4
The basic rule for dealing with weakref callbacks (and __del__ methods too,
5
for that matter) during cyclic gc:
7
Once gc has computed the set of unreachable objects, no Python-level
8
code can be allowed to access an unreachable object.
10
If that can happen, then the Python code can resurrect unreachable objects
11
too, and gc can't detect that without starting over. Since gc eventually
12
runs tp_clear on all unreachable objects, if an unreachable object is
13
resurrected then tp_clear will eventually be called on it (or may already
14
have been called before resurrection). At best (and this has been an
15
historically common bug), tp_clear empties an instance's __dict__, and
16
"impossible" AttributeErrors result. At worst, tp_clear leaves behind an
17
insane object at the C level, and segfaults result (historically, most
18
often by setting a class's mro pointer to NULL, after which attribute
19
lookups performed by the class can segfault).
21
OTOH, it's OK to run Python-level code that can't access unreachable
22
objects, and sometimes that's necessary. The chief example is the callback
23
attached to a reachable weakref W to an unreachable object O. Since O is
24
going away, and W is still alive, the callback must be invoked. Because W
25
is still alive, everything reachable from its callback is also reachable,
26
so it's also safe to invoke the callback (although that's trickier than it
27
sounds, since other reachable weakrefs to other unreachable objects may
28
still exist, and be accessible to the callback -- there are lots of painful
29
details like this covered in the rest of this file).
34
The "Before 2.3.3" section below turned out to be wrong in some ways, but
35
I'm leaving it as-is because it's more right than wrong, and serves as a
36
wonderful example of how painful analysis can miss not only the forest for
37
the trees, but also miss the trees for the aphids sucking the trees
40
The primary thing it missed is that when a weakref to a piece of cyclic
41
trash (CT) exists, then any call to any Python code whatsoever can end up
42
materializing a strong reference to that weakref's CT referent, and so
43
possibly resurrect an insane object (one for which cyclic gc has called-- or
44
will call before it's done --tp_clear()). It's not even necessarily that a
45
weakref callback or __del__ method does something nasty on purpose: as
46
soon as we execute Python code, threads other than the gc thread can run
47
too, and they can do ordinary things with weakrefs that end up resurrecting
48
CT while gc is running.
50
http://www.python.org/sf/1055820
52
shows how innocent it can be, and also how nasty. Variants of the three
53
focussed test cases attached to that bug report are now part of Python's
54
standard Lib/test/test_gc.py.
56
Jim Fulton gave the best nutshell summary of the new (in 2.4 and 2.3.5)
59
Clearing cyclic trash can call Python code. If there are weakrefs to
60
any of the cyclic trash, then those weakrefs can be used to resurrect
61
the objects. Therefore, *before* clearing cyclic trash, we need to
62
remove any weakrefs. If any of the weakrefs being removed have
63
callbacks, then we need to save the callbacks and call them *after* all
64
of the weakrefs have been cleared.
66
Alas, doing just that much doesn't work, because it overlooks what turned
67
out to be the much subtler problems that were fixed earlier, and described
68
below. We do clear all weakrefs to CT now before breaking cycles, but not
69
all callbacks encountered can be run later. That's explained in horrid
72
Older text follows, with a some later comments in [] brackets:
77
Before 2.3.3, Python's cyclic gc didn't pay any attention to weakrefs.
78
Segfaults in Zope3 resulted.
80
weakrefs in Python are designed to, at worst, let *other* objects learn
81
that a given object has died, via a callback function. The weakly
82
referenced object itself is not passed to the callback, and the presumption
83
is that the weakly referenced object is unreachable trash at the time the
86
That's usually true, but not always. Suppose a weakly referenced object
87
becomes part of a clump of cyclic trash. When enough cycles are broken by
88
cyclic gc that the object is reclaimed, the callback is invoked. If it's
89
possible for the callback to get at objects in the cycle(s), then it may be
90
possible for those objects to access (via strong references in the cycle)
91
the weakly referenced object being torn down, or other objects in the cycle
92
that have already suffered a tp_clear() call. There's no guarantee that an
93
object is in a sane state after tp_clear(). Bad things (including
94
segfaults) can happen right then, during the callback's execution, or can
95
happen at any later time if the callback manages to resurrect an insane
98
[That missed that, in addition, a weakref to CT can exist outside CT, and
99
any callback into Python can use such a non-CT weakref to resurrect its CT
100
referent. The same bad kinds of things can happen then.]
102
Note that if it's possible for the callback to get at objects in the trash
103
cycles, it must also be the case that the callback itself is part of the
104
trash cycles. Else the callback would have acted as an external root to
105
the current collection, and nothing reachable from it would be in cyclic
108
[Except that a non-CT callback can also use a non-CT weakref to get at
111
More, if the callback itself is in cyclic trash, then the weakref to which
112
the callback is attached must also be trash, and for the same kind of
113
reason: if the weakref acted as an external root, then the callback could
114
not have been cyclic trash.
