Intro

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Note: this document was written long ago, before PEP 442 (safe object

finalization) was implemented. While that has changed some things, this

document is still mostly accurate. Just note that the rules being discussed

here apply to the unreachable set of objects *after* non-legacy finalizers

have been called. Also, the clearing of weakrefs has been changed to happen

later in the collection (after running finalizers but before tp_clear is

called).

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The basic rule for dealing with weakref callbacks (and __del__ methods too,

for that matter) during cyclic gc:

Once gc has computed the set of unreachable objects, no Python-level

code can be allowed to access an unreachable object.

If that can happen, then the Python code can resurrect unreachable objects

too, and gc can't detect that without starting over. Since gc eventually

runs tp_clear on all unreachable objects, if an unreachable object is

resurrected then tp_clear will eventually be called on it (or may already

have been called before resurrection). At best (and this has been an

historically common bug), tp_clear empties an instance's __dict__, and

"impossible" AttributeErrors result. At worst, tp_clear leaves behind an

insane object at the C level, and segfaults result (historically, most

often by setting a class's mro pointer to NULL, after which attribute

lookups performed by the class can segfault).

OTOH, it's OK to run Python-level code that can't access unreachable

objects, and sometimes that's necessary. The chief example is the callback

attached to a reachable weakref W to an unreachable object O. Since O is

going away, and W is still alive, the callback must be invoked. Because W

is still alive, everything reachable from its callback is also reachable,

so it's also safe to invoke the callback (although that's trickier than it

sounds, since other reachable weakrefs to other unreachable objects may

still exist, and be accessible to the callback -- there are lots of painful

details like this covered in the rest of this file).

Python 2.4/2.3.5

================

The "Before 2.3.3" section below turned out to be wrong in some ways, but

I'm leaving it as-is because it's more right than wrong, and serves as a

wonderful example of how painful analysis can miss not only the forest for

the trees, but also miss the trees for the aphids sucking the trees

dry <wink>.

The primary thing it missed is that when a weakref to a piece of cyclic

trash (CT) exists, then any call to any Python code whatsoever can end up

materializing a strong reference to that weakref's CT referent, and so

possibly resurrect an insane object (one for which cyclic gc has called-- or

will call before it's done --tp_clear()). It's not even necessarily that a

weakref callback or __del__ method does something nasty on purpose: as

soon as we execute Python code, threads other than the gc thread can run

too, and they can do ordinary things with weakrefs that end up resurrecting

CT while gc is running.

https://bugs.python.org/issue1055820

shows how innocent it can be, and also how nasty. Variants of the three

focused test cases attached to that bug report are now part of Python's

standard Lib/test/test_gc.py.

Jim Fulton gave the best nutshell summary of the new (in 2.4 and 2.3.5)

approach:

Clearing cyclic trash can call Python code. If there are weakrefs to

any of the cyclic trash, then those weakrefs can be used to resurrect

the objects. Therefore, *before* clearing cyclic trash, we need to

remove any weakrefs. If any of the weakrefs being removed have

callbacks, then we need to save the callbacks and call them *after* all

of the weakrefs have been cleared.

Alas, doing just that much doesn't work, because it overlooks what turned

out to be the much subtler problems that were fixed earlier, and described

below. We do clear all weakrefs to CT now before breaking cycles, but not

all callbacks encountered can be run later. That's explained in horrid

detail below.

Older text follows, with a some later comments in [] brackets:

Before 2.3.3

============

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.

[That missed that, in addition, a weakref to CT can exist outside CT, and

any callback into Python can use such a non-CT weakref to resurrect its CT

referent. The same bad kinds of things can happen then.]

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.

[Except that a non-CT callback can also use a non-CT weakref to get at

CT objects.]

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 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.

[In 2.4/2.3.5, we first clear all weakrefs to CT objects, whether or not

those weakrefs are themselves CT, and whether or not they have callbacks.

The callbacks (if any) on non-CT weakrefs (if any) are invoked later,

after all weakrefs-to-CT have been cleared. The callbacks (if any) on CT

weakrefs (if any) are never invoked, for the excruciating reasons

explained here.]

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,

[That was always wrong: we can't stop Python code from running when gc

is breaking cycles. If an object with a __del__ method is not itself in

a cycle, but is reachable only from CT, then breaking cycles will, as a

matter of course, drop the refcount on that object to 0, and its __del__

will run right then. What we can and must stop is running any Python

code that could access CT.]

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.

[Although we may trigger such callbacks later, as explained below.]

Then we can call tp_clear on all the cyclic objects and never trigger

Python code.

[As above, not so: it means never trigger Python code that can access CT.]

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.

[That's so. In addition, now we also invoke (if any) the callbacks on

non-CT weakrefs to CT objects, during the same pass that decrefs the

callback objects.]

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.

[The callbacks (if any) on non-CT weakrefs to CT objects are also executed

in an arbitrary order now. But they were before too, depending on the

vagaries of when tp_clear() happened to break enough cycles to trigger

them. People simply shouldn't try to use __del__ or weakref callbacks to

do fancy stuff.]