multiprocessing.Process depends on sys.stdout being open

[progval]

BPO	28326
Nosy	@terryjreedy, @pitrou, @tiagoantao, @progval, @applio, @Mariatta, @glolol
PRs	bpo-28326: Fixes multiprocessing.Process depends on sys.stdout being open #1410 bpo-28326: Fix multiprocessing.Process when stdout and/or stderr is closed or None. #4073 [3.6] bpo-28326: Fix multiprocessing.Process when stdout and/or stderr is closed or None. (GH-4073).  #4075
Files	mp.patch: Patch with test case

^{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 2017-10-22.10:27:53.597>
created_at = <Date 2016-10-01.11:45:34.763>
labels = ['3.7', 'type-bug', 'library']
title = 'multiprocessing.Process depends on sys.stdout being open'
updated_at = <Date 2017-10-22.10:27:53.595>
user = 'https://github.com/ProgVal'

bugs.python.org fields:

activity = <Date 2017-10-22.10:27:53.595>
actor = 'pitrou'
assignee = 'none'
closed = True
closed_date = <Date 2017-10-22.10:27:53.597>
closer = 'pitrou'
components = ['Library (Lib)']
creation = <Date 2016-10-01.11:45:34.763>
creator = 'Valentin.Lorentz'
dependencies = []
files = ['44919']
hgrepos = []
issue_num = 28326
keywords = ['patch']
message_count = 8.0
messages = ['277807', '277849', '278271', '278273', '304713', '304737', '304740', '304741']
nosy_count = 7.0
nosy_names = ['terry.reedy', 'pitrou', 'tiagoantao', 'Valentin.Lorentz', 'davin', 'Mariatta', 'tacocat']
pr_nums = ['1410', '4073', '4075']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue28326'
versions = ['Python 3.6', 'Python 3.7']

[progval]

Hello,

The following code:

import sys
import multiprocessing

sys.stdout.close()

def foo():
    pass

p = multiprocessing.Process(target=foo)
p.start()

Crashes with:

Traceback (most recent call last):
  File "foo.py", line 10, in <module>
    p.start()
  File "/usr/lib/python3.5/multiprocessing/process.py", line 105, in start
    self._popen = self._Popen(self)
  File "/usr/lib/python3.5/multiprocessing/context.py", line 212, in _Popen
    return _default_context.get_context().Process._Popen(process_obj)
  File "/usr/lib/python3.5/multiprocessing/context.py", line 267, in _Popen
    return Popen(process_obj)
  File "/usr/lib/python3.5/multiprocessing/popen_fork.py", line 17, in __init__
    sys.stdout.flush()
ValueError: I/O operation on closed file.

This bug has been reported to me on a daemonized program (written prior to PEP-3143).

[tiagoantao]

I made a small patch for this. This is the first time I submit one, so please be careful with this...

[terryjreedy]

Patch looks plausible to me. The flush calls definitely need to be guarded.

[tiagoantao]

This is the first patch that I am submitting. More than willing to do any changes deemed necessary...

[pitrou]

In bpo-31804, we see that sys.stdout and sys.stderr can sometimes simply be None, so we must check for that too.

[pitrou]

New changeset daeefd2 by Antoine Pitrou in branch 'master':
bpo-28326: Fix multiprocessing.Process when stdout and/or stderr is closed or None. (bpo-4073)
daeefd2

[pitrou]

New changeset 34ef6da by Antoine Pitrou in branch '3.6':
[3.6] bpo-28326: Fix multiprocessing.Process when stdout and/or stderr is closed or None. (GH-4073). (bpo-4075)
34ef6da

[pitrou]

This is now fixed in 3.6 and 3.7. Closing!