Segfault when closing BufferedWriter from a different thread

[benfogle]

BPO	31976
Nosy	@pitrou, @benjaminp, @vadmium, @benfogle
PRs	bpo-31976: Fix race condition when flushing a file is slow. #4331

^{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 2017-11-10.21:05:15.294>
created_at = <Date 2017-11-08.04:27:27.470>
labels = ['interpreter-core', '3.7', 'type-crash']
title = 'Segfault when closing BufferedWriter from a different thread'
updated_at = <Date 2017-11-10.21:05:15.294>
user = 'https://github.com/benfogle'

bugs.python.org fields:

activity = <Date 2017-11-10.21:05:15.294>
actor = 'pitrou'
assignee = 'none'
closed = True
closed_date = <Date 2017-11-10.21:05:15.294>
closer = 'pitrou'
components = ['Interpreter Core']
creation = <Date 2017-11-08.04:27:27.470>
creator = 'benfogle'
dependencies = []
files = []
hgrepos = []
issue_num = 31976
keywords = ['patch']
message_count = 3.0
messages = ['305803', '306048', '306049']
nosy_count = 5.0
nosy_names = ['pitrou', 'benjamin.peterson', 'stutzbach', 'martin.panter', 'benfogle']
pr_nums = ['4331']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'crash'
url = 'https://bugs.python.org/issue31976'
versions = ['Python 3.7']

[benfogle]

To reproduce:

import threading
import io
import time
import _pyio

class MyFileIO(io.FileIO):
 def flush(self):
 # Simulate a slow flush. Slow disk, etc.
 time.sleep(0.25)
 super().flush()

raw = MyFileIO('test.dat', 'wb')
#fp = _pyio.BufferedWriter(raw)
fp = io.BufferedWriter(raw)
t1 = threading.Thread(target=fp.close)
t1.start()
time.sleep(0.1) # Ensure t1 is sleeping in fp.close()/raw.flush()
fp.write(b'test')
t1.join()

_pyio.BufferedWriter ignores the error completely rather than throwing a ValueError("write to closed file").

[pitrou]

New changeset 9703f09 by Antoine Pitrou (benfogle) in branch 'master':
bpo-31976: Fix race condition when flushing a file is slow. (bpo-4331)
9703f09

[pitrou]

Thank you for posting this Benjamin. As I said on the PR, I don't think I want to backport this to 3.6, as it is always delicate to reason about subclassing and threads.