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gcmodule.c
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/*
Reference Cycle Garbage Collection
==================================
Neil Schemenauer <[email protected]>
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 "internal/mem.h"
#include "internal/pystate.h"
#include "frameobject.h" /* for PyFrame_ClearFreeList */
#include "pydtrace.h"
#include "pytime.h" /* for _PyTime_GetMonotonicClock() */
/*[clinic input]
module gc
[clinic start generated code]*/
/*[clinic end generated code: output=da39a3ee5e6b4b0d input=b5c9690ecc842d79]*/
/* 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))
/* Python string to use if unhandled exception occurs */
static PyObject *gc_str = NULL;
/* set for debugging information */
#define DEBUG_STATS (1<<0) /* print collection statistics */
#define DEBUG_COLLECTABLE (1<<1) /* print collectable objects */
#define DEBUG_UNCOLLECTABLE (1<<2) /* print uncollectable objects */
#define DEBUG_SAVEALL (1<<5) /* save all garbage in gc.garbage */
#define DEBUG_LEAK DEBUG_COLLECTABLE | \
DEBUG_UNCOLLECTABLE | \
DEBUG_SAVEALL
#define GEN_HEAD(n) (&_PyRuntime.gc.generations[n].head)
void
_PyGC_Initialize(struct _gc_runtime_state *state)
{
state->enabled = 1; /* automatic collection enabled? */
#define _GEN_HEAD(n) (&state->generations[n].head)
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},
};
for (int i = 0; i < NUM_GENERATIONS; i++) {
state->generations[i] = generations[i];
};
state->generation0 = GEN_HEAD(0);
struct gc_generation permanent_generation = {
{{&state->permanent_generation.head, &state->permanent_generation.head, 0}}, 0, 0
};
state->permanent_generation = permanent_generation;
}
/*--------------------------------------------------------------------------
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) (_PyGC_REFS(o) != GC_UNTRACKED)
#define IS_REACHABLE(o) (_PyGC_REFS(o) == GC_REACHABLE)
#define IS_TENTATIVELY_UNREACHABLE(o) ( \
_PyGC_REFS(o) == 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(_PyGCHead_REFS(gc) == GC_REACHABLE);
_PyGCHead_SET_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?
*/
assert(_PyGCHead_REFS(gc) != 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(_PyGCHead_REFS(gc) != 0); /* else refcount was too small */
if (_PyGCHead_REFS(gc) > 0)
_PyGCHead_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 = 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 = _PyGCHead_REFS(gc);
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.
*/
_PyGCHead_SET_REFS(gc, 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);
_PyGCHead_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.
* 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 (_PyGCHead_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;
assert(_PyGCHead_REFS(gc) > 0);
_PyGCHead_SET_REFS(gc, GC_REACHABLE);
(void) traverse(op,
(visitproc)visit_reachable,
(void *)young);
next = gc->gc.gc_next;
if (PyTuple_CheckExact(op)) {
_PyTuple_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);
_PyGCHead_SET_REFS(gc, GC_TENTATIVELY_UNREACHABLE);
}
gc = next;
}
}
/* Try to untrack all currently tracked dictionaries */
static void
untrack_dicts(PyGC_Head *head)
{
PyGC_Head *next, *gc = head->gc.gc_next;
while (gc != head) {
PyObject *op = FROM_GC(gc);
next = gc->gc.gc_next;
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 op->ob_type->tp_del != NULL;
}
/* Move the objects in unreachable with tp_del slots 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_legacy_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_legacy_finalizer(op)) {
gc_list_move(gc, finalizers);
_PyGCHead_SET_REFS(gc, GC_REACHABLE);
}
}
}
/* A traversal callback for move_legacy_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);
_PyGCHead_SET_REFS(gc, GC_REACHABLE);
}
}
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 = 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(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.
