diff options
| -rw-r--r-- | Include/weakrefobject.h | 2 | ||||
| -rw-r--r-- | Lib/test/test_weakref.py | 205 | ||||
| -rw-r--r-- | Misc/NEWS | 21 | ||||
| -rw-r--r-- | Modules/gc_weakref.txt | 107 | ||||
| -rw-r--r-- | Modules/gcmodule.c | 142 | ||||
| -rw-r--r-- | Objects/weakrefobject.c | 34 |
6 files changed, 491 insertions, 20 deletions
diff --git a/Include/weakrefobject.h b/Include/weakrefobject.h index b6fc389..effa0ed 100644 --- a/Include/weakrefobject.h +++ b/Include/weakrefobject.h @@ -39,6 +39,8 @@ PyAPI_FUNC(PyObject *) PyWeakref_GetObject(PyObject *ref); PyAPI_FUNC(long) _PyWeakref_GetWeakrefCount(PyWeakReference *head); +PyAPI_FUNC(void) _PyWeakref_ClearRef(PyWeakReference *self); + #define PyWeakref_GET_OBJECT(ref) (((PyWeakReference *)(ref))->wr_object) diff --git a/Lib/test/test_weakref.py b/Lib/test/test_weakref.py index 0209faf..34831f1 100644 --- a/Lib/test/test_weakref.py +++ b/Lib/test/test_weakref.py @@ -337,6 +337,211 @@ class ReferencesTestCase(TestBase): # deallocation of c2. del c2 + def test_callback_in_cycle_1(self): + import gc + + class J(object): + pass + + class II(object): + def acallback(self, ignore): + self.J + + I = II() + I.J = J + I.wr = weakref.ref(J, I.acallback) + + # Now J and II are each in a self-cycle (as all new-style class + # objects are, since their __mro__ points back to them). I holds + # both a weak reference (I.wr) and a strong reference (I.J) to class + # J. I is also in a cycle (I.wr points to a weakref that references + # I.acallback). When we del these three, they all become trash, but + # the cycles prevent any of them from getting cleaned up immediately. + # Instead they have to wait for cyclic gc to deduce that they're + # trash. + # + # gc used to call tp_clear on all of them, and the order in which + # it does that is pretty accidental. The exact order in which we + # built up these things manages to provoke gc into running tp_clear + # in just the right order (I last). Calling tp_clear on II leaves + # behind an insane class object (its __mro__ becomes NULL). Calling + # tp_clear on J breaks its self-cycle, but J doesn't get deleted + # just then because of the strong reference from I.J. Calling + # tp_clear on I starts to clear I's __dict__, and just happens to + # clear I.J first -- I.wr is still intact. That removes the last + # reference to J, which triggers the weakref callback. The callback + # tries to do "self.J", and instances of new-style classes look up + # attributes ("J") in the class dict first. The class (II) wants to + # search II.__mro__, but that's NULL. The result was a segfault in + # a release build, and an assert failure in a debug build. + del I, J, II + gc.collect() + + def test_callback_in_cycle_2(self): + import gc + + # This is just like test_callback_in_cycle_1, except that II is an + # old-style class. The symptom is different then: an instance of an + # old-style class looks in its own __dict__ first. 'J' happens to + # get cleared from I.__dict__ before 'wr', and 'J' was never in II's + # __dict__, so the attribute isn't found. The difference is that + # the old-style II doesn't have a NULL __mro__ (it doesn't have any + # __mro__), so no segfault occurs. Instead it got: + # test_callback_in_cycle_2 (__main__.ReferencesTestCase) ... + # Exception exceptions.AttributeError: + # "II instance has no attribute 'J'" in <bound method II.acallback + # of <?.II instance at 0x00B9B4B8>> ignored + + class J(object): + pass + + class II: + def acallback(self, ignore): + self.J + + I = II() + I.J = J + I.wr = weakref.ref(J, I.acallback) + + del I, J, II + gc.collect() + + def test_callback_in_cycle_3(self): + import gc + + # This one broke the first patch that fixed the last two. In this + # case, the objects reachable from the callback aren't also reachable + # from the object (c1) *triggering* the callback: you can get to + # c1 from c2, but not vice-versa. The result was that c2's __dict__ + # got tp_clear'ed by the time the c2.cb callback got invoked. + + class C: + def cb(self, ignore): + self.me + self.c1 + self.wr + + c1, c2 = C(), C() + + c2.me = c2 + c2.c1 = c1 + c2.wr = weakref.ref(c1, c2.cb) + + del c1, c2 + gc.collect() + + def test_callback_in_cycle_4(self): + import gc + + # Like test_callback_in_cycle_3, except c2 and c1 have different + # classes. c2's class (C) isn't reachable from c1 then, so protecting + # objects reachable from the dying object (c1) isn't enough to stop + # c2's class (C) from getting tp_clear'ed before c2.cb is invoked. + # The result was a segfault (C.