add ydb deps

1 files changed, 2384 insertions, 0 deletions

diff --git a/contrib/tools/python3/src/Modules/gcmodule.c b/contrib/tools/python3/src/Modules/gcmodule.c
new file mode 100644
index 0000000000..95f5085edb
--- /dev/null
+++ b/contrib/tools/python3/src/Modules/gcmodule.c
@@ -0,0 +1,2384 @@
+/*
+
+  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 "pycore_context.h"
+#include "pycore_initconfig.h"
+#include "pycore_interp.h"      // PyInterpreterState.gc
+#include "pycore_object.h"
+#include "pycore_pyerrors.h"
+#include "pycore_pystate.h"     // _PyThreadState_GET()
+#include "pydtrace.h"
+
+typedef struct _gc_runtime_state GCState;
+
+/*[clinic input]
+module gc
+[clinic start generated code]*/
+/*[clinic end generated code: output=da39a3ee5e6b4b0d input=b5c9690ecc842d79]*/
+
+
+#ifdef Py_DEBUG
+#  ifndef GC_NDEBUG
+#    define GC_DEBUG
+#  endif
+#endif
+
+#define GC_NEXT _PyGCHead_NEXT
+#define GC_PREV _PyGCHead_PREV
+
+// update_refs() set this bit for all objects in current generation.
+// subtract_refs() and move_unreachable() uses this to distinguish
+// visited object is in GCing or not.
+//
+// move_unreachable() removes this flag from reachable objects.
+// Only unreachable objects have this flag.
+//
+// No objects in interpreter have this flag after GC ends.
+#define PREV_MASK_COLLECTING   _PyGC_PREV_MASK_COLLECTING
+
+// Lowest bit of _gc_next is used for UNREACHABLE flag.
+//
+// This flag represents the object is in unreachable list in move_unreachable()
+//
+// Although this flag is used only in move_unreachable(), move_unreachable()
+// doesn't clear this flag to skip unnecessary iteration.
+// move_legacy_finalizers() removes this flag instead.
+// Between them, unreachable list is not normal list and we can not use
+// most gc_list_* functions for it.
+#define NEXT_MASK_UNREACHABLE  (1)
+
+/* Get an object's GC head */
+#define AS_GC(o) ((PyGC_Head *)(((char *)(o))-sizeof(PyGC_Head)))
+
+/* Get the object given the GC head */
+#define FROM_GC(g) ((PyObject *)(((char *)(g))+sizeof(PyGC_Head)))
+
+static inline int
+gc_is_collecting(PyGC_Head *g)
+{
+    return (g->_gc_prev & PREV_MASK_COLLECTING) != 0;
+}
+
+static inline void
+gc_clear_collecting(PyGC_Head *g)
+{
+    g->_gc_prev &= ~PREV_MASK_COLLECTING;
+}
+
+static inline Py_ssize_t
+gc_get_refs(PyGC_Head *g)
+{
+    return (Py_ssize_t)(g->_gc_prev >> _PyGC_PREV_SHIFT);
+}
+
+static inline void
+gc_set_refs(PyGC_Head *g, Py_ssize_t refs)
+{
+    g->_gc_prev = (g->_gc_prev & ~_PyGC_PREV_MASK)
+        | ((uintptr_t)(refs) << _PyGC_PREV_SHIFT);
+}
+
+static inline void
+gc_reset_refs(PyGC_Head *g, Py_ssize_t refs)
+{
+    g->_gc_prev = (g->_gc_prev & _PyGC_PREV_MASK_FINALIZED)
+        | PREV_MASK_COLLECTING
+        | ((uintptr_t)(refs) << _PyGC_PREV_SHIFT);
+}
+
+static inline void
+gc_decref(PyGC_Head *g)
+{
+    _PyObject_ASSERT_WITH_MSG(FROM_GC(g),
+                              gc_get_refs(g) > 0,
+                              "refcount is too small");
+    g->_gc_prev -= 1 << _PyGC_PREV_SHIFT;
+}
+
+/* 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
+
+#define GEN_HEAD(gcstate, n) (&(gcstate)->generations[n].head)
+
+
+static GCState *
+get_gc_state(void)
+{
+    PyInterpreterState *interp = _PyInterpreterState_GET();
+    return &interp->gc;
+}
+
+
+void
+_PyGC_InitState(GCState *gcstate)
+{
+#define INIT_HEAD(GEN) \
+    do { \
+        GEN.head._gc_next = (uintptr_t)&GEN.head; \
+        GEN.head._gc_prev = (uintptr_t)&GEN.head; \
+    } while (0)
+
+    for (int i = 0; i < NUM_GENERATIONS; i++) {
+        assert(gcstate->generations[i].count == 0);
+        INIT_HEAD(gcstate->generations[i]);
+    };
+    gcstate->generation0 = GEN_HEAD(gcstate, 0);
+    INIT_HEAD(gcstate->permanent_generation);
+
+#undef INIT_HEAD
+}
+
+
+PyStatus
+_PyGC_Init(PyInterpreterState *interp)
+{
+    GCState *gcstate = &interp->gc;
+
+    gcstate->garbage = PyList_New(0);
+    if (gcstate->garbage == NULL) {
+        return _PyStatus_NO_MEMORY();
+    }
+
+    gcstate->callbacks = PyList_New(0);
+    if (gcstate->callbacks == NULL) {
+        return _PyStatus_NO_MEMORY();
+    }
+
+    return _PyStatus_OK();
+}
+
+
+/*
+_gc_prev values
+---------------
+
+Between collections, _gc_prev is used for doubly linked list.
+
+Lowest two bits of _gc_prev are used for flags.
+PREV_MASK_COLLECTING is used only while collecting and cleared before GC ends
+or _PyObject_GC_UNTRACK() is called.
+
+During a collection, _gc_prev is temporary used for gc_refs, and the gc list
+is singly linked until _gc_prev is restored.
+
+gc_refs
+    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.
+
+PREV_MASK_COLLECTING
+    Objects in generation being collected are marked PREV_MASK_COLLECTING in
+    update_refs().
+
+
+_gc_next values
+---------------
+
+_gc_next takes these values:
+
+0
+    The object is not tracked
+
+!= 0
+    Pointer to the next object in the GC list.
+    Additionally, lowest bit is used temporary for
+    NEXT_MASK_UNREACHABLE flag described below.
+
+NEXT_MASK_UNREACHABLE
+    move_unreachable() then moves objects not reachable (whether directly or
+    indirectly) from outside the generation into an "unreachable" set and
+    set this flag.
+
+    Objects that are found to be reachable have gc_refs set to 1.
+    When this flag is set for the reachable object, the object must be in
+    "unreachable" set.
+    The flag is unset and the object is moved back to "reachable" set.
+
+    move_legacy_finalizers() will remove this flag from "unreachable" set.
+*/
+
+/*** list functions ***/
+
+static inline void
+gc_list_init(PyGC_Head *list)
+{
+    // List header must not have flags.
+    // We can assign pointer by simple cast.
+    list->_gc_prev = (uintptr_t)list;
+    list->_gc_next = (uintptr_t)list;
+}
+
+static inline int
+gc_list_is_empty(PyGC_Head *list)
+{
+    return (list->_gc_next == (uintptr_t)list);
+}
+
+/* Append `node` to `list`. */
+static inline void
+gc_list_append(PyGC_Head *node, PyGC_Head *list)
+{
+    PyGC_Head *last = (PyGC_Head *)list->_gc_prev;
+
+    // last <-> node
+    _PyGCHead_SET_PREV(node, last);
+    _PyGCHead_SET_NEXT(last, node);
+
+    // node <-> list
+    _PyGCHead_SET_NEXT(node, list);
+    list->_gc_prev = (uintptr_t)node;
+}
+
+/* Remove `node` from the gc list it's currently in. */
+static inline void
+gc_list_remove(PyGC_Head *node)
+{
+    PyGC_Head *prev = GC_PREV(node);
+    PyGC_Head *next = GC_NEXT(node);
+
+    _PyGCHead_SET_NEXT(prev, next);
+    _PyGCHead_SET_PREV(next, prev);
+
+    node->_gc_next = 0; /* 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)
+{
+    /* Unlink from current list. */
+    PyGC_Head *from_prev = GC_PREV(node);
+    PyGC_Head *from_next = GC_NEXT(node);
+    _PyGCHead_SET_NEXT(from_prev, from_next);
+    _PyGCHead_SET_PREV(from_next, from_prev);
+
+    /* Relink at end of new list. */
+    // list must not have flags.  So we can skip macros.
