/* 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;
}
PyVarObject *
_PyObject_GC_Resize(PyVarObject *op, Py_ssize_t nitems)
{
const size_t basicsize = _PyObject_VAR_SIZE(op->ob_type, 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);
op->ob_size = nitems;
return op;
}
/* 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_move(gc, unreachable);
gc->gc.gc_refs = GC_TENTATIVELY_UNREACHABLE;
}
gc = next;
}
}
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 */
}
/* 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;
}
}
}
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;
}
/* 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;
}
/* 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 *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 (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)
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 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) {
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;
}
/* 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(op->ob_type))
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_CallFunction(callback, "O", wr);
if (temp == NULL)
PyErr_WriteUnraisable(callback);
else
Py_DECREF(temp);
/* Give up the reference we created in the first pass. When
* op's refcount hits 0 (which it may or may not do right now),
* op's tp_dealloc will decref op->wr_callback too. Note
* that the refcount probably will hit 0 now, and because this
* weakref was reachable to begin with, gc didn't already
* add it to its count of freed objects. Example: a reachable
* weak value dict maps some key to this reachable weakref.
* The callback removes this key->weakref mapping from the
* dict, leaving no other references to the weakref (excepting
* ours).
*/
Py_DECREF(op);
if (wrcb_to_call.gc.gc_next == gc) {
/* object is still alive -- move it */
gc_list_move(gc, old);
}
else
++num_freed;
}
return num_freed;
}
/* for debugging */
void
_PyGC_Dump(PyGC_Head *g)
{
_PyObject_Dump(FROM_GC(g));
}
