/* Reference Cycle Garbage Collection ================================== Neil Schemenauer Based on a post on the python-dev list. Ideas from Guido van Rossum, Eric Tiedemann, and various others. http://www.arctrix.com/nas/python/gc/ The following mailing list threads provide a historical perspective on the design of this module. Note that a fair amount of refinement has occurred since those discussions. http://mail.python.org/pipermail/python-dev/2000-March/002385.html http://mail.python.org/pipermail/python-dev/2000-March/002434.html http://mail.python.org/pipermail/python-dev/2000-March/002497.html For a highlevel view of the collection process, read the collect function. */ #include "Python.h" #include "frameobject.h" /* for PyFrame_ClearFreeList */ /* Get an object's GC head */ #define AS_GC(o) ((PyGC_Head *)(o)-1) /* Get the object given the GC head */ #define FROM_GC(g) ((PyObject *)(((PyGC_Head *)g)+1)) /*** Global GC state ***/ struct gc_generation { PyGC_Head head; int threshold; /* collection threshold */ int count; /* count of allocations or collections of younger generations */ }; #define NUM_GENERATIONS 3 #define GEN_HEAD(n) (&generations[n].head) /* linked lists of container objects */ static struct gc_generation generations[NUM_GENERATIONS] = { /* PyGC_Head, threshold, count */ {{{GEN_HEAD(0), GEN_HEAD(0), 0}}, 700, 0}, {{{GEN_HEAD(1), GEN_HEAD(1), 0}}, 10, 0}, {{{GEN_HEAD(2), GEN_HEAD(2), 0}}, 10, 0}, }; PyGC_Head *_PyGC_generation0 = GEN_HEAD(0); static int enabled = 1; /* automatic collection enabled? */ /* true if we are currently running the collector */ static int collecting = 0; /* list of uncollectable objects */ static PyObject *garbage = NULL; /* Python string to use if unhandled exception occurs */ static PyObject *gc_str = NULL; /* Python string used to look for __del__ attribute. */ static PyObject *delstr = NULL; /* set for debugging information */ #define DEBUG_STATS (1<<0) /* print collection statistics */ #define DEBUG_COLLECTABLE (1<<1) /* print collectable objects */ #define DEBUG_UNCOLLECTABLE (1<<2) /* print uncollectable objects */ #define DEBUG_SAVEALL (1<<5) /* save all garbage in gc.garbage */ #define DEBUG_LEAK DEBUG_COLLECTABLE | \ DEBUG_UNCOLLECTABLE | \ DEBUG_SAVEALL static int debug; static PyObject *tmod = NULL; /*-------------------------------------------------------------------------- gc_refs values. Between collections, every gc'ed object has one of two gc_refs values: GC_UNTRACKED The initial state; objects returned by PyObject_GC_Malloc are in this state. The object doesn't live in any generation list, and its tp_traverse slot must not be called. GC_REACHABLE The object lives in some generation list, and its tp_traverse is safe to call. An object transitions to GC_REACHABLE when PyObject_GC_Track is called. During a collection, gc_refs can temporarily take on other states: >= 0 At the start of a collection, update_refs() copies the true refcount to gc_refs, for each object in the generation being collected. subtract_refs() then adjusts gc_refs so that it equals the number of times an object is referenced directly from outside the generation being collected. gc_refs remains >= 0 throughout these steps. GC_TENTATIVELY_UNREACHABLE move_unreachable() then moves objects not reachable (whether directly or indirectly) from outside the generation into an "unreachable" set. Objects that are found to be reachable have gc_refs set to GC_REACHABLE again. Objects that are found to be unreachable have gc_refs set to GC_TENTATIVELY_UNREACHABLE. It's "tentatively" because the pass doing this can't be sure until it ends, and GC_TENTATIVELY_UNREACHABLE may transition back to GC_REACHABLE. Only objects with GC_TENTATIVELY_UNREACHABLE still set are candidates for collection. If it's decided not to collect such an object (e.g., it has a __del__ method), its gc_refs is restored to GC_REACHABLE again. ---------------------------------------------------------------------------- */ #define GC_UNTRACKED _PyGC_REFS_UNTRACKED #define GC_REACHABLE _PyGC_REFS_REACHABLE #define GC_TENTATIVELY_UNREACHABLE _PyGC_REFS_TENTATIVELY_UNREACHABLE #define IS_TRACKED(o) ((AS_GC(o))->gc.gc_refs != GC_UNTRACKED) #define IS_REACHABLE(o) ((AS_GC(o))->gc.gc_refs == GC_REACHABLE) #define IS_TENTATIVELY_UNREACHABLE(o) ( \ (AS_GC(o))->gc.gc_refs == GC_TENTATIVELY_UNREACHABLE) /*** list functions ***/ static void gc_list_init(PyGC_Head *list) { list->gc.gc_prev = list; list->gc.gc_next = list; } static int gc_list_is_empty(PyGC_Head *list) { return (list->gc.gc_next == list); } #if 0 /* This became unused after gc_list_move() was introduced. */ /* Append `node` to `list`. */ static void gc_list_append(PyGC_Head *node, PyGC_Head *list) { node->gc.gc_next = list; node->gc.gc_prev = list->gc.gc_prev; node->gc.gc_prev->gc.gc_next = node; list->gc.gc_prev = node; } #endif /* Remove `node` from the gc list it's currently in. */ static void gc_list_remove(PyGC_Head *node) { node->gc.gc_prev->gc.gc_next = node->gc.gc_next; node->gc.gc_next->gc.gc_prev = node->gc.gc_prev; node->gc.gc_next = NULL; /* object is not currently tracked */ } /* Move `node` from the gc list it's currently in (which is not explicitly * named here) to the end of `list`. This is semantically the same as * gc_list_remove(node) followed by gc_list_append(node, list). */ static void gc_list_move(PyGC_Head *node, PyGC_Head *list) { PyGC_Head *new_prev; PyGC_Head *current_prev = node->gc.gc_prev; PyGC_Head *current_next = node->gc.gc_next; /* Unlink from current list. */ current_prev->gc.gc_next = current_next; current_next->gc.gc_prev = current_prev; /* Relink at end of new list. */ new_prev = node->gc.gc_prev = list->gc.gc_prev; new_prev->gc.gc_next = list->gc.gc_prev = node; node->gc.gc_next = list; } /* append list `from` onto list `to`; `from` becomes an empty list */ static void gc_list_merge(PyGC_Head *from, PyGC_Head *to) { PyGC_Head *tail; assert(from != to); if (!gc_list_is_empty(from)) { tail = to->gc.gc_prev; tail->gc.gc_next = from->gc.gc_next; tail->gc.gc_next->gc.gc_prev = tail; to->gc.gc_prev = from->gc.gc_prev; to->gc.gc_prev->gc.gc_next = to; } gc_list_init(from); } static Py_ssize_t gc_list_size(PyGC_Head *list) { PyGC_Head *gc; Py_ssize_t n = 0; for (gc = list->gc.gc_next; gc != list; gc = gc->gc.gc_next) { n++; } return n; } /* Append objects in a GC list to a Python list. * Return 0 if all OK, < 0 if error (out of memory for list). */ static int append_objects(PyObject *py_list, PyGC_Head *gc_list) { PyGC_Head *gc; for (gc = gc_list->gc.gc_next; gc != gc_list; gc = gc->gc.gc_next) { PyObject *op = FROM_GC(gc); if (op != py_list) { if (PyList_Append(py_list, op)) { return -1; /* exception */ } } } return 0; } /*** end of list stuff ***/ /* Set all gc_refs = ob_refcnt. After this, gc_refs is > 0 for all objects * in containers, and is GC_REACHABLE for all tracked gc objects not in * containers. */ static void update_refs(PyGC_Head *containers) { PyGC_Head *gc = containers->gc.gc_next; for (; gc != containers; gc = gc->gc.gc_next) { assert(gc->gc.gc_refs == GC_REACHABLE); gc->gc.gc_refs = Py_REFCNT(FROM_GC(gc)); /* Python's cyclic gc should never see an incoming refcount * of 0: if something decref'ed to 0, it should have been * deallocated immediately at that time. * Possible cause (if the assert triggers): a tp_dealloc * routine left a gc-aware object tracked during its teardown * phase, and did something-- or allowed something to happen -- * that called back into Python. gc can trigger then, and may * see the still-tracked dying object. Before this assert * was added, such mistakes went on to allow gc to try to * delete the object again. In a debug build, that caused * a mysterious segfault, when _Py_ForgetReference tried * to remove the object from the doubly-linked list of all * objects a second time. In a release build, an actual * double deallocation occurred, which leads to corruption * of the allocator's internal bookkeeping pointers. That's * so serious that maybe this should be a release-build * check instead of an assert? */ assert(gc->gc.gc_refs != 0); } } /* A traversal callback for subtract_refs. */ static int visit_decref(PyObject *op, void *data) { assert(op != NULL); if (PyObject_IS_GC(op)) { PyGC_Head *gc = AS_GC(op); /* We're only interested in gc_refs for objects in the * generation being collected, which can be recognized * because only they have positive gc_refs. */ assert(gc->gc.gc_refs != 0); /* else refcount was too small */ if (gc->gc.gc_refs > 0) gc->gc.gc_refs--; } return 0; } /* Subtract internal references from gc_refs. After this, gc_refs is >= 0 * for all objects in