+    PyGC_Head *to_prev = (PyGC_Head*)list->_gc_prev;
+    _PyGCHead_SET_PREV(node, to_prev);
+    _PyGCHead_SET_NEXT(to_prev, node);
+    list->_gc_prev = (uintptr_t)node;
+    _PyGCHead_SET_NEXT(node, list);
+}
+
+/* append list `from` onto list `to`; `from` becomes an empty list */
+static void
+gc_list_merge(PyGC_Head *from, PyGC_Head *to)
+{
+    assert(from != to);
+    if (!gc_list_is_empty(from)) {
+        PyGC_Head *to_tail = GC_PREV(to);
+        PyGC_Head *from_head = GC_NEXT(from);
+        PyGC_Head *from_tail = GC_PREV(from);
+        assert(from_head != from);
+        assert(from_tail != from);
+
+        _PyGCHead_SET_NEXT(to_tail, from_head);
+        _PyGCHead_SET_PREV(from_head, to_tail);
+
+        _PyGCHead_SET_NEXT(from_tail, to);
+        _PyGCHead_SET_PREV(to, from_tail);
+    }
+    gc_list_init(from);
+}
+
+static Py_ssize_t
+gc_list_size(PyGC_Head *list)
+{
+    PyGC_Head *gc;
+    Py_ssize_t n = 0;
+    for (gc = GC_NEXT(list); gc != list; gc = GC_NEXT(gc)) {
+        n++;
+    }
+    return n;
+}
+
+/* Walk the list and mark all objects as non-collecting */
+static inline void
+gc_list_clear_collecting(PyGC_Head *collectable)
+{
+    PyGC_Head *gc;
+    for (gc = GC_NEXT(collectable); gc != collectable; gc = GC_NEXT(gc)) {
+        gc_clear_collecting(gc);
+    }
+}
+
+/* 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_NEXT(gc_list); gc != gc_list; gc = GC_NEXT(gc)) {
+        PyObject *op = FROM_GC(gc);
+        if (op != py_list) {
+            if (PyList_Append(py_list, op)) {
+                return -1; /* exception */
+            }
+        }
+    }
+    return 0;
+}
+
+// Constants for validate_list's flags argument.
+enum flagstates {collecting_clear_unreachable_clear,
+                 collecting_clear_unreachable_set,
+                 collecting_set_unreachable_clear,
+                 collecting_set_unreachable_set};
+
+#ifdef GC_DEBUG
+// validate_list checks list consistency.  And it works as document
+// describing when flags are expected to be set / unset.
+// `head` must be a doubly-linked gc list, although it's fine (expected!) if
+// the prev and next pointers are "polluted" with flags.
+// What's checked:
+// - The `head` pointers are not polluted.
+// - The objects' PREV_MASK_COLLECTING and NEXT_MASK_UNREACHABLE flags are all
+//   `set or clear, as specified by the 'flags' argument.
+// - The prev and next pointers are mutually consistent.
+static void
+validate_list(PyGC_Head *head, enum flagstates flags)
+{
+    assert((head->_gc_prev & PREV_MASK_COLLECTING) == 0);
+    assert((head->_gc_next & NEXT_MASK_UNREACHABLE) == 0);
+    uintptr_t prev_value = 0, next_value = 0;
+    switch (flags) {
+        case collecting_clear_unreachable_clear:
+            break;
+        case collecting_set_unreachable_clear:
+            prev_value = PREV_MASK_COLLECTING;
+            break;
+        case collecting_clear_unreachable_set:
+            next_value = NEXT_MASK_UNREACHABLE;
+            break;
+        case collecting_set_unreachable_set:
+            prev_value = PREV_MASK_COLLECTING;
+            next_value = NEXT_MASK_UNREACHABLE;
+            break;
+        default:
+            assert(! "bad internal flags argument");
+    }
+    PyGC_Head *prev = head;
+    PyGC_Head *gc = GC_NEXT(head);
+    while (gc != head) {
+        PyGC_Head *trueprev = GC_PREV(gc);
+        PyGC_Head *truenext = (PyGC_Head *)(gc->_gc_next  & ~NEXT_MASK_UNREACHABLE);
+        assert(truenext != NULL);
+        assert(trueprev == prev);
+        assert((gc->_gc_prev & PREV_MASK_COLLECTING) == prev_value);
+        assert((gc->_gc_next & NEXT_MASK_UNREACHABLE) == next_value);
+        prev = gc;
+        gc = truenext;
+    }
+    assert(prev == GC_PREV(head));
+}
+#else
+#define validate_list(x, y) do{}while(0)
+#endif
+
+/*** end of list stuff ***/
+
+
+/* Set all gc_refs = ob_refcnt.  After this, gc_refs is > 0 and
+ * PREV_MASK_COLLECTING bit is set for all objects in containers.
+ */
+static void
+update_refs(PyGC_Head *containers)
+{
+    PyGC_Head *gc = GC_NEXT(containers);
+    for (; gc != containers; gc = GC_NEXT(gc)) {
+        gc_reset_refs(gc, 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?
+         */
+        _PyObject_ASSERT(FROM_GC(gc), gc_get_refs(gc) != 0);
+    }
+}
+
+/* A traversal callback for subtract_refs. */
+static int
+visit_decref(PyObject *op, void *parent)
+{
+    _PyObject_ASSERT(_PyObject_CAST(parent), !_PyObject_IsFreed(op));
+
+    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.
+         */
+        if (gc_is_collecting(gc)) {
+            gc_decref(gc);
+        }
+    }
+    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 = GC_NEXT(containers);
+    for (; gc != containers; gc = GC_NEXT(gc)) {
+        PyObject *op = FROM_GC(gc);
+        traverse = Py_TYPE(op)->tp_traverse;
+        (void) traverse(op,
+                        (visitproc)visit_decref,
+                        op);
+    }
+}
+
+/* A traversal callback for move_unreachable. */
+static int
+visit_reachable(PyObject *op, PyGC_Head *reachable)
+{
+    if (!_PyObject_IS_GC(op)) {
+        return 0;
+    }
+
+    PyGC_Head *gc = AS_GC(op);
+    const Py_ssize_t gc_refs = gc_get_refs(gc);
+
+    // Ignore objects in other generation.
+    // This also skips objects "to the left" of the current position in
+    // move_unreachable's scan of the 'young' list - they've already been
+    // traversed, and no longer have the PREV_MASK_COLLECTING flag.
+    if (! gc_is_collecting(gc)) {
+        return 0;
+    }
+    // It would be a logic error elsewhere if the collecting flag were set on
+    // an untracked object.
+    assert(gc->_gc_next != 0);
+
+    if (gc->_gc_next & NEXT_MASK_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.
+         */
+        // Manually unlink gc from unreachable list because the list functions
+        // don't work right in the presence of NEXT_MASK_UNREACHABLE flags.
+        PyGC_Head *prev = GC_PREV(gc);
+        PyGC_Head *next = (PyGC_Head*)(gc->_gc_next & ~NEXT_MASK_UNREACHABLE);
+        _PyObject_ASSERT(FROM_GC(prev),
+                         prev->_gc_next & NEXT_MASK_UNREACHABLE);
+        _PyObject_ASSERT(FROM_GC(next),
+                         next->_gc_next & NEXT_MASK_UNREACHABLE);
+        prev->_gc_next = gc->_gc_next;  // copy NEXT_MASK_UNREACHABLE
+        _PyGCHead_SET_PREV(next, prev);
+
+        gc_list_append(gc, reachable);
+        gc_set_refs(gc, 1);
+    }
+    else 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_set_refs(gc, 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.
+     */
+    else {
+        _PyObject_ASSERT_WITH_MSG(op, gc_refs > 0, "refcount is too small");
+    }
+    return 0;
+}
+
+/* Move the unreachable objects from young to unreachable.  After this,
+ * all objects in young don't have PREV_MASK_COLLECTING flag and
+ * unreachable have the flag.
+ * All objects in young after this are directly or indirectly reachable
+ * from outside the original young; and all objects in unreachable are
+ * not.
+ *
+ * This function restores _gc_prev pointer.  young and unreachable are
+ * doubly linked list after this function.
+ * But _gc_next in unreachable list has NEXT_MASK_UNREACHABLE flag.
+ * So we can not gc_list_* functions for unreachable until we remove the flag.
+ */
+static void
+move_unreachable(PyGC_Head *young, PyGC_Head *unreachable)
+{
+    // previous elem in the young list, used for restore gc_prev.
+    PyGC_Head *prev = young;
+    PyGC_Head *gc = GC_NEXT(young);
+
+    /* Invariants:  all objects "to the left" of us in young are 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 NEXT_MASK_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) {
+        if (gc_get_refs(gc)) {
+            /* 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;
+            _PyObject_ASSERT_WITH_MSG(op, gc_get_refs(gc) > 0,
+                                      "refcount is too small");
+            // NOTE: visit_reachable may change gc->_gc_next when
+            // young->_gc_prev == gc.  Don't do gc = GC_NEXT(gc) before!
+            (void) traverse(op,
+                    (visitproc)visit_reachable,
+                    (void *)young);
+            // relink gc_prev to prev element.
+            _PyGCHead_SET_PREV(gc, prev);
+            // gc is not COLLECTING state after here.
+            gc_clear_collecting(gc);
+            prev = gc;
+        }
+        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.
