--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/cos/python/Modules/gcmodule.c	Tue Dec 27 18:59:02 2011 +0100
@@ -0,0 +1,1513 @@
+/*
+
+  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/
+
+  The following mailing list threads provide a historical perspective on
+  the design of this module.  Note that a fair amount of refinement has
+  occurred since those discussions.
+
+  http://mail.python.org/pipermail/python-dev/2000-March/002385.html
+  http://mail.python.org/pipermail/python-dev/2000-March/002434.html
+  http://mail.python.org/pipermail/python-dev/2000-March/002497.html
+
+  For a highlevel view of the collection process, read the collect
+  function.
+
+*/
+
+#include "Python.h"
+#include "frameobject.h"        /* for PyFrame_ClearFreeList */
+
+/* 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 = 0;
+
+/* list of uncollectable objects */
+static PyObject *garbage = NULL;
+
+/* Python string to use if unhandled exception occurs */
+static PyObject *gc_str = NULL;
+
+/* Python string used to look for __del__ attribute. */
+static PyObject *delstr = NULL;
+
+/* This is the number of objects who survived the last full collection. It
+   approximates the number of long lived objects tracked by the GC.
+
+   (by "full collection", we mean a collection of the oldest generation).
+*/
+static Py_ssize_t long_lived_total = 0;
+
+/* This is the number of objects who survived all "non-full" collections,
+   and are awaiting to undergo a full collection for the first time.
+
+*/
+static Py_ssize_t long_lived_pending = 0;
+
+/*
+   NOTE: about the counting of long-lived objects.
+
+   To limit the cost of garbage collection, there are two strategies;
+     - make each collection faster, e.g. by scanning fewer objects
+     - do less collections
+   This heuristic is about the latter strategy.
+
+   In addition to the various configurable thresholds, we only trigger a
+   full collection if the ratio
+    long_lived_pending / long_lived_total
+   is above a given value (hardwired to 25%).
+
+   The reason is that, while "non-full" collections (i.e., collections of
+   the young and middle generations) will always examine roughly the same
+   number of objects -- determined by the aforementioned thresholds --,
+   the cost of a full collection is proportional to the total number of
+   long-lived objects, which is virtually unbounded.
+
+   Indeed, it has been remarked that doing a full collection every
+   <constant number> of object creations entails a dramatic performance
+   degradation in workloads which consist in creating and storing lots of
+   long-lived objects (e.g. building a large list of GC-tracked objects would
+   show quadratic performance, instead of linear as expected: see issue #4074).
+
+   Using the above ratio, instead, yields amortized linear performance in
+   the total number of objects (the effect of which can be summarized
+   thusly: "each full garbage collection is more and more costly as the
+   number of objects grows, but we do fewer and fewer of them").
+
+   This heuristic was suggested by Martin von Löwis on python-dev in
+   June 2008. His original analysis and proposal can be found at:
+    http://mail.python.org/pipermail/python-dev/2008-June/080579.html
+*/
+
+
+/* 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_SAVEALL           (1<<5) /* save all garbage in gc.garbage */
+#define DEBUG_LEAK              DEBUG_COLLECTABLE | \
+                DEBUG_UNCOLLECTABLE | \
+                DEBUG_SAVEALL
+static int debug;
+static PyObject *tmod = NULL;
+
+/*--------------------------------------------------------------------------
+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);
+}
+
+#if 0
+/* This became unused after gc_list_move() was introduced. */
+/* Append `node` to `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;
+}
+#endif
+
+/* Remove `node` from the gc list it's currently in. */
+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 */
+}
+
+/* Move `node` from the gc list it's currently in (which is not explicitly
+ * named here) to the end of `list`.  This is semantically the same as
+ * gc_list_remove(node) followed by gc_list_append(node, list).