__mro__ was NULL when the callback + # tried to look up self.me). + + class C(object): + def cb(self, ignore): + self.me + self.c1 + self.wr + + class D: + pass + + c1, c2 = D(), C() + + c2.me = c2 + c2.c1 = c1 + c2.wr = weakref.ref(c1, c2.cb) + + del c1, c2, C, D + gc.collect() + + def test_callback_in_cycle_resurrection(self): + import gc + + # Do something nasty in a weakref callback: resurrect objects + # from dead cycles. For this to be attempted, the weakref and + # its callback must also be part of the cyclic trash (else the + # objects reachable via the callback couldn't be in cyclic trash + # to begin with -- the callback would act like an external root). + # But gc clears trash weakrefs with callbacks early now, which + # disables the callbacks, so the callbacks shouldn't get called + # at all (and so nothing actually gets resurrected). + + alist = [] + class C(object): + def __init__(self, value): + self.attribute = value + + def acallback(self, ignore): + alist.append(self.c) + + c1, c2 = C(1), C(2) + c1.c = c2 + c2.c = c1 + c1.wr = weakref.ref(c2, c1.acallback) + c2.wr = weakref.ref(c1, c2.acallback) + + def C_went_away(ignore): + alist.append("C went away") + wr = weakref.ref(C, C_went_away) + + del c1, c2, C # make them all trash + self.assertEqual(alist, []) # del isn't enough to reclaim anything + + gc.collect() + # c1.wr and c2.wr were part of the cyclic trash, so should have + # been cleared without their callbacks executing. OTOH, the weakref + # to C is bound to a function local (wr), and wasn't trash, so that + # callback should have been invoked when C went away. + self.assertEqual(alist, ["C went away"]) + # The remaining weakref should be dead now (its callback ran). + self.assertEqual(wr(), None) + + del alist[:] + gc.collect() + self.assertEqual(alist, []) + + def test_callbacks_on_callback(self): + import gc + + # Set up weakref callbacks *on* weakref callbacks. + alist = [] + def safe_callback(ignore): + alist.append("safe_callback called") + + class C(object): + def cb(self, ignore): + alist.append("cb called") + + c, d = C(), C() + c.other = d + d.other = c + callback = c.cb + c.wr = weakref.ref(d, callback) # this won't trigger + d.wr = weakref.ref(callback, d.cb) # ditto + external_wr = weakref.ref(callback, safe_callback) # but this will + self.assert_(external_wr() is callback) + + # The weakrefs attached to c and d should get cleared, so that + # C.cb is never called. But external_wr isn't part of the cyclic + # trash, and no cyclic trash is reachable from it, so safe_callback + # should get invoked when the bound method object callback (c.cb) + # -- which is itself a callback, and also part of the cyclic trash -- + # gets reclaimed at the end of gc. + + del callback, c, d, C + self.assertEqual(alist, []) # del isn't enough to clean up cycles + gc.collect() + self.assertEqual(alist, ["safe_callback called"]) + self.assertEqual(external_wr(), None) + + del alist[:] + gc.collect() + self.assertEqual(alist, []) + class Object: def __init__(self, arg): self.arg = arg @@ -12,9 +12,20 @@ What's New in Python 2.4 alpha 1? Core and builtins ----------------- -- Compiler flags set in PYTHONSTARTUP are now active in __main__. - -- Added two builtin types, set() and frozenset(). +- Critical bugfix, for SF bug 839548: if a weakref with a callback, + its callback, and its weakly referenced object, all became part of + cyclic garbage during a single run of garbage collection, the order + in which they were torn down was unpredictable. It was possible for + the callback to see partially-torn-down objects, leading to immediate + segfaults, or, if the callback resurrected garbage objects, to + resurrect insane objects that caused segfaults (or other surprises) + later. In one sense this wasn't surprising, because Python's cyclic gc + had no knowledge of Python's weakref objects. It does now. When + weakrefs with callbacks become part of cyclic garbage now, those + weakrefs are cleared first. The callbacks don't trigger then, + preventing the problems. If you need callbacks to trigger, then just + as when cyclic gc is not involved, you need to write your code so + that weakref objects outlive the objects they weakly reference. - Critical bugfix, for SF bug 840829: if cyclic garbage collection happened to occur during a weakref callback for a new-style class @@ -22,6 +33,10 @@ Core and builtins in a debug build, a segfault occurred reliably very soon after). This has been repaired. +- Compiler flags set in PYTHONSTARTUP are now active in __main__. + +- Added two builtin types, set() and frozenset(). + - Added a reversed() builtin function that returns a reverse iterator over a sequence. diff --git a/Modules/gc_weakref.txt b/Modules/gc_weakref.txt new file mode 100644 index 0000000..b07903b --- /dev/null +++ b/Modules/gc_weakref.txt @@ -0,0 +1,107 @@ +Before 2.3.3, Python's cyclic gc didn't pay any attention to weakrefs. +Segfaults in Zope3 resulted. + +weakrefs in Python are designed to, at worst, let *other* objects learn +that a given object has died, via a callback function. The weakly +referenced object itself is not passed to the callback, and the presumption +is that the weakly referenced object is unreachable trash at the time the +callback is invoked. + +That's usually true, but not always. Suppose a weakly referenced object +becomes part of a clump of cyclic trash. When