Modules/gcmodule.c

/*

  Reference Cycle Garbage Collection
  ==================================

  Neil Schemenauer <nas@arctrix.com>

  Based on a post on the python-dev list.  Ideas from Guido van Rossum,
  Eric Tiedemann, and various others.

  http://www.arctrix.com/nas/python/gc/
  http://www.python.org/pipermail/python-dev/2000-March/003869.html
  http://www.python.org/pipermail/python-dev/2000-March/004010.html
  http://www.python.org/pipermail/python-dev/2000-March/004022.html

  For a highlevel view of the collection process, read the collect
  function.

*/

#include "Python.h"

/* Get an object's GC head */
#define AS_GC(o) ((PyGC_Head *)(o)-1)

/* Get the object given the GC head */
#define FROM_GC(g) ((PyObject *)(((PyGC_Head *)g)+1))

/*** Global GC state ***/

struct gc_generation {
	PyGC_Head head;
	int threshold; /* collection threshold */
	int count; /* count of allocations or collections of younger
		      generations */
};

#define NUM_GENERATIONS 3
#define GEN_HEAD(n) (&generations[n].head)

/* linked lists of container objects */
static struct gc_generation generations[NUM_GENERATIONS] = {
	/* PyGC_Head,				threshold,	count */
	{{{GEN_HEAD(0), GEN_HEAD(0), 0}},	700,		0},
	{{{GEN_HEAD(1), GEN_HEAD(1), 0}},	10,		0},
	{{{GEN_HEAD(2), GEN_HEAD(2), 0}},	10,		0},
};

PyGC_Head *_PyGC_generation0 = GEN_HEAD(0);

static int enabled = 1; /* automatic collection enabled? */

/* true if we are currently running the collector */
static int collecting;

/* list of uncollectable objects */
static PyObject *garbage;

/* Python string to use if unhandled exception occurs */
static PyObject *gc_str;

/* Python string used to look for __del__ attribute. */
static PyObject *delstr = NULL;

/* set for debugging information */
#define DEBUG_STATS		(1<<0) /* print collection statistics */
#define DEBUG_COLLECTABLE	(1<<1) /* print collectable objects */
#define DEBUG_UNCOLLECTABLE	(1<<2) /* print uncollectable objects */
#define DEBUG_INSTANCES		(1<<3) /* print instances */
#define DEBUG_OBJECTS		(1<<4) /* print other objects */
#define DEBUG_SAVEALL		(1<<5) /* save all garbage in gc.garbage */
#define DEBUG_LEAK		DEBUG_COLLECTABLE | \
				DEBUG_UNCOLLECTABLE | \
				DEBUG_INSTANCES | \
				DEBUG_OBJECTS | \
				DEBUG_SAVEALL
static int debug;

/*--------------------------------------------------------------------------
gc_refs values.

Between collections, every gc'ed object has one of two gc_refs values:

GC_UNTRACKED
    The initial state; objects returned by PyObject_GC_Malloc are in this
    state.  The object doesn't live in any generation list, and its
    tp_traverse slot must not be called.

GC_REACHABLE
    The object lives in some generation list, and its tp_traverse is safe to
    call.  An object transitions to GC_REACHABLE when PyObject_GC_Track
    is called.

During a collection, gc_refs can temporarily take on other states:

>= 0
    At the start of a collection, update_refs() copies the true refcount
    to gc_refs, for each object in the generation being collected.
    subtract_refs() then adjusts gc_refs so that it equals the number of
    times an object is referenced directly from outside the generation
    being collected.
    gc_refs remains >= 0 throughout these steps.

GC_TENTATIVELY_UNREACHABLE
    move_unreachable() then moves objects not reachable (whether directly or
    indirectly) from outside the generation into an "unreachable" set.
    Objects that are found to be reachable have gc_refs set to GC_REACHABLE
    again.  Objects that are found to be unreachable have gc_refs set to
    GC_TENTATIVELY_UNREACHABLE.  It's "tentatively" because the pass doing
    this can't be sure until it ends, and GC_TENTATIVELY_UNREACHABLE may
    transition back to GC_REACHABLE.