+ */
+static void
+gc_list_move(PyGC_Head *node, PyGC_Head *list)
+{
+    PyGC_Head *new_prev;
+    PyGC_Head *current_prev = node->gc.gc_prev;
+    PyGC_Head *current_next = node->gc.gc_next;
+    /* Unlink from current list. */
+    current_prev->gc.gc_next = current_next;
+    current_next->gc.gc_prev = current_prev;
+    /* Relink at end of new list. */
+    new_prev = node->gc.gc_prev = list->gc.gc_prev;
+    new_prev->gc.gc_next = list->gc.gc_prev = node;
+    node->gc.gc_next = list;
+}
+
+/* append list `from` onto list `to`; `from` becomes an empty list */
+static void
+gc_list_merge(PyGC_Head *from, PyGC_Head *to)
+{
+    PyGC_Head *tail;
+    assert(from != to);
+    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 Py_ssize_t
+gc_list_size(PyGC_Head *list)
+{
+    PyGC_Head *gc;
+    Py_ssize_t n = 0;
+    for (gc = list->gc.gc_next; gc != list; gc = gc->gc.gc_next) {
+        n++;
+    }
+    return n;
+}
+
+/* Append objects in a GC list to a Python list.
+ * Return 0 if all OK, < 0 if error (out of memory for 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;
+}
+
+/*** 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 = Py_REFCNT(FROM_GC(gc));
+        /* Python's cyclic gc should never see an incoming refcount
+         * of 0:  if something decref'ed to 0, it should have been
+         * deallocated immediately at that time.
+         * Possible cause (if the assert triggers):  a tp_dealloc
+         * routine left a gc-aware object tracked during its teardown
+         * phase, and did something-- or allowed something to happen --
+         * that called back into Python.  gc can trigger then, and may
+         * see the still-tracked dying object.  Before this assert
+         * was added, such mistakes went on to allow gc to try to
+         * delete the object again.  In a debug build, that caused
+         * a mysterious segfault, when _Py_ForgetReference tried
+         * to remove the object from the doubly-linked list of all
+         * objects a second time.  In a release build, an actual
+         * double deallocation occurred, which leads to corruption
+         * of the allocator's internal bookkeeping pointers.  That's
+         * so serious that maybe this should be a release-build
+         * check instead of an assert?
+         */
+        assert(gc->gc.gc_refs != 0);
+    }
+}
+
+/* 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 = Py_TYPE(FROM_GC(gc))->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 Py_ssize_t 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_move(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 = Py_TYPE(op)->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;
+            if (PyTuple_CheckExact(op)) {
+                _PyTuple_MaybeUntrack(op);
+            }
+            else if (PyDict_CheckExact(op)) {
+                _PyDict_MaybeUntrack(op);
+            }
+        }
+        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_move(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)
+{
+    if (PyGen_CheckExact(op))
+        return PyGen_NeedsFinalizing((PyGenObject *)op);
+    else
+        return op->ob_type->tp_del != NULL;
+}
+
+/* Move the objects in unreachable with __del__ methods into `finalizers`.
+ * Objects moved into `finalizers` have gc_refs set to GC_REACHABLE; the
+ * objects remaining in unreachable are left at GC_TENTATIVELY_UNREACHABLE.
+ */
+static void
+move_finalizers(PyGC_Head *unreachable, PyGC_Head *finalizers)
+{
+    PyGC_Head *gc;
+    PyGC_Head *next;
+
+    /* March over unreachable.  Move objects with finalizers into
+     * `finalizers`.
