[New-bugs-announce] [issue32270] subprocess closes redirected fds even if they are in pass_fds

[Alexey Izbyshev]

New submission from Alexey Izbyshev <izbyshev at ispras.ru>:

Demonstration:

$ cat test.py
import os
import subprocess
import sys

fd = os.dup(sys.stdout.fileno())
subprocess.call([sys.executable, '-c',
                 'import sys;'
                 'print("Hello stdout");'
                 'print("Hello fd", file=open(int(sys.argv[1]), "w"))',
                 str(fd)],
                stdout=fd,
                pass_fds=[fd])


$ python3 test.py
Hello stdout
Traceback (most recent call last):
  File "<string>", line 1, in <module>
OSError: [Errno 9] Bad file descriptor

I see two issues here:

1. The fact that file descriptors specified for redirection are closed (even with close_fds=False) unless in range [0, 2] is not documented and not tested. If I change the corresponding code in _posixsubprocess.c to close them only if they are ends of pipes created by subprocess itself, all tests still pass. Also, shells behave differently: e.g., in command 'echo 3>&1' fd 3 is duplicated but not closed in the child, so subprocess behavior may be not what people expect.

2. pass_fds interaction with (1) should be fixed and documented. We may either raise ValueError if pass_fds contains a redirected fd or don't close such a redirected fd.

Please provide your thoughts.

----------
components: Library (Lib)
files: test.py
messages: 307962
nosy: gregory.p.smith, izbyshev, pitrou, vstinner
priority: normal
severity: normal
status: open
title: subprocess closes redirected fds even if they are in pass_fds
type: behavior
versions: Python 3.6, Python 3.7
Added file: https://bugs.python.org/file47329/test.py

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue32270>
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