Issue 19542: WeakValueDictionary bug in setdefault()&pop()

Messages (15) msg202510 - (view) Author: Armin Rigo (arigo) * Date: 2013-11-10 08:51

WeakValueDictionary.setdefault() contains a bug that shows up in multithreaded situations using reference cycles.  Attached a test case: it is possible for 'setdefault(key, default)' to return None, although None is never put as a value in the dictionary.  (Actually, being a WeakValueDictionary, None is not allowed as a value.)

The reason is that the code in setdefault() ends in "return wr()", but the weakref "wr" might have gone invalid between the time it was fetched from "self.data" and the "wr()" itself, thus returning None.

A similar problem occurs in pop(), leading it to possibly raise KeyError even if it is called with a default argument.

msg205603 - (view) Author: Antoine Pitrou (pitrou) * Date: 2013-12-08 20:30

I think the underlying question is: are weak dicts otherwise MT-safe?

msg205609 - (view) Author: Armin Rigo (arigo) * Date: 2013-12-08 21:57

As you can see in x.py, the underlying question is rather: are weakdicts usable in a single thread of a multithreaded program?  I believe that this question cannot reasonably be answered No, independently on the answer you want to give to your own question.

msg205614 - (view) Author: Antoine Pitrou (pitrou) * Date: 2013-12-08 22:19

Ah, you're right. We just need to cook up a patch then.

msg205645 - (view) Author: Tim Peters (tim.peters) * Date: 2013-12-09 03:55

The weakref.slice fix looks solid to me, although it appears to be specific to 2.7 (the methods are fancier on the current default branch, fiddling with self._pending_removals too).

Does anyone know why the signature of pop is:

    def pop(self, key, *args)

?  It doesn't make sense to me to accept any number of arguments following `key`.  Perhaps it's just an obscure way to avoid doing:

_noarg = object()  # unique sentinel

def pop(self, key, default=_noarg):

?

msg205674 - (view) Author: Antoine Pitrou (pitrou) * Date: 2013-12-09 10:41

Changeset ea70032a24b1 is where the pop(*args) thing comes from.

msg235301 - (view) Author: Armin Rigo (arigo) * Date: 2015-02-02 23:42

Converted the test into an official test, which fails; and wrote the patch for CPython "3.trunk" and for CPython 2.7.  Please review and commit.

msg245470 - (view) Author: Tin Tvrtković (tinchester) * Date: 2015-06-18 15:23

We're actually getting bitten by this in production through the Riak Python client, so this isn't a strictly theoretical problem.

msg245471 - (view) Author: Antoine Pitrou (pitrou) * Date: 2015-06-18 15:24

Yes, we need to fix this. Sorry for being a bit slow.

msg279455 - (view) Author: Armin Rigo (arigo) * Date: 2016-10-25 20:39

ping

msg283599 - (view) Author: Roundup Robot (python-dev) Date: 2016-12-19 10:01

New changeset 5a542a2bca08 by Antoine Pitrou in branch '3.5':
Issue #19542: Fix bugs in WeakValueDictionary.setdefault() and WeakValueDictionary.pop()
https://hg.python.org/cpython/rev/5a542a2bca08

New changeset f3706a9430da by Antoine Pitrou in branch '3.6':
Issue #19542: Fix bugs in WeakValueDictionary.setdefault() and WeakValueDictionary.pop()
https://hg.python.org/cpython/rev/f3706a9430da

New changeset ac2715d04119 by Antoine Pitrou in branch 'default':
Issue #19542: Fix bugs in WeakValueDictionary.setdefault() and WeakValueDictionary.pop()
https://hg.python.org/cpython/rev/ac2715d04119

msg283601 - (view) Author: Roundup Robot (python-dev) Date: 2016-12-19 10:14

New changeset bcb1f0698401 by Antoine Pitrou in branch '2.7':
Issue #19542: Fix bugs in WeakValueDictionary.setdefault() and WeakValueDictionary.pop()
https://hg.python.org/cpython/rev/bcb1f0698401

msg283602 - (view) Author: Antoine Pitrou (pitrou) * Date: 2016-12-19 10:14

This is finally fixed!

msg284040 - (view) Author: Arfrever Frehtes Taifersar Arahesis (Arfrever) * Date: 2016-12-26 20:06

Newly added test_threaded_weak_valued_pop() triggers ignored KeyError exceptions (on all branches):