+     */
+    for (gc = unreachable->gc.gc_next; gc != unreachable; gc = next) {
+        PyObject *op = FROM_GC(gc);
+
+        assert(IS_TENTATIVELY_UNREACHABLE(op));
+        next = gc->gc.gc_next;
+
+        if (has_finalizer(op)) {
+            gc_list_move(gc, finalizers);
+            gc->gc.gc_refs = GC_REACHABLE;
+        }
+    }
+}
+
+/* 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_move(gc, tolist);
+            gc->gc.gc_refs = GC_REACHABLE;
+        }
+    }
+    return 0;
+}
+
+/* Move objects that are reachable from finalizers, from the unreachable set
+ * into 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) {
+        /* Note that the finalizers list may grow during this. */
+        traverse = Py_TYPE(FROM_GC(gc))->tp_traverse;
+        (void) traverse(FROM_GC(gc),
+                        (visitproc)visit_move,
+                        (void *)finalizers);
+    }
+}
+
+/* Clear all weakrefs to unreachable objects, and if such a weakref has a
+ * callback, invoke it if necessary.  Note that it's possible for such
+ * weakrefs to be outside the unreachable set -- indeed, those are precisely
+ * the weakrefs whose callbacks must be invoked.  See gc_weakref.txt for
+ * overview & some details.  Some weakrefs with callbacks may be reclaimed
+ * directly by this routine; the number reclaimed is the return value.  Other
+ * weakrefs with callbacks may be moved into the `old` generation.  Objects
+ * moved into `old` have gc_refs set to GC_REACHABLE; the objects remaining in
+ * unreachable are left at GC_TENTATIVELY_UNREACHABLE.  When this returns,
+ * no object in `unreachable` is weakly referenced anymore.
+ */
+static int
+handle_weakrefs(PyGC_Head *unreachable, PyGC_Head *old)
+{
+    PyGC_Head *gc;
+    PyObject *op;               /* generally FROM_GC(gc) */
+    PyWeakReference *wr;        /* generally a cast of op */
+    PyGC_Head wrcb_to_call;     /* weakrefs with callbacks to call */
+    PyGC_Head *next;
+    int num_freed = 0;
+
+    gc_list_init(&wrcb_to_call);
+
+    /* Clear all weakrefs to the objects in unreachable.  If such a weakref
+     * also has a callback, move it into `wrcb_to_call` if the callback
+     * needs to be invoked.  Note that we cannot invoke any callbacks until
+     * all weakrefs to unreachable objects are cleared, lest the callback
+     * resurrect an unreachable object via a still-active weakref.  We
+     * make another pass over wrcb_to_call, invoking callbacks, after this
+     * pass completes.
+     */
+    for (gc = unreachable->gc.gc_next; gc != unreachable; gc = next) {
+        PyWeakReference **wrlist;
+
+        op = FROM_GC(gc);
+        assert(IS_TENTATIVELY_UNREACHABLE(op));
+        next = gc->gc.gc_next;
+
+        if (! PyType_SUPPORTS_WEAKREFS(Py_TYPE(op)))
+            continue;
+
+        /* It supports weakrefs.  Does it have any? */
+        wrlist = (PyWeakReference **)
+                                PyObject_GET_WEAKREFS_LISTPTR(op);
+
+        /* `op` may have some weakrefs.  March over the list, clear
+         * all the weakrefs, and move the weakrefs with callbacks
+         * that must be called into wrcb_to_call.
+         */
+        for (wr = *wrlist; wr != NULL; wr = *wrlist) {
+            PyGC_Head *wrasgc;                  /* AS_GC(wr) */
+
+            /* _PyWeakref_ClearRef clears the weakref but leaves
+             * the callback pointer intact.  Obscure:  it also
+             * changes *wrlist.
+             */
+            assert(wr->wr_object == op);
+            _PyWeakref_ClearRef(wr);
+            assert(wr->wr_object == Py_None);
+            if (wr->wr_callback == NULL)
+                continue;                       /* no callback */
+
+    /* Headache time.  `op` is going away, and is weakly referenced by
+     * `wr`, which has a callback.  Should the callback be invoked?  If wr
+     * is also trash, no:
+     *
+     * 1. There's no need to call it.  The object and the weakref are
+     *    both going away, so it's legitimate to pretend the weakref is
+     *    going away first.  The user has to ensure a weakref outlives its
+     *    referent if they want a guarantee that the wr callback will get
+     *    invoked.