    Only objects with GC_TENTATIVELY_UNREACHABLE still set are candidates
    for collection.  If it's decided not to collect such an object (e.g.,
    it has a __del__ method), its gc_refs is restored to GC_REACHABLE again.
----------------------------------------------------------------------------
*/
#define GC_UNTRACKED			_PyGC_REFS_UNTRACKED
#define GC_REACHABLE			_PyGC_REFS_REACHABLE
#define GC_TENTATIVELY_UNREACHABLE	_PyGC_REFS_TENTATIVELY_UNREACHABLE

#define IS_TRACKED(o) ((AS_GC(o))->gc.gc_refs != GC_UNTRACKED)
#define IS_REACHABLE(o) ((AS_GC(o))->gc.gc_refs == GC_REACHABLE)
#define IS_TENTATIVELY_UNREACHABLE(o) ( \
	(AS_GC(o))->gc.gc_refs == GC_TENTATIVELY_UNREACHABLE)

/*** list functions ***/

static void
gc_list_init(PyGC_Head *list)
{
	list->gc.gc_prev = list;
	list->gc.gc_next = list;
}

static int
gc_list_is_empty(PyGC_Head *list)
{
	return (list->gc.gc_next == list);
}

static void
gc_list_append(PyGC_Head *node, PyGC_Head *list)
{
	node->gc.gc_next = list;
	node->gc.gc_prev = list->gc.gc_prev;
	node->gc.gc_prev->gc.gc_next = node;
	list->gc.gc_prev = node;
}

static void
gc_list_remove(PyGC_Head *node)
{
	node->gc.gc_prev->gc.gc_next = node->gc.gc_next;
	node->gc.gc_next->gc.gc_prev = node->gc.gc_prev;
	node->gc.gc_next = NULL; /* object is not currently tracked */
}

/* append a list onto another list, from becomes an empty list */
static void
gc_list_merge(PyGC_Head *from, PyGC_Head *to)
{
	PyGC_Head *tail;
	if (!gc_list_is_empty(from)) {
		tail = to->gc.gc_prev;
		tail->gc.gc_next = from->gc.gc_next;
		tail->gc.gc_next->gc.gc_prev = tail;
		to->gc.gc_prev = from->gc.gc_prev;
		to->gc.gc_prev->gc.gc_next = to;
	}
	gc_list_init(from);
}

static long
gc_list_size(PyGC_Head *list)
{
	PyGC_Head *gc;
	long n = 0;
	for (gc = list->gc.gc_next; gc != list; gc = gc->gc.gc_next) {
		n++;
	}
	return n;
}

/*** end of list stuff ***/


/* Set all gc_refs = ob_refcnt.  After this, gc_refs is > 0 for all objects
 * in containers, and is GC_REACHABLE for all tracked gc objects not in
 * containers.
 */
static void
update_refs(PyGC_Head *containers)
{
	PyGC_Head *gc = containers->gc.gc_next;
	for (; gc != containers; gc = gc->gc.gc_next) {
		assert(gc->gc.gc_refs == GC_REACHABLE);
		gc->gc.gc_refs = FROM_GC(gc)->ob_refcnt;
	}
}

/* A traversal callback for subtract_refs. */
static int
visit_decref(PyObject *op, void *data)
{
        assert(op != NULL);
	if (PyObject_IS_GC(op)) {
		PyGC_Head *gc = AS_GC(op);
		/* We're only interested in gc_refs for objects in the
		 * generation being collected, which can be recognized
		 * because only they have positive gc_refs.
		 */
		assert(gc->gc.gc_refs != 0); /* else refcount was too small */
		if (gc->gc.gc_refs > 0)
			gc->gc.gc_refs--;
	}
	return 0;
}

/* Subtract internal references from gc_refs.  After this, gc_refs is >= 0
 * for all objects in containers, and is GC_REACHABLE for all tracked gc
 * objects not in containers.  The ones with gc_refs > 0 are directly
 * reachable from outside containers, and so can't be collected.
 */
static void
subtract_refs(PyGC_Head *containers)
{
	traverseproc traverse;
	PyGC_Head *gc = containers->gc.gc_next;
	for (; gc != containers; gc=gc->gc.gc_next) {
		traverse = FROM_GC(gc)->ob_type->tp_traverse;
		(void) traverse(FROM_GC(gc),
			       (visitproc)visit_decref,
			       NULL);
	}
}

/* A traversal callback for move_unreachable. */
static int
visit_reachable(PyObject *op, PyGC_Head *reachable)
{
	if (PyObject_IS_GC(op)) {
		PyGC_Head *gc = AS_GC(op);
		const int gc_refs = gc->gc.gc_refs;

		if (gc_refs == 0) {
			/* This is in move_unreachable's 'young' list, but
			 * the traversal hasn't yet gotten to it.  All
			 * we need to do is tell move_unreachable that it's
			 * reachable.
			 */
			gc->gc.gc_refs = 1;
		}
		else if (gc_refs == GC_TENTATIVELY_UNREACHABLE) {
			/* This had gc_refs = 0 when move_unreachable got
			 * to it, but turns out it's reachable after all.
			 * Move it back to move_unreachable's 'young' list,
			 * and move_unreachable will eventually get to it
			 * again.
			 */
			gc_list_remove(gc);
			gc_list_append(gc, reachable);
			gc->gc.gc_refs = 1;
		}
		/* Else there's nothing to do.
		 * If gc_refs > 0, it must be in move_unreachable's 'young'
		 * list, and move_unreachable will eventually get to it.
		 * If gc_refs == GC_REACHABLE, it's either in some other
		 * generation so we don't care about it, or move_unreachable
		 * already dealt with it.
		 * If gc_refs == GC_UNTRACKED, it must be ignored.
		 */
		 else {
		 	assert(gc_refs > 0
		 	       || gc_refs == GC_REACHABLE
		 	       || gc_refs == GC_UNTRACKED);
		 }
	}
	return 0;
}