116
So a problem here requires that a weakref, that weakref's callback, and the
117
weakly referenced object, all be in cyclic trash at the same time. This
118
isn't easy to stumble into by accident while Python is running, and, indeed,
119
it took quite a while to dream up failing test cases. Zope3 saw segfaults
120
during shutdown, during the second call of gc in Py_Finalize, after most
121
modules had been torn down. That creates many trash cycles (esp. those
122
involving classes), making the problem much more likely. Once you
123
know what's required to provoke the problem, though, it's easy to create
124
tests that segfault before shutdown.
126
In 2.3.3, before breaking cycles, we first clear all the weakrefs with
127
callbacks in cyclic trash. Since the weakrefs *are* trash, and there's no
128
defined-- or even predictable --order in which tp_clear() gets called on
129
cyclic trash, it's defensible to first clear weakrefs with callbacks. It's
130
a feature of Python's weakrefs too that when a weakref goes away, the
131
callback (if any) associated with it is thrown away too, unexecuted.
133
[In 2.4/2.3.5, we first clear all weakrefs to CT objects, whether or not
134
those weakrefs are themselves CT, and whether or not they have callbacks.
135
The callbacks (if any) on non-CT weakrefs (if any) are invoked later,
136
after all weakrefs-to-CT have been cleared. The callbacks (if any) on CT
137
weakrefs (if any) are never invoked, for the excruciating reasons
140
Just that much is almost enough to prevent problems, by throwing away
141
*almost* all the weakref callbacks that could get triggered by gc. The
142
problem remaining is that clearing a weakref with a callback decrefs the
143
callback object, and the callback object may *itself* be weakly referenced,
144
via another weakref with another callback. So the process of clearing
145
weakrefs can trigger callbacks attached to other weakrefs, and those
146
latter weakrefs may or may not be part of cyclic trash.
148
So, to prevent any Python code from running while gc is invoking tp_clear()
149
on all the objects in cyclic trash,
151
[That was always wrong: we can't stop Python code from running when gc
152
is breaking cycles. If an object with a __del__ method is not itself in
153
a cycle, but is reachable only from CT, then breaking cycles will, as a
154
matter of course, drop the refcount on that object to 0, and its __del__
155
will run right then. What we can and must stop is running any Python
156
code that could access CT.]
157
it's not quite enough just to invoke
158
tp_clear() on weakrefs with callbacks first. Instead the weakref module
159
grew a new private function (_PyWeakref_ClearRef) that does only part of
160
tp_clear(): it removes the weakref from the weakly-referenced object's list
161
of weakrefs, but does not decref the callback object. So calling
162
_PyWeakref_ClearRef(wr) ensures that wr's callback object will never
163
trigger, and (unlike weakref's tp_clear()) also prevents any callback
164
associated *with* wr's callback object from triggering.
166
[Although we may trigger such callbacks later, as explained below.]
168
Then we can call tp_clear on all the cyclic objects and never trigger
171
[As above, not so: it means never trigger Python code that can access CT.]
173
After we do that, the callback objects still need to be decref'ed. Callbacks
174
(if any) *on* the callback objects that were also part of cyclic trash won't
175
get invoked, because we cleared all trash weakrefs with callbacks at the
176
start. Callbacks on the callback objects that were not part of cyclic trash
177
acted as external roots to everything reachable from them, so nothing
178
reachable from them was part of cyclic trash, so gc didn't do any damage to
179
objects reachable from them, and it's safe to call them at the end of gc.
181
[That's so. In addition, now we also invoke (if any) the callbacks on
182
non-CT weakrefs to CT objects, during the same pass that decrefs the
185
An alternative would have been to treat objects with callbacks like objects
186
with __del__ methods, refusing to collect them, appending them to gc.garbage
187
instead. That would have been much easier. Jim Fulton gave a strong
188
argument against that (on Python-Dev):
190
There's a big difference between __del__ and weakref callbacks.
191
The __del__ method is "internal" to a design. When you design a
192
class with a del method, you know you have to avoid including the
195
Now, suppose you have a design that makes has no __del__ methods but
196
that does use cyclic data structures. You reason about the design,
197
run tests, and convince yourself you don't have a leak.
199
Now, suppose some external code creates a weakref to one of your
200
objects. All of a sudden, you start leaking. You can look at your
201
code all you want and you won't find a reason for the leak.
203
IOW, a class designer can out-think __del__ problems, but has no control
204
over who creates weakrefs to his classes or class instances. The class
205
user has little chance either of predicting when the weakrefs he creates
206
may end up in cycles.
208
Callbacks on weakref callbacks are executed in an arbitrary order, and
209
that's not good (a primary reason not to collect cycles with objects with
210
__del__ methods is to avoid running finalizers in an arbitrary order).
211
However, a weakref callback on a weakref callback has got to be rare.
212
It's possible to do such a thing, so gc has to be robust against it, but
213
I doubt anyone has done it outside the test case I wrote for it.
215
[The callbacks (if any) on non-CT weakrefs to CT objects are also executed
216
in an arbitrary order now. But they were before too, depending on the
217
vagaries of when tp_clear() happened to break enough cycles to trigger
218
them. People simply shouldn't try to use __del__ or weakref callbacks to