[issue27275] KeyError thrown by optimised collections.OrderedDict.popitem()

[Łukasz Langa]

Change by Łukasz Langa :


--
resolution:  -> fixed
stage: patch review -> resolved
status: open -> closed

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[issue27275] KeyError thrown by optimised collections.OrderedDict.popitem()

[Łukasz Langa]

Łukasz Langa  added the comment:


New changeset 8c9f847997196aa76500d1ae104cbe7fe2a467ed by Serhiy Storchaka in 
branch 'main':
bpo-27275: Change popitem() and pop() methods of collections.OrderedDict 
(GH-27530)
https://github.com/python/cpython/commit/8c9f847997196aa76500d1ae104cbe7fe2a467ed


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[issue27275] KeyError thrown by optimised collections.OrderedDict.popitem()

[Raymond Hettinger]

Raymond Hettinger  added the comment:

> I am not sure how far we should backport these changes
> if backport them.

We've had no reports of the current code causing problems for any existing 
applications (except the LRU recipe in the docs), so there is likely no value 
in making backports.  Instead, we can clean it up so there won't be new issues 
going forward.

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versions:  -Python 3.10, Python 3.9

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[issue27275] KeyError thrown by optimised collections.OrderedDict.popitem()

[Serhiy Storchaka]

Serhiy Storchaka  added the comment:

PR 27528 makes the C implementation of OrderedDict.popitem() consistent with 
the Python implementation (do not call overridden __getitem__ and __setitem__).

PR 27530 changes also both implementations of OrderedDict.pop(). It simplifies 
the C code, but adds a duplication of the code in Python.

I am not sure how far we should backport these changes if backport them.

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[issue27275] KeyError thrown by optimised collections.OrderedDict.popitem()

[Serhiy Storchaka]

Change by Serhiy Storchaka :


--
pull_requests: +26045
pull_request: https://github.com/python/cpython/pull/27530

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[issue27275] KeyError thrown by optimised collections.OrderedDict.popitem()

[Raymond Hettinger]

Raymond Hettinger  added the comment:

Let's do the right thing and fix the pure python OrderedDict.pop() method as 
well.

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[issue27275] KeyError thrown by optimised collections.OrderedDict.popitem()

[Serhiy Storchaka]

Change by Serhiy Storchaka :


--
pull_requests: +26043
pull_request: https://github.com/python/cpython/pull/27528

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[issue27275] KeyError thrown by optimised collections.OrderedDict.popitem()

[Serhiy Storchaka]

Serhiy Storchaka  added the comment:

It is complicated. The pure Python implementation of OrderedDict.popitem() and 
OrderedDict.pop() are not consistent. The former uses dict.pop() which doesn't 
call __getitem__ and __setitem__. The latter calls __getitem__ and __setitem__. 
The C implementation shared code between popitem() and pop(), therefore it will 
differ from the pure Python implementation until we write separate code for 
popitem() and pop().

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[issue27275] KeyError thrown by optimised collections.OrderedDict.popitem()

[Serhiy Storchaka]

Change by Serhiy Storchaka :


--
versions: +Python 3.10, Python 3.11, Python 3.9 -Python 3.5, Python 3.6, Python 
3.7

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[issue27275] KeyError thrown by optimised collections.OrderedDict.popitem()

[Serhiy Storchaka]

Serhiy Storchaka  added the comment:

It was reverted because it did not keep the C version in harmony with the pure 
Python version. In the pure Python version pop() calls __getitem__ and 
__delitem__ which can be overridden in subclasses of OrederedDict. My patch 
always called dict.__getitem__ and dict.__delitem__.

But I see now clearer what is the problem with the current C code. It removes 
the key from the linked list before calling __delitem__ which itself removes 
the key from the linked list. Perhaps I can fix it correctly this time.

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[issue27275] KeyError thrown by optimised collections.OrderedDict.popitem()

[Raymond Hettinger]

Raymond Hettinger  added the comment:

> I think Serhiy's patch here (before revert) may be a 
> good idea (to re-apply).

That seems sensible to me as well.  It keeps the C version in harmony with the 
pure python version and it follows how regular dict's are implemented.