+     *
+     * 2. It may be catastrophic to call it.  If the callback is also in
+     *    cyclic trash (CT), then although the CT is unreachable from
+     *    outside the current generation, CT may be reachable from the
+     *    callback.  Then the callback could resurrect insane objects.
+     *
+     * Since the callback is never needed and may be unsafe in this case,
+     * wr is simply left in the unreachable set.  Note that because we
+     * already called _PyWeakref_ClearRef(wr), its callback will never
+     * trigger.
+     *
+     * OTOH, if wr isn't part of CT, we should invoke the callback:  the
+     * weakref outlived the trash.  Note that since wr isn't CT in this
+     * case, its callback can't be CT either -- wr acted as an external
+     * root to this generation, and therefore its callback did too.  So
+     * nothing in CT is reachable from the callback either, so it's hard
+     * to imagine how calling it later could create a problem for us.  wr
+     * is moved to wrcb_to_call in this case.
+     */
+            if (IS_TENTATIVELY_UNREACHABLE(wr))
+                continue;
+            assert(IS_REACHABLE(wr));
+
+            /* Create a new reference so that wr can't go away
+             * before we can process it again.
+             */
+            Py_INCREF(wr);
+
+            /* Move wr to wrcb_to_call, for the next pass. */
+            wrasgc = AS_GC(wr);
+            assert(wrasgc != next); /* wrasgc is reachable, but
+                                       next isn't, so they can't
+                                       be the same */
+            gc_list_move(wrasgc, &wrcb_to_call);
+        }
+    }
+
+    /* Invoke the callbacks we decided to honor.  It's safe to invoke them
+     * because they can't reference unreachable objects.
+     */
+    while (! gc_list_is_empty(&wrcb_to_call)) {
+        PyObject *temp;
+        PyObject *callback;
+
+        gc = wrcb_to_call.gc.gc_next;
+        op = FROM_GC(gc);
+        assert(IS_REACHABLE(op));
+        assert(PyWeakref_Check(op));
+        wr = (PyWeakReference *)op;
+        callback = wr->wr_callback;
+        assert(callback != NULL);
+
+        /* copy-paste of weakrefobject.c's handle_callback() */
+        temp = PyObject_CallFunctionObjArgs(callback, wr, NULL);
+        if (temp == NULL)
+            PyErr_WriteUnraisable(callback);
+        else
+            Py_DECREF(temp);
+
+        /* Give up the reference we created in the first pass.  When
+         * op's refcount hits 0 (which it may or may not do right now),
+         * op's tp_dealloc will decref op->wr_callback too.  Note
+         * that the refcount probably will hit 0 now, and because this
+         * weakref was reachable to begin with, gc didn't already
+         * add it to its count of freed objects.  Example:  a reachable
+         * weak value dict maps some key to this reachable weakref.
+         * The callback removes this key->weakref mapping from the
+         * dict, leaving no other references to the weakref (excepting
+         * ours).
+         */
+        Py_DECREF(op);
+        if (wrcb_to_call.gc.gc_next == gc) {
+            /* object is still alive -- move it */
+            gc_list_move(gc, old);
+        }
+        else
+            ++num_freed;
+    }
+
+    return num_freed;
+}
+
+static void
+debug_cycle(char *msg, PyObject *op)
+{
+    PySys_WriteStderr("gc: %.100s <%.100s %p>\n",
+                      msg, Py_TYPE(op)->tp_name, op);
+}
+
+/* Handle uncollectable garbage (cycles with finalizers, and stuff reachable
+ * only from such cycles).
+ * If DEBUG_SAVEALL, all objects in finalizers are appended to the module
+ * garbage list (a Python list), else only the objects in finalizers with
+ * __del__ methods are appended to garbage.  All objects in finalizers are
+ * merged into the old list regardless.
+ * Returns 0 if all OK, <0 on error (out of memory to grow the garbage list).
+ * The finalizers list is made empty on a successful return.
