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cpython/Modules/gcmodule.c

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/*
Reference Cycle Garbage Collection
==================================
Neil Schemenauer <nas@arctrix.com>
Based on a post on the python-dev list. Ideas from Guido van Rossum,
Eric Tiedemann, and various others.
http://www.arctrix.com/nas/python/gc/
http://www.python.org/pipermail/python-dev/2000-March/003869.html
http://www.python.org/pipermail/python-dev/2000-March/004010.html
http://www.python.org/pipermail/python-dev/2000-March/004022.html
For a highlevel view of the collection process, read the collect
function.
*/
#include "Python.h"
/* Get an object's GC head */
#define AS_GC(o) ((PyGC_Head *)(o)-1)
/* Get the object given the GC head */
#define FROM_GC(g) ((PyObject *)(((PyGC_Head *)g)+1))
/*** Global GC state ***/
struct gc_generation {
PyGC_Head head;
int threshold; /* collection threshold */
int count; /* count of allocations or collections of younger
generations */
};
#define NUM_GENERATIONS 3
#define GEN_HEAD(n) (&generations[n].head)
/* linked lists of container objects */
static struct gc_generation generations[NUM_GENERATIONS] = {
/* PyGC_Head, threshold, count */
{{{GEN_HEAD(0), GEN_HEAD(0), 0}}, 700, 0},
{{{GEN_HEAD(1), GEN_HEAD(1), 0}}, 10, 0},
{{{GEN_HEAD(2), GEN_HEAD(2), 0}}, 10, 0},
};
PyGC_Head *_PyGC_generation0 = GEN_HEAD(0);
static int enabled = 1; /* automatic collection enabled? */
/* true if we are currently running the collector */
static int collecting = 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;
/* set for debugging information */
#define DEBUG_STATS (1<<0) /* print collection statistics */
#define DEBUG_COLLECTABLE (1<<1) /* print collectable objects */
#define DEBUG_UNCOLLECTABLE (1<<2) /* print uncollectable objects */
#define DEBUG_INSTANCES (1<<3) /* print instances */
#define DEBUG_OBJECTS (1<<4) /* print other objects */
#define DEBUG_SAVEALL (1<<5) /* save all garbage in gc.garbage */
#define DEBUG_LEAK DEBUG_COLLECTABLE | \
DEBUG_UNCOLLECTABLE | \
DEBUG_INSTANCES | \
DEBUG_OBJECTS | \
DEBUG_SAVEALL
static int debug;
/*--------------------------------------------------------------------------
gc_refs values.
Between collections, every gc'ed object has one of two gc_refs values:
GC_UNTRACKED
The initial state; objects returned by PyObject_GC_Malloc are in this
state. The object doesn't live in any generation list, and its
tp_traverse slot must not be called.
GC_REACHABLE
The object lives in some generation list, and its tp_traverse is safe to
call. An object transitions to GC_REACHABLE when PyObject_GC_Track
is called.
During a collection, gc_refs can temporarily take on other states:
>= 0
At the start of a collection, update_refs() copies the true refcount
to gc_refs, for each object in the generation being collected.
subtract_refs() then adjusts gc_refs so that it equals the number of
times an object is referenced directly from outside the generation
being collected.
gc_refs remains >= 0 throughout these steps.
GC_TENTATIVELY_UNREACHABLE
move_unreachable() then moves objects not reachable (whether directly or
indirectly) from outside the generation into an "unreachable" set.
Objects that are found to be reachable have gc_refs set to GC_REACHABLE
again. Objects that are found to be unreachable have gc_refs set to
GC_TENTATIVELY_UNREACHABLE. It's "tentatively" because the pass doing
this can't be sure until it ends, and GC_TENTATIVELY_UNREACHABLE may
transition back to GC_REACHABLE.
Only objects with GC_TENTATIVELY_UNREACHABLE still set are candidates
for collection. If it's decided not to collect such an object (e.g.,
it has a __del__ method), its gc_refs is restored to GC_REACHABLE again.