/* Move the unreachable objects from young to unreachable.  After this,
 * all objects in young have gc_refs = GC_REACHABLE, and all objects in
 * unreachable have gc_refs = GC_TENTATIVELY_UNREACHABLE.  All tracked
 * gc objects not in young or unreachable still have gc_refs = GC_REACHABLE.
 * All objects in young after this are directly or indirectly reachable
 * from outside the original young; and all objects in unreachable are
 * not.
 */
static void
move_unreachable(PyGC_Head *young, PyGC_Head *unreachable)
{
	PyGC_Head *gc = young->gc.gc_next;

	/* Invariants:  all objects "to the left" of us in young have gc_refs
	 * = GC_REACHABLE, and are indeed reachable (directly or indirectly)
	 * from outside the young list as it was at entry.  All other objects
	 * from the original young "to the left" of us are in unreachable now,
	 * and have gc_refs = GC_TENTATIVELY_UNREACHABLE.  All objects to the
	 * left of us in 'young' now have been scanned, and no objects here
	 * or to the right have been scanned yet.
	 */

	while (gc != young) {
		PyGC_Head *next;

		if (gc->gc.gc_refs) {
                        /* gc is definitely reachable from outside the
                         * original 'young'.  Mark it as such, and traverse
                         * its pointers to find any other objects that may
                         * be directly reachable from it.  Note that the
                         * call to tp_traverse may append objects to young,
                         * so we have to wait until it returns to determine
                         * the next object to visit.
                         */
                        PyObject *op = FROM_GC(gc);
                        traverseproc traverse = op->ob_type->tp_traverse;
                        assert(gc->gc.gc_refs > 0);
                        gc->gc.gc_refs = GC_REACHABLE;
                        (void) traverse(op,
                                        (visitproc)visit_reachable,
                                        (void *)young);
                        next = gc->gc.gc_next;
		}
		else {
			/* This *may* be unreachable.  To make progress,
			 * assume it is.  gc isn't directly reachable from
			 * any object we've already traversed, but may be
			 * reachable from an object we haven't gotten to yet.
			 * visit_reachable will eventually move gc back into
			 * young if that's so, and we'll see it again.
			 */
			next = gc->gc.gc_next;
			gc_list_remove(gc);
			gc_list_append(gc, unreachable);
			gc->gc.gc_refs = GC_TENTATIVELY_UNREACHABLE;
		}
		gc = next;
	}
}

/* return true if object has a finalization method */
static int
has_finalizer(PyObject *op)
{
	return PyInstance_Check(op) ? PyObject_HasAttr(op, delstr) :
	        PyType_HasFeature(op->ob_type, Py_TPFLAGS_HEAPTYPE) ?
		op->ob_type->tp_del != NULL : 0;
}

/* Move all objects out of unreachable and into collectable or finalizers.
 */
static void
move_finalizers(PyGC_Head *unreachable, PyGC_Head *collectable,
		PyGC_Head *finalizers)
{
	while (!gc_list_is_empty(unreachable)) {
		PyGC_Head *gc = unreachable->gc.gc_next;
		PyObject *op = FROM_GC(gc);
		int finalizer;

		if (PyInstance_Check(op)) {
			/* The HasAttr() check may run enough Python
			   code to deallocate the object or make it
			   reachable again.  INCREF the object before
			   calling HasAttr() to guard against the client
			   code deallocating the object.
			*/
			Py_INCREF(op);
			finalizer = PyObject_HasAttr(op, delstr);
			if (op->ob_refcnt == 1) {
				/* The object will be deallocated.
				   Nothing left to do.
				 */
				Py_DECREF(op);
				continue;
			}
			Py_DECREF(op);
		}
		else
			finalizer = has_finalizer(op);
		if (finalizer) {
			gc_list_remove(gc);
			gc_list_append(gc, finalizers);
			gc->gc.gc_refs = GC_REACHABLE;
		}
		else {
			gc_list_remove(gc);
			gc_list_append(gc, collectable);
			/* XXX change gc_refs? */
		}
	}
}

