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cpython/Modules/gcmodule.c

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/*
Reference Cycle Garbage Collection
==================================
Neil Schemenauer <nascheme@enme.ucalgary.ca>
Based on a post on the python-dev list. Ideas from Guido van Rossum,
Eric Tiedemann, and various others.
http://www.enme.calgary.ca/~nascheme/python/gc.html
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.
TODO:
use a different interface for set_debug() (keywords)?
tune parameters
*/
#include "Python.h"
#ifdef WITH_CYCLE_GC
/* magic gc_refs value */
#define GC_MOVED -1
/*** Global GC state ***/
/* linked lists of container objects */
static PyGC_Head generation0 = {&generation0, &generation0, 0};
static PyGC_Head generation1 = {&generation1, &generation1, 0};
static PyGC_Head generation2 = {&generation2, &generation2, 0};
static int generation = 0; /* current generation being collected */
/* collection frequencies, XXX tune these */
static int enabled = 1; /* automatic collection enabled? */
static int threshold0 = 5000; /* net new containers before collection */
static int threshold1 = 10; /* generation0 collections before collecting 1 */
static int threshold2 = 10; /* generation1 collections before collecting 2 */
/* net new objects allocated since last collection */
static int allocated;
/* 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_LEAK DEBUG_COLLECTABLE | \
DEBUG_UNCOLLECTABLE | \
DEBUG_INSTANCES | \
DEBUG_OBJECTS
static int debug;
/* list of uncollectable objects */
static PyObject *garbage;
/* Python string to use if unhandled exception occurs */
static PyObject *gc_str;
/*** list functions ***/
static void
gc_list_init(PyGC_Head *list)
{
list->gc_prev = list;
list->gc_next = list;
}
static void
gc_list_append(PyGC_Head *node, PyGC_Head *list)
{
node->gc_next = list;
node->gc_prev = list->gc_prev;
node->gc_prev->gc_next = node;
list->gc_prev = node;
}
static void
gc_list_remove(PyGC_Head *node)
{
node->gc_prev->gc_next = node->gc_next;
node->gc_next->gc_prev = node->gc_prev;
#ifdef Py_DEBUG
node->gc_prev = NULL;
node->gc_next = NULL;
#endif
}
static void
gc_list_move(PyGC_Head *from, PyGC_Head *to)
{
if (from->gc_next == from) {
/* empty from list */
gc_list_init(to);
} else {
to->gc_next = from->gc_next;
to->gc_next->gc_prev = to;
to->gc_prev = from->gc_prev;
to->gc_prev->gc_next = to;
}
gc_list_init(from);
}
/* 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 (from->gc_next != from) {
tail = to->gc_prev;
tail->gc_next = from->gc_next;
tail->gc_next->gc_prev = tail;
to->gc_prev = from->gc_prev;
to->gc_prev->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_next; gc != list; gc = gc->gc_next) {
n++;
}
return n;
}
/*** end of list stuff ***/
/* Set all gc_refs = ob_refcnt */
static void
update_refs(PyGC_Head *containers)
{
PyGC_Head *gc = containers->gc_next;
for (; gc != containers; gc=gc->gc_next) {
gc->gc_refs = PyObject_FROM_GC(gc)->ob_refcnt;
}
}
static int
visit_decref(PyObject *op, void *data)
{
if (op && PyObject_IS_GC(op)) {
PyObject_AS_GC(op)->gc_refs--;
}
return 0;
}
/* Subtract internal references from gc_refs */
static void
subtract_refs(PyGC_Head *containers)
{
traverseproc traverse;
PyGC_Head *gc = containers->gc_next;
for (; gc != containers; gc=gc->gc_next) {
traverse = PyObject_FROM_GC(gc)->ob_type->tp_traverse;
(void) traverse(PyObject_FROM_GC(gc),
(visitproc)visit_decref,
NULL);
}
}
/* Append objects with gc_refs > 0 to roots list */
static void
move_roots(PyGC_Head *containers, PyGC_Head *roots)
