Add Garbage Collection support to new-style classes (not yet to their

instances).

Also added GC support to various auxiliary types: super, property,
descriptors, wrappers, dictproxy.  (Only type objects have a tp_clear
field; the other types are.)

One change was necessary to the GC infrastructure.  We have statically
allocated type objects that don't have a GC header (and can't easily
be given one) and heap-allocated type objects that do have a GC
header.  Giving these different metatypes would be really ugly: I
tried, and I had to modify pickle.py, cPickle.c, copy.py, add a new
invent a new name for the new metatype and make it a built-in, change
affected tests...  In short, a mess.  So instead, we add a new type
slot tp_is_gc, which is a simple Boolean function that determines
whether a particular instance has GC headers or not.  This slot is
only relevant for types that have the (new) GC flag bit set.  If the
tp_is_gc slot is NULL (by far the most common case), all instances of
the type are deemed to have GC headers.  This slot is called by the
PyObject_IS_GC() macro (which is only used twice, both times in
gcmodule.c).

I also changed the extern declarations for a bunch of GC-related
functions (_PyObject_GC_Del etc.): these always exist but objimpl.h
only declared them when WITH_CYCLE_GC was defined, but I needed to be
able to reference them without #ifdefs.  (When WITH_CYCLE_GC is not
defined, they do the same as their non-GC counterparts anyway.)

2 files changed, 9 insertions, 3 deletions

diff --git a/Include/object.h b/Include/object.h
index 4529b61..400e65751 100644
--- a/Include/object.h
+++ b/Include/object.h
@@ -285,6 +285,7 @@ typedef struct _typeobject {
 	allocfunc tp_alloc;
 	newfunc tp_new;
 	destructor tp_free; /* Low-level free-memory routine */
+	inquiry tp_is_gc; /* For PyObject_IS_GC */
 	PyObject *tp_bases;
 	PyObject *tp_mro; /* method resolution order */
 	PyObject *tp_defined;
diff --git a/Include/objimpl.h b/Include/objimpl.h
index 89e1c0a..0fd6652 100644
--- a/Include/objimpl.h
+++ b/Include/objimpl.h
@@ -217,13 +217,18 @@ extern DL_IMPORT(void) _PyObject_Del(PyObject *);
 /*
  * Garbage Collection Support
  * ==========================
+ *
+ * Some of the functions and macros below are always defined; when
+ * WITH_CYCLE_GC is undefined, they simply don't do anything different
+ * than their non-GC counterparts.
  */
 
 /* 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 */
-#define PyObject_IS_GC(o) PyType_IS_GC((o)->ob_type)
+#define PyObject_IS_GC(o) (PyType_IS_GC((o)->ob_type) && \
+	((o)->ob_type->tp_is_gc == NULL || (o)->ob_type->tp_is_gc(o)))
 
 extern DL_IMPORT(PyObject *) _PyObject_GC_Malloc(PyTypeObject *, int);
 extern DL_IMPORT(PyVarObject *) _PyObject_GC_Resize(PyVarObject *, int);
@@ -231,14 +236,14 @@ extern DL_IMPORT(PyVarObject *) _PyObject_GC_Resize(PyVarObject *, int);
 #define PyObject_GC_Resize(type, op, n) \
 		( (type *) _PyObject_GC_Resize((PyVarObject *)(op), (n)) )
 
-#ifdef WITH_CYCLE_GC
-
 extern DL_IMPORT(PyObject *) _PyObject_GC_New(PyTypeObject *);
 extern DL_IMPORT(PyVarObject *) _PyObject_GC_NewVar(PyTypeObject *, int);
 extern DL_IMPORT(void) _PyObject_GC_Del(PyObject *);
 extern DL_IMPORT(void) _PyObject_GC_Track(PyObject *);
 extern DL_IMPORT(void) _PyObject_GC_UnTrack(PyObject *);
 
+#ifdef WITH_CYCLE_GC
+
 /* GC information is stored BEFORE the object structure */
 typedef struct _gc_head {
 	struct _gc_head *gc_next; /* not NULL if object is tracked */