+ */
+static int
+handle_finalizers(PyGC_Head *finalizers, PyGC_Head *old)
+{
+    PyGC_Head *gc = finalizers->gc.gc_next;
+
+    if (garbage == NULL) {
+        garbage = PyList_New(0);
+        if (garbage == NULL)
+            Py_FatalError("gc couldn't create gc.garbage list");
+    }
+    for (; gc != finalizers; gc = gc->gc.gc_next) {
+        PyObject *op = FROM_GC(gc);
+
+        if ((debug & DEBUG_SAVEALL) || has_finalizer(op)) {
+            if (PyList_Append(garbage, op) < 0)
+                return -1;
+        }
+    }
+
+    gc_list_merge(finalizers, old);
+    return 0;
+}
+
+/* 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 = Py_TYPE(op)->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_move(gc, old);
+            gc->gc.gc_refs = GC_REACHABLE;
+        }
+    }
+}
+
+/* Clear all free lists
+ * All free lists are cleared during the collection of the highest generation.
+ * Allocated items in the free list may keep a pymalloc arena occupied.
+ * Clearing the free lists may give back memory to the OS earlier.
+ */
+static void
+clear_freelists(void)
+{
+    (void)PyMethod_ClearFreeList();
+    (void)PyFrame_ClearFreeList();
+    (void)PyCFunction_ClearFreeList();
+    (void)PyTuple_ClearFreeList();
+    (void)PyUnicode_ClearFreeList();
+    (void)PyFloat_ClearFreeList();
+}
+
+static double
+get_time(void)
+{
+    double result = 0;
+    if (tmod != NULL) {
+        PyObject *f = PyObject_CallMethod(tmod, "time", NULL);
+        if (f == NULL) {
+            PyErr_Clear();
+        }
+        else {
+            if (PyFloat_Check(f))
+                result = PyFloat_AsDouble(f);
+            Py_DECREF(f);
+        }
+    }
+    return result;
+}
+
+/* This is the main function.  Read this to understand how the
+ * collection process works. */
+static Py_ssize_t
+collect(int generation)
+{
+    int i;
+    Py_ssize_t m = 0; /* # objects collected */
+    Py_ssize_t 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; /* non-problematic unreachable trash */
+    PyGC_Head finalizers;  /* objects with, & reachable from, __del__ */
+    PyGC_Head *gc;
+    double t1 = 0.0;
+
+    if (delstr == NULL) {
+        delstr = PyUnicode_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(" %" PY_FORMAT_SIZE_T "d",
+                              gc_list_size(GEN_HEAD(i)));
+        t1 = get_time();
+        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 (i.e., 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) {
+        if (generation == NUM_GENERATIONS - 2) {
+            long_lived_pending += gc_list_size(young);
+        }
+        gc_list_merge(young, old);
+    }
+    else {
+        long_lived_pending = 0;
+        long_lived_total = gc_list_size(young);
+    }
+
+    /* 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.  Weakrefs with callbacks
+     * can also call arbitrary Python code but they will be dealt with by
+     * handle_weakrefs().
+     */
+    gc_list_init(&finalizers);
+    move_finalizers(&unreachable, &finalizers);
+    /* finalizers contains the unreachable objects with a finalizer;
+     * unreachable objects reachable *from* those are also uncollectable,
+     * and we move those into the finalizers list too.
+     */
+    move_finalizer_reachable(&finalizers);
+
+    /* Collect statistics on collectable objects found and print
+     * debugging information.
+     */
+    for (gc = unreachable.gc.gc_next; gc != &unreachable;
+                    gc = gc->gc.gc_next) {
+        m++;
+        if (debug & DEBUG_COLLECTABLE) {
+            debug_cycle("collectable", FROM_GC(gc));
+        }
+    }
+
+    /* Clear weakrefs and invoke callbacks as necessary. */
+    m += handle_weakrefs(&unreachable, old);
+
+    /* Call tp_clear on objects in the unreachable set.  This will cause
+     * the reference cycles to be broken.  It may also cause some objects
+     * in finalizers to be freed.
