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.
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 @@
# 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