containers, and is GC_REACHABLE for all tracked gc * objects not in containers. The ones with gc_refs > 0 are directly * reachable from outside containers, and so can't be collected. */ static void subtract_refs(PyGC_Head *containers) { traverseproc traverse; PyGC_Head *gc = containers->gc.gc_next; for (; gc != containers; gc=gc->gc.gc_next) { traverse = Py_TYPE(FROM_GC(gc))->tp_traverse; (void) traverse(FROM_GC(gc), (visitproc)visit_decref, NULL); } } /* A traversal callback for move_unreachable. */ static int visit_reachable(PyObject *op, PyGC_Head *reachable) { if (PyObject_IS_GC(op)) { PyGC_Head *gc = AS_GC(op); const Py_ssize_t gc_refs = gc->gc.gc_refs; if (gc_refs == 0) { /* This is in move_unreachable's 'young' list, but * the traversal hasn't yet gotten to it. All * we need to do is tell move_unreachable that it's * reachable. */ gc->gc.gc_refs = 1; } else if (gc_refs == GC_TENTATIVELY_UNREACHABLE) { /* This had gc_refs = 0 when move_unreachable got * to it, but turns out it's reachable after all. * Move it back to move_unreachable's 'young' list, * and move_unreachable will eventually get to it * again. */ gc_list_move(gc, reachable); gc->gc.gc_refs = 1; } /* Else there's nothing to do. * If gc_refs > 0, it must be in move_unreachable's 'young' * list, and move_unreachable will eventually get to it. * If gc_refs == GC_REACHABLE, it's either in some other * generation so we don't care about it, or move_unreachable * already dealt with it. * If gc_refs == GC_UNTRACKED, it must be ignored. */ else { assert(gc_refs > 0 || gc_refs == GC_REACHABLE || gc_refs == GC_UNTRACKED); } } return 0; } /* Move the unreachable objects from young to unreachable. After this, * all objects in young have gc_refs = GC_REACHABLE, and all objects in * unreachable have gc_refs = GC_TENTATIVELY_UNREACHABLE. All tracked * gc objects not in young or unreachable still have gc_refs = GC_REACHABLE. * All objects in young after this are directly or indirectly reachable * from outside the original young; and all objects in unreachable are * not. */ static void move_unreachable(PyGC_Head *young, PyGC_Head *unreachable) { PyGC_Head *gc = young->gc.gc_next; /* Invariants: all objects "to the left" of us in young have gc_refs * = GC_REACHABLE, and are indeed reachable (directly or indirectly) * from outside the young list as it was at entry. All other objects * from the original young "to the left" of us are in unreachable now, * and have gc_refs = GC_TENTATIVELY_UNREACHABLE. All objects to the * left of us in 'young' now have been scanned, and no objects here * or to the right have been scanned yet. */ while (gc != young) { PyGC_Head *next; if (gc->gc.gc_refs) { /* gc is definitely reachable from outside the * original 'young'. Mark it as such, and traverse * its pointers to find any other objects that may * be directly reachable from it. Note that the * call to tp_traverse may append objects to young, * so we have to wait until it returns to determine * the next object to visit. */ PyObject *op = FROM_GC(gc); traverseproc traverse = Py_TYPE(op)->tp_traverse; assert(gc->gc.gc_refs > 0); gc->gc.gc_refs = GC_REACHABLE; (void) traverse(op, (visitproc)visit_reachable, (void *)young); next = gc->gc.gc_next; } else { /* This *may* be unreachable. To make progress, * assume it is. gc isn't directly reachable from * any object we've already traversed, but may be * reachable from an object we haven't gotten to yet. * visit_reachable will eventually move gc back into * young if that's so, and we'll see it again. */ next = gc->gc.gc_next; gc_list_move(gc, unreachable); gc->gc.gc_refs = GC_TENTATIVELY_UNREACHABLE; } gc = next; } } /* Return true if object has a finalization method. */ static int has_finalizer(PyObject *op) { if (PyGen_CheckExact(op)) return PyGen_NeedsFinalizing((PyGenObject *)op); else return op->ob_type->tp_del != NULL; } /* Move the objects in unreachable with __del__ methods into `finalizers`. * Objects moved into `finalizers` have gc_refs set to GC_REACHABLE; the * objects remaining in unreachable are left at GC_TENTATIVELY_UNREACHABLE. */ static void move_finalizers(PyGC_Head *unreachable, PyGC_Head *finalizers) { PyGC_Head *gc; PyGC_Head *next; /* March over unreachable. Move objects with finalizers into * `finalizers`. */ for (gc = unreachable->gc.gc_next; gc != unreachable; gc = next) { PyObject *op = FROM_GC(gc); assert(IS_TENTATIVELY_UNREACHABLE(op)); next = gc->gc.gc_next; if (has_finalizer(op)) { gc_list_move(gc, finalizers); gc->gc.gc_refs = GC_REACHABLE; } } } /* A traversal callback for move_finalizer_reachable. */ static int visit_move(PyObject *op, PyGC_Head *tolist) { if (PyObject_IS_GC(op)) { if (IS_TENTATIVELY_UNREACHABLE(op)) { PyGC_Head *gc = AS_GC(op); gc_list_move(gc, tolist); gc->gc.gc_refs = GC_REACHABLE; } } return 0; } /* Move objects that are reachable from finalizers, from the unreachable set * into finalizers set. */ static void move_finalizer_reachable(PyGC_Head *finalizers) { traverseproc traverse; PyGC_Head *gc = finalizers->gc.gc_next; for (; gc != finalizers; gc = gc->gc.gc_next) { /* Note that the finalizers list may grow during this. */ traverse = Py_TYPE(FROM_GC(gc))->tp_traverse; (void) traverse(FROM_GC(gc), (visitproc)visit_move, (void *)finalizers); } } /* Clear all weakrefs to unreachable objects, and if such a weakref has a * callback, invoke it if necessary. Note that it's possible for such * weakrefs to be outside the unreachable set -- indeed, those are precisely * the weakrefs whose callbacks must be invoked. See gc_weakref.txt for * overview & some details. Some weakrefs with callbacks may be reclaimed * directly by this routine; the number reclaimed is the return value. Other * weakrefs with callbacks may be moved into the `old` generation. Objects * moved into `old` have gc_refs set to GC_REACHABLE; the objects remaining in * unreachable are left at GC_TENTATIVELY_UNREACHABLE. When this returns, * no object in `unreachable` is weakly referenced anymore. */ static int handle_weakrefs(PyGC_Head *unreachable, PyGC_Head *old) { PyGC_Head *gc; PyObject *op; /* generally FROM_GC(gc) */ PyWeakReference *wr; /* generally a cast of op */ PyGC_Head wrcb_to_call; /* weakrefs with callbacks to call */ PyGC_Head *next; int num_freed = 0; gc_list_init(&wrcb_to_call); /* Clear all weakrefs to the objects in unreachable. If such a weakref * also has a callback, move it into `wrcb_to_call` if the callback * needs to be invoked. Note that we cannot invoke any callbacks until * all weakrefs to unreachable objects are cleared, lest the callback * resurrect an unreachable object via a still-active weakref. We * make another pass over wrcb_to_call, invoking callbacks, after this * pass completes. */ for (gc = unreachable->gc.gc_next; gc != unreachable; gc = next) { PyWeakReference **wrlist; op = FROM_GC(gc); assert(IS_TENTATIVELY_UNREACHABLE(op)); next = gc->gc.gc_next; if (! PyType_SUPPORTS_WEAKREFS(Py_TYPE(op))) continue; /* It supports weakrefs. Does it have any? */ wrlist = (PyWeakReference **) PyObject_GET_WEAKREFS_LISTPTR(op); /* `op` may have some weakrefs. March over the list, clear * all the weakrefs, and move the weakrefs with callbacks * that must be called into wrcb_to_call. */ for (wr = *wrlist; wr != NULL; wr = *wrlist) { PyGC_Head *wrasgc; /* AS_GC(wr) */ /* _PyWeakref_ClearRef clears the weakref but leaves * the callback pointer intact. Obscure: it also * changes *wrlist. */ assert(wr->wr_object == op); _PyWeakref_ClearRef(wr); assert(wr->wr_object == Py_None); if (wr->wr_callback == NULL) continue; /* no callback */ /* Headache time. `op` is going away, and is weakly referenced by * `wr`, which has a callback. Should the callback be invoked? If wr * is also trash, no: * * 1. There's no need to call it. The object and the weakref are * both going away, so it's legitimate to pretend the weakref is * going away first. The user has to ensure a weakref outlives its * referent if they want a guarantee that the wr callback will get * invoked. * * 2. It may be catastrophic to call it. If the callback is also in * cyclic trash (CT), then although the CT is unreachable from * outside the current generation, CT may be reachable from the * callback. Then the callback could resurrect insane objects. * * Since the callback is never needed and may be unsafe in this case, * wr is simply left in the unreachable set. Note that because we * already called _PyWeakref_ClearRef(wr), its callback will never * trigger. * * OTOH, if wr isn't part of CT, we should invoke the callback: the * weakref outlived the trash. Note that since wr isn't CT in this * case, its callback can't be CT either -- wr acted as an external * root to this generation, and therefore its callback did too. So * nothing in CT is reachable from the callback either, so it's hard * to imagine how calling it later could create a problem for us. wr * is moved to wrcb_to_call in this case. */ if (IS_TENTATIVELY_UNREACHABLE(wr)) continue; assert(IS_REACHABLE(wr)); /* Create a new reference so that wr can't go away * before we can process it again. */ Py_INCREF(wr); /* Move wr to wrcb_to_call, for the next pass. */ wrasgc = AS_GC(wr); assert(wrasgc != next); /* wrasgc is reachable, but next isn't, so they can't be the same */ gc_list_move(wrasgc, &wrcb_to_call); } } /* Invoke the callbacks we decided to honor. It's safe to invoke them * because they can't reference unreachable objects. */ while (! gc_list_is_empty(&wrcb_to_call)) { PyObject *temp; PyObject *callback; gc = wrcb_to_call.gc.gc_next; op = FROM_GC(gc); assert(IS_REACHABLE(op)); assert(PyWeakref_Check(op)); wr = (PyWeakReference *)op; callback = wr->wr_callback; assert(callback != NULL); /* copy-paste of weakrefobject.c's handle_callback() */ temp = PyObject_CallFunctionObjArgs(callback, wr, NULL); if (temp == NULL) PyErr_WriteUnraisable(callback); else Py_DECREF(temp); /* Give up the reference we created in the first pass. When * op's refcount hits 0 (which it may or may not do right now), * op's tp_dealloc will decref op->wr_callback too. Note * that the refcount probably will hit 0 now, and because this * weakref was reachable to begin with, gc didn't already * add it to its count of freed objects. Example: a reachable * weak value dict maps some key to this reachable weakref. * The callback removes this key->weakref mapping from the * dict, leaving no other references to the weakref (excepting * ours). */ Py_DECREF(op); if (wrcb_to_call.gc.gc_next == gc) { /* object is still alive -- move it */ gc_list_move(gc, old); } else ++num_freed; } return num_freed; } static void debug_cycle(char *msg, PyObject *op) { PySys_WriteStderr("gc: %.100s <%.100s %p>\n", msg, Py_TYPE(op)->tp_name, op); } /* Handle uncollectable garbage (cycles with finalizers, and stuff reachable * only from such cycles). * If DEBUG_SAVEALL, all objects in finalizers are appended to the module * garbage list (a Python list), else only the objects in finalizers with * __del__ methods are appended to garbage. All objects in finalizers are * merged into the old list regardless. * Returns 0 if all OK, <0 on error (out of memory to grow the garbage list). * The finalizers list is made empty on a successful return. */ static int handle_finalizers(PyGC_Head *finalizers, PyGC_Head *old) { PyGC_Head *gc = finalizers->gc.gc_next; if (garbage == NULL) { garbage = PyList_New(0); if (garbage == NULL) Py_FatalError("gc couldn't create gc.garbage list"); } for (; gc != finalizers; gc = gc->gc.gc_next) { PyObject *op = FROM_GC(gc); if ((debug & DEBUG_SAVEALL) || has_finalizer(op)) { if (PyList_Append(garbage, op) < 0) return -1; } } gc_list_merge(finalizers, old); return 0; } /* Break reference cycles by clearing the containers involved. This is * tricky business as the lists can be changing and we don't know which * objects may be freed. It is possible I screwed something up here. */ static void delete_garbage(PyGC_Head *collectable, PyGC_Head *old) { inquiry clear; while (!gc_list_is_empty(collectable)) { PyGC_Head *gc = collectable->gc.gc_next; PyObject *op = FROM_GC(gc); assert(IS_TENTATIVELY_UNREACHABLE(op)); if (debug & DEBUG_SAVEALL) { PyList_Append(garbage, op); } else { if ((clear = Py_TYPE(op)->tp_clear) != NULL) { Py_INCREF(op); clear(op); Py_DECREF(op); } } if (collectable->gc.gc_next == gc) { /* object is still alive, move it, it may die later */ gc_list_move(gc, old); gc->gc.gc_refs = GC_REACHABLE; } } } /* Clear all free lists * All free lists are cleared during the collection of the highest generation. * Allocated items in the free list may keep a pymalloc arena occupied. * Clearing the free lists may give back memory to the OS earlier. */ static void clear_freelists(void) { (void)PyMethod_ClearFreeList(); (void)PyFrame_ClearFreeList(); (void)PyCFunction_ClearFreeList(); (void)PyTuple_ClearFreeList(); (void)PyUnicode_ClearFreeList(); (void)PyFloat_ClearFreeList(); } static double get_time(void) { double result = 0; if (tmod != NULL) { PyObject *f = PyObject_CallMethod(tmod, "time", NULL); if (f == NULL) { PyErr_Clear(); } else { if (PyFloat_Check(f)) result = PyFloat_AsDouble(f); Py_DECREF(f); } } return result; } /* This is the main function. Read this to understand how the * collection process works. */ static Py_ssize_t collect(int generation) { int i; Py_ssize_t m = 0; /* # objects collected */ Py_ssize_t n = 0; /* # unreachable objects that couldn't be collected */ PyGC_Head *young; /* the generation we are examining */ PyGC_Head *old; /* next older generation */ PyGC_Head unreachable; /* non-problematic unreachable trash */ PyGC_Head finalizers; /* objects with, & reachable from, __del__ */ PyGC_Head *gc; double t1 = 0.0; if (delstr == NULL) { delstr = PyUnicode_InternFromString("__del__"); if (delstr == NULL) Py_FatalError("gc couldn't allocate \"__del__\""); } if (debug & DEBUG_STATS) { t1 = get_time(); PySys_WriteStderr("gc: collecting generation %d...\n", generation); PySys_WriteStderr("gc: objects in each generation:"); for (i = 0; i < NUM_GENERATIONS; i++) PySys_WriteStderr(" %" PY_FORMAT_SIZE_T "d", gc_list_size(GEN_HEAD(i))); 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) { double t2 = get_time(); if (m == 0 && n == 0) PySys_WriteStderr("gc: done"); else PySys_WriteStderr( "gc: done, " "%" PY_FORMAT_SIZE_T "d unreachable, " "%" PY_FORMAT_SIZE_T "d uncollectable", n+m, n); if (t1 && t2) { PySys_WriteStderr(", %.4fs elapsed", t2-t1); } PySys_WriteStderr(".\n"); } /* Append instances in the uncollectable set to a Python * reachable list of garbage. The programmer has to deal with * this if they insist on creating this type of structure. */ (void)handle_finalizers(&finalizers, old); /* Clear free list only during the collection of the higest * generation */ if (generation == NUM_GENERATIONS-1) { clear_freelists(); } if (PyErr_Occurred()) { if (gc_str == NULL) gc_str = PyUnicode_FromString("garbage collection"); PyErr_WriteUnraisable(gc_str); Py_FatalError("unexpected exception during garbage collection"); } return n+m; } static Py_ssize_t collect_generations(void) { int i; Py_ssize_t n = 0; /* Find the oldest generation (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 PyBool_FromLong((long)enabled); } PyDoc_STRVAR(gc_collect__doc__, "collect([generation]) -> n\n" "\n" "With no arguments, run a full collection. The optional argument\n" "may be an integer specifying which generation to collect. A ValueError\n" "is raised if the generation number is invalid.\n\n" "The number of unreachable objects is returned.\n"); static PyObject * gc_collect(PyObject *self, PyObject *args, PyObject *kws) { static char *keywords[] = {"generation", NULL}; int genarg = NUM_GENERATIONS - 1; Py_ssize_t n; if (!PyArg_ParseTupleAndKeywords(args, kws, "|i", keywords, &genarg)) return NULL; else if (genarg < 0 || genarg >= NUM_GENERATIONS) { PyErr_SetString(PyExc_ValueError, "invalid generation"); return NULL; } if (collecting) n = 0; /* already collecting, don't do anything */ else { collecting = 1; n = collect(genarg); collecting = 0; } return PyLong_FromSsize_t(n); } PyDoc_STRVAR(gc_set_debug__doc__, "set_debug(flags) -> None\n" "\n" "Set the garbage collection debugging flags. Debugging information is\n" "written to sys.stderr.\n" "\n" "flags is an integer and can have the following bits turned on:\n" "\n" " DEBUG_STATS - Print statistics during collection.\n" " DEBUG_COLLECTABLE - Print collectable objects found.\n" " DEBUG_UNCOLLECTABLE - Print unreachable but uncollectable objects found.\n" " DEBUG_SAVEALL - Save objects to gc.garbage rather than freeing them.\n" " DEBUG_LEAK - Debug leaking programs (everything but STATS).\n"); static PyObject * gc_set_debug(PyObject *self, PyObject *args) { if (!PyArg_ParseTuple(args, "i:set_debug", &debug)) return NULL; Py_INCREF(Py_None); return Py_None; } PyDoc_STRVAR(gc_get_debug__doc__, "get_debug() -> flags\n" "\n" "Get the garbage collection debugging flags.\n"); static PyObject * gc_get_debug(PyObject *self, PyObject *noargs) { return Py_BuildValue("i", debug); } PyDoc_STRVAR(gc_set_thresh__doc__, "set_threshold(threshold0, [threshold1, threshold2]) -> None\n" "\n" "Sets the collection thresholds. Setting threshold0 to zero disables\n" "collection.