Serhiy, do you remember why your patch was reverted?

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[issue27275] KeyError thrown by optimised collections.OrderedDict.popitem()

[Dennis Sweeney]

Dennis Sweeney  added the comment:

bpo-44782 was opened about the `class LRU(OrderedDict)` in the OrderedDict 
docs, and its pop() method failing.

I think Serhiy's patch here (before revert) may be a good idea (to re-apply).
I think it is reasonable to ignore user-implemented dunder methods from 
subclasses.
Concrete type implementations generally do not behave as mix-ins:

def never_called(self, *args):
print("Never called.")
raise ZeroDivisionError

class MyList(list):
__setitem__ = __delitem__ = __getitem__ = __len__ = __iter__ = 
__contains__ = never_called

class MyDict(dict):
__setitem__ = __delitem__ = __getitem__ = __len__ = __iter__ = 
__contains__ = never_called

class MySet(set):
__setitem__ = __delitem__ = __getitem__ = __len__ = __iter__ = 
__contains__ = never_called

L = MyList([5, 4, 3, 2])
L.sort()
L.pop(1)
L.insert(0, 42)
L.pop()
L.reverse()
assert type(L) is MyList

D = MyDict({"a": 1, "b": 2, "c": 3})
assert D.get(0) is None
assert D.get("a") == 1
assert D.pop("b") == 2
assert D.popitem() == ("c", 3)
assert type(D) is MyDict

S = MySet({"a", "b", "c"})
S.discard("a")
S.remove("b")
S.isdisjoint(S)
S |= S
S &= S
S ^= S
assert type(S) is MySet

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[issue27275] KeyError thrown by optimised collections.OrderedDict.popitem()

[Roundup Robot]

Roundup Robot added the comment:

New changeset 3f816eecc53e by Serhiy Storchaka in branch '3.5':
Backed out changeset 9f7505019767 (issue #27275).
https://hg.python.org/cpython/rev/3f816eecc53e

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[issue27275] KeyError thrown by optimised collections.OrderedDict.popitem()

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

In issue28014 __getitem__() is idempotent. Multiple calls of __getitem__() 
return the same result and keep the OrderedDict in the same state.

> I'd be perfectly happy with making popitem implemented in terms of pop on 
> subclasses when pop is overridden (if pop isn't overridden though, that's 
> effectively what popitem already does).

I like this idea.

> Note: In the expiring case, the fix is still "wrong" if someone used popitem 
> for the intended purpose (to get and delete).

Good catch! But old implementation still looks doubtful to me.

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[issue27275] KeyError thrown by optimised collections.OrderedDict.popitem()

[Josh Rosenberg]

Josh Rosenberg added the comment:

The Python implementation of OrderedDict breaks for issue28014, at least on 
3.4.3 (it doesn't raise KeyError, but if you check the repr, it's only showing 
one of the two entries, because calling __getitem__ is rearranging the 
OrderedDict).

>>> s = SimpleLRUCache(2)
>>> s['t1'] = 1
>>> s
SimpleLRUCache([('t1', 1)])
>>> s['t2'] = 2
>>> s
SimpleLRUCache([('t1', 1)])
>>> s
SimpleLRUCache([('t2', 2)])

Again, the OrderedDict code (in the Python case, __repr__, in the C case, 
popitem) assumes __getitem__ is idempotent, and again, the violation of that 
constraint makes things break. They break differently in the Python 
implementation and the C implementation, but they still break, because people 
are trying to force OrderedDict to do unnatural things without implementing 
their own logic to ensure their violations of the dict pseudo-contract actually 
works.

popitem happens to be a common cause of problems because it's logically a get 
and delete combined. People aren't using it for the get feature, it's just a 
convenient way to remove items from the end; if they bypassed getting and just 
deleted it would work, but it's a more awkward construction, so they don't. If 
they implemented their own popitem that avoided their own non-idempotent 
__getitem__, that would also work.

I'd be perfectly happy with making popitem implemented in terms of pop on 
subclasses when pop is overridden (if pop isn't overridden though, that's 
effectively what popitem already does).

