Fixed various refcount issues in ElementTree iteration

[serhiy-storchaka]

BPO	25902
Nosy	@vadmium, @serhiy-storchaka
Files	etree_iter_refcount.patch etree_iter_refcount_2.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 2015-12-21.10:40:14.398>
created_at = <Date 2015-12-18.12:04:44.709>
labels = ['extension-modules', 'expert-XML', 'type-crash']
title = 'Fixed various refcount issues in ElementTree iteration'
updated_at = <Date 2015-12-21.10:40:14.396>
user = 'https://github.com/serhiy-storchaka'

bugs.python.org fields:

activity = <Date 2015-12-21.10:40:14.396>
actor = 'serhiy.storchaka'
assignee = 'serhiy.storchaka'
closed = True
closed_date = <Date 2015-12-21.10:40:14.398>
closer = 'serhiy.storchaka'
components = ['Extension Modules', 'XML']
creation = <Date 2015-12-18.12:04:44.709>
creator = 'serhiy.storchaka'
dependencies = []
files = ['41352', '41373']
hgrepos = []
issue_num = 25902
keywords = ['patch']
message_count = 5.0
messages = ['256673', '256767', '256782', '256786', '256794']
nosy_count = 3.0
nosy_names = ['python-dev', 'martin.panter', 'serhiy.storchaka']
pr_nums = []
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'crash'
url = 'https://bugs.python.org/issue25902'
versions = ['Python 3.5', 'Python 3.6']

[serhiy-storchaka]

ElementTree iteration code holds borrowed references when call user code (comparison with the tag, concatenating text and tail lazy lists, testing text and tail for empty). User code can make borrowed references invalid and this can lead to a crash. One example is provided by Martin in msg256495. Other possible cases can be more hard to reproduce.

Proposed patch makes iterating code to own references that it operates.

[serhiy-storchaka]

Added tests.

[vadmium]

Left some comments about possibly unnecessary ref counting. Other than that it seems okay.

[python-dev]

New changeset 00b6a13cfd70 by Serhiy Storchaka in branch '3.5':
Issue bpo-25902: Fixed various refcount issues in ElementTree iteration.
https://hg.python.org/cpython/rev/00b6a13cfd70

New changeset e09cb2af3092 by Serhiy Storchaka in branch 'default':
Issue bpo-25902: Fixed various refcount issues in ElementTree iteration.
https://hg.python.org/cpython/rev/e09cb2af3092

[serhiy-storchaka]

Thank you Martin.

Actually this patch is for 3.5 only. In 3.6 the code will be rewrote (bpo-25873).