+     */
+    delete_garbage(&unreachable, 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) {
+        double t2 = get_time();
+        if (m == 0 && n == 0)
+            PySys_WriteStderr("gc: done");
+        else
+            PySys_WriteStderr(
+                "gc: done, "
+                "%" PY_FORMAT_SIZE_T "d unreachable, "
+                "%" PY_FORMAT_SIZE_T "d uncollectable",
+                n+m, n);
+        if (t1 && t2) {
+            PySys_WriteStderr(", %.4fs elapsed", t2-t1);
+        }
+        PySys_WriteStderr(".\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.
+     */
+    (void)handle_finalizers(&finalizers, old);
+
+    /* Clear free list only during the collection of the highest
+     * generation */
+    if (generation == NUM_GENERATIONS-1) {
+        clear_freelists();
+    }
+
+    if (PyErr_Occurred()) {
+        if (gc_str == NULL)
+            gc_str = PyUnicode_FromString("garbage collection");
+        PyErr_WriteUnraisable(gc_str);
+        Py_FatalError("unexpected exception during garbage collection");
+    }
+    return n+m;
+}
+
+static Py_ssize_t
+collect_generations(void)
+{
+    int i;
+    Py_ssize_t n = 0;
+
+    /* Find the oldest generation (highest 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) {
+            /* Avoid quadratic performance degradation in number
+               of tracked objects. See comments at the beginning
+               of this file, and issue #4074.
+            */
+            if (i == NUM_GENERATIONS - 1
+                && long_lived_pending < long_lived_total / 4)
+                continue;
+            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 *noargs)
+{
+    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 *noargs)
+{
+    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 *noargs)
+{
+    return PyBool_FromLong((long)enabled);
+}
+
+PyDoc_STRVAR(gc_collect__doc__,
+"collect([generation]) -> n\n"
+"\n"
+"With no arguments, run a full collection.  The optional argument\n"
+"may be an integer specifying which generation to collect.  A ValueError\n"
+"is raised if the generation number is invalid.\n\n"
+"The number of unreachable objects is returned.\n");
+
+static PyObject *
+gc_collect(PyObject *self, PyObject *args, PyObject *kws)
+{
+    static char *keywords[] = {"generation", NULL};
+    int genarg = NUM_GENERATIONS - 1;
+    Py_ssize_t n;
+
+    if (!PyArg_ParseTupleAndKeywords(args, kws, "|i", keywords, &genarg))
+        return NULL;
+
+    else if (genarg < 0 || genarg >= NUM_GENERATIONS) {
+        PyErr_SetString(PyExc_ValueError, "invalid generation");
+        return NULL;
+    }
+
+    if (collecting)
+        n = 0; /* already collecting, don't do anything */
+    else {
+        collecting = 1;
+        n = collect(genarg);
+        collecting = 0;
+    }
+
+    return PyLong_FromSsize_t(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_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 *noargs)
+{
+    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 *noargs)
+{
+    return Py_BuildValue("(iii)",
+                         generations[0].threshold,
+                         generations[1].threshold,
+                         generations[2].threshold);
+}
+
+PyDoc_STRVAR(gc_get_count__doc__,
+"get_count() -> (count0, count1, count2)\n"
+"\n"
+"Return the current collection counts\n");
+
+static PyObject *
+gc_get_count(PyObject *self, PyObject *noargs)
+{
+    return Py_BuildValue("(iii)",
+                         generations[0].count,
+                         generations[1].count,
+                         generations[2].count);
+}
+
+static int
+referrersvisit(PyObject* obj, PyObject *objs)
+{
+    Py_ssize_t 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 = Py_TYPE(obj)->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);
+    if (!result) return NULL;
+
+    for (i = 0; i < NUM_GENERATIONS; i++) {
+        if (!(gc_referrers_for(args, GEN_HEAD(i), result))) {
+            Py_DECREF(result);
