assertion failure in FutureObj_finalize() after setting _log_traceback to True

[orenmn]

BPO	31721
Nosy	@asvetlov, @1st1, @orenmn
PRs	bpo-31721: Allow Future._log_traceback to only be set to False #5009

^{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 2017-12-25.21:16:59.889>
created_at = <Date 2017-10-07.14:19:24.158>
labels = ['3.7', 'type-crash', 'expert-asyncio']
title = 'assertion failure in FutureObj_finalize() after setting _log_traceback to True'
updated_at = <Date 2017-12-25.21:16:59.888>
user = 'https://github.com/orenmn'

bugs.python.org fields:

activity = <Date 2017-12-25.21:16:59.888>
actor = 'yselivanov'
assignee = 'none'
closed = True
closed_date = <Date 2017-12-25.21:16:59.889>
closer = 'yselivanov'
components = ['asyncio']
creation = <Date 2017-10-07.14:19:24.158>
creator = 'Oren Milman'
dependencies = []
files = []
hgrepos = []
issue_num = 31721
keywords = []
message_count = 4.0
messages = ['303878', '309036', '309038', '309042']
nosy_count = 3.0
nosy_names = ['asvetlov', 'yselivanov', 'Oren Milman']
pr_nums = ['5009']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'crash'
url = 'https://bugs.python.org/issue31721'
versions = ['Python 3.7']

[orenmn]

The following code causes an assertion failure in FutureObj_finalize() (in
Modules/_asynciomodule.c):

import asyncio
asyncio.Future()._log_traceback = True

Maybe we should allow Python code to only set it to False, and raise a
ValueError in case Python code tries to set it to True?

(PR 2050 made _log_traceback writable. Are there any usecases for setting it to
True from Python code?)

[asvetlov]

_log_traceback is a private property and asyncio implementation detail, you should never touch it.

[1st1]

_log_traceback is a private property and asyncio implementation detail, you should never touch it.

But still, we shouldn't let Python crash if someone sets it. I'll make a PR to fix this the way Oren proposed.

Andrew, let's discuss later with what we want to replace _log_traceback. I don't like that it's a semi-public thing.

[1st1]

New changeset e0aef4f by Yury Selivanov in branch 'master':
bpo-31721: Allow Future._log_traceback to only be set to False (bpo-5009)
e0aef4f