WeakKeyDictionary.__len__ fragile w/ _IterationGuards

[pjenvey]

BPO	21173
Nosy	@pitrou, @pjenvey
Files	issue21173-test.diff weakdictlen.patch

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2014-10-05.18:10:44.398>
created_at = <Date 2014-04-07.20:17:27.795>
labels = ['type-bug', 'library']
title = 'WeakKeyDictionary.__len__ fragile w/ _IterationGuards'
updated_at = <Date 2014-10-05.18:10:44.396>
user = 'https://github.com/pjenvey'

bugs.python.org fields:

activity = <Date 2014-10-05.18:10:44.396>
actor = 'python-dev'
assignee = 'none'
closed = True
closed_date = <Date 2014-10-05.18:10:44.398>
closer = 'python-dev'
components = ['Library (Lib)']
creation = <Date 2014-04-07.20:17:27.795>
creator = 'pjenvey'
dependencies = []
files = ['34751', '36802']
hgrepos = []
issue_num = 21173
keywords = ['patch']
message_count = 3.0
messages = ['215716', '228495', '228606']
nosy_count = 3.0
nosy_names = ['pitrou', 'pjenvey', 'python-dev']
pr_nums = []
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue21173'
versions = ['Python 3.4', 'Python 3.5']

[pjenvey]

len() on WeakKeyDictionarys can fail with ValueErrors when _IterationGuards are kept alive

Attached is a test showing this:

======================================================================
ERROR: test_weak_keys_len_destroy_while_iterating (main.MappingTestCase)
----------------------------------------------------------------------

Traceback (most recent call last):
  File "Lib/test/test_weakref.py", line 1336, in test_weak_keys_len_destroy_while_iterating
    self.assertEqual(len(dict), 0)
ValueError: __len__() should return >= 0

One probably shouldn't keep them alive like this, but __len__ shouldn't be blowing up either. On non ref counting GC platforms this situation is easier to trigger unintentionally

[pitrou]

Thank you for reporting this. Here is a patch with tests.

[python-dev]

New changeset 657d21b4b121 by Antoine Pitrou in branch '3.4':
Closes bpo-21173: Fix len() on a WeakKeyDictionary when .clear() was called with an iterator alive.
https://hg.python.org/cpython/rev/657d21b4b121

New changeset 27533444b964 by Antoine Pitrou in branch 'default':
Closes bpo-21173: Fix len() on a WeakKeyDictionary when .clear() was called with an iterator alive.
https://hg.python.org/cpython/rev/27533444b964