TextIOWrapper assertion failure after read() and SEEK_CUR

[vadmium]

BPO	25862
Nosy	@vadmium, @serhiy-storchaka, @ZackerySpytz, @miss-islington
PRs	bpo-25862: Fix an assertion failure in io.TextIOWrapper.tell(). #3918 [3.7] bpo-25862: Fix assertion failures in io.TextIOWrapper.tell(). (GH-3918) #8007 [3.6] bpo-25862: Fix assertion failures in io.TextIOWrapper.tell(). (GH-3918) #8012 [2.7] bpo-25862: Fix assertion failures in io.TextIOWrapper.tell(). (GH-3918) #8013 bpo-25862: Fix several bugs in the _io module. #8026 [3.7] bpo-25862: Fix several bugs in the _io module. (GH-8026) #8028 [3.6] bpo-25862: Fix several bugs in the _io module. (GH-8026) #8029 [2.7] bpo-25862: Fix several bugs in the _io module. (GH-8026) #8033

^{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 2018-12-04.10:47:52.634>
created_at = <Date 2015-12-15.01:43:10.182>
labels = ['3.7', '3.8', 'expert-IO', 'type-crash']
title = 'TextIOWrapper assertion failure after read() and SEEK_CUR'
updated_at = <Date 2019-02-08.23:20:42.391>
user = 'https://github.com/vadmium'

bugs.python.org fields:

activity = <Date 2019-02-08.23:20:42.391>
actor = 'martin.panter'
assignee = 'serhiy.storchaka'
closed = True
closed_date = <Date 2018-12-04.10:47:52.634>
closer = 'serhiy.storchaka'
components = ['IO']
creation = <Date 2015-12-15.01:43:10.182>
creator = 'martin.panter'
dependencies = []
files = []
hgrepos = []
issue_num = 25862
keywords = ['patch']
message_count = 12.0
messages = ['256430', '320312', '320709', '320710', '320732', '320737', '320780', '320798', '320802', '320803', '331024', '335118']
nosy_count = 4.0
nosy_names = ['martin.panter', 'serhiy.storchaka', 'ZackerySpytz', 'miss-islington']
pr_nums = ['3918', '8007', '8012', '8013', '8026', '8028', '8029', '8033']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'crash'
url = 'https://bugs.python.org/issue25862'
versions = ['Python 2.7', 'Python 3.6', 'Python 3.7', 'Python 3.8']

[vadmium]

Python 3.5.1+ (3.5:014e6f7d7c1a, Dec 14 2015, 11:20:58) 
[GCC 5.3.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> from io import *
>>> text = TextIOWrapper(BytesIO(b"x"), "ascii")
>>> text.read(1)
'x'
>>> text.read()
''
>>> text.seek(0, SEEK_CUR)
python: ./Modules/_io/textio.c:2293: _io_TextIOWrapper_tell_impl: Assertion `self->decoded_chars == ((void *)0) || PyUnicode_GetLength(self->decoded_chars) == 0' failed.
Aborted (core dumped)
[Exit 134]

May affect other versions; I haven’t looked.

[serhiy-storchaka]

Similar case:

text.read(1); text.write(b'x'); text.tell()

And there may be crashes after setting self->snapshot to NULL due to the failure of Py_BuildValue().

[serhiy-storchaka]

New changeset 23db935 by Serhiy Storchaka (Zackery Spytz) in branch 'master':
bpo-25862: Fix assertion failures in io.TextIOWrapper.tell(). (GH-3918)
23db935

[miss-islington]

New changeset eabebbb by Miss Islington (bot) in branch '3.7':
bpo-25862: Fix assertion failures in io.TextIOWrapper.tell(). (GH-3918)
eabebbb

[serhiy-storchaka]

New changeset d6a283b by Serhiy Storchaka (Zackery Spytz) in branch '3.6':
[3.6] bpo-25862: Fix assertion failures in io.TextIOWrapper.tell(). (GH-3918). (GH-8012)
d6a283b

[serhiy-storchaka]

New changeset 0464de0 by Serhiy Storchaka (Zackery Spytz) in branch '2.7':
[2.7] bpo-25862: Fix assertion failures in io.TextIOWrapper.tell(). (GH-3918). (GH-8013)
0464de0

[serhiy-storchaka]

The assertion failure can be also caused by setting self->snapshot to NULL when Py_BuildValue() fails due to lack of memory. It is very difficult to reproduce this. PR 8026 fixes this and other difficulty reproducible refcount bugs in the _io module.

[serhiy-storchaka]

New changeset fdb5a50 by Serhiy Storchaka in branch 'master':
bpo-25862: Fix several bugs in the _io module. (GH-8026)
fdb5a50

[miss-islington]

New changeset dedb28e by Miss Islington (bot) in branch '3.7':
bpo-25862: Fix several bugs in the _io module. (GH-8026)
dedb28e

[miss-islington]

New changeset e5153d0 by Miss Islington (bot) in branch '3.6':
bpo-25862: Fix several bugs in the _io module. (GH-8026)
e5153d0

[serhiy-storchaka]

New changeset eab421b by Serhiy Storchaka in branch '2.7':
[2.7] bpo-25862: Fix several bugs in the _io module. (GH-8026) (GH-8033)
eab421b

[vadmium]

I haven’t confirmed, but it looks like the original change here broke bidirectional reading and writing with a single TextIOWrapper object; see bpo-35928.