/* A traversal callback for move_finalizer_reachable. */
static int
visit_move(PyObject *op, PyGC_Head *tolist)
{
	if (PyObject_IS_GC(op)) {
		if (IS_TENTATIVELY_UNREACHABLE(op)) {
			PyGC_Head *gc = AS_GC(op);
			gc_list_remove(gc);
			gc_list_append(gc, tolist);
			gc->gc.gc_refs = GC_REACHABLE;
		}
	}
	return 0;
}

/* Move objects that are reachable from finalizers, from the unreachable set
 * into the finalizers set.
 */
static void
move_finalizer_reachable(PyGC_Head *finalizers)
{
	traverseproc traverse;
	PyGC_Head *gc = finalizers->gc.gc_next;
	for (; gc != finalizers; gc=gc->gc.gc_next) {
		/* careful, finalizers list is growing here */
		traverse = FROM_GC(gc)->ob_type->tp_traverse;
		(void) traverse(FROM_GC(gc),
			       (visitproc)visit_move,
			       (void *)finalizers);
	}
}

static void
debug_instance(char *msg, PyInstanceObject *inst)
{
	char *cname;
	/* simple version of instance_repr */
	PyObject *classname = inst->in_class->cl_name;
	if (classname != NULL && PyString_Check(classname))
		cname = PyString_AsString(classname);
	else
		cname = "?";
	PySys_WriteStderr("gc: %.100s <%.100s instance at %p>\n",
			  msg, cname, inst);
}

static void
debug_cycle(char *msg, PyObject *op)
{
	if ((debug & DEBUG_INSTANCES) && PyInstance_Check(op)) {
		debug_instance(msg, (PyInstanceObject *)op);
	}
	else if (debug & DEBUG_OBJECTS) {
		PySys_WriteStderr("gc: %.100s <%.100s %p>\n",
				  msg, op->ob_type->tp_name, op);
	}
}

/* Handle uncollectable garbage (cycles with finalizers). */
static void
handle_finalizers(PyGC_Head *finalizers, PyGC_Head *old)
{
	PyGC_Head *gc;
	if (garbage == NULL) {
		garbage = PyList_New(0);
	}
	for (gc = finalizers->gc.gc_next; gc != finalizers;
			gc = finalizers->gc.gc_next) {
		PyObject *op = FROM_GC(gc);
		/* XXX has_finalizer() is not safe here. */
		if ((debug & DEBUG_SAVEALL) || has_finalizer(op)) {
			/* If SAVEALL is not set then just append objects with
			 * finalizers to the list of garbage.  All objects in
			 * the finalizers list are reachable from those
			 * objects.
			 */
			PyList_Append(garbage, op);
		}
		/* object is now reachable again */
		assert(IS_REACHABLE(op));
		gc_list_remove(gc);
		gc_list_append(gc, old);
	}
}

/* Break reference cycles by clearing the containers involved.	This is
 * tricky business as the lists can be changing and we don't know which
 * objects may be freed.  It is possible I screwed something up here.
 */
static void
delete_garbage(PyGC_Head *collectable, PyGC_Head *old)
{
	inquiry clear;

	while (!gc_list_is_empty(collectable)) {
		PyGC_Head *gc = collectable->gc.gc_next;
		PyObject *op = FROM_GC(gc);

		assert(IS_TENTATIVELY_UNREACHABLE(op));
		if (debug & DEBUG_SAVEALL) {
			PyList_Append(garbage, op);
		}
		else {
			if ((clear = op->ob_type->tp_clear) != NULL) {
				Py_INCREF(op);
				clear(op);
				Py_DECREF(op);
			}
		}
		if (collectable->gc.gc_next == gc) {
			/* object is still alive, move it, it may die later */
			gc_list_remove(gc);
			gc_list_append(gc, old);
			gc->gc.gc_refs = GC_REACHABLE;
		}
	}
}

/* This is the main function.  Read this to understand how the
 * collection process works. */
static long
collect(int generation)
{
	int i;
	long m = 0;	/* # objects collected */
	long n = 0;	/* # unreachable objects that couldn't be collected */
	PyGC_Head *young; /* the generation we are examining */
	PyGC_Head *old; /* next older generation */
	PyGC_Head unreachable;
	PyGC_Head collectable;
	PyGC_Head finalizers;
	PyGC_Head *gc;

	if (delstr == NULL) {
		delstr = PyString_InternFromString("__del__");
		if (delstr == NULL)
			Py_FatalError("gc couldn't allocate \"__del__\"");
	}

	if (debug & DEBUG_STATS) {
		PySys_WriteStderr("gc: collecting generation %d...\n",
				  generation);
		PySys_WriteStderr("gc: objects in each generation:");
		for (i = 0; i < NUM_GENERATIONS; i++) {
			PySys_WriteStderr(" %ld", gc_list_size(GEN_HEAD(i)));
		}
		PySys_WriteStderr("\n");
	}