I just don't think we should be making the decision that popitem *requires* 
inheritance for all dict subclasses that have (normal) idempotent __contains__ 
and __getitem__ because classes that violate the usual expectations of 
__contains__ and __getitem__ have (non-segfaulting) problems.

Note: In the expiring case, the fix is still "wrong" if someone used popitem 
for the intended purpose (to get and delete). The item popped might have 
expired an hour ago, but because the fixed code bypasses __getitem__, it will 
happily return the expired a long expired item (having bypassed expiration 
checking). It also breaks encapsulation, returning the expiry time that is 
supposed to be stripped on pop. By fixing one logic flaw on behalf of a 
fundamentally broken subclass, we introduced another.

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[issue27275] KeyError thrown by optimised collections.OrderedDict.popitem()

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

Ah, what is the reason for this code!

But Python implementation of popitem() don't call overridden 
__getitem__/__delitem__. It uses dict.pop(). Simplified C implementation is 
closer to Python implementation.

expiringdict is not the only implementation broken by accelerated OrderedDict. 
See other example in issue28014.

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[issue27275] KeyError thrown by optimised collections.OrderedDict.popitem()

[Josh Rosenberg]

Josh Rosenberg added the comment:

Explaining expiringdict's issue: It's two race conditions, with itself, not 
another thread.

Example scenario (narrow race window):

1. An entry has an expiry time of X (so it will self-delete at time X or later)
2. At time X-1, the PySequence_Contains check is run, and it returns 1 (true)
3. Because contains returned True, at time X PyObject_GetItem is run, but 
because it's time X, expiringdict's __getitem__ deletes the entry and raises 
KeyError

An alternative scenario with a *huge* race window is:

1. An entry has an expiry time of X (so it will self-delete at time X or later)
2. No lookups or membership tests or any other operations that implicitly clean 
the expiring dict occur for a while
3. At time X+1000, _odict_FIRST (in popitem) grabs the first entry in the 
OrderedDict without invoking the __contains__ machinery that would delete the 
entry
3. At time X+1000 or so, the PySequence_Contains check is run, and it returns 0 
(false), because the __contains__ machinery is invoked, and again, because no 
default is provided for popitem, a KeyError is raised (this time by the popitem 
machinery, not __getitem__)

expiringdict is unusually good at bringing this on itself. The failing popitem 
call is in __setitem__ for limited length expiringdicts, 
self.popitem(last=False), where they're intentionally removing the oldest 
entry, when the oldest entry is the most likely to have expired (and since 
__len__ isn't overridden to expire old entries, it may have been expired for 
quite a while).

The del self[next(OrderedDict.__iter__(self))] works because they didn't 
override __iter__, so it's not expiring anything to get the first item, and 
therefore only __delitem__ is involved, not __contains__ or __getitem__ (note: 
This is also why the bug they reference has an issue with "OrderedDict mutated 
during iteration"; iteration returns long expired keys, but looking the expired 
keys up deletes them, causing the mutation issue).


Possible correct fixes:

1. Make popitem _more_ subclass friendly; when it's an OrderedDict subclass, 
instead of using _odict_LAST and _odict_FIRST, use (C equivalent) of 
`next(reversed(self))` and `next(iter(self))` respectively. This won't fix 
expiringdict as is (because it's broken by design; it doesn't override 
__iter__, so it will happily return long expired keys that disappear on 
access), but if we're going for subclass friendliness and allowing 
non-idempotent __contains__/__getitem__/__iter__ implementations, it's the 
right thing to do. If expiringdict implemented __iter__ to copy the keys, then 
loop over the copy, deleting expired values and yielding unexpired values, this 
would at least reduce the huge race window to a narrow window (because it could 
still yield a key that's almost expired)
2. Check for the presence of __missing__ on the type and only perform the 
PySequence_Contains check if __missing__ is defined (to avoid 
autovivification). This fixes the narrow race condition for subclasses without 
__missing__, but not the huge race condition
3. Explicitly document the idempotence assumptions made by OrderedDict 
(specifically, that all non-mutating methods of OrderedDict must not be made 
mutating in subclasses unless the caller also overrides all multistep 
operations, e.g. pop/popitem/setdefault).