----------------------------------------------------------------------------
*/
#define GC_UNTRACKED _PyGC_REFS_UNTRACKED
#define GC_REACHABLE _PyGC_REFS_REACHABLE
#define GC_TENTATIVELY_UNREACHABLE _PyGC_REFS_TENTATIVELY_UNREACHABLE
#define IS_TRACKED(o) ((AS_GC(o))->gc.gc_refs != GC_UNTRACKED)
#define IS_REACHABLE(o) ((AS_GC(o))->gc.gc_refs == GC_REACHABLE)
#define IS_TENTATIVELY_UNREACHABLE(o) ( \
(AS_GC(o))->gc.gc_refs == GC_TENTATIVELY_UNREACHABLE)
/*** list functions ***/
static void
gc_list_init(PyGC_Head *list)
{
list->gc.gc_prev = list;
list->gc.gc_next = list;
}
static int
gc_list_is_empty(PyGC_Head *list)
{
return (list->gc.gc_next == list);
}
static void
gc_list_append(PyGC_Head *node, PyGC_Head *list)
{
node->gc.gc_next = list;
node->gc.gc_prev = list->gc.gc_prev;
node->gc.gc_prev->gc.gc_next = node;
list->gc.gc_prev = node;
}
static void
gc_list_remove(PyGC_Head *node)
{
node->gc.gc_prev->gc.gc_next = node->gc.gc_next;
node->gc.gc_next->gc.gc_prev = node->gc.gc_prev;
node->gc.gc_next = NULL; /* object is not currently tracked */
}
/* append a list onto another list, from becomes an empty list */
static void
gc_list_merge(PyGC_Head *from, PyGC_Head *to)
{
PyGC_Head *tail;
if (!gc_list_is_empty(from)) {
tail = to->gc.gc_prev;
tail->gc.gc_next = from->gc.gc_next;
tail->gc.gc_next->gc.gc_prev = tail;
to->gc.gc_prev = from->gc.gc_prev;
to->gc.gc_prev->gc.gc_next = to;
}
gc_list_init(from);
}
static long
gc_list_size(PyGC_Head *list)
{
PyGC_Head *gc;
long n = 0;
for (gc = list->gc.gc_next; gc != list; gc = gc->gc.gc_next) {
n++;
}
return n;
}
/* 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 = FROM_GC(gc)->ob_refcnt;
/* 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 = FROM_GC(gc)->ob_type->tp_traverse;
(void) traverse(FROM_GC(gc),
(visitproc)visit_decref,
NULL);
}
}
/* A traversal callback for move_unreachable. */
static int
visit_reachable(PyObject *op, PyGC_Head *reachable)
{
if (PyObject_IS_GC(op)) {
PyGC_Head *gc = AS_GC(op);
const int gc_refs = gc->gc.gc_refs;
if (gc_refs == 0) {
/* This is in move_unreachable's 'young' list, but
* the traversal hasn't yet gotten to it. All
* we need to do is tell move_unreachable that it's
* reachable.
*/
gc->gc.gc_refs = 1;
}
else if (gc_refs == GC_TENTATIVELY_UNREACHABLE) {
/* This had gc_refs = 0 when move_unreachable got
* to it, but turns out it's reachable after all.
* Move it back to move_unreachable's 'young' list,
* and move_unreachable will eventually get to it
* again.
*/
gc_list_remove(gc);
gc_list_append(gc, reachable);
gc->gc.gc_refs = 1;
}
/* Else there's nothing to do.
* If gc_refs > 0, it must be in move_unreachable's 'young'
* list, and move_unreachable will eventually get to it.
* If gc_refs == GC_REACHABLE, it's either in some other
* generation so we don't care about it, or move_unreachable
* already dealt with it.
* If gc_refs == GC_UNTRACKED, it must be ignored.
*/
else {
assert(gc_refs > 0
|| gc_refs == GC_REACHABLE
|| gc_refs == GC_UNTRACKED);
}
}
return 0;
}
/* Move the unreachable objects from young to unreachable. After this,
* all objects in young have gc_refs = GC_REACHABLE, and all objects in
* unreachable have gc_refs = GC_TENTATIVELY_UNREACHABLE. All tracked
* gc objects not in young or unreachable still have gc_refs = GC_REACHABLE.
* All objects in young after this are directly or indirectly reachable
* from outside the original young; and all objects in unreachable are
* not.