	/* update collection and allocation counters */
	if (generation+1 < NUM_GENERATIONS)
		generations[generation+1].count += 1;
	for (i = 0; i <= generation; i++)
		generations[i].count = 0;

	/* merge younger generations with one we are currently collecting */
	for (i = 0; i < generation; i++) {
		gc_list_merge(GEN_HEAD(i), GEN_HEAD(generation));
	}

	/* handy references */
	young = GEN_HEAD(generation);
	if (generation < NUM_GENERATIONS-1)
		old = GEN_HEAD(generation+1);
	else
		old = young;

	/* Using ob_refcnt and gc_refs, calculate which objects in the
	 * container set are reachable from outside the set (ie. have a
	 * refcount greater than 0 when all the references within the
	 * set are taken into account
	 */
	update_refs(young);
	subtract_refs(young);

	/* Leave everything reachable from outside young in young, and move
	 * everything else (in young) to unreachable.
	 * NOTE:  This used to move the reachable objects into a reachable
	 * set instead.  But most things usually turn out to be reachable,
	 * so it's more efficient to move the unreachable things.
	 */
	gc_list_init(&unreachable);
	move_unreachable(young, &unreachable);

	/* Move reachable objects to next generation. */
	if (young != old)
		gc_list_merge(young, old);

	/* All objects in unreachable are trash, but objects reachable from
	 * finalizers can't safely be deleted.  Python programmers should take
	 * care not to create such things.  For Python, finalizers means
	 * instance objects with __del__ methods.
	 *
	 * Move each object into the collectable set or the finalizers set.
	 * It's possible that a classic class with a getattr() hook will
	 * be revived or deallocated in this step.
	 */
	gc_list_init(&collectable);
	gc_list_init(&finalizers);
	move_finalizers(&unreachable, &collectable, &finalizers);
	move_finalizer_reachable(&finalizers);

	/* Collect statistics on collectable objects found and print
	 * debugging information. */
	for (gc = collectable.gc.gc_next; gc != &collectable;
			gc = gc->gc.gc_next) {
		m++;
		if (debug & DEBUG_COLLECTABLE) {
			debug_cycle("collectable", FROM_GC(gc));
		}
	}
	/* Call tp_clear on objects in the collectable set.  This will cause
	 * the reference cycles to be broken. It may also cause some objects in
	 * finalizers to be freed */
	delete_garbage(&collectable, old);

	/* Collect statistics on uncollectable objects found and print
	 * debugging information. */
	for (gc = finalizers.gc.gc_next; gc != &finalizers;
			gc = gc->gc.gc_next) {
		n++;
		if (debug & DEBUG_UNCOLLECTABLE) {
			debug_cycle("uncollectable", FROM_GC(gc));
		}
	}
	if (debug & DEBUG_STATS) {
		if (m == 0 && n == 0) {
			PySys_WriteStderr("gc: done.\n");
		}
		else {
			PySys_WriteStderr(
			    "gc: done, %ld unreachable, %ld uncollectable.\n",
			    n+m, n);
		}
	}

	/* Append instances in the uncollectable set to a Python
	 * reachable list of garbage.  The programmer has to deal with
	 * this if they insist on creating this type of structure. */
	handle_finalizers(&finalizers, old);

	if (PyErr_Occurred()) {
		if (gc_str == NULL) {
		    gc_str = PyString_FromString("garbage collection");
		}
		PyErr_WriteUnraisable(gc_str);
		Py_FatalError("unexpected exception during garbage collection");
	}
	return n+m;
}

static long
collect_generations(void)
{
	int i;
	long n = 0;

	/* Find the oldest generation (higest numbered) where the count
	 * exceeds the threshold.  Objects in the that generation and
	 * generations younger than it will be collected. */
	for (i = NUM_GENERATIONS-1; i >= 0; i--) {
		if (generations[i].count > generations[i].threshold) {
			n = collect(i);
			break;
		}
	}
	return n;
}

PyDoc_STRVAR(gc_enable__doc__,
"enable() -> None\n"
"\n"
"Enable automatic garbage collection.\n");

static PyObject *
gc_enable(PyObject *self, PyObject *args)
{

	if (!PyArg_ParseTuple(args, ":enable"))	/* check no args */
		return NULL;

	enabled = 1;

	Py_INCREF(Py_None);
	return Py_None;
}

PyDoc_STRVAR(gc_disable__doc__,
"disable() -> None\n"
"\n"
"Disable automatic garbage collection.\n");

static PyObject *
gc_disable(PyObject *self, PyObject *args)
{

	if (!PyArg_ParseTuple(args, ":disable"))	/* check no args */
		return NULL;

	enabled = 0;