$ python3.7 -m test -v test_weakref
...
0:00:00 [1/1] test_weakref
...
test_make_weak_keyed_dict_from_dict (test.test_weakref.MappingTestCase) ... ok
test_make_weak_keyed_dict_from_weak_keyed_dict (test.test_weakref.MappingTestCase) ... ok
test_make_weak_keyed_dict_repr (test.test_weakref.MappingTestCase) ... ok
test_make_weak_valued_dict_from_dict (test.test_weakref.MappingTestCase) ... ok
test_make_weak_valued_dict_from_weak_valued_dict (test.test_weakref.MappingTestCase) ... ok
test_make_weak_valued_dict_misc (test.test_weakref.MappingTestCase) ... ok
test_make_weak_valued_dict_repr (test.test_weakref.MappingTestCase) ... ok
test_threaded_weak_valued_pop (test.test_weakref.MappingTestCase) ... Exception ignored in: <function WeakValueDictionary.__init__.<locals>.remove at 0x7fb20612bb70>
Traceback (most recent call last):
  File "/usr/lib64/python3.7/weakref.py", line 114, in remove
    del self.data[wr.key]
KeyError: (10,)
Exception ignored in: <function WeakValueDictionary.__init__.<locals>.remove at 0x7fb20612bb70>
Traceback (most recent call last):
  File "/usr/lib64/python3.7/weakref.py", line 114, in remove
    del self.data[wr.key]
KeyError: (10,)
Exception ignored in: <function WeakValueDictionary.__init__.<locals>.remove at 0x7fb20612bb70>
Traceback (most recent call last):
  File "/usr/lib64/python3.7/weakref.py", line 114, in remove
    del self.data[wr.key]
KeyError: (10,)
Exception ignored in: <function WeakValueDictionary.__init__.<locals>.remove at 0x7fb20612bb70>
Traceback (most recent call last):
  File "/usr/lib64/python3.7/weakref.py", line 114, in remove
    del self.data[wr.key]
KeyError: (10,)
Exception ignored in: <function WeakValueDictionary.__init__.<locals>.remove at 0x7fb20612bb70>
Traceback (most recent call last):
  File "/usr/lib64/python3.7/weakref.py", line 114, in remove
    del self.data[wr.key]
KeyError: (10,)
Exception ignored in: <function WeakValueDictionary.__init__.<locals>.remove at 0x7fb20612bb70>
Traceback (most recent call last):
  File "/usr/lib64/python3.7/weakref.py", line 114, in remove
    del self.data[wr.key]
KeyError: (10,)
Exception ignored in: <function WeakValueDictionary.__init__.<locals>.remove at 0x7fb20612bb70>
Traceback (most recent call last):
  File "/usr/lib64/python3.7/weakref.py", line 114, in remove
    del self.data[wr.key]
KeyError: (10,)
ok
test_threaded_weak_valued_setdefault (test.test_weakref.MappingTestCase) ... ok
test_weak_keyed_bad_delitem (test.test_weakref.MappingTestCase) ... ok
test_weak_keyed_cascading_deletes (test.test_weakref.MappingTestCase) ... ok
test_weak_keyed_delitem (test.test_weakref.MappingTestCase) ... ok
test_weak_keyed_dict_popitem (test.test_weakref.MappingTestCase) ... ok
test_weak_keyed_dict_setdefault (test.test_weakref.MappingTestCase) ... ok
test_weak_keyed_dict_update (test.test_weakref.MappingTestCase) ... ok
test_weak_keyed_iters (test.test_weakref.MappingTestCase) ... ok
test_weak_keyed_len_cycles (test.test_weakref.MappingTestCase) ... ok
test_weak_keyed_len_race (test.test_weakref.MappingTestCase) ... ok
test_weak_keys (test.test_weakref.MappingTestCase) ... ok
test_weak_keys_destroy_while_iterating (test.test_weakref.MappingTestCase) ... ok
test_weak_valued_delitem (test.test_weakref.MappingTestCase) ... ok
test_weak_valued_dict_popitem (test.test_weakref.MappingTestCase) ... ok
test_weak_valued_dict_setdefault (test.test_weakref.MappingTestCase) ... ok
test_weak_valued_dict_update (test.test_weakref.MappingTestCase) ... ok
test_weak_valued_iters (test.test_weakref.MappingTestCase) ... ok
test_weak_valued_len_cycles (test.test_weakref.MappingTestCase) ... ok
test_weak_valued_len_race (test.test_weakref.MappingTestCase) ... ok
test_weak_values (test.test_weakref.MappingTestCase) ... ok
test_weak_values_destroy_while_iterating (test.test_weakref.MappingTestCase) ... ok
...

----------------------------------------------------------------------
Ran 116 tests in 3.641s
...

msg284046 - (view) Author: Antoine Pitrou (pitrou) * Date: 2016-12-26 20:58

This is expected, because of https://bugs.python.org/issue28427

History Date User Action Args 2022-04-11 14:57:53adminsetgithub: 63741 2017-03-31 16:36:07dstufftsetpull_requests: + pull_request828 2016-12-26 20:58:59pitrousetstatus: open -> closed
resolution: fixed 2016-12-26 20:58:47pitrousetmessages: + msg284046 2016-12-26 20:06:10Arfreversetstatus: closed -> open
versions: + Python 3.7, - Python 3.4
nosy: + Arfrever

messages: + msg284040

resolution: fixed -> (no value)

2016-12-19 10:14:27pitrousetstatus: open -> closed
resolution: fixed
messages: + msg283602

stage: patch review -> resolved

2016-12-19 10:14:00python-devsetmessages: + msg283601 2016-12-19 10:01:24python-devsetnosy: + python-dev
messages: + msg283599
2016-10-25 21:11:58serhiy.storchakasetnosy: + serhiy.storchaka
2016-10-25 20:39:44arigosetmessages: + msg279455 2015-06-18 15:24:27pitrousetmessages: + msg245471
versions: + Python 3.5, Python 3.6, - Python 3.3 2015-06-18 15:23:11tinchestersetnosy: + tinchester
messages: + msg245470
2015-02-02 23:42:34arigosetkeywords: + needs review, - patch

messages: + msg235301
stage: needs patch -> patch review

2015-02-02 23:40:50arigosetfiles: + fix-weakvaluedict-2.7.diff 2015-02-02 23:40:35arigosetfiles: + fix-weakvaluedict.diff
keywords: + patch 2013-12-09 10:41:07pitrousetmessages: + msg205674 2013-12-09 03:55:35tim.peterssetnosy: + tim.peters
messages: + msg205645
2013-12-08 22:19:31pitrousettype: behavior
messages: + msg205614
stage: needs patch 2013-12-08 21:57:12arigosetmessages: + msg205609 2013-12-08 20:30:23pitrousetmessages: + msg205603
versions: + Python 3.4 2013-12-08 20:13:53ned.deilysetnosy: + fdrake, pitrou
2013-11-10 08:57:24arigosetfiles: + weakref.slice 2013-11-10 08:55:17arigosetfiles: + x.py 2013-11-10 08:51:04arigocreate