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).

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

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

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

In issue31804, we see that sys.stdout and sys.stderr can sometimes simply be None, so we must check for that too.

New changeset daeefd2e049b74340307481112a39f77de0f4769 by Antoine Pitrou in branch 'master':
bpo-28326: Fix multiprocessing.Process when stdout and/or stderr is closed or None. (#4073)
https://github.com/python/cpython/commit/daeefd2e049b74340307481112a39f77de0f4769

New changeset 34ef6da8f5fb03b83268bd35b77fb2183c748b70 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).  (#4075)
https://github.com/python/cpython/commit/34ef6da8f5fb03b83268bd35b77fb2183c748b70

This is now fixed in 3.6 and 3.7.  Closing!