* 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_legacy_finalizers(PyGC_Head *finalizers, PyGC_Head *old)
{
PyGC_Head *gc = finalizers->gc.gc_next;
if (_PyRuntime.gc.garbage == NULL) {
_PyRuntime.gc.garbage = PyList_New(0);
if (_PyRuntime.gc.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 ((_PyRuntime.gc.debug & DEBUG_SAVEALL) || has_legacy_finalizer(op)) {
if (PyList_Append(_PyRuntime.gc.garbage, op) < 0)
return -1;
}
}
gc_list_merge(finalizers, old);
return 0;
}
/* 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(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 = collectable->gc.gc_next;
PyObject *op = FROM_GC(gc);
gc_list_move(gc, &seen);
if (!_PyGCHead_FINALIZED(gc) &&
PyType_HasFeature(Py_TYPE(op), Py_TPFLAGS_HAVE_FINALIZE) &&
(finalize = Py_TYPE(op)->tp_finalize) != NULL) {
_PyGCHead_SET_FINALIZED(gc, 1);
Py_INCREF(op);
finalize(op);
Py_DECREF(op);
}
}
gc_list_merge(&seen, collectable);
}
/* Walk the collectable list and check that they are really unreachable
from the outside (some objects could have been resurrected by a
finalizer). */
static int
check_garbage(PyGC_Head *collectable)
{
PyGC_Head *gc;
for (gc = collectable->gc.gc_next; gc != collectable;
gc = gc->gc.gc_next) {
_PyGCHead_SET_REFS(gc, Py_REFCNT(FROM_GC(gc)));
assert(_PyGCHead_REFS(gc) != 0);
}
subtract_refs(collectable);
for (gc = collectable->gc.gc_next; gc != collectable;
gc = gc->gc.gc_next) {
assert(_PyGCHead_REFS(gc) >= 0);
if (_PyGCHead_REFS(gc) != 0)
return -1;
}
return 0;
}
static void
revive_garbage(PyGC_Head *collectable)
{
PyGC_Head *gc;
for (gc = collectable->gc.gc_next; gc != collectable;
gc = gc->gc.gc_next) {
_PyGCHead_SET_REFS(gc, GC_REACHABLE);
}
}
/* 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);
if (_PyRuntime.gc.debug & DEBUG_SAVEALL) {
PyList_Append(_PyRuntime.gc.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);
_PyGCHead_SET_REFS(gc, 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();
(void)PyList_ClearFreeList();
(void)PyDict_ClearFreeList();
(void)PySet_ClearFreeList();
(void)PyAsyncGen_ClearFreeLists();
}
/* This is the main function. Read this to understand how the
* collection process works. */
static Py_ssize_t
collect(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 */
struct gc_generation_stats *stats = &_PyRuntime.gc.generation_stats[generation];
if (_PyRuntime.gc.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_FormatStderr(" %zd",
gc_list_size(GEN_HEAD(i)));
PySys_WriteStderr("\ngc: objects in permanent generation: %zd",
gc_list_size(&_PyRuntime.gc.permanent_generation.head));
t1 = _PyTime_GetMonotonicClock();
PySys_WriteStderr("\n");
}
if (PyDTrace_GC_START_ENABLED())
PyDTrace_GC_START(generation);
/* update collection and allocation counters */
if (generation+1 < NUM_GENERATIONS)
_PyRuntime.gc.generations[generation+1].count += 1;
for (i = 0; i <= generation; i++)
_PyRuntime.gc.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) {
_PyRuntime.gc.long_lived_pending += gc_list_size(young);
}
gc_list_merge(young, old);
}
else {
/* We only untrack dicts in full collections, to avoid quadratic
dict build-up. See issue #14775. */
untrack_dicts(young);
_PyRuntime.gc.long_lived_pending = 0;
_PyRuntime.gc.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);
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);
/* 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 (_PyRuntime.gc.debug & DEBUG_COLLECTABLE) {
debug_cycle("collectable", FROM_GC(gc));
}
}
/* Clear weakrefs and invoke callbacks as necessary. */
m += handle_weakrefs(&unreachable, old);
/* Call tp_finalize on objects which have one. */
finalize_garbage(&unreachable);
if (check_garbage(&unreachable)) {
revive_garbage(&unreachable);
gc_list_merge(&unreachable, old);
}
else {
/* 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 (_PyRuntime.gc.debug & DEBUG_UNCOLLECTABLE)
debug_cycle("uncollectable", FROM_GC(gc));
}
if (_PyRuntime.gc.debug & DEBUG_STATS) {
_PyTime_t t2 = _PyTime_GetMonotonicClock();
if (m == 0 && n == 0)
PySys_WriteStderr("gc: done");
else
PySys_FormatStderr(
"gc: done, %zd unreachable, %zd uncollectable",
n+m, n);
PySys_WriteStderr(", %.4fs elapsed\n",
_PyTime_AsSecondsDouble(t2 - t1));
}
/* 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_legacy_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 (nofail) {
PyErr_Clear();
}
else {
if (gc_str == NULL)
gc_str = PyUnicode_FromString("garbage collection");
PyErr_WriteUnraisable(gc_str);
Py_FatalError("unexpected exception during garbage collection");
}
}
/* Update stats */
if (n_collected)
*n_collected = m;
if (n_uncollectable)
*n_uncollectable = n;
stats->collections++;
stats->collected += m;
stats->uncollectable += n;
if (PyDTrace_GC_DONE_ENABLED())
PyDTrace_GC_DONE(n+m);
return n+m;
}
/* Invoke progress callbacks to notify clients that garbage collection
* is starting or stopping
*/
static void
invoke_gc_callback(const char *phase, int generation,
Py_ssize_t collected, Py_ssize_t uncollectable)
{
Py_ssize_t i;
PyObject *info = NULL;
/* we may get called very early */
if (_PyRuntime.gc.callbacks == NULL)
return;
/* The local variable cannot be rebound, check it for sanity */
assert(_PyRuntime.gc.callbacks != NULL && PyList_CheckExact(_PyRuntime.gc.callbacks));
if (PyList_GET_SIZE(_PyRuntime.gc.callbacks) != 0) {
info = Py_BuildValue("{sisnsn}",
"generation", generation,
"collected", collected,
"uncollectable", uncollectable);