+            return NULL;
+        }
+    }
+    return result;
+}
+
+/* Append obj to list; return true if error (out of memory), false if OK. */
+static int
+referentsvisit(PyObject *obj, PyObject *list)
+{
+    return PyList_Append(list, obj) < 0;
+}
+
+PyDoc_STRVAR(gc_get_referents__doc__,
+"get_referents(*objs) -> list\n\
+Return the list of objects that are directly referred to by objs.");
+
+static PyObject *
+gc_get_referents(PyObject *self, PyObject *args)
+{
+    Py_ssize_t i;
+    PyObject *result = PyList_New(0);
+
+    if (result == NULL)
+        return NULL;
+
+    for (i = 0; i < PyTuple_GET_SIZE(args); i++) {
+        traverseproc traverse;
+        PyObject *obj = PyTuple_GET_ITEM(args, i);
+
+        if (! PyObject_IS_GC(obj))
+            continue;
+        traverse = Py_TYPE(obj)->tp_traverse;
+        if (! traverse)
+            continue;
+        if (traverse(obj, (visitproc)referentsvisit, result)) {
+            Py_DECREF(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");
+
+static PyObject *
+gc_get_objects(PyObject *self, PyObject *noargs)
+{
+    int i;
+    PyObject* result;
+
+    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_is_tracked__doc__,
+"is_tracked(obj) -> bool\n"
+"\n"
+"Returns true if the object is tracked by the garbage collector.\n"
+"Simple atomic objects will return false.\n"
+);
+
+static PyObject *
+gc_is_tracked(PyObject *self, PyObject *obj)
+{
+    PyObject *result;
+
+    if (PyObject_IS_GC(obj) && IS_TRACKED(obj))
+        result = Py_True;
+    else
+        result = Py_False;
+    Py_INCREF(result);
+    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"
+"get_count() -- Return the current collection counts.\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"
+"is_tracked() -- Returns true if a given object is tracked.\n"
+"get_referrers() -- Return the list of objects that refer to an object.\n"
+"get_referents() -- Return the list of objects that an object refers to.\n");
+
+static PyMethodDef GcMethods[] = {
+    {"enable",             gc_enable,     METH_NOARGS,  gc_enable__doc__},
+    {"disable",            gc_disable,    METH_NOARGS,  gc_disable__doc__},
+    {"isenabled",          gc_isenabled,  METH_NOARGS,  gc_isenabled__doc__},
+    {"set_debug",          gc_set_debug,  METH_VARARGS, gc_set_debug__doc__},
+    {"get_debug",          gc_get_debug,  METH_NOARGS,  gc_get_debug__doc__},
+    {"get_count",          gc_get_count,  METH_NOARGS,  gc_get_count__doc__},
+    {"set_threshold",  gc_set_thresh, METH_VARARGS, gc_set_thresh__doc__},
+    {"get_threshold",  gc_get_thresh, METH_NOARGS,  gc_get_thresh__doc__},
+    {"collect",            (PyCFunction)gc_collect,
+        METH_VARARGS | METH_KEYWORDS,           gc_collect__doc__},
+    {"get_objects",    gc_get_objects,METH_NOARGS,  gc_get_objects__doc__},
+    {"is_tracked",     gc_is_tracked, METH_O,       gc_is_tracked__doc__},
+    {"get_referrers",  gc_get_referrers, METH_VARARGS,
+        gc_get_referrers__doc__},
+    {"get_referents",  gc_get_referents, METH_VARARGS,
+        gc_get_referents__doc__},
+    {NULL,      NULL}           /* Sentinel */
+};
+
+static struct PyModuleDef gcmodule = {
+    PyModuleDef_HEAD_INIT,
+    "gc",              /* m_name */
+    gc__doc__,         /* m_doc */
+    -1,                /* m_size */
+    GcMethods,         /* m_methods */
+    NULL,              /* m_reload */
+    NULL,              /* m_traverse */
+    NULL,              /* m_clear */
+    NULL               /* m_free */
+};
+
+PyMODINIT_FUNC
+PyInit_gc(void)
+{
+    PyObject *m;
+
+    m = PyModule_Create(&gcmodule);
+
+    if (m == NULL)
+        return NULL;
+
+    if (garbage == NULL) {
+        garbage = PyList_New(0);
+        if (garbage == NULL)
+            return NULL;
+    }
+    Py_INCREF(garbage);
+    if (PyModule_AddObject(m, "garbage", garbage) < 0)
+        return NULL;
+
+    /* Importing can't be done in collect() because collect()
+     * can be called via PyGC_Collect() in Py_Finalize().