+             */
+            // Move gc to unreachable.
+            // No need to gc->next->prev = prev because it is single linked.
+            prev->_gc_next = gc->_gc_next;
+
+            // We can't use gc_list_append() here because we use
+            // NEXT_MASK_UNREACHABLE here.
+            PyGC_Head *last = GC_PREV(unreachable);
+            // NOTE: Since all objects in unreachable set has
+            // NEXT_MASK_UNREACHABLE flag, we set it unconditionally.
+            // But this may pollute the unreachable list head's 'next' pointer
+            // too. That's semantically senseless but expedient here - the
+            // damage is repaired when this function ends.
+            last->_gc_next = (NEXT_MASK_UNREACHABLE | (uintptr_t)gc);
+            _PyGCHead_SET_PREV(gc, last);
+            gc->_gc_next = (NEXT_MASK_UNREACHABLE | (uintptr_t)unreachable);
+            unreachable->_gc_prev = (uintptr_t)gc;
+        }
+        gc = (PyGC_Head*)prev->_gc_next;
+    }
+    // young->_gc_prev must be last element remained in the list.
+    young->_gc_prev = (uintptr_t)prev;
+    // don't let the pollution of the list head's next pointer leak
+    unreachable->_gc_next &= ~NEXT_MASK_UNREACHABLE;
+}
+
+static void
+untrack_tuples(PyGC_Head *head)
+{
+    PyGC_Head *next, *gc = GC_NEXT(head);
+    while (gc != head) {
+        PyObject *op = FROM_GC(gc);
+        next = GC_NEXT(gc);
+        if (PyTuple_CheckExact(op)) {
+            _PyTuple_MaybeUntrack(op);
+        }
+        gc = next;
+    }
+}
+
+/* Try to untrack all currently tracked dictionaries */
+static void
+untrack_dicts(PyGC_Head *head)
+{
+    PyGC_Head *next, *gc = GC_NEXT(head);
+    while (gc != head) {
+        PyObject *op = FROM_GC(gc);
+        next = GC_NEXT(gc);
+        if (PyDict_CheckExact(op)) {
+            _PyDict_MaybeUntrack(op);
+        }
+        gc = next;
+    }
+}
+
+/* Return true if object has a pre-PEP 442 finalization method. */
+static int
+has_legacy_finalizer(PyObject *op)
+{
+    return Py_TYPE(op)->tp_del != NULL;
+}
+
+/* Move the objects in unreachable with tp_del slots into `finalizers`.
+ *
+ * This function also removes NEXT_MASK_UNREACHABLE flag
+ * from _gc_next in unreachable.
+ */
+static void
+move_legacy_finalizers(PyGC_Head *unreachable, PyGC_Head *finalizers)
+{
+    PyGC_Head *gc, *next;
+    assert((unreachable->_gc_next & NEXT_MASK_UNREACHABLE) == 0);
+
+    /* March over unreachable.  Move objects with finalizers into
+     * `finalizers`.
+     */
+    for (gc = GC_NEXT(unreachable); gc != unreachable; gc = next) {
+        PyObject *op = FROM_GC(gc);
+
+        _PyObject_ASSERT(op, gc->_gc_next & NEXT_MASK_UNREACHABLE);
+        gc->_gc_next &= ~NEXT_MASK_UNREACHABLE;
+        next = (PyGC_Head*)gc->_gc_next;
+
+        if (has_legacy_finalizer(op)) {
+            gc_clear_collecting(gc);
+            gc_list_move(gc, finalizers);
+        }
+    }
+}
+
+static inline void
+clear_unreachable_mask(PyGC_Head *unreachable)
+{
+    /* Check that the list head does not have the unreachable bit set */
+    assert(((uintptr_t)unreachable & NEXT_MASK_UNREACHABLE) == 0);
+
+    PyGC_Head *gc, *next;
+    assert((unreachable->_gc_next & NEXT_MASK_UNREACHABLE) == 0);
+    for (gc = GC_NEXT(unreachable); gc != unreachable; gc = next) {
+        _PyObject_ASSERT((PyObject*)FROM_GC(gc), gc->_gc_next & NEXT_MASK_UNREACHABLE);
+        gc->_gc_next &= ~NEXT_MASK_UNREACHABLE;
+        next = (PyGC_Head*)gc->_gc_next;
+    }
+    validate_list(unreachable, collecting_set_unreachable_clear);
+}
+
+/* A traversal callback for move_legacy_finalizer_reachable. */
+static int
+visit_move(PyObject *op, PyGC_Head *tolist)
+{
+    if (_PyObject_IS_GC(op)) {
+        PyGC_Head *gc = AS_GC(op);
+        if (gc_is_collecting(gc)) {
+            gc_list_move(gc, tolist);
+            gc_clear_collecting(gc);
+        }
+    }
+    return 0;
+}
+
+/* Move objects that are reachable from finalizers, from the unreachable set
+ * into finalizers set.
+ */
+static void
+move_legacy_finalizer_reachable(PyGC_Head *finalizers)
+{
+    traverseproc traverse;
+    PyGC_Head *gc = GC_NEXT(finalizers);
+    for (; gc != finalizers; gc = GC_NEXT(gc)) {
+        /* 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 = GC_NEXT(unreachable); gc != unreachable; gc = next) {
+        PyWeakReference **wrlist;
+
+        op = FROM_GC(gc);
+        next = GC_NEXT(gc);
+
+        if (PyWeakref_Check(op)) {
+            /* A weakref inside the unreachable set must be cleared.  If we
+             * allow its callback to execute inside delete_garbage(), it
+             * could expose objects that have tp_clear already called on
+             * them.  Or, it could resurrect unreachable objects.  One way
+             * this can happen is if some container objects do not implement
+             * tp_traverse.  Then, wr_object can be outside the unreachable
+             * set but can be deallocated as a result of breaking the
+             * reference cycle.  If we don't clear the weakref, the callback
+             * will run and potentially cause a crash.  See bpo-38006 for
+             * one example.
+             */
+            _PyWeakref_ClearRef((PyWeakReference *)op);
+        }
+
+        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.
+             */
+            _PyObject_ASSERT((PyObject *)wr, wr->wr_object == op);
+            _PyWeakref_ClearRef(wr);
+            _PyObject_ASSERT((PyObject *)wr, wr->wr_object == Py_None);
+            if (wr->wr_callback == NULL) {
+                /* no callback */
+                continue;
+            }
+
+            /* 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 (gc_is_collecting(AS_GC(wr))) {
+                /* it should already have been cleared above */
+                assert(wr->wr_object == Py_None);
+                continue;
+            }
+
+            /* 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 = (PyGC_Head*)wrcb_to_call._gc_next;
+        op = FROM_GC(gc);
+        _PyObject_ASSERT(op, PyWeakref_Check(op));
+        wr = (PyWeakReference *)op;
+        callback = wr->wr_callback;
+        _PyObject_ASSERT(op, callback != NULL);
+
+        /* copy-paste of weakrefobject.c's handle_callback() */
+        temp = PyObject_CallOneArg(callback, (PyObject *)wr);
+        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_next == (uintptr_t)gc) {
+            /* object is still alive -- move it */
+            gc_list_move(gc, old);
+        }
+        else {
+            ++num_freed;
+        }
+    }
+
+    return num_freed;
+}
+
+static void
+debug_cycle(const char *msg, PyObject *op)
+{
+    PySys_FormatStderr("gc: %s <%s %p>\n",
+                       msg, Py_TYPE(op)->tp_name, op);
+}
+
+/* Handle uncollectable garbage (cycles with tp_del slots, 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.
+ */
+static void
+handle_legacy_finalizers(PyThreadState *tstate,
+                         GCState *gcstate,
+                         PyGC_Head *finalizers, PyGC_Head *old)
+{
+    assert(!_PyErr_Occurred(tstate));
+    assert(gcstate->garbage != NULL);
+
+    PyGC_Head *gc = GC_NEXT(finalizers);
+    for (; gc != finalizers; gc = GC_NEXT(gc)) {
+        PyObject *op = FROM_GC(gc);
+
+        if ((gcstate->debug & DEBUG_SAVEALL) || has_legacy_finalizer(op)) {
+            if (PyList_Append(gcstate->garbage, op) < 0) {
+                _PyErr_Clear(tstate);
+                break;
+            }
+        }
+    }
+
+    gc_list_merge(finalizers, old);
+}
+
+/* Run first-time finalizers (if any) on all the objects in collectable.
+ * Note that this may remove some (or even all) of the objects from the
+ * list, due to refcounts falling to 0.
+ */
+static void
+finalize_garbage(PyThreadState *tstate, PyGC_Head *collectable)
+{
+    destructor finalize;
+    PyGC_Head seen;
+
+    /* While we're going through the loop, `finalize(op)` may cause op, or
+     * other objects, to be reclaimed via refcounts falling to zero.  So
+     * there's little we can rely on about the structure of the input
+     * `collectable` list across iterations.  For safety, we always take the
+     * first object in that list and move it to a temporary `seen` list.