{
PyGC_Head *next;
PyGC_Head *gc = containers->gc_next;
while (gc != containers) {
next = gc->gc_next;
if (gc->gc_refs > 0) {
gc_list_remove(gc);
gc_list_append(gc, roots);
gc->gc_refs = GC_MOVED;
}
gc = next;
}
}
static int
visit_reachable(PyObject *op, PyGC_Head *roots)
{
if (PyObject_IS_GC(op)) {
PyGC_Head *gc = PyObject_AS_GC(op);
if (gc && gc->gc_refs != GC_MOVED) {
gc_list_remove(gc);
gc_list_append(gc, roots);
gc->gc_refs = GC_MOVED;
}
}
return 0;
}
/* Move objects referenced from reachable to reachable set. */
static void
move_root_reachable(PyGC_Head *reachable)
{
traverseproc traverse;
PyGC_Head *gc = reachable->gc_next;
for (; gc != reachable; gc=gc->gc_next) {
/* careful, reachable list is growing here */
PyObject *op = PyObject_FROM_GC(gc);
traverse = op->ob_type->tp_traverse;
(void) traverse(op,
(visitproc)visit_reachable,
(void *)reachable);
}
}
/* move all objects with finalizers (instances with __del__) */
static void
move_finalizers(PyGC_Head *unreachable, PyGC_Head *finalizers)
{
PyGC_Head *next;
PyGC_Head *gc = unreachable->gc_next;
static PyObject *delstr = NULL;
if (delstr == NULL) {
delstr = PyString_InternFromString("__del__");
if (delstr == NULL)
Py_FatalError("PyGC: can't initialize __del__ string");
}
for (; gc != unreachable; gc=next) {
PyObject *op = PyObject_FROM_GC(gc);
next = gc->gc_next;
if (PyInstance_Check(op) && PyObject_HasAttr(op, delstr)) {
gc_list_remove(gc);
gc_list_append(gc, finalizers);
}
}
}
/* called by tp_traverse */
static int
visit_finalizer_reachable(PyObject *op, PyGC_Head *finalizers)
{
if (PyObject_IS_GC(op)) {
PyGC_Head *gc = PyObject_AS_GC(op);
if (gc && gc->gc_refs != GC_MOVED) {
gc_list_remove(gc);
gc_list_append(gc, finalizers);
gc->gc_refs = GC_MOVED;
}
}
return 0;
}
/* Move objects referenced from roots to roots */
static void
move_finalizer_reachable(PyGC_Head *finalizers)
{
traverseproc traverse;
PyGC_Head *gc = finalizers->gc_next;
for (; gc != finalizers; gc=gc->gc_next) {
/* careful, finalizers list is growing here */
traverse = PyObject_FROM_GC(gc)->ob_type->tp_traverse;
(void) traverse(PyObject_FROM_GC(gc),
(visitproc)visit_finalizer_reachable,
(void *)finalizers);
}
}
static void
debug_instance(char *msg, PyInstanceObject *inst)
{
char *cname;
/* be careful not to create new dictionaries */
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). */
static void
handle_finalizers(PyGC_Head *finalizers, PyGC_Head *old)
{
PyGC_Head *gc;
if (garbage == NULL) {
garbage = PyList_New(0);
}
for (gc = finalizers->gc_next; gc != finalizers;
gc = finalizers->gc_next) {
PyObject *op = PyObject_FROM_GC(gc);
/* Add all instances to a Python accessible list of garbage */
if (PyInstance_Check(op)) {
PyList_Append(garbage, op);
}
/* We assume that all objects in finalizers are reachable from
* instances. Once we add the instances to the garbage list
* everything is reachable from Python again. */
gc_list_remove(gc);
gc_list_append(gc, old);
}
}
/* 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 *unreachable, PyGC_Head *old)
{
inquiry clear;
while (unreachable->gc_next != unreachable) {
PyGC_Head *gc = unreachable->gc_next;
PyObject *op = PyObject_FROM_GC(gc);
/*
PyList_Append(garbage, op);
*/
if ((clear = op->ob_type->tp_clear) != NULL) {
Py_INCREF(op);
clear((PyObject *)op);
Py_DECREF(op);
}
/* only try to call tp_clear once for each object */
if (unreachable->gc_next == gc) {
/* still alive, move it, it may die later */
gc_list_remove(gc);
gc_list_append(gc, old);
}
}
}
/* This is the main function. Read this to understand how the