TL;DR: expiringdict is doing terrible things, assuming the superclass will 
handle them even though the superclass has completely different assumptions, 
and therefore expiringdict has only itself to blame.

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[issue27275] KeyError thrown by optimised collections.OrderedDict.popitem()

[Josh Rosenberg]

Josh Rosenberg added the comment:

Serhiy, doesn't this patch "fix" the issue by making subclasses with custom 
__getitem__/__delitem__ implementations not have them invoked by the 
superclass's pop/popitem?

The old code meant that pop and popitem didn't need to be overridden even if 
you overrode __getitem__/__delitem__ in a way that differed from the default 
(e.g. __setitem__ might add some tracking data to the value that __getitem__ 
strips). Now they must be overwritten.

The expiringdict's flaw seems to be that its __contains__ call and its 
__getitem__ are not idempotent, which the original code assumed (reasonably) 
they would be.

The original code should probably be restored here. The general 
PyObject_GetItem/DelItem are needed to work with arbitrary subclasses 
correctly. The Sequence_Contains check is needed to avoid accidentally invoking 
__missing__ (though if __missing__ is not defined for the subclass, the 
Sequence_Contains check could be skipped).

The only reason OrderedDict has the problem and dict doesn't is that 
OrderedDict was trying to be subclassing friendly (perhaps to ensure it remains 
compatible with code that subclassed the old Python implementation), while dict 
makes no such efforts. dict happily bypasses custom __getitem__/__delitem__ 
calls when it uses pop/popitem.

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[issue27275] KeyError thrown by optimised collections.OrderedDict.popitem()

[Roundup Robot]

Roundup Robot added the comment:

New changeset 9f7505019767 by Serhiy Storchaka in branch '3.5':
Issue #27275: Fixed implementation of pop() and popitem() methods in
https://hg.python.org/cpython/rev/9f7505019767

New changeset 2def8a24c299 by Serhiy Storchaka in branch '3.6':
Issue #27275: Fixed implementation of pop() and popitem() methods in
https://hg.python.org/cpython/rev/2def8a24c299

New changeset 19e199038704 by Serhiy Storchaka in branch 'default':
Issue #27275: Fixed implementation of pop() and popitem() methods in
https://hg.python.org/cpython/rev/19e199038704

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[issue27275] KeyError thrown by optimised collections.OrderedDict.popitem()

[Serhiy Storchaka]

Changes by Serhiy Storchaka :


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[issue27275] KeyError thrown by optimised collections.OrderedDict.popitem()

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

Eric, could you please look at the patch? Maybe I missed some reasons for 
current implementation.

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[issue27275] KeyError thrown by optimised collections.OrderedDict.popitem()

[INADA Naoki]

INADA Naoki added the comment:

lgtm

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[issue27275] KeyError thrown by optimised collections.OrderedDict.popitem()

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

Proposed patch makes the implementation of pop() and popitem() methods of the C 
implementation of OrderedDict matching the Python implementation. This fixes 
issue28014 and I suppose this fixes this issue too.

--
keywords: +patch
stage: test needed -> patch review
versions: +Python 3.7
Added file: http://bugs.python.org/file44842/ordered_dict_subclass_pop.patch

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[issue27275] KeyError thrown by optimised collections.OrderedDict.popitem()

[Zachary Ware]

Zachary Ware added the comment:

Attaching test case from #28014 here since this issue looks close enough to 
that one to be caused by the same thing.

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Added file: http://bugs.python.org/file44459/simple_lru.py

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[issue27275] KeyError thrown by optimised collections.OrderedDict.popitem()

[Xiang Zhang]

Xiang Zhang added the comment:

There seems to be some difference behaviours between C version and pure Python 
version when it comes to subclass. Except popitem, the constructor also goes 
different code path. There may be more. Should these differences be eliminated 
or they are accepted?

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[issue27275] KeyError thrown by optimised collections.OrderedDict.popitem()

[William Pitcock]

William Pitcock added the comment:

It seems to me that calling __contains__() (PySequence_Contains()) isn't 
necessary, as the first and last elements of the list are already known, and 
therefore known to be in the list.  Revising the behaviour of popitem() to 
avoid calling _odict_popkey_hash() seems like it may provide a marginal 
performance benefit as well as fix the problem.  Calling PyObject_DelItem() 
directly on the node should work fine I believe.