+     * If objects vanish from the `collectable` and `seen` lists we don't
+     * care.
+     */
+    gc_list_init(&seen);
+
+    while (!gc_list_is_empty(collectable)) {
+        PyGC_Head *gc = GC_NEXT(collectable);
+        PyObject *op = FROM_GC(gc);
+        gc_list_move(gc, &seen);
+        if (!_PyGCHead_FINALIZED(gc) &&
+                (finalize = Py_TYPE(op)->tp_finalize) != NULL) {
+            _PyGCHead_SET_FINALIZED(gc);
+            Py_INCREF(op);
+            finalize(op);
+            assert(!_PyErr_Occurred(tstate));
+            Py_DECREF(op);
+        }
+    }
+    gc_list_merge(&seen, collectable);
+}
+
+/* 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(PyThreadState *tstate, GCState *gcstate,
+               PyGC_Head *collectable, PyGC_Head *old)
+{
+    assert(!_PyErr_Occurred(tstate));
+
+    while (!gc_list_is_empty(collectable)) {
+        PyGC_Head *gc = GC_NEXT(collectable);
+        PyObject *op = FROM_GC(gc);
+
+        _PyObject_ASSERT_WITH_MSG(op, Py_REFCNT(op) > 0,
+                                  "refcount is too small");
+
+        if (gcstate->debug & DEBUG_SAVEALL) {
+            assert(gcstate->garbage != NULL);
+            if (PyList_Append(gcstate->garbage, op) < 0) {
+                _PyErr_Clear(tstate);
+            }
+        }
+        else {
+            inquiry clear;
+            if ((clear = Py_TYPE(op)->tp_clear) != NULL) {
+                Py_INCREF(op);
+                (void) clear(op);
+                if (_PyErr_Occurred(tstate)) {
+                    _PyErr_WriteUnraisableMsg("in tp_clear of",
+                                              (PyObject*)Py_TYPE(op));
+                }
+                Py_DECREF(op);
+            }
+        }
+        if (GC_NEXT(collectable) == gc) {
+            /* object is still alive, move it, it may die later */
+            gc_clear_collecting(gc);
+            gc_list_move(gc, old);
+        }
+    }
+}
+
+/* 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(PyInterpreterState *interp)
+{
+    _PyTuple_ClearFreeList(interp);
+    _PyFloat_ClearFreeList(interp);
+    _PyList_ClearFreeList(interp);
+    _PyDict_ClearFreeList(interp);
+    _PyAsyncGen_ClearFreeLists(interp);
+    _PyContext_ClearFreeList(interp);
+}
+
+// Show stats for objects in each generations
+static void
+show_stats_each_generations(GCState *gcstate)
+{
+    char buf[100];
+    size_t pos = 0;
+
+    for (int i = 0; i < NUM_GENERATIONS && pos < sizeof(buf); i++) {
+        pos += PyOS_snprintf(buf+pos, sizeof(buf)-pos,
+                             " %zd",
+                             gc_list_size(GEN_HEAD(gcstate, i)));
+    }
+
+    PySys_FormatStderr(
+        "gc: objects in each generation:%s\n"
+        "gc: objects in permanent generation: %zd\n",
+        buf, gc_list_size(&gcstate->permanent_generation.head));
+}
+
+/* Deduce which objects among "base" are unreachable from outside the list
+   and move them to 'unreachable'. The process consist in the following steps:
+
+1. Copy all reference counts to a different field (gc_prev is used to hold
+   this copy to save memory).
+2. Traverse all objects in "base" and visit all referred objects using
+   "tp_traverse" and for every visited object, subtract 1 to the reference
+   count (the one that we copied in the previous step). After this step, all
+   objects that can be reached directly from outside must have strictly positive
+   reference count, while all unreachable objects must have a count of exactly 0.
+3. Identify all unreachable objects (the ones with 0 reference count) and move
+   them to the "unreachable" list. This step also needs to move back to "base" all
+   objects that were initially marked as unreachable but are referred transitively
+   by the reachable objects (the ones with strictly positive reference count).
+
+Contracts:
+
+    * The "base" has to be a valid list with no mask set.
+
+    * The "unreachable" list must be uninitialized (this function calls
+      gc_list_init over 'unreachable').
+
+IMPORTANT: This function leaves 'unreachable' with the NEXT_MASK_UNREACHABLE
+flag set but it does not clear it to skip unnecessary iteration. Before the
+flag is cleared (for example, by using 'clear_unreachable_mask' function or
+by a call to 'move_legacy_finalizers'), the 'unreachable' list is not a normal
+list and we can not use most gc_list_* functions for it. */
+static inline void
+deduce_unreachable(PyGC_Head *base, PyGC_Head *unreachable) {
+    validate_list(base, collecting_clear_unreachable_clear);
+    /* 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(base);  // gc_prev is used for gc_refs
+    subtract_refs(base);
+
+    /* Leave everything reachable from outside base in base, and move
+     * everything else (in base) 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.  It "sounds slick"
+     * to move the unreachable objects, until you think about it - the reason it
+     * pays isn't actually obvious.
+     *
+     * Suppose we create objects A, B, C in that order.  They appear in the young
+     * generation in the same order.  If B points to A, and C to B, and C is
+     * reachable from outside, then the adjusted refcounts will be 0, 0, and 1
+     * respectively.
+     *
+     * When move_unreachable finds A, A is moved to the unreachable list.  The
+     * same for B when it's first encountered.  Then C is traversed, B is moved
+     * _back_ to the reachable list.  B is eventually traversed, and then A is
+     * moved back to the reachable list.
+     *
+     * So instead of not moving at all, the reachable objects B and A are moved
+     * twice each.  Why is this a win?  A straightforward algorithm to move the
+     * reachable objects instead would move A, B, and C once each.
+     *
+     * The key is that this dance leaves the objects in order C, B, A - it's
+     * reversed from the original order.  On all _subsequent_ scans, none of
+     * them will move.  Since most objects aren't in cycles, this can save an
+     * unbounded number of moves across an unbounded number of later collections.
+     * It can cost more only the first time the chain is scanned.
+     *
+     * Drawback:  move_unreachable is also used to find out what's still trash
+     * after finalizers may resurrect objects.  In _that_ case most unreachable
+     * objects will remain unreachable, so it would be more efficient to move
+     * the reachable objects instead.  But this is a one-time cost, probably not
+     * worth complicating the code to speed just a little.
+     */
+    gc_list_init(unreachable);
+    move_unreachable(base, unreachable);  // gc_prev is pointer again
+    validate_list(base, collecting_clear_unreachable_clear);
+    validate_list(unreachable, collecting_set_unreachable_set);
+}
+
+/* Handle objects that may have resurrected after a call to 'finalize_garbage', moving
+   them to 'old_generation' and placing the rest on 'still_unreachable'.
+
+   Contracts:
+       * After this function 'unreachable' must not be used anymore and 'still_unreachable'
+         will contain the objects that did not resurrect.
+
+       * The "still_unreachable" list must be uninitialized (this function calls
+         gc_list_init over 'still_unreachable').
+
+IMPORTANT: After a call to this function, the 'still_unreachable' set will have the
+PREV_MARK_COLLECTING set, but the objects in this set are going to be removed so
+we can skip the expense of clearing the flag to avoid extra iteration. */
+static inline void
+handle_resurrected_objects(PyGC_Head *unreachable, PyGC_Head* still_unreachable,
+                           PyGC_Head *old_generation)
+{
+    // Remove the PREV_MASK_COLLECTING from unreachable
+    // to prepare it for a new call to 'deduce_unreachable'
+    gc_list_clear_collecting(unreachable);
+
+    // After the call to deduce_unreachable, the 'still_unreachable' set will
+    // have the PREV_MARK_COLLECTING set, but the objects are going to be
+    // removed so we can skip the expense of clearing the flag.
+    PyGC_Head* resurrected = unreachable;
+    deduce_unreachable(resurrected, still_unreachable);
+    clear_unreachable_mask(still_unreachable);
+
+    // Move the resurrected objects to the old generation for future collection.