	Py_INCREF(Py_None);
	return Py_None;
}

PyDoc_STRVAR(gc_isenabled__doc__,
"isenabled() -> status\n"
"\n"
"Returns true if automatic garbage collection is enabled.\n");

static PyObject *
gc_isenabled(PyObject *self, PyObject *args)
{

	if (!PyArg_ParseTuple(args, ":isenabled"))	/* check no args */
		return NULL;

	return Py_BuildValue("i", enabled);
}

PyDoc_STRVAR(gc_collect__doc__,
"collect() -> n\n"
"\n"
"Run a full collection.  The number of unreachable objects is returned.\n");

static PyObject *
gc_collect(PyObject *self, PyObject *args)
{
	long n;

	if (!PyArg_ParseTuple(args, ":collect"))	/* check no args */
		return NULL;

	if (collecting) {
		n = 0; /* already collecting, don't do anything */
	}
	else {
		collecting = 1;
		n = collect(NUM_GENERATIONS - 1);
		collecting = 0;
	}

	return Py_BuildValue("l", n);
}

PyDoc_STRVAR(gc_set_debug__doc__,
"set_debug(flags) -> None\n"
"\n"
"Set the garbage collection debugging flags. Debugging information is\n"
"written to sys.stderr.\n"
"\n"
"flags is an integer and can have the following bits turned on:\n"
"\n"
"  DEBUG_STATS - Print statistics during collection.\n"
"  DEBUG_COLLECTABLE - Print collectable objects found.\n"
"  DEBUG_UNCOLLECTABLE - Print unreachable but uncollectable objects found.\n"
"  DEBUG_INSTANCES - Print instance objects.\n"
"  DEBUG_OBJECTS - Print objects other than instances.\n"
"  DEBUG_SAVEALL - Save objects to gc.garbage rather than freeing them.\n"
"  DEBUG_LEAK - Debug leaking programs (everything but STATS).\n");

static PyObject *
gc_set_debug(PyObject *self, PyObject *args)
{
	if (!PyArg_ParseTuple(args, "i:set_debug", &debug))
		return NULL;

	Py_INCREF(Py_None);
	return Py_None;
}

PyDoc_STRVAR(gc_get_debug__doc__,
"get_debug() -> flags\n"
"\n"
"Get the garbage collection debugging flags.\n");

static PyObject *
gc_get_debug(PyObject *self, PyObject *args)
{
	if (!PyArg_ParseTuple(args, ":get_debug"))	/* no args */
		return NULL;

	return Py_BuildValue("i", debug);
}

PyDoc_STRVAR(gc_set_thresh__doc__,
"set_threshold(threshold0, [threshold1, threshold2]) -> None\n"
"\n"
"Sets the collection thresholds.  Setting threshold0 to zero disables\n"
"collection.\n");

static PyObject *
gc_set_thresh(PyObject *self, PyObject *args)
{
	int i;
	if (!PyArg_ParseTuple(args, "i|ii:set_threshold",
			      &generations[0].threshold,
			      &generations[1].threshold,
			      &generations[2].threshold))
		return NULL;
	for (i = 2; i < NUM_GENERATIONS; i++) {
 		/* generations higher than 2 get the same threshold */
		generations[i].threshold = generations[2].threshold;
	}

	Py_INCREF(Py_None);
	return Py_None;
}

PyDoc_STRVAR(gc_get_thresh__doc__,
"get_threshold() -> (threshold0, threshold1, threshold2)\n"
"\n"
"Return the current collection thresholds\n");

static PyObject *
gc_get_thresh(PyObject *self, PyObject *args)
{
	if (!PyArg_ParseTuple(args, ":get_threshold"))	/* no args */
		return NULL;

	return Py_BuildValue("(iii)",
			     generations[0].threshold,
			     generations[1].threshold,
			     generations[2].threshold);
}

static int
referrersvisit(PyObject* obj, PyObject *objs)
{
	int i;
	for (i = 0; i < PyTuple_GET_SIZE(objs); i++)
		if (PyTuple_GET_ITEM(objs, i) == obj)
			return 1;
	return 0;
}

static int
gc_referrers_for(PyObject *objs, PyGC_Head *list, PyObject *resultlist)
{
	PyGC_Head *gc;
	PyObject *obj;
	traverseproc traverse;
	for (gc = list->gc.gc_next; gc != list; gc = gc->gc.gc_next) {
		obj = FROM_GC(gc);
		traverse = obj->ob_type->tp_traverse;
		if (obj == objs || obj == resultlist)
			continue;
		if (traverse(obj, (visitproc)referrersvisit, objs)) {
			if (PyList_Append(resultlist, obj) < 0)
				return 0; /* error */
		}
	}
	return 1; /* no error */
}