*/
static void
move_unreachable(PyGC_Head *young, PyGC_Head *unreachable)
{
PyGC_Head *gc = young->gc.gc_next;
/* Invariants: all objects "to the left" of us in young have gc_refs
* = GC_REACHABLE, and are indeed reachable (directly or indirectly)
* from outside the young list as it was at entry. All other objects
* from the original young "to the left" of us are in unreachable now,
* and have gc_refs = GC_TENTATIVELY_UNREACHABLE. All objects to the
* left of us in 'young' now have been scanned, and no objects here
* or to the right have been scanned yet.
*/
while (gc != young) {
PyGC_Head *next;
if (gc->gc.gc_refs) {
/* gc is definitely reachable from outside the
* original 'young'. Mark it as such, and traverse
* its pointers to find any other objects that may
* be directly reachable from it. Note that the
* call to tp_traverse may append objects to young,
* so we have to wait until it returns to determine
* the next object to visit.
*/
PyObject *op = FROM_GC(gc);
traverseproc traverse = op->ob_type->tp_traverse;
assert(gc->gc.gc_refs > 0);
gc->gc.gc_refs = GC_REACHABLE;
(void) traverse(op,
(visitproc)visit_reachable,
(void *)young);
next = gc->gc.gc_next;
}
else {
/* This *may* be unreachable. To make progress,
* assume it is. gc isn't directly reachable from
* any object we've already traversed, but may be
* reachable from an object we haven't gotten to yet.
* visit_reachable will eventually move gc back into
* young if that's so, and we'll see it again.
*/
next = gc->gc.gc_next;
gc_list_remove(gc);
gc_list_append(gc, unreachable);
gc->gc.gc_refs = GC_TENTATIVELY_UNREACHABLE;
}
gc = next;
}
}
/* Return true if object has a finalization method.
* CAUTION: An instance of an old-style class has to be checked for a
*__del__ method, and earlier versions of this used to call PyObject_HasAttr,
* which in turn could call the class's __getattr__ hook (if any). That
* could invoke arbitrary Python code, mutating the object graph in arbitrary
* ways, and that was the source of some excruciatingly subtle bugs.
*/
static int
has_finalizer(PyObject *op)
{
if (PyInstance_Check(op)) {
assert(delstr != NULL);
return _PyInstance_Lookup(op, delstr) != NULL;
}
else if (PyType_HasFeature(op->ob_type, Py_TPFLAGS_HEAPTYPE))
return op->ob_type->tp_del != NULL;
else
return 0;
}
/* Move the objects in unreachable with __del__ methods into finalizers,
* and weakrefs with callbacks into wr_callbacks.
* The objects remaining in unreachable do not have __del__ methods, and are
* not weakrefs with callbacks.
* The objects moved have gc_refs changed to GC_REACHABLE; the objects
* remaining in unreachable are left at GC_TENTATIVELY_UNREACHABLE.
*/
static void
move_troublemakers(PyGC_Head *unreachable,
PyGC_Head *finalizers,
PyGC_Head *wr_callbacks)
{
PyGC_Head *gc = unreachable->gc.gc_next;
while (gc != unreachable) {
PyObject *op = FROM_GC(gc);
PyGC_Head *next = gc->gc.gc_next;
assert(IS_TENTATIVELY_UNREACHABLE(op));
if (has_finalizer(op)) {
gc_list_remove(gc);
gc_list_append(gc, finalizers);
gc->gc.gc_refs = GC_REACHABLE;
}
else if (PyWeakref_Check(op) &&
((PyWeakReference *)op)->wr_callback) {
gc_list_remove(gc);
gc_list_append(gc, wr_callbacks);
gc->gc.gc_refs = GC_REACHABLE;
}
gc = next;
}
}
/* A traversal callback for move_finalizer_reachable. */
static int
visit_move(PyObject *op, PyGC_Head *tolist)
{
if (PyObject_IS_GC(op)) {
if (IS_TENTATIVELY_UNREACHABLE(op)) {
PyGC_Head *gc = AS_GC(op);
gc_list_remove(gc);
gc_list_append(gc, tolist);
gc->gc.gc_refs = GC_REACHABLE;
}
}
return 0;
}
/* Move objects that are reachable from finalizers, from the unreachable set
* into 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 = FROM_GC(gc)->ob_type->tp_traverse;
(void) traverse(FROM_GC(gc),
(visitproc)visit_move,
(void *)finalizers);
}
}
/* Clear all trash weakrefs with callbacks. This clears weakrefs first,
* which has the happy result of disabling the callbacks without executing
* them. A nasty technical complication: a weakref callback can itself be
* the target of a weakref, in which case decrefing the callback can cause
* another callback to trigger. But we can't allow arbitrary Python code to
* get executed at this point (the callback on the callback may try to muck
* with other cyclic trash we're trying to collect, even resurrecting it
* while we're in the middle of doing tp_clear() on the trash).