+     * This wouldn't be a problem, except that <initialized> is
+     * reset to 0 before calling collect which trips up
+     * the import and triggers an assertion.
+     */
+    if (tmod == NULL) {
+        tmod = PyImport_ImportModuleNoBlock("time");
+        if (tmod == NULL)
+            PyErr_Clear();
+    }
+
+#define ADD_INT(NAME) if (PyModule_AddIntConstant(m, #NAME, NAME) < 0) return NULL
+    ADD_INT(DEBUG_STATS);
+    ADD_INT(DEBUG_COLLECTABLE);
+    ADD_INT(DEBUG_UNCOLLECTABLE);
+    ADD_INT(DEBUG_SAVEALL);
+    ADD_INT(DEBUG_LEAK);
+#undef ADD_INT
+    return m;
+}
+
+/* API to invoke gc.collect() from C */
+Py_ssize_t
+PyGC_Collect(void)
+{
+    Py_ssize_t n;
+
+    if (collecting)
+        n = 0; /* already collecting, don't do anything */
+    else {
+        collecting = 1;
+        n = collect(NUM_GENERATIONS - 1);
+        collecting = 0;
+    }
+
+    return n;
+}
+
+void
+_PyGC_Fini(void)
+{
+    if (!(debug & DEBUG_SAVEALL)
+        && garbage != NULL && PyList_GET_SIZE(garbage) > 0) {
+        char *message;
+        if (debug & DEBUG_UNCOLLECTABLE)
+            message = "gc: %zd uncollectable objects at " \
+                "shutdown";
+        else
+            message = "gc: %zd uncollectable objects at " \
+                "shutdown; use gc.set_debug(gc.DEBUG_UNCOLLECTABLE) to list them";
+        if (PyErr_WarnFormat(PyExc_ResourceWarning, 0, message,
+                             PyList_GET_SIZE(garbage)) < 0)
+            PyErr_WriteUnraisable(NULL);
+        if (debug & DEBUG_UNCOLLECTABLE) {
+            PyObject *repr = NULL, *bytes = NULL;
+            repr = PyObject_Repr(garbage);
+            if (!repr || !(bytes = PyUnicode_EncodeFSDefault(repr)))
+                PyErr_WriteUnraisable(garbage);
+            else {
+                PySys_WriteStderr(
+                    "    %s\n",
+                    PyBytes_AS_STRING(bytes)
+                    );
+            }
+            Py_XDECREF(repr);
+            Py_XDECREF(bytes);
+        }
+    }
+}
+
+/* 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;
+    if (basicsize > PY_SSIZE_T_MAX - sizeof(PyGC_Head))
+        return PyErr_NoMemory();
+    g = (PyGC_Head *)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, Py_ssize_t 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, Py_ssize_t nitems)
+{
+    const size_t basicsize = _PyObject_VAR_SIZE(Py_TYPE(op), nitems);
+    PyGC_Head *g = AS_GC(op);
+    if (basicsize > PY_SSIZE_T_MAX - sizeof(PyGC_Head))
+        return (PyVarObject *)PyErr_NoMemory();
+    g = (PyGC_Head *)PyObject_REALLOC(g,  sizeof(PyGC_Head) + basicsize);
+    if (g == NULL)
+        return (PyVarObject *)PyErr_NoMemory();
+    op = (PyVarObject *) FROM_GC(g);
+    Py_SIZE(op) = 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);
+}