[issue10806] Subprocess error if fds 0,1,2 are closed

[Ross Lagerwall]

New submission from Ross Lagerwall <rosslagerwall at gmail.com>:

There is an issue where if a python program closes all the std. file descriptors (e.g. a daemon) and then uses the subprocess module, the file descriptors may not be set up properly in the subprocess. This may actually be a fairly common use case in a daemon program that needs to run a subprocess and set up pipes to it.

Here is an example:

import os, subprocess, sys

x=os.open('/dev/null', os.O_RDWR)
os.close(0)
os.close(1)
os.close(2)

res = subprocess.Popen([sys.executable, "-c",
                      'import sys;'
                      'sys.stdout.write("apple");'
                      'sys.stdout.flush();'
                      'sys.stderr.write("orange")'],
           stdin=x,
           stdout=subprocess.PIPE,
           stderr=subprocess.PIPE).communicate()
with open('/tmp/out', 'w') as f:
    f.write(repr(res) + '\n')
    f.write(repr((b'apple', b'orange')) + '\n')

The expected output in /tmp/out is:
('apple', 'orange')
('apple', 'orange')

but we get:
(b'', b"Fatal Python error: Py_Initialize: can't initialize sys standard streams\nOSError: [Errno 9] Bad file descriptor\n")
(b'apple', b'orange')

The problem comes about where the calls are made (this applies to the python & c versions):
 os.dup2(p2cread, 0)
 os.dup2(c2pwrite, 1)
 os.dup2(errwrite, 2)

if c2pwrite or p2cread or errwrite is the same as what it's being dupped() to (eg if c2pwrite == 1) then the dup2 call does nothing. But, if we're using pipes, the close-on-exec flags are set initially and the dup2() call would normally remove the flag but it doesn't.

Attached is a patch which basically uses fcntl if necessary to remove the close-on-exec flag, and tests.

----------
components: Library (Lib)
files: subprocess.patch
keywords: patch
messages: 125067
nosy: georg.brandl, giampaolo.rodola, gregory.p.smith, pitrou, rosslagerwall
priority: normal
severity: normal
status: open
title: Subprocess error if fds 0,1,2 are closed
type: behavior
versions: Python 3.2
Added file: http://bugs.python.org/file20223/subprocess.patch

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