*
* The private _PyWeakref_ClearRef() function exists so that we can clear
* the reference in a weakref without triggering a callback on the callback.
*
* We have to save the callback objects and decref them later. But we can't
* allocate new memory to save them (if we can't get new memory, we're dead).
* So we grab a new reference on the clear'ed weakref, which prevents the
* rest of gc from reclaiming it. _PyWeakref_ClearRef() leaves the
* weakref's wr_callback member intact.
*
* In the end, then, wr_callbacks consists of cleared weakrefs that are
* immune from collection. Near the end of gc, after collecting all the
* cyclic trash, we call release_weakrefs(). That releases our references
* to the cleared weakrefs, which in turn may trigger callbacks on their
* callbacks.
*/
static void
clear_weakrefs(PyGC_Head *wr_callbacks)
{
PyGC_Head *gc = wr_callbacks->gc.gc_next;
for (; gc != wr_callbacks; gc = gc->gc.gc_next) {
PyObject *op = FROM_GC(gc);
PyWeakReference *wr;
assert(IS_REACHABLE(op));
assert(PyWeakref_Check(op));
wr = (PyWeakReference *)op;
assert(wr->wr_callback != NULL);
Py_INCREF(op);
_PyWeakref_ClearRef(wr);
}
}
/* Called near the end of gc. This gives up the references we own to
* cleared weakrefs, allowing them to get collected, and in turn decref'ing
* their callbacks.
*
* If a callback object is itself the target of a weakref callback,
* decref'ing the callback object may trigger that other callback. If
* that other callback was part of the cyclic trash in this generation,
* that won't happen, since we cleared *all* trash-weakref callbacks near
* the start of gc. If that other callback was not part of the cyclic trash
* in this generation, then it acted like an external root to this round
* of gc, so all the objects reachable from that callback are still alive.
*
* Giving up the references to the weakref objects will probably make
* them go away too. However, if a weakref is reachable from finalizers,
* it won't go away. We move it to the old generation then. Since a
* weakref object doesn't have a finalizer, that's the right thing to do (it
* doesn't belong in gc.garbage).
*
* We return the number of weakref objects freed (those not appended to old).
*/
static int
release_weakrefs(PyGC_Head *wr_callbacks, PyGC_Head *old)
{
int num_freed = 0;
while (! gc_list_is_empty(wr_callbacks)) {
PyGC_Head *gc = wr_callbacks->gc.gc_next;
PyObject *op = FROM_GC(gc);
assert(IS_REACHABLE(op));
assert(PyWeakref_Check(op));
assert(((PyWeakReference *)op)->wr_callback != NULL);
Py_DECREF(op);
if (wr_callbacks->gc.gc_next == gc) {
/* object is still alive -- move it */
gc_list_remove(gc);
gc_list_append(gc, old);
}
else
++num_freed;
}
return num_freed;
}
static void
debug_instance(char *msg, PyInstanceObject *inst)
{
char *cname;
/* simple version of instance_repr */
PyObject *classname = inst->in_class->cl_name;
if (classname != NULL && PyString_Check(classname))
cname = PyString_AsString(classname);
else
cname = "?";
PySys_WriteStderr("gc: %.100s <%.100s instance at %p>\n",
msg, cname, inst);
}
static void
debug_cycle(char *msg, PyObject *op)
{
if ((debug & DEBUG_INSTANCES) && PyInstance_Check(op)) {
debug_instance(msg, (PyInstanceObject *)op);
}
else if (debug & DEBUG_OBJECTS) {
PySys_WriteStderr("gc: %.100s <%.100s %p>\n",
msg, op->ob_type->tp_name, op);
}
}
/* Handle uncollectable garbage (cycles with finalizers, 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 = op->ob_type->tp_clear) != NULL) {
Py_INCREF(op);
clear(op);
Py_DECREF(op);
}
}
if (collectable->gc.gc_next == gc) {
/* object is still alive, move it, it may die later */
gc_list_remove(gc);
gc_list_append(gc, old);
gc->gc.gc_refs = GC_REACHABLE;
}
}
}
/* This is the main function. Read this to understand how the
* collection process works. */
static long
collect(int generation)
{
int i;
long m = 0; /* # objects collected */