\n"); static PyObject * gc_set_thresh(PyObject *self, PyObject *args) { int i; if (!PyArg_ParseTuple(args, "i|ii:set_threshold", &generations[0].threshold, &generations[1].threshold, &generations[2].threshold)) return NULL; for (i = 2; i < NUM_GENERATIONS; i++) { /* generations higher than 2 get the same threshold */ generations[i].threshold = generations[2].threshold; } Py_INCREF(Py_None); return Py_None; } PyDoc_STRVAR(gc_get_thresh__doc__, "get_threshold() -> (threshold0, threshold1, threshold2)\n" "\n" "Return the current collection thresholds\n"); static PyObject * gc_get_thresh(PyObject *self, PyObject *noargs) { return Py_BuildValue("(iii)", generations[0].threshold, generations[1].threshold, generations[2].threshold); } PyDoc_STRVAR(gc_get_count__doc__, "get_count() -> (count0, count1, count2)\n" "\n" "Return the current collection counts\n"); static PyObject * gc_get_count(PyObject *self, PyObject *noargs) { return Py_BuildValue("(iii)", generations[0].count, generations[1].count, generations[2].count); } static int referrersvisit(PyObject* obj, PyObject *objs) { Py_ssize_t i; for (i = 0; i < PyTuple_GET_SIZE(objs); i++) if (PyTuple_GET_ITEM(objs, i) == obj) return 1; return 0; } static int gc_referrers_for(PyObject *objs, PyGC_Head *list, PyObject *resultlist) { PyGC_Head *gc; PyObject *obj; traverseproc traverse; for (gc = list->gc.gc_next; gc != list; gc = gc->gc.gc_next) { obj = FROM_GC(gc); traverse = Py_TYPE(obj)->tp_traverse; if (obj == objs || obj == resultlist) continue; if (traverse(obj, (visitproc)referrersvisit, objs)) { if (PyList_Append(resultlist, obj) < 0) return 0; /* error */ } } return 1; /* no error */ } PyDoc_STRVAR(gc_get_referrers__doc__, "get_referrers(*objs) -> list\n\ Return the list of objects that directly refer to any of objs."); static PyObject * gc_get_referrers(PyObject *self, PyObject *args) { int i; PyObject *result = PyList_New(0); if (!result) return NULL; for (i = 0; i < NUM_GENERATIONS; i++) { if (!(gc_referrers_for(args, GEN_HEAD(i), result))) { Py_DECREF(result); return NULL; } } return result; } /* Append obj to list; return true if error (out of memory), false if OK. */ static int referentsvisit(PyObject *obj, PyObject *list) { return PyList_Append(list, obj) < 0; } PyDoc_STRVAR(gc_get_referents__doc__, "get_referents(*objs) -> list\n\ Return the list of objects that are directly referred to by objs."); static PyObject * gc_get_referents(PyObject *self, PyObject *args) { Py_ssize_t i; PyObject *result = PyList_New(0); if (result == NULL) return NULL; for (i = 0; i < PyTuple_GET_SIZE(args); i++) { traverseproc traverse; PyObject *obj = PyTuple_GET_ITEM(args, i); if (! PyObject_IS_GC(obj)) continue; traverse = Py_TYPE(obj)->tp_traverse; if (! traverse) continue; if (traverse(obj, (visitproc)referentsvisit, result)) { Py_DECREF(result); return NULL; } } return result; } PyDoc_STRVAR(gc_get_objects__doc__, "get_objects() -> [...]\n" "\n" "Return a list of objects tracked by the collector (excluding the list\n" "returned).\n"); static PyObject * gc_get_objects(PyObject *self, PyObject *noargs) { int i; PyObject* result; result = PyList_New(0); if (result == NULL) return NULL; for (i = 0; i < NUM_GENERATIONS; i++) { if (append_objects(result, GEN_HEAD(i))) { Py_DECREF(result); return NULL; } } return result; } PyDoc_STRVAR(gc__doc__, "This module provides access to the garbage collector for reference cycles.\n" "\n" "enable() -- Enable automatic garbage collection.\n" "disable() -- Disable automatic garbage collection.\n" "isenabled() -- Returns true if automatic collection is enabled.\n" "collect() -- Do a full collection right now.\n" "get_count() -- Return the current collection counts.\n" "set_debug() -- Set debugging flags.\n" "get_debug() -- Get debugging flags.\n" "set_threshold() -- Set the collection thresholds.\n" "get_threshold() -- Return the current the collection thresholds.\n" "get_objects() -- Return a list of all objects tracked by the collector.\n" "get_referrers() -- Return the list of objects that refer to an object.\n" "get_referents() -- Return the list of objects that an object refers to.