enough cycles are broken by +cyclic gc that the object is reclaimed, the callback is invoked. If it's +possible for the callback to get at objects in the cycle(s), then it may be +possible for those objects to access (via strong references in the cycle) +the weakly referenced object being torn down, or other objects in the cycle +that have already suffered a tp_clear() call. There's no guarantee that an +object is in a sane state after tp_clear(). Bad things (including +segfaults) can happen right then, during the callback's execution, or can +happen at any later time if the callback manages to resurrect an insane +object. + +Note that if it's possible for the callback to get at objects in the trash +cycles, it must also be the case that the callback itself is part of the +trash cycles. Else the callback would have acted as an external root to +the current collection, and nothing reachable from it would be in cyclic +trash either. + +More, if the callback itself is in cyclic trash, then the weakref to which +the callback is attached must also be trash, and for the same kind of +reason: if the weakref acted as an external root, then the callback could +not have been cyclic trash. + +So a problem here requires that a weakref, that weakref's callback, and the +weakly referenced object, all be in cyclic trash at the same time. This +isn't easy to stumble into by accident while Python is running, and, indeed, +it took quite a while to dream up failing test cases. Zope3 saw segfaults +during shutdown, during the second call of gc in Py_Finalize, after most +modules had been torn down. That creates many trash cycles (esp. those +involving new-style classes), making the problem much more likely. Once you +know what's required to provoke the problem, though, it's easy to create +tests that segfault before shutdown. + +In 2.3.3, before breaking cycles, we first clear all the weakrefs with +callbacks in cyclic trash. Since the weakrefs *are* trash, and there's no +defined-- or even predictable --order in which tp_clear() gets called on +cyclic trash, it's defensible to first clear weakrefs with callbacks. It's +a feature of Python's weakrefs too that when a weakref goes away, the +callback (if any) associated with it is thrown away too, unexecuted. + +Just that much is almost enough to prevent problems, by throwing away +*almost* all the weakref callbacks that could get triggered by gc. The +problem remaining is that clearing a weakref with a callback decrefs the +callback object, and the callback object may *itself* be weakly referenced, +via another weakref with another callback. So the process of clearing +weakrefs can trigger callbacks attached to other weakrefs, and those +latter weakrefs may or may not be part of cyclic trash. + +So, to prevent any Python code from running while gc is invoking tp_clear() +on all the objects in cyclic trash, it's not quite enough just to invoke +tp_clear() on weakrefs with callbacks first. Instead the weakref module +grew a new private function (_PyWeakref_ClearRef) that does only part of +tp_clear(): it removes the weakref from the weakly-referenced object's list +of weakrefs, but does not decref the callback object. So calling +_PyWeakref_ClearRef(wr) ensures that wr's callback object will never +trigger, and (unlike weakref's tp_clear()) also prevents any callback +associated *with* wr's callback object from triggering. + +Then we can call tp_clear on all the cyclic objects and never trigger +Python code. + +After we do that, the callback objects still need to be decref'ed. Callbacks +(if any) *on* the callback objects that were also part of cyclic trash won't +get invoked, because we cleared all trash weakrefs with callbacks at the +start. Callbacks on the callback objects that were not part of cyclic trash +acted as external roots to everything reachable from them, so nothing +reachable from them was part of cyclic trash, so gc didn't do any damage to +objects reachable from them, and it's safe to call them at the end of gc. + +An alternative would have been to treat objects with callbacks like objects +with __del__ methods, refusing to collect them, appending them to gc.garbage +instead. That would have been much easier. Jim Fulton gave a strong +argument against that (on Python-Dev): + + There's a big difference between __del__ and weakref callbacks. + The __del__ method is "internal" to a design. When you design a + class with a del method, you know you have to avoid including the + class in cycles. + + Now, suppose you have a design that makes has no __del__ methods but + that does use cyclic data structures. You reason about the design, + run tests, and convince yourself you don't have a leak. + + Now, suppose some external code creates a weakref to one of your + objects. All of a sudden, you start leaking. You can look at your + code all you want and you won't find a reason for the leak. + +IOW, a class designer can out-think __del__ problems, but