long n = 0; /* # unreachable objects that couldn't be collected */
PyGC_Head *young; /* the generation we are examining */
PyGC_Head *old; /* next older generation */
PyGC_Head unreachable; /* non-problematic unreachable trash */
PyGC_Head finalizers; /* objects with, & reachable from, __del__ */
PyGC_Head wr_callbacks; /* weakrefs with callbacks */
PyGC_Head *gc;
if (delstr == NULL) {
delstr = PyString_InternFromString("__del__");
if (delstr == NULL)
Py_FatalError("gc couldn't allocate \"__del__\"");
}
if (debug & DEBUG_STATS) {
PySys_WriteStderr("gc: collecting generation %d...\n",
generation);
PySys_WriteStderr("gc: objects in each generation:");
for (i = 0; i < NUM_GENERATIONS; i++) {
PySys_WriteStderr(" %ld", gc_list_size(GEN_HEAD(i)));
}
PySys_WriteStderr("\n");
}
/* update collection and allocation counters */
if (generation+1 < NUM_GENERATIONS)
generations[generation+1].count += 1;
for (i = 0; i <= generation; i++)
generations[i].count = 0;
/* merge younger generations with one we are currently collecting */
for (i = 0; i < generation; i++) {
gc_list_merge(GEN_HEAD(i), GEN_HEAD(generation));
}
/* handy references */
young = GEN_HEAD(generation);
if (generation < NUM_GENERATIONS-1)
old = GEN_HEAD(generation+1);
else
old = young;
/* Using ob_refcnt and gc_refs, calculate which objects in the
* container set are reachable from outside the set (ie. have a
* refcount greater than 0 when all the references within the
* set are taken into account
*/
update_refs(young);
subtract_refs(young);
/* Leave everything reachable from outside young in young, and move
* everything else (in young) to unreachable.
* NOTE: This used to move the reachable objects into a reachable
* set instead. But most things usually turn out to be reachable,
* so it's more efficient to move the unreachable things.
*/
gc_list_init(&unreachable);
move_unreachable(young, &unreachable);
/* Move reachable objects to next generation. */
if (young != old)
gc_list_merge(young, old);
/* All objects in unreachable are trash, but objects reachable from
* finalizers can't safely be deleted. Python programmers should take
* care not to create such things. For Python, finalizers means
* instance objects with __del__ methods. Weakrefs with callbacks
* can call arbitrary Python code, so those are special-cased too.
*
* Move unreachable objects with finalizers, and weakrefs with
* callbacks, into different lists.
*/
gc_list_init(&finalizers);
gc_list_init(&wr_callbacks);
move_troublemakers(&unreachable, &finalizers, &wr_callbacks);
/* Clear the trash weakrefs with callbacks. This prevents their
* callbacks from getting invoked (when a weakref goes away, so does
* its callback).
* We do this even if the weakrefs are reachable from finalizers.
* If we didn't, breaking cycles in unreachable later could trigger
* deallocation of objects in finalizers, which could in turn
* cause callbacks to trigger. This may not be ideal behavior.
*/
clear_weakrefs(&wr_callbacks);
/* 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));
}
}
/* 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);
/* Now that we're done analyzing stuff and breaking cycles, let
* delayed weakref callbacks run.
*/
m += release_weakrefs(&wr_callbacks, old);
/* Collect statistics on uncollectable objects found and print
* debugging information. */
for (gc = finalizers.gc.gc_next;
gc != &finalizers;
gc = gc->gc.gc_next) {
n++;
if (debug & DEBUG_UNCOLLECTABLE)
debug_cycle("uncollectable", FROM_GC(gc));
}
if (debug & DEBUG_STATS) {
if (m == 0 && n == 0) {
PySys_WriteStderr("gc: done.\n");
}
else {
PySys_WriteStderr(
"gc: done, %ld unreachable, %ld uncollectable.\n",
n+m, n);
}
}
/* Append instances in the uncollectable set to a Python
* reachable list of garbage. The programmer has to deal with
* this if they insist on creating this type of structure.