* collection process works. */
static long
collect(PyGC_Head *young, PyGC_Head *old)
{
long n = 0;
long m = 0;
PyGC_Head reachable;
PyGC_Head unreachable;
PyGC_Head finalizers;
PyGC_Head *gc;
if (debug & DEBUG_STATS) {
PySys_WriteStderr(
"gc: collecting generation %d...\n"
"gc: objects in each generation: %ld %ld %ld\n",
generation,
gc_list_size(&generation0),
gc_list_size(&generation1),
gc_list_size(&generation2));
}
/* 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);
/* Move everything reachable from outside the set into the
* reachable set (ie. gc_refs > 0). Next, move everything
* reachable from objects in the reachable set. */
gc_list_init(&reachable);
move_roots(young, &reachable);
move_root_reachable(&reachable);
/* move unreachable objects to a temporary list, new objects can be
* allocated after this point */
gc_list_init(&unreachable);
gc_list_move(young, &unreachable);
/* move reachable objects to next generation */
gc_list_merge(&reachable, old);
/* Move objects reachable from finalizers, we can't safely delete
* them. Python programmers should take care not to create such
* things. For Python finalizers means instance objects with
* __del__ methods. */
gc_list_init(&finalizers);
move_finalizers(&unreachable, &finalizers);
move_finalizer_reachable(&finalizers);
/* Collect statistics on collectable objects found and print
* debugging information. */
for (gc = unreachable.gc_next; gc != &unreachable;
gc = gc->gc_next) {
m++;
if (debug & DEBUG_COLLECTABLE) {
debug_cycle("collectable", PyObject_FROM_GC(gc));
}
}
/* call tp_clear on objects in the collectable 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_next; gc != &finalizers;
gc = gc->gc_next) {
n++;
if (debug & DEBUG_UNCOLLECTABLE) {
debug_cycle("uncollectable", PyObject_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. */
handle_finalizers(&finalizers, old);
if (PyErr_Occurred()) {
PyErr_WriteUnraisable(gc_str);
Py_FatalError("unexpected exception during garbage collection");
}
allocated = 0;
return n+m;
}
static long
collect_generations(void)
{
static long collections0 = 0;
static long collections1 = 0;
long n = 0;
if (collections1 > threshold2) {
generation = 2;
gc_list_merge(&generation0, &generation2);
gc_list_merge(&generation1, &generation2);
if (generation2.gc_next != &generation2) {
n = collect(&generation2, &generation2);
}
collections1 = 0;
} else if (collections0 > threshold1) {
generation = 1;
collections1++;
gc_list_merge(&generation0, &generation1);
if (generation1.gc_next != &generation1) {
n = collect(&generation1, &generation2);
}
collections0 = 0;
} else {
generation = 0;
collections0++;
if (generation0.gc_next != &generation0) {
n = collect(&generation0, &generation1);
}
}
return n;
}
void
_PyGC_Insert(PyObject *op)
{
/* collection lock since collecting may cause allocations */
static int collecting = 0;
#ifdef Py_DEBUG
if (!PyObject_IS_GC(op)) {
abort();
}
#endif
if (allocated > threshold0 && enabled && threshold0 && !collecting) {
collecting++;
collect_generations();
collecting--;
}
allocated++;
gc_list_append(PyObject_AS_GC(op), &generation0);
}
void
_PyGC_Remove(PyObject *op)
{
PyGC_Head *g = PyObject_AS_GC(op);
#ifdef Py_DEBUG
if (!PyObject_IS_GC(op)) {
abort();
}
#endif
gc_list_remove(g);
if (allocated > 0) {
allocated--;
}
}
static char gc_enable__doc__[] =
"enable() -> None\n"
"\n"
"Enable automatic garbage collection.\n"
;
static PyObject *
gc_enable(PyObject *self, PyObject *args)
{
if (!PyArg_ParseTuple(args, ":enable")) /* check no args */
return NULL;