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[issue27275] KeyError thrown by optimised collections.OrderedDict.popitem()

[Xiang Zhang]

Xiang Zhang added the comment:

Raymond, In single threaded case popitem may still fail.

I want to correct my last message that popitem does not fail in this case 
because it calls __getitem__ but instead it calls __contains__[1]. In 
__contains__ it deletes the item since it expires, and finally emit a 
KeyError[2]. Even if it passes __contains__, it will call __getitem__[3].

[1] https://hg.python.org/cpython/file/tip/Objects/odictobject.c#l1115
[2] https://hg.python.org/cpython/file/tip/Objects/odictobject.c#l1135
[3] https://hg.python.org/cpython/file/tip/Objects/odictobject.c#l1119

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[issue27275] KeyError thrown by optimised collections.OrderedDict.popitem()

[Serhiy Storchaka]

Changes by Serhiy Storchaka :


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[issue27275] KeyError thrown by optimised collections.OrderedDict.popitem()

[William Pitcock]

William Pitcock added the comment:

At least in my case, the application is single-threaded.  I don't think this is 
a locking-related issue as the expiringdict test case itself fails which is 
also single-threaded.

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[issue27275] KeyError thrown by optimised collections.OrderedDict.popitem()

[Raymond Hettinger]

Raymond Hettinger added the comment:

I'm wondering if the expiringdict(1) needs to have locked wrappers for the 
inherited methods:

def __delitem__(self, key):
with self.lock:
OrderedDict.__delitem__(self, key)

Otherwise, there is a risk that one thread is deleting a key with no lock held, 
while another thread is running expiringdict.popitem() which holds a lock while 
calling both __getitem__ and del.  If the first thread runs between the two 
steps in the second, the race condition would cause a KeyError.

This might explain why you've observed, '''Replacing use of popitem() with "del 
self[next(OrderedDict.__iter__(self))]" removes the KeyErrors and the structure 
otherwise works fine.'''

(1) https://github.com/mailgun/expiringdict/blob/master/expiringdict/__init__.py

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[issue27275] KeyError thrown by optimised collections.OrderedDict.popitem()

[Xiang Zhang]

Xiang Zhang added the comment:

I think your expiringdict seems not work with the C version OrderedDict, you 
may need to change your implementation or clarify that :(.

The C version's OrderedDict.popitem may call your __getitem__ which then does 
deletion and emit KeyError when expires. I think the new OrderedDict may call 
your __getitem__ even in iteration which leads to the 'RuntimeError: 
OrderedDict mutated during iteration'. I haven't checked that.

So a simple working example in Py3.4:

d = ExpiringDict(max_len=3, max_age_seconds=0.01)
d['a'] = 'z'
sleep(1)
d.popitem()

will fail in Py3.5+.

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[issue27275] KeyError thrown by optimised collections.OrderedDict.popitem()

[William Pitcock]

William Pitcock added the comment:

A frequent reproducer is to run the expiringdict tests on Python 3.5.1, 
unfortunately I cannot come up with a testcase.

Replacing use of popitem() with "del self[next(OrderedDict.__iter__(self))]" 
removes the KeyErrors and the structure otherwise works fine.

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[issue27275] KeyError thrown by optimised collections.OrderedDict.popitem()

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

Could you please provide short example?

--
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stage:  -> test needed

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[issue27275] KeyError thrown by optimised collections.OrderedDict.popitem()

[William Pitcock]

New submission from William Pitcock:

The C-based optimised version of collections.OrderedDict occasionally throws 
KeyErrors when deleting items.

See https://github.com/mailgun/expiringdict/issues/16 for an example of this 
regression.

Backporting 3.6's patches to 3.5.1 does not resolve the issue. :(

--
components: Library (Lib)
messages: 267957
nosy: kaniini
priority: normal
severity: normal
status: open
title: KeyError thrown by optimised collections.OrderedDict.popitem()
type: behavior
versions: Python 3.5, Python 3.6

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