*/
(void)handle_finalizers(&finalizers, old);
if (PyErr_Occurred()) {
if (gc_str == NULL)
gc_str = PyString_FromString("garbage collection");
PyErr_WriteUnraisable(gc_str);
Py_FatalError("unexpected exception during garbage collection");
}
return n+m;
}
static long
collect_generations(void)
{
int i;
long n = 0;
/* Find the oldest generation (higest numbered) where the count
* exceeds the threshold. Objects in the that generation and
* generations younger than it will be collected. */
for (i = NUM_GENERATIONS-1; i >= 0; i--) {
if (generations[i].count > generations[i].threshold) {
n = collect(i);
break;
}
}
return n;
}
PyDoc_STRVAR(gc_enable__doc__,
"enable() -> None\n"
"\n"
"Enable automatic garbage collection.\n");
static PyObject *
gc_enable(PyObject *self, PyObject *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 Py_BuildValue("i", enabled);
}
PyDoc_STRVAR(gc_collect__doc__,
"collect() -> n\n"
"\n"
"Run a full collection. The number of unreachable objects is returned.\n");
static PyObject *
gc_collect(PyObject *self, PyObject *noargs)
{
long n;
if (collecting)
n = 0; /* already collecting, don't do anything */
else {
collecting = 1;
n = collect(NUM_GENERATIONS - 1);
collecting = 0;
}
return Py_BuildValue("l", n);
}
PyDoc_STRVAR(gc_set_debug__doc__,
"set_debug(flags) -> None\n"
"\n"
"Set the garbage collection debugging flags. Debugging information is\n"
"written to sys.stderr.\n"
"\n"
"flags is an integer and can have the following bits turned on:\n"
"\n"
" DEBUG_STATS - Print statistics during collection.\n"
" DEBUG_COLLECTABLE - Print collectable objects found.\n"
" DEBUG_UNCOLLECTABLE - Print unreachable but uncollectable objects found.\n"
" DEBUG_INSTANCES - Print instance objects.\n"
" DEBUG_OBJECTS - Print objects other than instances.\n"
" DEBUG_SAVEALL - Save objects to gc.garbage rather than freeing them.\n"
" DEBUG_LEAK - Debug leaking programs (everything but STATS).\n");
static PyObject *
gc_set_debug(PyObject *self, PyObject *args)
{
if (!PyArg_ParseTuple(args, "i:set_debug", &debug))
return NULL;
Py_INCREF(Py_None);
return Py_None;
}
PyDoc_STRVAR(gc_get_debug__doc__,
"get_debug() -> flags\n"
"\n"
"Get the garbage collection debugging flags.\n");
static PyObject *
gc_get_debug(PyObject *self, PyObject *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);
}
static int
referrersvisit(PyObject* obj, PyObject *objs)
{
int i;
for (i = 0; i < PyTuple_GET_SIZE(objs); i++)
if (PyTuple_GET_ITEM(objs, i) == obj)
return 1;
return 0;
}
static int
gc_referrers_for(PyObject *objs, PyGC_Head *list, PyObject *resultlist)
{
PyGC_Head *gc;
PyObject *obj;
traverseproc traverse;
for (gc = list->gc.gc_next; gc != list; gc = gc->gc.gc_next) {
obj = FROM_GC(gc);
traverse = obj->ob_type->tp_traverse;
if (obj == objs || obj == resultlist)
continue;
if (traverse(obj, (visitproc)referrersvisit, objs)) {
if (PyList_Append(resultlist, obj) < 0)
return 0; /* error */
}
}
return 1; /* no error */
}
PyDoc_STRVAR(gc_get_referrers__doc__,
"get_referrers(*objs) -> list\n\
Return the list of objects that directly refer to any of objs.");
static PyObject *
gc_get_referrers(PyObject *self, PyObject *args)
{
int i;
PyObject *result = PyList_New(0);
for (i = 0; i < NUM_GENERATIONS; i++) {
if (!(gc_referrers_for(args, GEN_HEAD(i), result))) {
Py_DECREF(result);
return NULL;
}
}
return result;
}