has no control +over who creates weakrefs to his classes or class instances. The class +user has little chance either of predicting when the weakrefs he creates +may end up in cycles. + +Callbacks on weakref callbacks are executed in an arbitrary order, and +that's not good (a primary reason not to collect cycles with objects with +__del__ methods is to avoid running finalizers in an arbitrary order). +However, a weakref callback on a weakref callback has got to be rare. +It's possible to do such a thing, so gc has to be robust against it, but +I doubt anyone has done it outside the test case I wrote for it. diff --git a/Modules/gcmodule.c b/Modules/gcmodule.c index e6aabe4..7976b40 100644 --- a/Modules/gcmodule.c +++ b/Modules/gcmodule.c @@ -396,13 +396,17 @@ has_finalizer(PyObject *op) return 0; } -/* Move the objects in unreachable with __del__ methods into finalizers. - * The objects remaining in unreachable do not have __del__ methods, and - * gc_refs remains GC_TENTATIVELY_UNREACHABLE for them. The objects - * moved into finalizers have gc_refs changed to GC_REACHABLE. +/* 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_finalizers(PyGC_Head *unreachable, PyGC_Head *finalizers) +move_troublemakers(PyGC_Head *unreachable, + PyGC_Head *finalizers, + PyGC_Head *wr_callbacks) { PyGC_Head *gc = unreachable->gc.gc_next; @@ -417,6 +421,12 @@ move_finalizers(PyGC_Head *unreachable, PyGC_Head *finalizers) 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; } } @@ -453,6 +463,93 @@ move_finalizer_reachable(PyGC_Head *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); + PyWeakReference *wr = (PyWeakReference *)op; + + assert(IS_REACHABLE(op)); + assert(PyWeakref_Check(op)); + assert(wr->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) { @@ -554,8 +651,9 @@ collect(int generation) 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; - PyGC_Head finalizers; + 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) { @@ -616,20 +714,33 @@ collect(int generation) /* 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. + * instance objects with __del__ methods. Weakrefs with callbacks + * can call arbitrary Python code, so those are special-cased too. * - * Move unreachable objects with finalizers into a different list. + * Move unreachable objects with finalizers, and weakrefs with + * callbacks, into different lists. */ gc_list_init(&finalizers); - move_finalizers(&unreachable, &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 only *from* those are also - * uncollectable, and we move those into the finalizers list too. + * 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. */ + * debugging information. + */ for (gc = unreachable.gc.gc_next; gc != &unreachable; gc = gc->gc.gc_next) { m++; @@ -643,6 +754,11 @@ collect(int generation) */ 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; diff --git a/Objects/weakrefobject.c b/Objects/weakrefobject.c index f5be759..db1f8d1 100644 --- a/Objects/weakrefobject.c +++ b/Objects/weakrefobject.c @@ -53,17 +53,43 @@ clear_weakref(PyWeakReference *self) if (*list == self) *list = self->wr_next; self->wr_object = Py_None; - self->wr_callback = NULL; if (self->wr_prev != NULL) self->wr_prev->wr_next = self->wr_next; if (self->wr_next != NULL) self->wr_next->wr_prev = self->wr_prev; self->wr_prev = NULL; self->wr_next = NULL; - Py_XDECREF(callback); + } + if (callback != NULL) { + Py_DECREF(callback); + self->wr_callback = NULL; } } +/* Cyclic gc uses this to *just* clear the passed-in reference, leaving + * the callback intact and uncalled. It must be possible to call self's + * tp_dealloc() after calling this, so self has to be left in a sane enough + * state for that to work. We expect tp_dealloc to decref the callback + * then. The reason for not letting clear_weakref() decref the callback + * right now is that if the callback goes away, that may in turn trigger + * another callback (if a weak reference to the callback exists) -- running + * arbitrary Python code in the middle of gc is a disaster. The convolution + * here allows gc to delay triggering such callbacks until the world is in + * a sane state again. + */ +void +_PyWeakref_ClearRef(PyWeakReference *self) +{ + PyObject *callback; + + assert(self != NULL); + assert(PyWeakref_Check(self)); + /* Preserve and restore the callback around clear_weakref. */ + callback = self->wr_callback; + self->wr_callback = NULL; + clear_weakref(self); + self->wr_callback = callback; +} static void weakref_dealloc(PyWeakReference *self) @@ -117,7 +143,7 @@ weakref_hash(PyWeakReference *self) self->hash = PyObject_Hash(PyWeakref_GET_OBJECT(self)); return self->hash; } - + static PyObject * weakref_repr(PyWeakReference *self) @@ -324,7 +350,7 @@ WRAP_BINARY(proxy_iand, PyNumber_InPlaceAnd) WRAP_BINARY(proxy_ixor, PyNumber_InPlaceXor) WRAP_BINARY(proxy_ior, PyNumber_InPlaceOr) -static int +static int proxy_nonzero(PyWeakReference *proxy) { PyObject *o = PyWeakref_GET_OBJECT(proxy); |