIZED(o) \
+    _PyGCHead_FINALIZED(_Py_AS_GC(o))
+#define _PyGC_SET_FINALIZED(o) \
+    _PyGCHead_SET_FINALIZED(_Py_AS_GC(o))
 
 /* Tell the GC to track this object.
  *
  * NB: While the object is tracked by the collector, it must be safe to call the
  * ob_traverse method.
  *
- * Internal note: _PyGC_generation0->_gc_prev doesn't have any bit flags
+ * Internal note: _PyRuntime.gc.generation0->_gc_prev doesn't have any bit flags
  * because it's not object header.  So we don't use _PyGCHead_PREV() and
  * _PyGCHead_SET_PREV() for it to avoid unnecessary bitwise operations.
+ *
+ * The PyObject_GC_Track() function is the public version of this macro.
  */
 #define _PyObject_GC_TRACK(o) do { \
     PyGC_Head *g = _Py_AS_GC(o); \
@@ -325,17 +342,19 @@ extern PyGC_Head *_PyGC_generation0;
         Py_FatalError("GC object already tracked"); \
     } \
     assert((g->_gc_prev & _PyGC_PREV_MASK_COLLECTING) == 0); \
-    PyGC_Head *last = (PyGC_Head*)(_PyGC_generation0->_gc_prev); \
+    PyGC_Head *last = (PyGC_Head*)(_PyRuntime.gc.generation0->_gc_prev); \
     _PyGCHead_SET_NEXT(last, g); \
     _PyGCHead_SET_PREV(g, last); \
-    _PyGCHead_SET_NEXT(g, _PyGC_generation0); \
-    _PyGC_generation0->_gc_prev = (uintptr_t)g; \
+    _PyGCHead_SET_NEXT(g, _PyRuntime.gc.generation0); \
+    _PyRuntime.gc.generation0->_gc_prev = (uintptr_t)g; \
     } while (0);
 
 /* Tell the GC to stop tracking this object.
  *
  * Internal note: This may be called while GC.  So _PyGC_PREV_MASK_COLLECTING must
  * be cleared.  But _PyGC_PREV_MASK_FINALIZED bit is kept.
+ *
+ * The PyObject_GC_UnTrack() function is the public version of this macro.
  */
 #define _PyObject_GC_UNTRACK(o) do { \
     PyGC_Head *g = _Py_AS_GC(o); \
@@ -347,16 +366,7 @@ extern PyGC_Head *_PyGC_generation0;
     g->_gc_next = 0; \
     g->_gc_prev &= _PyGC_PREV_MASK_FINALIZED; \
     } while (0);
-
-/* True if the object is currently tracked by the GC. */
-#define _PyObject_GC_IS_TRACKED(o) (_Py_AS_GC(o)->_gc_next != 0)
-
-/* True if the object may be tracked by the GC in the future, or already is.
-   This can be useful to implement some optimizations. */
-#define _PyObject_GC_MAY_BE_TRACKED(obj) \
-    (PyObject_IS_GC(obj) && \
-        (!PyTuple_CheckExact(obj) || _PyObject_GC_IS_TRACKED(obj)))
-#endif /* Py_LIMITED_API */
+#endif   /* defined(Py_BUILD_CORE) || defined(Py_BUILD_CORE_BUILTIN) */
 
 #ifndef Py_LIMITED_API
 PyAPI_FUNC(PyObject *) _PyObject_GC_Malloc(size_t size);
@@ -364,8 +374,17 @@ PyAPI_FUNC(PyObject *) _PyObject_GC_Calloc(size_t size);
 #endif /* !Py_LIMITED_API */
 PyAPI_FUNC(PyObject *) _PyObject_GC_New(PyTypeObject *);
 PyAPI_FUNC(PyVarObject *) _PyObject_GC_NewVar(PyTypeObject *, Py_ssize_t);
+
+/* Tell the GC to track this object.
+ *
+ * See also private _PyObject_GC_TRACK() macro. */
 PyAPI_FUNC(void) PyObject_GC_Track(void *);
+
+/* Tell the GC to stop tracking this object.
+ *
+ * See also private _PyObject_GC_UNTRACK() macro. */
 PyAPI_FUNC(void) PyObject_GC_UnTrack(void *);
+
 PyAPI_FUNC(void) PyObject_GC_Del(void *);
 
 #define PyObject_GC_New(type, typeobj) \

Misc/NEWS.d/next/C API/2018-11-13-12-13-04.bpo-35081.gFd85N.rst

@@ -0,0 +1,2 @@
+The :c:func:`_PyObject_GC_TRACK` and :c:func:`_PyObject_GC_UNTRACK` macros
+have been removed from the public C API.

Modules/_queuemodule.c

@@ -26,7 +26,7 @@ typedef struct {
 static void
 simplequeue_dealloc(simplequeueobject *self)
 {
-    _PyObject_GC_UNTRACK(self);
+    PyObject_GC_UnTrack(self);
     if (self->lock != NULL) {
         /* Unlock the lock so it's safe to free it */
         if (self->locked > 0)