PyDoc_STRVAR(gc_get_referrers__doc__,
"get_referrers(*objs) -> list\n\
Return the list of objects that directly refer to any of objs.");

static PyObject *
gc_get_referrers(PyObject *self, PyObject *args)
{
	int i;
	PyObject *result = PyList_New(0);
	for (i = 0; i < NUM_GENERATIONS; i++) {
		if (!(gc_referrers_for(args, GEN_HEAD(i), result))) {
			Py_DECREF(result);
			return NULL;
		}
	}
	return result;
}

static int
referrentsvisit(PyObject *obj, PyObject *list)
{
	if (PyList_Append(list, obj) < 0)
		return 1;
	return 0;
}

PyDoc_STRVAR(gc_get_referrents__doc__,
"get_referrents(*objs) -> list\n\
Return the list of objects that are directly referred to by objs.");

static PyObject *
gc_get_referrents(PyObject *self, PyObject *args)
{
	int i;
	PyObject *result = PyList_New(0);
	for (i = 0; i < PyTuple_GET_SIZE(args); i++) {
		PyObject *obj = PyTuple_GET_ITEM(args, i);
		traverseproc traverse = obj->ob_type->tp_traverse;
		if (!traverse)
			continue;
		if (traverse(obj, (visitproc)referrentsvisit, result))
			return NULL;
	}
	return result;
}

PyDoc_STRVAR(gc_get_objects__doc__,
"get_objects() -> [...]\n"
"\n"
"Return a list of objects tracked by the collector (excluding the list\n"
"returned).\n");

/* appending objects in a GC list to a Python list */
static int
append_objects(PyObject *py_list, PyGC_Head *gc_list)
{
	PyGC_Head *gc;
	for (gc = gc_list->gc.gc_next; gc != gc_list; gc = gc->gc.gc_next) {
		PyObject *op = FROM_GC(gc);
		if (op != py_list) {
			if (PyList_Append(py_list, op)) {
				return -1; /* exception */
			}
		}
	}
	return 0;
}

static PyObject *
gc_get_objects(PyObject *self, PyObject *args)
{
	int i;
	PyObject* result;

	if (!PyArg_ParseTuple(args, ":get_objects")) /* check no args */
		return NULL;
	result = PyList_New(0);
	if (result == NULL) {
		return NULL;
	}
	for (i = 0; i < NUM_GENERATIONS; i++) {
		if (append_objects(result, GEN_HEAD(i))) {
			Py_DECREF(result);
			return NULL;
		}
	}
	return result;
}


PyDoc_STRVAR(gc__doc__,
"This module provides access to the garbage collector for reference cycles.\n"
"\n"
"enable() -- Enable automatic garbage collection.\n"
"disable() -- Disable automatic garbage collection.\n"
"isenabled() -- Returns true if automatic collection is enabled.\n"
"collect() -- Do a full collection right now.\n"
"set_debug() -- Set debugging flags.\n"
"get_debug() -- Get debugging flags.\n"
"set_threshold() -- Set the collection thresholds.\n"
"get_threshold() -- Return the current the collection thresholds.\n"
"get_objects() -- Return a list of all objects tracked by the collector.\n"
"get_referrers() -- Return the list of objects that refer to an object.\n"
"get_referrents() -- Return the list of objects that an object refers to.\n");

static PyMethodDef GcMethods[] = {
	{"enable",	   gc_enable,	  METH_VARARGS, gc_enable__doc__},
	{"disable",	   gc_disable,	  METH_VARARGS, gc_disable__doc__},
	{"isenabled",	   gc_isenabled,  METH_VARARGS, gc_isenabled__doc__},
	{"set_debug",	   gc_set_debug,  METH_VARARGS, gc_set_debug__doc__},
	{"get_debug",	   gc_get_debug,  METH_VARARGS, gc_get_debug__doc__},
	{"set_threshold",  gc_set_thresh, METH_VARARGS, gc_set_thresh__doc__},
	{"get_threshold",  gc_get_thresh, METH_VARARGS, gc_get_thresh__doc__},
	{"collect",	   gc_collect,	  METH_VARARGS, gc_collect__doc__},
	{"get_objects",    gc_get_objects,METH_VARARGS, gc_get_objects__doc__},
	{"get_referrers",  gc_get_referrers, METH_VARARGS,
		gc_get_referrers__doc__},
	{"get_referrents",  gc_get_referrents, METH_VARARGS,
		gc_get_referrents__doc__},
	{NULL,	NULL}		/* Sentinel */
};

