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author | Tim Peters <tim.peters@gmail.com> | 2003-11-20 21:21:46 (GMT) |
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committer | Tim Peters <tim.peters@gmail.com> | 2003-11-20 21:21:46 (GMT) |
commit | 403a2032230087943a52453deef93769f22461f8 (patch) | |
tree | fb760ac52c5fc2d9a83dd444f0d8dd15d28b50ce /Lib/test | |
parent | 901dc983168d4ca706ed664a8f5134ee3add26ed (diff) | |
download | cpython-403a2032230087943a52453deef93769f22461f8.zip cpython-403a2032230087943a52453deef93769f22461f8.tar.gz cpython-403a2032230087943a52453deef93769f22461f8.tar.bz2 |
SF bug 839548: Bug in type's GC handling causes segfaults.
Also SF patch 843455.
This is a critical bugfix.
I'll backport to 2.3 maint, but not beyond that. The bugs this fixes
have been there since weakrefs were introduced.
Diffstat (limited to 'Lib/test')
-rw-r--r-- | Lib/test/test_weakref.py | 205 |
1 files changed, 205 insertions, 0 deletions
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 |