/* 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)
{
int 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 = obj->ob_type->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__doc__,
"This module provides access to the garbage collector for reference cycles.\n"
"\n"
"enable() -- Enable automatic garbage collection.\n"
"disable() -- Disable automatic garbage collection.\n"
"isenabled() -- Returns true if automatic collection is enabled.\n"
"collect() -- Do a full collection right now.\n"
"set_debug() -- Set debugging flags.\n"
"get_debug() -- Get debugging flags.\n"
"set_threshold() -- Set the collection thresholds.\n"
"get_threshold() -- Return the current the collection thresholds.\n"
"get_objects() -- Return a list of all objects tracked by the collector.\n"
"get_referrers() -- Return the list of objects that refer to an object.\n"
"get_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__},
{"set_threshold", gc_set_thresh, METH_VARARGS, gc_set_thresh__doc__},
{"get_threshold", gc_get_thresh, METH_NOARGS, gc_get_thresh__doc__},
{"collect", gc_collect, METH_NOARGS, gc_collect__doc__},
{"get_objects", gc_get_objects,METH_NOARGS, gc_get_objects__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 */
};
PyMODINIT_FUNC
initgc(void)
{
PyObject *m;
m = Py_InitModule4("gc",
GcMethods,
gc__doc__,
NULL,
PYTHON_API_VERSION);
if (garbage == NULL) {
garbage = PyList_New(0);
if (garbage == NULL)
return;
}
if (PyModule_AddObject(m, "garbage", garbage) < 0)
return;
#define ADD_INT(NAME) if (PyModule_AddIntConstant(m, #NAME, NAME) < 0) return
ADD_INT(DEBUG_STATS);
ADD_INT(DEBUG_COLLECTABLE);
ADD_INT(DEBUG_UNCOLLECTABLE);
ADD_INT(DEBUG_INSTANCES);
ADD_INT(DEBUG_OBJECTS);
ADD_INT(DEBUG_SAVEALL);
ADD_INT(DEBUG_LEAK);
#undef ADD_INT
}
/* API to invoke gc.collect() from C */
long
PyGC_Collect(void)
{
long 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 = PyObject_MALLOC(sizeof(PyGC_Head) + basicsize);
if (g == NULL)
return PyErr_NoMemory();
g->gc.gc_refs = GC_UNTRACKED;
generations[0].count++; /* number of allocated GC objects */
if (generations[0].count > generations[0].threshold &&
enabled &&
generations[0].threshold &&
!collecting &&
!PyErr_Occurred()) {
collecting = 1;
collect_generations();
collecting = 0;
}
op = FROM_GC(g);
return op;
}
PyObject *
_PyObject_GC_New(PyTypeObject *tp)
{
PyObject *op = _PyObject_GC_Malloc(_PyObject_SIZE(tp));
if (op != NULL)
op = PyObject_INIT(op, tp);
return op;
}
PyVarObject *
_PyObject_GC_NewVar(PyTypeObject *tp, int nitems)
{
const size_t size = _PyObject_VAR_SIZE(tp, nitems);
PyVarObject *op = (PyVarObject *) _PyObject_GC_Malloc(size);
if (op != NULL)
op = PyObject_INIT_VAR(op, tp, nitems);
return op;
}
PyVarObject *
_PyObject_GC_Resize(PyVarObject *op, int nitems)
{
const size_t basicsize = _PyObject_VAR_SIZE(op->ob_type, nitems);
PyGC_Head *g = AS_GC(op);
g = PyObject_REALLOC(g, sizeof(PyGC_Head) + basicsize);
if (g == NULL)
return (PyVarObject *)PyErr_NoMemory();
op = (PyVarObject *) FROM_GC(g);
op->ob_size = nitems;
return op;
}
void
PyObject_GC_Del(void *op)
{
PyGC_Head *g = AS_GC(op);
if (IS_TRACKED(op))
gc_list_remove(g);
if (generations[0].count > 0) {
generations[0].count--;
}
PyObject_FREE(g);
}
/* for binary compatibility with 2.2 */
#undef _PyObject_GC_Del
void
_PyObject_GC_Del(PyObject *op)
{
PyObject_GC_Del(op);
}
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