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The current subprocess.Popen() has a boolean close_fds parameter, which
cannot satisfy all the requirements. Eg. want to pass specific fd to
child process, but close others.
This patch adds a extra parameter pass_fds to subprocess.Popen's
__init__(). This parameter only effect when close_fds=True. When
close_fds=True, all fds in pass_fds will not closed before exec.

A couple of unit tests would be great, as well as a paragraph for the
documentation.

The reason os.closerange() is used is that it can be slow to call
os.close() on lots of fds (I suppose this depends on the OS). See
http://code.python.org/hg/trunk/rev/43caff85ec85
Therefore, the patch should be smart enough to continue using
os.closerange() on ranges of contiguous FDs to be closed.

I've committed this feature just in time for 3.2beta1 (so it can't be said i'm adding a feature after the beta ;). r87026
It still needs tests and documentation. It doesn't break any existing tests.
I'll take care of that after some sleep.

The patch doesn't seem to work.
I added this before closerange in _close_all_but_a_sorted_few_fds:
print("Closing", start_fd, "up to", fd, "exclusive")
And used the attached script to run as a subprocess to check for open fds (taken from my tests patch for issue 7213).
Here's the result:
Python 3.2b1 (py3k:87158M, Dec 11 2010, 02:55:28) 
[GCC 4.4.5] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import sys
>>> import os
>>> import subprocess
>>> subprocess.Popen([sys.executable, 'fd_status.py'], close_fds=False).wait()
0,1,2
0
>>> os.pipe()
(3, 4)
>>> os.pipe()
(5, 6)
>>> subprocess.Popen([sys.executable, 'fd_status.py'], close_fds=False).wait()
0,1,2,3,4,5,6
0
>>> subprocess.Popen([sys.executable, 'fd_status.py'], close_fds=True).wait()
0,1,2
0
>>> subprocess.Popen([sys.executable, 'fd_status.py'], close_fds=True, pass_fds=(6,)).wait()
0,1,2,6
0
>>> subprocess.Popen([sys.executable, 'fd_status.py'], close_fds=True, pass_fds=(3,)).wait()
0,1,2
0
>>> subprocess._posixsubprocess = None
>>> subprocess.Popen([sys.executable, 'fd_status.py'], close_fds=True, pass_fds=(6,)).wait()
Closing 3 up to 6 exclusive
Closing 7 up to 8 exclusive
0,1,2,6
0
>>> subprocess.Popen([sys.executable, 'fd_status.py'], close_fds=True, pass_fds=(3,)).wait()
Closing 3 up to 8 exclusive
0,1,2
0
I also attach a possible test for pass_fds, and an example fix for Python-only implementation. The test requires either my tests patch for issue 7213, or the attached fd_status.py to be put in subprocessdata subdir of Lib/test. The fixed Python implementation passes my test and works fine in the console, I haven't tried the C one. (I don't have a patch for the fix, since it would conflict with the patches for issue 7213.)

Bug fix, unittest and documentation added in r87229. Thanks!

It seems there are (intermittent?) test failures:
======================================================================
FAIL: test_pass_fds (test.test_subprocess.POSIXProcessTestCase)
----------------------------------------------------------------------
Traceback (most recent call last):
 File "/home2/buildbot/slave/3.x.loewis-sun/build/Lib/test/test_subprocess.py", line 1067, in test_pass_fds
 "fd to be closed passed")
AssertionError: {5} is not False : fd to be closed passed
======================================================================
FAIL: test_pass_fds (test.test_subprocess.ProcessTestCasePOSIXPurePython)
----------------------------------------------------------------------
Traceback (most recent call last):
 File "/home2/buildbot/slave/3.x.loewis-sun/build/Lib/test/test_subprocess.py", line 1067, in test_pass_fds
 "fd to be closed passed")
AssertionError: {5} is not False : fd to be closed passed
(from http://www.python.org/dev/buildbot/all/builders/sparc%20solaris10%20gcc%203.x/builds/2337/steps/test/logs/stdio )

Just noticed this patch... aside from liking the intention, the api for this is going to grow tiresome quick since it expects the FDs to already be in place; is there any reasons a mapping wasn't used here, specifically of (src_fd|src_fileobj) -> target_fd ?
If that was fed in, post fork the client can shuffle around the fd's into appropriate positions- something the parent may not be able to do (threaded environment for example, or async in some respect).
I've had similar functionality in my own process code for a while, and have found it to be generally pretty easy to deal with- for subprocess it has the added benefit that the existing stdin/stdout/stderr bits could be pretty easily folded directly into it.
So... any reason this route wasn't considered, or just wasn't thought about?

Wasn't thought about. I have seen something similar to that done in another c++ subprocess implementation since. If you have suggestions for a more useful API, feel free to propose them in a new issue.

_posixsubprocess.fork_exec docstring was not updated.

New changeset 77abf23c67c1a465a8899666c69f6bcd6930e003 by Miss Islington (bot) (Orivej Desh) in branch 'master':
bpo-6559: Update _posixsubprocess.fork_exec doc (GH-16283)
https://github.com/python/cpython/commit/77abf23c67c1a465a8899666c69f6bcd6930e003