+    gc_list_merge(resurrected, old_generation);
+}
+
+/* This is the main function.  Read this to understand how the
+ * collection process works. */
+static Py_ssize_t
+gc_collect_main(PyThreadState *tstate, int generation,
+                Py_ssize_t *n_collected, Py_ssize_t *n_uncollectable,
+                int nofail)
+{
+    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;
+    _PyTime_t t1 = 0;   /* initialize to prevent a compiler warning */
+    GCState *gcstate = &tstate->interp->gc;
+
+    // gc_collect_main() must not be called before _PyGC_Init
+    // or after _PyGC_Fini()
+    assert(gcstate->garbage != NULL);
+    assert(!_PyErr_Occurred(tstate));
+
+    if (gcstate->debug & DEBUG_STATS) {
+        PySys_WriteStderr("gc: collecting generation %d...\n", generation);
+        show_stats_each_generations(gcstate);
+        t1 = _PyTime_GetPerfCounter();
+    }
+
+    if (PyDTrace_GC_START_ENABLED())
+        PyDTrace_GC_START(generation);
+
+    /* update collection and allocation counters */
+    if (generation+1 < NUM_GENERATIONS)
+        gcstate->generations[generation+1].count += 1;
+    for (i = 0; i <= generation; i++)
+        gcstate->generations[i].count = 0;
+
+    /* merge younger generations with one we are currently collecting */
+    for (i = 0; i < generation; i++) {
+        gc_list_merge(GEN_HEAD(gcstate, i), GEN_HEAD(gcstate, generation));
+    }
+
+    /* handy references */
+    young = GEN_HEAD(gcstate, generation);
+    if (generation < NUM_GENERATIONS-1)
+        old = GEN_HEAD(gcstate, generation+1);
+    else
+        old = young;
+    validate_list(old, collecting_clear_unreachable_clear);
+
+    deduce_unreachable(young, &unreachable);
+
+    untrack_tuples(young);
+    /* Move reachable objects to next generation. */
+    if (young != old) {
+        if (generation == NUM_GENERATIONS - 2) {
+            gcstate->long_lived_pending += gc_list_size(young);
+        }
+        gc_list_merge(young, old);
+    }
+    else {
+        /* We only un-track dicts in full collections, to avoid quadratic
+           dict build-up. See issue #14775. */
+        untrack_dicts(young);
+        gcstate->long_lived_pending = 0;
+        gcstate->long_lived_total = gc_list_size(young);
+    }
+
+    /* All objects in unreachable are trash, but objects reachable from
+     * legacy finalizers (e.g. tp_del) can't safely be deleted.
+     */
+    gc_list_init(&finalizers);
+    // NEXT_MASK_UNREACHABLE is cleared here.
+    // After move_legacy_finalizers(), unreachable is normal list.
+    move_legacy_finalizers(&unreachable, &finalizers);
+    /* finalizers contains the unreachable objects with a legacy finalizer;
+     * unreachable objects reachable *from* those are also uncollectable,
+     * and we move those into the finalizers list too.
+     */
+    move_legacy_finalizer_reachable(&finalizers);
+
+    validate_list(&finalizers, collecting_clear_unreachable_clear);
+    validate_list(&unreachable, collecting_set_unreachable_clear);
+
+    /* Print debugging information. */
+    if (gcstate->debug & DEBUG_COLLECTABLE) {
+        for (gc = GC_NEXT(&unreachable); gc != &unreachable; gc = GC_NEXT(gc)) {
+            debug_cycle("collectable", FROM_GC(gc));
+        }
+    }
+
+    /* Clear weakrefs and invoke callbacks as necessary. */
+    m += handle_weakrefs(&unreachable, old);
+
+    validate_list(old, collecting_clear_unreachable_clear);
+    validate_list(&unreachable, collecting_set_unreachable_clear);
+
+    /* Call tp_finalize on objects which have one. */
+    finalize_garbage(tstate, &unreachable);
+
+    /* Handle any objects that may have resurrected after the call
+     * to 'finalize_garbage' and continue the collection with the
+     * objects that are still unreachable */
+    PyGC_Head final_unreachable;
+    handle_resurrected_objects(&unreachable, &final_unreachable, old);
+
+    /* Call tp_clear on objects in the final_unreachable set.  This will cause
+    * the reference cycles to be broken.  It may also cause some objects
+    * in finalizers to be freed.
+    */
+    m += gc_list_size(&final_unreachable);
+    delete_garbage(tstate, gcstate, &final_unreachable, old);
+
+    /* Collect statistics on uncollectable objects found and print
+     * debugging information. */
+    for (gc = GC_NEXT(&finalizers); gc != &finalizers; gc = GC_NEXT(gc)) {
+        n++;
+        if (gcstate->debug & DEBUG_UNCOLLECTABLE)
+            debug_cycle("uncollectable", FROM_GC(gc));
+    }
+    if (gcstate->debug & DEBUG_STATS) {
+        double d = _PyTime_AsSecondsDouble(_PyTime_GetPerfCounter() - t1);
+        PySys_WriteStderr(
+            "gc: done, %zd unreachable, %zd uncollectable, %.4fs elapsed\n",
+            n+m, n, d);
+    }
+
+    /* 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_legacy_finalizers(tstate, gcstate, &finalizers, old);
+    validate_list(old, collecting_clear_unreachable_clear);
+
+    /* Clear free list only during the collection of the highest
+     * generation */
+    if (generation == NUM_GENERATIONS-1) {
+        clear_freelists(tstate->interp);
+    }
+
+    if (_PyErr_Occurred(tstate)) {
+        if (nofail) {
+            _PyErr_Clear(tstate);
+        }
+        else {
+            _PyErr_WriteUnraisableMsg("in garbage collection", NULL);
+        }
+    }
+
+    /* Update stats */
+    if (n_collected) {
+        *n_collected = m;
+    }
+    if (n_uncollectable) {
+        *n_uncollectable = n;
+    }
+
+    struct gc_generation_stats *stats = &gcstate->generation_stats[generation];
+    stats->collections++;
+    stats->collected += m;
+    stats->uncollectable += n;
+
+    if (PyDTrace_GC_DONE_ENABLED()) {
+        PyDTrace_GC_DONE(n + m);
+    }
+
+    assert(!_PyErr_Occurred(tstate));
+    return n + m;
+}
+
+/* Invoke progress callbacks to notify clients that garbage collection
+ * is starting or stopping
+ */
+static void
+invoke_gc_callback(PyThreadState *tstate, const char *phase,
+                   int generation, Py_ssize_t collected,
+                   Py_ssize_t uncollectable)
+{
+    assert(!_PyErr_Occurred(tstate));
+
+    /* we may get called very early */
+    GCState *gcstate = &tstate->interp->gc;
+    if (gcstate->callbacks == NULL) {
+        return;
+    }
+
+    /* The local variable cannot be rebound, check it for sanity */
+    assert(PyList_CheckExact(gcstate->callbacks));
+    PyObject *info = NULL;
+    if (PyList_GET_SIZE(gcstate->callbacks) != 0) {
+        info = Py_BuildValue("{sisnsn}",
+            "generation", generation,
+            "collected", collected,
+            "uncollectable", uncollectable);
+        if (info == NULL) {
+            PyErr_WriteUnraisable(NULL);
+            return;
+        }
+    }
+    for (Py_ssize_t i=0; i<PyList_GET_SIZE(gcstate->callbacks); i++) {
+        PyObject *r, *cb = PyList_GET_ITEM(gcstate->callbacks, i);
+        Py_INCREF(cb); /* make sure cb doesn't go away */
+        r = PyObject_CallFunction(cb, "sO", phase, info);
+        if (r == NULL) {
+            PyErr_WriteUnraisable(cb);
+        }
+        else {
+            Py_DECREF(r);
+        }
+        Py_DECREF(cb);
+    }
+    Py_XDECREF(info);
+    assert(!_PyErr_Occurred(tstate));
+}
+
+/* Perform garbage collection of a generation and invoke
+ * progress callbacks.
+ */
+static Py_ssize_t
+gc_collect_with_callback(PyThreadState *tstate, int generation)
+{
+    assert(!_PyErr_Occurred(tstate));
+    Py_ssize_t result, collected, uncollectable;
+    invoke_gc_callback(tstate, "start", generation, 0, 0);
+    result = gc_collect_main(tstate, generation, &collected, &uncollectable, 0);
+    invoke_gc_callback(tstate, "stop", generation, collected, uncollectable);
+    assert(!_PyErr_Occurred(tstate));
+    return result;
+}
+
+static Py_ssize_t
+gc_collect_generations(PyThreadState *tstate)
+{
+    GCState *gcstate = &tstate->interp->gc;
+    /* 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. */
+    Py_ssize_t n = 0;
+    for (int i = NUM_GENERATIONS-1; i >= 0; i--) {
+        if (gcstate->generations[i].count > gcstate->generations[i].threshold) {
+            /* Avoid quadratic performance degradation in number
+               of tracked objects (see also issue #4074):
+
+               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
+            */
+            if (i == NUM_GENERATIONS - 1
+                && gcstate->long_lived_pending < gcstate->long_lived_total / 4)
+                continue;
+            n = gc_collect_with_callback(tstate, i);
+            break;
+        }
+    }
+    return n;
+}
+
+#include "clinic/gcmodule.c.h"
+
+/*[clinic input]
+gc.enable
+
+Enable automatic garbage collection.
+[clinic start generated code]*/
+
+static PyObject *
+gc_enable_impl(PyObject *module)
+/*[clinic end generated code: output=45a427e9dce9155c input=81ac4940ca579707]*/
+{
+    PyGC_Enable();
+    Py_RETURN_NONE;
+}
+
+/*[clinic input]
+gc.disable
+
+Disable automatic garbage collection.