void
initgc(void)
{
	PyObject *m;
	PyObject *d;

	m = Py_InitModule4("gc",
			      GcMethods,
			      gc__doc__,
			      NULL,
			      PYTHON_API_VERSION);
	d = PyModule_GetDict(m);
	if (garbage == NULL) {
		garbage = PyList_New(0);
	}
	PyDict_SetItemString(d, "garbage", garbage);
	PyDict_SetItemString(d, "DEBUG_STATS",
			PyInt_FromLong(DEBUG_STATS));
	PyDict_SetItemString(d, "DEBUG_COLLECTABLE",
			PyInt_FromLong(DEBUG_COLLECTABLE));
	PyDict_SetItemString(d, "DEBUG_UNCOLLECTABLE",
			PyInt_FromLong(DEBUG_UNCOLLECTABLE));
	PyDict_SetItemString(d, "DEBUG_INSTANCES",
			PyInt_FromLong(DEBUG_INSTANCES));
	PyDict_SetItemString(d, "DEBUG_OBJECTS",
			PyInt_FromLong(DEBUG_OBJECTS));
	PyDict_SetItemString(d, "DEBUG_SAVEALL",
			PyInt_FromLong(DEBUG_SAVEALL));
	PyDict_SetItemString(d, "DEBUG_LEAK",
			PyInt_FromLong(DEBUG_LEAK));
}

/* for debugging */
void _PyGC_Dump(PyGC_Head *g)
{
	_PyObject_Dump(FROM_GC(g));
}

/* extension modules might be compiled with GC support so these
   functions must always be available */

#undef PyObject_GC_Track
#undef PyObject_GC_UnTrack
#undef PyObject_GC_Del
#undef _PyObject_GC_Malloc

void
PyObject_GC_Track(void *op)
{
	_PyObject_GC_TRACK(op);
}

/* for binary compatibility with 2.2 */
void
_PyObject_GC_Track(PyObject *op)
{
    PyObject_GC_Track(op);
}

void
PyObject_GC_UnTrack(void *op)
{
	/* Obscure:  the Py_TRASHCAN mechanism requires that we be able to
	 * call PyObject_GC_UnTrack twice on an object.
	 */
	if (IS_TRACKED(op))
		_PyObject_GC_UNTRACK(op);
}

/* for binary compatibility with 2.2 */
void
_PyObject_GC_UnTrack(PyObject *op)
{
    PyObject_GC_UnTrack(op);
}

PyObject *
_PyObject_GC_Malloc(size_t basicsize)
{
	PyObject *op;
	PyGC_Head *g = PyObject_MALLOC(sizeof(PyGC_Head) + basicsize);
	if (g == NULL)
		return PyErr_NoMemory();
	g->gc.gc_refs = GC_UNTRACKED;
	generations[0].count++; /* number of allocated GC objects */
 	if (generations[0].count > generations[0].threshold &&
 	    enabled &&
 	    generations[0].threshold &&
 	    !collecting &&
 	    !PyErr_Occurred()) {
		collecting = 1;
		collect_generations();
		collecting = 0;
	}
	op = FROM_GC(g);
	return op;
}

PyObject *
_PyObject_GC_New(PyTypeObject *tp)
{
	PyObject *op = _PyObject_GC_Malloc(_PyObject_SIZE(tp));
	if (op != NULL)
		op = PyObject_INIT(op, tp);
	return op;
}

PyVarObject *
_PyObject_GC_NewVar(PyTypeObject *tp, int nitems)
{
	const size_t size = _PyObject_VAR_SIZE(tp, nitems);
	PyVarObject *op = (PyVarObject *) _PyObject_GC_Malloc(size);
	if (op != NULL)
		op = PyObject_INIT_VAR(op, tp, nitems);
	return op;
}

PyVarObject *
_PyObject_GC_Resize(PyVarObject *op, int nitems)
{
	const size_t basicsize = _PyObject_VAR_SIZE(op->ob_type, nitems);
	PyGC_Head *g = AS_GC(op);
	g = PyObject_REALLOC(g,  sizeof(PyGC_Head) + basicsize);
	if (g == NULL)
		return (PyVarObject *)PyErr_NoMemory();
	op = (PyVarObject *) FROM_GC(g);
	op->ob_size = nitems;
	return op;
}

void
PyObject_GC_Del(void *op)
{
	PyGC_Head *g = AS_GC(op);
	if (IS_TRACKED(op))
		gc_list_remove(g);
	if (generations[0].count > 0) {
		generations[0].count--;
	}
	PyObject_FREE(g);
}

/* for binary compatibility with 2.2 */
#undef _PyObject_GC_Del
void
_PyObject_GC_Del(PyObject *op)
{
    PyObject_GC_Del(op);
}