enabled = 1;
Py_INCREF(Py_None);
return Py_None;
}
static char gc_disable__doc__[] =
"disable() -> None\n"
"\n"
"Disable automatic garbage collection.\n"
;
static PyObject *
gc_disable(PyObject *self, PyObject *args)
{
if (!PyArg_ParseTuple(args, ":disable")) /* check no args */
return NULL;
enabled = 0;
Py_INCREF(Py_None);
return Py_None;
}
static char gc_isenabled__doc__[] =
"isenabled() -> status\n"
"\n"
"Returns true if automatic garbage collection is enabled.\n"
;
static PyObject *
gc_isenabled(PyObject *self, PyObject *args)
{
if (!PyArg_ParseTuple(args, ":isenabled")) /* check no args */
return NULL;
return Py_BuildValue("i", enabled);
}
static char 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 *args)
{
long n;
if (!PyArg_ParseTuple(args, ":collect")) /* check no args */
return NULL;
generation = 2;
gc_list_merge(&generation0, &generation2);
gc_list_merge(&generation1, &generation2);
n = collect(&generation2, &generation2);
return Py_BuildValue("i", n);
}
static char 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_LEAK - Debug leaking programs (everything but STATS).\n"
;
static PyObject *
gc_set_debug(PyObject *self, PyObject *args)
{
if (!PyArg_ParseTuple(args, "l:get_debug", &debug))
return NULL;
Py_INCREF(Py_None);
return Py_None;
}
static char gc_get_debug__doc__[] =
"get_debug() -> flags\n"
"\n"
"Get the garbage collection debugging flags.\n"
;
static PyObject *
gc_get_debug(PyObject *self, PyObject *args)
{
if (!PyArg_ParseTuple(args, ":get_debug")) /* no args */
return NULL;
return Py_BuildValue("i", debug);
}
static char gc_set_thresh__doc__[] =
"set_threshold(threshold0, [threhold1, 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)
{
if (!PyArg_ParseTuple(args, "i|ii:set_threshold", &threshold0,
&threshold1, &threshold2))
return NULL;
Py_INCREF(Py_None);
return Py_None;
}
static char 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 *args)
{
if (!PyArg_ParseTuple(args, ":get_threshold")) /* no args */
return NULL;
return Py_BuildValue("(iii)", threshold0, threshold1, threshold2);
}
static char 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"
;
static PyMethodDef GcMethods[] = {
{"enable", gc_enable, METH_VARARGS, gc_enable__doc__},
{"disable", gc_disable, METH_VARARGS, gc_disable__doc__},
{"isenabled", gc_isenabled, METH_VARARGS, gc_isenabled__doc__},
{"set_debug", gc_set_debug, METH_VARARGS, gc_set_debug__doc__},
{"get_debug", gc_get_debug, METH_VARARGS, gc_get_debug__doc__},
{"set_threshold", gc_set_thresh, METH_VARARGS, gc_set_thresh__doc__},
{"get_threshold", gc_get_thresh, METH_VARARGS, gc_get_thresh__doc__},
{"collect", gc_collect, METH_VARARGS, gc_collect__doc__},
{NULL, NULL} /* Sentinel */
};
void
initgc(void)
{
PyObject *m;
PyObject *d;
m = Py_InitModule4("gc",
GcMethods,
gc__doc__,
NULL,
PYTHON_API_VERSION);
d = PyModule_GetDict(m);
if (garbage == NULL) {
garbage = PyList_New(0);
}
if (gc_str == NULL) {
gc_str = PyString_FromString("garbage collection");
}
PyDict_SetItemString(d, "garbage", garbage);
PyDict_SetItemString(d, "DEBUG_STATS",
PyInt_FromLong(DEBUG_STATS));
PyDict_SetItemString(d, "DEBUG_COLLECTABLE",
PyInt_FromLong(DEBUG_COLLECTABLE));
PyDict_SetItemString(d, "DEBUG_UNCOLLECTABLE",
PyInt_FromLong(DEBUG_UNCOLLECTABLE));
PyDict_SetItemString(d, "DEBUG_INSTANCES",
PyInt_FromLong(DEBUG_INSTANCES));
PyDict_SetItemString(d, "DEBUG_OBJECTS",
PyInt_FromLong(DEBUG_OBJECTS));
PyDict_SetItemString(d, "DEBUG_LEAK",
PyInt_FromLong(DEBUG_LEAK));
}
#endif /* WITH_CYCLE_GC */
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