+[clinic start generated code]*/
+
+static PyObject *
+gc_disable_impl(PyObject *module)
+/*[clinic end generated code: output=97d1030f7aa9d279 input=8c2e5a14e800d83b]*/
+{
+    PyGC_Disable();
+    Py_RETURN_NONE;
+}
+
+/*[clinic input]
+gc.isenabled -> bool
+
+Returns true if automatic garbage collection is enabled.
+[clinic start generated code]*/
+
+static int
+gc_isenabled_impl(PyObject *module)
+/*[clinic end generated code: output=1874298331c49130 input=30005e0422373b31]*/
+{
+    return PyGC_IsEnabled();
+}
+
+/*[clinic input]
+gc.collect -> Py_ssize_t
+
+    generation: int(c_default="NUM_GENERATIONS - 1") = 2
+
+Run the garbage collector.
+
+With no arguments, run a full collection.  The optional argument
+may be an integer specifying which generation to collect.  A ValueError
+is raised if the generation number is invalid.
+
+The number of unreachable objects is returned.
+[clinic start generated code]*/
+
+static Py_ssize_t
+gc_collect_impl(PyObject *module, int generation)
+/*[clinic end generated code: output=b697e633043233c7 input=40720128b682d879]*/
+{
+    PyThreadState *tstate = _PyThreadState_GET();
+
+    if (generation < 0 || generation >= NUM_GENERATIONS) {
+        _PyErr_SetString(tstate, PyExc_ValueError, "invalid generation");
+        return -1;
+    }
+
+    GCState *gcstate = &tstate->interp->gc;
+    Py_ssize_t n;
+    if (gcstate->collecting) {
+        /* already collecting, don't do anything */
+        n = 0;
+    }
+    else {
+        gcstate->collecting = 1;
+        n = gc_collect_with_callback(tstate, generation);
+        gcstate->collecting = 0;
+    }
+    return n;
+}
+
+/*[clinic input]
+gc.set_debug
+
+    flags: int
+        An integer that can have the following bits turned on:
+          DEBUG_STATS - Print statistics during collection.
+          DEBUG_COLLECTABLE - Print collectable objects found.
+          DEBUG_UNCOLLECTABLE - Print unreachable but uncollectable objects
+            found.
+          DEBUG_SAVEALL - Save objects to gc.garbage rather than freeing them.
+          DEBUG_LEAK - Debug leaking programs (everything but STATS).
+    /
+
+Set the garbage collection debugging flags.
+
+Debugging information is written to sys.stderr.
+[clinic start generated code]*/
+
+static PyObject *
+gc_set_debug_impl(PyObject *module, int flags)
+/*[clinic end generated code: output=7c8366575486b228 input=5e5ce15e84fbed15]*/
+{
+    GCState *gcstate = get_gc_state();
+    gcstate->debug = flags;
+    Py_RETURN_NONE;
+}
+
+/*[clinic input]
+gc.get_debug -> int
+
+Get the garbage collection debugging flags.
+[clinic start generated code]*/
+
+static int
+gc_get_debug_impl(PyObject *module)
+/*[clinic end generated code: output=91242f3506cd1e50 input=91a101e1c3b98366]*/
+{
+    GCState *gcstate = get_gc_state();
+    return gcstate->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_threshold(PyObject *self, PyObject *args)
+{
+    GCState *gcstate = get_gc_state();
+    if (!PyArg_ParseTuple(args, "i|ii:set_threshold",
+                          &gcstate->generations[0].threshold,
+                          &gcstate->generations[1].threshold,
+                          &gcstate->generations[2].threshold))
+        return NULL;
+    for (int i = 3; i < NUM_GENERATIONS; i++) {
+        /* generations higher than 2 get the same threshold */
+        gcstate->generations[i].threshold = gcstate->generations[2].threshold;
+    }
+    Py_RETURN_NONE;
+}
+
+/*[clinic input]
+gc.get_threshold
+
+Return the current collection thresholds.
+[clinic start generated code]*/
+
+static PyObject *
+gc_get_threshold_impl(PyObject *module)
+/*[clinic end generated code: output=7902bc9f41ecbbd8 input=286d79918034d6e6]*/
+{
+    GCState *gcstate = get_gc_state();
+    return Py_BuildValue("(iii)",
+                         gcstate->generations[0].threshold,
+                         gcstate->generations[1].threshold,
+                         gcstate->generations[2].threshold);
+}
+
+/*[clinic input]
+gc.get_count
+
+Return a three-tuple of the current collection counts.
+[clinic start generated code]*/
+
+static PyObject *
+gc_get_count_impl(PyObject *module)
+/*[clinic end generated code: output=354012e67b16398f input=a392794a08251751]*/
+{
+    GCState *gcstate = get_gc_state();
+    return Py_BuildValue("(iii)",
+                         gcstate->generations[0].count,
+                         gcstate->generations[1].count,
+                         gcstate->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 = GC_NEXT(list); gc != list; gc = GC_NEXT(gc)) {
+        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)
+{
+    if (PySys_Audit("gc.get_referrers", "(O)", args) < 0) {
+        return NULL;
+    }
+
+    PyObject *result = PyList_New(0);
+    if (!result) {
+        return NULL;
+    }
+
+    GCState *gcstate = get_gc_state();
+    for (int i = 0; i < NUM_GENERATIONS; i++) {
+        if (!(gc_referrers_for(args, GEN_HEAD(gcstate, 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;
+    if (PySys_Audit("gc.get_referents", "(O)", args) < 0) {
+        return NULL;
+    }
+    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;
+}
+
+/*[clinic input]
+gc.get_objects
+    generation: Py_ssize_t(accept={int, NoneType}, c_default="-1") = None
+        Generation to extract the objects from.
+
+Return a list of objects tracked by the collector (excluding the list returned).
+
+If generation is not None, return only the objects tracked by the collector
+that are in that generation.
+[clinic start generated code]*/
+
+static PyObject *
+gc_get_objects_impl(PyObject *module, Py_ssize_t generation)
+/*[clinic end generated code: output=48b35fea4ba6cb0e input=ef7da9df9806754c]*/
+{
+    PyThreadState *tstate = _PyThreadState_GET();
+    int i;
+    PyObject* result;
+    GCState *gcstate = &tstate->interp->gc;
+
+    if (PySys_Audit("gc.get_objects", "n", generation) < 0) {
+        return NULL;
+    }
+
+    result = PyList_New(0);
+    if (result == NULL) {
+        return NULL;
+    }
+
+    /* If generation is passed, we extract only that generation */
+    if (generation != -1) {
+        if (generation >= NUM_GENERATIONS) {
+            _PyErr_Format(tstate, PyExc_ValueError,
+                          "generation parameter must be less than the number of "
+                          "available generations (%i)",
+                           NUM_GENERATIONS);
+            goto error;
+        }
+
+        if (generation < 0) {
+            _PyErr_SetString(tstate, PyExc_ValueError,
+                             "generation parameter cannot be negative");
+            goto error;
+        }
+
+        if (append_objects(result, GEN_HEAD(gcstate, generation))) {
+            goto error;
+        }
+
+        return result;
+    }
+
+    /* If generation is not passed or None, get all objects from all generations */
+    for (i = 0; i < NUM_GENERATIONS; i++) {
+        if (append_objects(result, GEN_HEAD(gcstate, i))) {
+            goto error;
+        }
+    }
+    return result;
+
+error:
+    Py_DECREF(result);
+    return NULL;
+}
+
+/*[clinic input]
+gc.get_stats
+
+Return a list of dictionaries containing per-generation statistics.
+[clinic start generated code]*/
+
+static PyObject *
+gc_get_stats_impl(PyObject *module)
+/*[clinic end generated code: output=a8ab1d8a5d26f3ab input=1ef4ed9d17b1a470]*/
+{
+    int i;
+    struct gc_generation_stats stats[NUM_GENERATIONS], *st;
+
+    /* To get consistent values despite allocations while constructing
+       the result list, we use a snapshot of the running stats. */
+    GCState *gcstate = get_gc_state();
+    for (i = 0; i < NUM_GENERATIONS; i++) {
+        stats[i] = gcstate->generation_stats[i];
+    }
+
+    PyObject *result = PyList_New(0);
+    if (result == NULL)
+        return NULL;
+
+    for (i = 0; i < NUM_GENERATIONS; i++) {
+        PyObject *dict;
+        st = &stats[i];
+        dict = Py_BuildValue("{snsnsn}",
+                             "collections", st->collections,
+                             "collected", st->collected,
+                             "uncollectable", st->uncollectable
+                            );
+        if (dict == NULL)
+            goto error;
+        if (PyList_Append(result, dict)) {
+            Py_DECREF(dict);
+            goto error;
+        }
+        Py_DECREF(dict);
+    }
+    return result;
+
+error:
+    Py_XDECREF(result);
+    return NULL;
+}
+
+
+/*[clinic input]
+gc.is_tracked
+
+    obj: object
+    /
+
+Returns true if the object is tracked by the garbage collector.