\n"); static PyMethodDef GcMethods[] = { {"enable", gc_enable, METH_NOARGS, gc_enable__doc__}, {"disable", gc_disable, METH_NOARGS, gc_disable__doc__}, {"isenabled", gc_isenabled, METH_NOARGS, gc_isenabled__doc__}, {"set_debug", gc_set_debug, METH_VARARGS, gc_set_debug__doc__}, {"get_debug", gc_get_debug, METH_NOARGS, gc_get_debug__doc__}, {"get_count", gc_get_count, METH_NOARGS, gc_get_count__doc__}, {"set_threshold", gc_set_thresh, METH_VARARGS, gc_set_thresh__doc__}, {"get_threshold", gc_get_thresh, METH_NOARGS, gc_get_thresh__doc__}, {"collect", (PyCFunction)gc_collect, METH_VARARGS | METH_KEYWORDS, gc_collect__doc__}, {"get_objects", gc_get_objects,METH_NOARGS, gc_get_objects__doc__}, {"get_referrers", gc_get_referrers, METH_VARARGS, gc_get_referrers__doc__}, {"get_referents", gc_get_referents, METH_VARARGS, gc_get_referents__doc__}, {NULL, NULL} /* Sentinel */ }; static struct PyModuleDef gcmodule = { PyModuleDef_HEAD_INIT, "gc", gc__doc__, -1, GcMethods, NULL, NULL, NULL, NULL }; PyMODINIT_FUNC PyInit_gc(void) { PyObject *m; m = PyModule_Create(&gcmodule); if (m == NULL) return NULL; if (garbage == NULL) { garbage = PyList_New(0); if (garbage == NULL) return NULL; } Py_INCREF(garbage); if (PyModule_AddObject(m, "garbage", garbage) < 0) return NULL; /* Importing can't be done in collect() because collect() * can be called via PyGC_Collect() in Py_Finalize(). * This wouldn't be a problem, except that is * reset to 0 before calling collect which trips up * the import and triggers an assertion. */ if (tmod == NULL) { tmod = PyImport_ImportModuleNoBlock("time"); if (tmod == NULL) PyErr_Clear(); } #define ADD_INT(NAME) if (PyModule_AddIntConstant(m, #NAME, NAME) < 0) return NULL ADD_INT(DEBUG_STATS); ADD_INT(DEBUG_COLLECTABLE); ADD_INT(DEBUG_UNCOLLECTABLE); ADD_INT(DEBUG_SAVEALL); ADD_INT(DEBUG_LEAK); #undef ADD_INT return m; } /* API to invoke gc.collect() from C */ Py_ssize_t PyGC_Collect(void) { Py_ssize_t n; if (collecting) n = 0; /* already collecting, don't do anything */ else { collecting = 1; n = collect(NUM_GENERATIONS - 1); collecting = 0; } return n; } /* for debugging */ void _PyGC_Dump(PyGC_Head *g) { _PyObject_Dump(FROM_GC(g)); } /* extension modules might be compiled with GC support so these functions must always be available */ #undef PyObject_GC_Track #undef PyObject_GC_UnTrack #undef PyObject_GC_Del #undef _PyObject_GC_Malloc void PyObject_GC_Track(void *op) { _PyObject_GC_TRACK(op); } /* for binary compatibility with 2.2 */ void _PyObject_GC_Track(PyObject *op) { PyObject_GC_Track(op); } void PyObject_GC_UnTrack(void *op) { /* Obscure: the Py_TRASHCAN mechanism requires that we be able to * call PyObject_GC_UnTrack twice on an object. */ if (IS_TRACKED(op)) _PyObject_GC_UNTRACK(op); } /* for binary compatibility with 2.2 */ void _PyObject_GC_UnTrack(PyObject *op) { PyObject_GC_UnTrack(op); } PyObject * _PyObject_GC_Malloc(size_t basicsize) { PyObject *op; PyGC_Head *g; if (basicsize > PY_SSIZE_T_MAX - sizeof(PyGC_Head)) return PyErr_NoMemory(); g = (PyGC_Head *)PyObject_MALLOC( sizeof(PyGC_Head) + basicsize); if (g == NULL) return PyErr_NoMemory(); g->gc.gc_refs = GC_UNTRACKED; generations[0].count++; /* number of allocated GC objects */ if (generations[0].count > generations[0].threshold && enabled && generations[0].threshold && !collecting && !PyErr_Occurred()) { collecting = 1; collect_generations(); collecting = 0; } op = FROM_GC(g); return op; } PyObject * _PyObject_GC_New(PyTypeObject *tp) { PyObject *op = _PyObject_GC_Malloc(_PyObject_SIZE(tp)); if (op != NULL) op = PyObject_INIT(op, tp); return op; } PyVarObject * _PyObject_GC_NewVar(PyTypeObject *tp, Py_ssize_t nitems) { const size_t size = _PyObject_VAR_SIZE(tp, nitems); PyVarObject *op = (PyVarObject *) _PyObject_GC_Malloc(size); if (op != NULL) op = PyObject_INIT_VAR(op, tp, nitems); return op; } PyVarObject * _PyObject_GC_Resize(PyVarObject *op, Py_ssize_t nitems) { const size_t basicsize = _PyObject_VAR_SIZE(Py_TYPE(op), nitems); PyGC_Head *g = AS_GC(op); if (basicsize > PY_SSIZE_T_MAX - sizeof(PyGC_Head)) return (PyVarObject *)PyErr_NoMemory(); g = (PyGC_Head *)PyObject_REALLOC(g, sizeof(PyGC_Head) + basicsize); if (g == NULL) return (PyVarObject *)PyErr_NoMemory(); op = (PyVarObject *) FROM_GC(g); Py_SIZE(op) = nitems; return op; } void PyObject_GC_Del(void *op) { PyGC_Head *g = AS_GC(op); if (IS_TRACKED(op)) gc_list_remove(g); if (generations[0].count > 0) { generations[0].count--; } PyObject_FREE(g); } /* for binary compatibility with 2.2 */ #undef _PyObject_GC_Del void _PyObject_GC_Del(PyObject *op) { PyObject_GC_Del(op); }