cpython/Modules/gcmodule.c

/*

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

  Neil Schemenauer <nas@arctrix.com>

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

  http://www.arctrix.com/nas/python/gc/

  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",
    gc__doc__,
    -1,
    GcMethods,
    NULL,
    NULL,
    NULL,
    NULL
};


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;
}

/* 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);
}

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