+
+Simple atomic objects will return false.
+[clinic start generated code]*/
+
+static PyObject *
+gc_is_tracked(PyObject *module, PyObject *obj)
+/*[clinic end generated code: output=14f0103423b28e31 input=d83057f170ea2723]*/
+{
+    PyObject *result;
+
+    if (_PyObject_IS_GC(obj) && _PyObject_GC_IS_TRACKED(obj))
+        result = Py_True;
+    else
+        result = Py_False;
+    Py_INCREF(result);
+    return result;
+}
+
+/*[clinic input]
+gc.is_finalized
+
+    obj: object
+    /
+
+Returns true if the object has been already finalized by the GC.
+[clinic start generated code]*/
+
+static PyObject *
+gc_is_finalized(PyObject *module, PyObject *obj)
+/*[clinic end generated code: output=e1516ac119a918ed input=201d0c58f69ae390]*/
+{
+    if (_PyObject_IS_GC(obj) && _PyGCHead_FINALIZED(AS_GC(obj))) {
+         Py_RETURN_TRUE;
+    }
+    Py_RETURN_FALSE;
+}
+
+/*[clinic input]
+gc.freeze
+
+Freeze all current tracked objects and ignore them for future collections.
+
+This can be used before a POSIX fork() call to make the gc copy-on-write friendly.
+Note: collection before a POSIX fork() call may free pages for future allocation
+which can cause copy-on-write.
+[clinic start generated code]*/
+
+static PyObject *
+gc_freeze_impl(PyObject *module)
+/*[clinic end generated code: output=502159d9cdc4c139 input=b602b16ac5febbe5]*/
+{
+    GCState *gcstate = get_gc_state();
+    for (int i = 0; i < NUM_GENERATIONS; ++i) {
+        gc_list_merge(GEN_HEAD(gcstate, i), &gcstate->permanent_generation.head);
+        gcstate->generations[i].count = 0;
+    }
+    Py_RETURN_NONE;
+}
+
+/*[clinic input]
+gc.unfreeze
+
+Unfreeze all objects in the permanent generation.
+
+Put all objects in the permanent generation back into oldest generation.
+[clinic start generated code]*/
+
+static PyObject *
+gc_unfreeze_impl(PyObject *module)
+/*[clinic end generated code: output=1c15f2043b25e169 input=2dd52b170f4cef6c]*/
+{
+    GCState *gcstate = get_gc_state();
+    gc_list_merge(&gcstate->permanent_generation.head,
+                  GEN_HEAD(gcstate, NUM_GENERATIONS-1));
+    Py_RETURN_NONE;
+}
+
+/*[clinic input]
+gc.get_freeze_count -> Py_ssize_t
+
+Return the number of objects in the permanent generation.
+[clinic start generated code]*/
+
+static Py_ssize_t
+gc_get_freeze_count_impl(PyObject *module)
+/*[clinic end generated code: output=61cbd9f43aa032e1 input=45ffbc65cfe2a6ed]*/
+{
+    GCState *gcstate = get_gc_state();
+    return gc_list_size(&gcstate->permanent_generation.head);
+}
+
+
+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"
+"get_stats() -- Return list of dictionaries containing per-generation stats.\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"
+"is_finalized() -- Returns true if a given object has been already finalized.\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"
+"freeze() -- Freeze all tracked objects and ignore them for future collections.\n"
+"unfreeze() -- Unfreeze all objects in the permanent generation.\n"
+"get_freeze_count() -- Return the number of objects in the permanent generation.\n");
+
+static PyMethodDef GcMethods[] = {
+    GC_ENABLE_METHODDEF
+    GC_DISABLE_METHODDEF
+    GC_ISENABLED_METHODDEF
+    GC_SET_DEBUG_METHODDEF
+    GC_GET_DEBUG_METHODDEF
+    GC_GET_COUNT_METHODDEF
+    {"set_threshold",  gc_set_threshold, METH_VARARGS, gc_set_thresh__doc__},
+    GC_GET_THRESHOLD_METHODDEF
+    GC_COLLECT_METHODDEF
+    GC_GET_OBJECTS_METHODDEF
+    GC_GET_STATS_METHODDEF
+    GC_IS_TRACKED_METHODDEF
+    GC_IS_FINALIZED_METHODDEF
+    {"get_referrers",  gc_get_referrers, METH_VARARGS,
+        gc_get_referrers__doc__},
+    {"get_referents",  gc_get_referents, METH_VARARGS,
+        gc_get_referents__doc__},
+    GC_FREEZE_METHODDEF
+    GC_UNFREEZE_METHODDEF
+    GC_GET_FREEZE_COUNT_METHODDEF
+    {NULL,      NULL}           /* Sentinel */
+};
+
+static int
+gcmodule_exec(PyObject *module)
+{
+    GCState *gcstate = get_gc_state();
+
+    /* garbage and callbacks are initialized by _PyGC_Init() early in
+     * interpreter lifecycle. */
+    assert(gcstate->garbage != NULL);
+    if (PyModule_AddObjectRef(module, "garbage", gcstate->garbage) < 0) {
+        return -1;
+    }
+    assert(gcstate->callbacks != NULL);
+    if (PyModule_AddObjectRef(module, "callbacks", gcstate->callbacks) < 0) {
+        return -1;
+    }
+
+#define ADD_INT(NAME) if (PyModule_AddIntConstant(module, #NAME, NAME) < 0) { return -1; }
+    ADD_INT(DEBUG_STATS);
+    ADD_INT(DEBUG_COLLECTABLE);
+    ADD_INT(DEBUG_UNCOLLECTABLE);
+    ADD_INT(DEBUG_SAVEALL);
+    ADD_INT(DEBUG_LEAK);
+#undef ADD_INT
+    return 0;
+}
+
+static PyModuleDef_Slot gcmodule_slots[] = {
+    {Py_mod_exec, gcmodule_exec},
+    {0, NULL}
+};
+
+static struct PyModuleDef gcmodule = {
+    PyModuleDef_HEAD_INIT,
+    .m_name = "gc",
+    .m_doc = gc__doc__,
+    .m_size = 0,  // per interpreter state, see: get_gc_state()
+    .m_methods = GcMethods,
+    .m_slots = gcmodule_slots
+};
+
+PyMODINIT_FUNC
+PyInit_gc(void)
+{
+    return PyModuleDef_Init(&gcmodule);
+}
+
+/* C API for controlling the state of the garbage collector */
+int
+PyGC_Enable(void)
+{
+    GCState *gcstate = get_gc_state();
+    int old_state = gcstate->enabled;
+    gcstate->enabled = 1;
+    return old_state;
+}
+
+int
+PyGC_Disable(void)
+{
+    GCState *gcstate = get_gc_state();
+    int old_state = gcstate->enabled;
+    gcstate->enabled = 0;
+    return old_state;
+}
+
+int
+PyGC_IsEnabled(void)
+{
+    GCState *gcstate = get_gc_state();
+    return gcstate->enabled;
+}
+
+/* Public API to invoke gc.collect() from C */
+Py_ssize_t
+PyGC_Collect(void)
+{
+    PyThreadState *tstate = _PyThreadState_GET();
+    GCState *gcstate = &tstate->interp->gc;
+
+    if (!gcstate->enabled) {
+        return 0;
+    }
+
+    Py_ssize_t n;
+    if (gcstate->collecting) {
+        /* already collecting, don't do anything */
+        n = 0;
+    }
+    else {
+        PyObject *exc, *value, *tb;
+        gcstate->collecting = 1;
+        _PyErr_Fetch(tstate, &exc, &value, &tb);
+        n = gc_collect_with_callback(tstate, NUM_GENERATIONS - 1);
+        _PyErr_Restore(tstate, exc, value, tb);
+        gcstate->collecting = 0;
+    }
+
+    return n;
+}
+
+Py_ssize_t
+_PyGC_CollectNoFail(PyThreadState *tstate)
+{
+    /* Ideally, this function is only called on interpreter shutdown,
+       and therefore not recursively.  Unfortunately, when there are daemon
+       threads, a daemon thread can start a cyclic garbage collection
+       during interpreter shutdown (and then never finish it).
+       See http://bugs.python.org/issue8713#msg195178 for an example.
