OrderedDict prevents garbage collection if a circulary referenced class is used as key

[charettes]

BPO	25935
Nosy	@ericsnowcurrently, @serhiy-storchaka, @charettes, @timgraham
Files	test.py: Reproduction test case odict_traverse.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 = 'https://github.com/serhiy-storchaka'
closed_at = <Date 2016-01-19.13:50:20.825>
created_at = <Date 2015-12-23.23:39:59.235>
labels = ['extension-modules', 'type-bug']
title = 'OrderedDict prevents garbage collection if a circulary referenced class is used as key'
updated_at = <Date 2016-01-19.13:50:20.824>
user = 'https://github.com/charettes'

bugs.python.org fields:

activity = <Date 2016-01-19.13:50:20.824>
actor = 'serhiy.storchaka'
assignee = 'serhiy.storchaka'
closed = True
closed_date = <Date 2016-01-19.13:50:20.825>
closer = 'serhiy.storchaka'
components = ['Extension Modules']
creation = <Date 2015-12-23.23:39:59.235>
creator = 'charettes'
dependencies = []
files = ['41399', '41441']
hgrepos = []
issue_num = 25935
keywords = ['patch']
message_count = 4.0
messages = ['256945', '257130', '258590', '258592']
nosy_count = 5.0
nosy_names = ['python-dev', 'eric.snow', 'serhiy.storchaka', 'charettes', 'Tim.Graham']
pr_nums = []
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue25935'
versions = ['Python 3.5', 'Python 3.6']

[charettes]

I attached a file with a reproduction test case that passes on Python 2.7 and 3.4 but fails on 3.5.0 and 3.5.1

This might be solved by the improvement planed in bpo-25410.

[serhiy-storchaka]

An OrderedDict owns two references to every key (as a dict and from a linked list). Therefore it should visit it twice in tp_traverse.

Proposed patch fixes garbage collection for OrderedDict. It also rewrites OrderedDict clearing in more reentrant form (a linked list is detached from an OrderedDict before deallocating its nodes that can trigger executing user code).

[python-dev]

New changeset 627575e9a7aa by Serhiy Storchaka in branch '3.5':
Issue bpo-25935: Garbage collector now breaks reference loops with OrderedDict.
https://hg.python.org/cpython/rev/627575e9a7aa

New changeset 5f0a08cfc4f6 by Serhiy Storchaka in branch 'default':
Issue bpo-25935: Garbage collector now breaks reference loops with OrderedDict.
https://hg.python.org/cpython/rev/5f0a08cfc4f6

[serhiy-storchaka]

In some cases (as in committed test) garbage collector was not able to collect a loop with OrderedDict at all.