+       */
+    GCState *gcstate = &tstate->interp->gc;
+    if (gcstate->collecting) {
+        return 0;
+    }
+
+    Py_ssize_t n;
+    gcstate->collecting = 1;
+    n = gc_collect_main(tstate, NUM_GENERATIONS - 1, NULL, NULL, 1);
+    gcstate->collecting = 0;
+    return n;
+}
+
+void
+_PyGC_DumpShutdownStats(PyInterpreterState *interp)
+{
+    GCState *gcstate = &interp->gc;
+    if (!(gcstate->debug & DEBUG_SAVEALL)
+        && gcstate->garbage != NULL && PyList_GET_SIZE(gcstate->garbage) > 0) {
+        const char *message;
+        if (gcstate->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";
+        /* PyErr_WarnFormat does too many things and we are at shutdown,
+           the warnings module's dependencies (e.g. linecache) may be gone
+           already. */
+        if (PyErr_WarnExplicitFormat(PyExc_ResourceWarning, "gc", 0,
+                                     "gc", NULL, message,
+                                     PyList_GET_SIZE(gcstate->garbage)))
+            PyErr_WriteUnraisable(NULL);
+        if (gcstate->debug & DEBUG_UNCOLLECTABLE) {
+            PyObject *repr = NULL, *bytes = NULL;
+            repr = PyObject_Repr(gcstate->garbage);
+            if (!repr || !(bytes = PyUnicode_EncodeFSDefault(repr)))
+                PyErr_WriteUnraisable(gcstate->garbage);
+            else {
+                PySys_WriteStderr(
+                    "      %s\n",
+                    PyBytes_AS_STRING(bytes)
+                    );
+            }
+            Py_XDECREF(repr);
+            Py_XDECREF(bytes);
+        }
+    }
+}
+
+
+static void
+gc_fini_untrack(PyGC_Head *list)
+{
+    PyGC_Head *gc;
+    for (gc = GC_NEXT(list); gc != list; gc = GC_NEXT(list)) {
+        PyObject *op = FROM_GC(gc);
+        _PyObject_GC_UNTRACK(op);
+        // gh-92036: If a deallocator function expect the object to be tracked
+        // by the GC (ex: func_dealloc()), it can crash if called on an object
+        // which is no longer tracked by the GC. Leak one strong reference on
+        // purpose so the object is never deleted and its deallocator is not
+        // called.
+        Py_INCREF(op);
+    }
+}
+
+
+void
+_PyGC_Fini(PyInterpreterState *interp)
+{
+    GCState *gcstate = &interp->gc;
+    Py_CLEAR(gcstate->garbage);
+    Py_CLEAR(gcstate->callbacks);
+
+    if (!_Py_IsMainInterpreter(interp)) {
+        // bpo-46070: Explicitly untrack all objects currently tracked by the
+        // GC. Otherwise, if an object is used later by another interpreter,
+        // calling PyObject_GC_UnTrack() on the object crashs if the previous
+        // or the next object of the PyGC_Head structure became a dangling
+        // pointer.
+        for (int i = 0; i < NUM_GENERATIONS; i++) {
+            PyGC_Head *gen = GEN_HEAD(gcstate, i);
+            gc_fini_untrack(gen);
+        }
+    }
+}
+
+/* for debugging */
+void
+_PyGC_Dump(PyGC_Head *g)
+{
+    _PyObject_Dump(FROM_GC(g));
+}
+
+
+#ifdef Py_DEBUG
+static int
+visit_validate(PyObject *op, void *parent_raw)
+{
+    PyObject *parent = _PyObject_CAST(parent_raw);
+    if (_PyObject_IsFreed(op)) {
+        _PyObject_ASSERT_FAILED_MSG(parent,
+                                    "PyObject_GC_Track() object is not valid");
+    }
+    return 0;
+}
+#endif
+
+
+/* extension modules might be compiled with GC support so these
+   functions must always be available */
+
+void
+PyObject_GC_Track(void *op_raw)
+{
+    PyObject *op = _PyObject_CAST(op_raw);
+    if (_PyObject_GC_IS_TRACKED(op)) {
+        _PyObject_ASSERT_FAILED_MSG(op,
+                                    "object already tracked "
+                                    "by the garbage collector");
+    }
+    _PyObject_GC_TRACK(op);
+
+#ifdef Py_DEBUG
+    /* Check that the object is valid: validate objects traversed
+       by tp_traverse() */
+    traverseproc traverse = Py_TYPE(op)->tp_traverse;
+    (void)traverse(op, visit_validate, op);
+#endif
+}
+
+void
+PyObject_GC_UnTrack(void *op_raw)
+{
+    PyObject *op = _PyObject_CAST(op_raw);
+    /* Obscure:  the Py_TRASHCAN mechanism requires that we be able to
+     * call PyObject_GC_UnTrack twice on an object.
+     */
+    if (_PyObject_GC_IS_TRACKED(op)) {
+        _PyObject_GC_UNTRACK(op);
+    }
+}
+
+int
+PyObject_IS_GC(PyObject *obj)
+{
+    return _PyObject_IS_GC(obj);
+}
+
+void
+_PyObject_GC_Link(PyObject *op)
+{
+    PyGC_Head *g = AS_GC(op);
+    assert(((uintptr_t)g & (sizeof(uintptr_t)-1)) == 0);  // g must be correctly aligned
+
+    PyThreadState *tstate = _PyThreadState_GET();
+    GCState *gcstate = &tstate->interp->gc;
+    g->_gc_next = 0;
+    g->_gc_prev = 0;
+    gcstate->generations[0].count++; /* number of allocated GC objects */
+    if (gcstate->generations[0].count > gcstate->generations[0].threshold &&
+        gcstate->enabled &&
+        gcstate->generations[0].threshold &&
+        !gcstate->collecting &&
+        !_PyErr_Occurred(tstate))
+    {
+        gcstate->collecting = 1;
+        gc_collect_generations(tstate);
+        gcstate->collecting = 0;
+    }
+}
+
+static PyObject *
+gc_alloc(size_t basicsize, size_t presize)
+{
+    PyThreadState *tstate = _PyThreadState_GET();
+    if (basicsize > PY_SSIZE_T_MAX - presize) {
+        return _PyErr_NoMemory(tstate);
+    }
+    size_t size = presize + basicsize;
+    char *mem = PyObject_Malloc(size);
+    if (mem == NULL) {
+        return _PyErr_NoMemory(tstate);
+    }
+    ((PyObject **)mem)[0] = NULL;
+    ((PyObject **)mem)[1] = NULL;
+    PyObject *op = (PyObject *)(mem + presize);
+    _PyObject_GC_Link(op);
+    return op;
+}
+
+PyObject *
+_PyObject_GC_New(PyTypeObject *tp)
+{
+    size_t presize = _PyType_PreHeaderSize(tp);
+    PyObject *op = gc_alloc(_PyObject_SIZE(tp), presize);
+    if (op == NULL) {
+        return NULL;
+    }
+    _PyObject_Init(op, tp);
+    return op;
+}
+
+PyVarObject *
+_PyObject_GC_NewVar(PyTypeObject *tp, Py_ssize_t nitems)
+{
+    size_t size;
+    PyVarObject *op;
+
+    if (nitems < 0) {
+        PyErr_BadInternalCall();
+        return NULL;
+    }
+    size_t presize = _PyType_PreHeaderSize(tp);
+    size = _PyObject_VAR_SIZE(tp, nitems);
+    op = (PyVarObject *)gc_alloc(size, presize);
+    if (op == NULL) {
+        return NULL;
+    }
+    _PyObject_InitVar(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);
+    _PyObject_ASSERT((PyObject *)op, !_PyObject_GC_IS_TRACKED(op));
+    if (basicsize > PY_SSIZE_T_MAX - sizeof(PyGC_Head)) {
+        return (PyVarObject *)PyErr_NoMemory();
+    }
+
+    PyGC_Head *g = AS_GC(op);
+    g = (PyGC_Head *)PyObject_Realloc(g,  sizeof(PyGC_Head) + basicsize);
+    if (g == NULL)
+        return (PyVarObject *)PyErr_NoMemory();
+    op = (PyVarObject *) FROM_GC(g);
+    Py_SET_SIZE(op, nitems);
+    return op;
+}
+
+void
+PyObject_GC_Del(void *op)
+{
+    size_t presize = _PyType_PreHeaderSize(((PyObject *)op)->ob_type);
+    PyGC_Head *g = AS_GC(op);
+    if (_PyObject_GC_IS_TRACKED(op)) {
+#ifdef Py_DEBUG
+        if (PyErr_WarnExplicitFormat(PyExc_ResourceWarning, "gc", 0,
+                                     "gc", NULL, "Object of type %s is not untracked before destruction",
+                                     ((PyObject*)op)->ob_type->tp_name)) {
+            PyErr_WriteUnraisable(NULL);
+        }
+#endif
+        gc_list_remove(g);
+    }
+    GCState *gcstate = get_gc_state();
+    if (gcstate->generations[0].count > 0) {
+        gcstate->generations[0].count--;
+    }
+    PyObject_Free(((char *)op)-presize);
+}
+
+int
+PyObject_GC_IsTracked(PyObject* obj)
+{
+    if (_PyObject_IS_GC(obj) && _PyObject_GC_IS_TRACKED(obj)) {
+        return 1;
+    }
+    return 0;
+}
+
+int
+PyObject_GC_IsFinalized(PyObject *obj)
+{
+    if (_PyObject_IS_GC(obj) && _PyGCHead_FINALIZED(AS_GC(obj))) {
+         return 1;
+    }
+    return 0;
+}