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The following program hangs on Debian, Python 2.6.6:
import subprocess
proc1 = subprocess.Popen(['cat'], stdin=subprocess.PIPE)
proc2 = subprocess.Popen(['cat'], stdin=subprocess.PIPE)
proc1.stdin.close()
proc1.wait()
Changing the last two lines to:
proc2.stdin.close()
proc2.wait()
Doesn't hang. The guys at #python-dev confirmed the same happens on 2.7 but not on 3.x.

"The guys at #python-dev confirmed the same happens on 2.7 but not on 3.x."
Really? This is on gentoo, not debian, admittedly:
coelacanth ~ 11:12:36 $ python3
Python 3.1.3 (r313:86834, May 1 2011, 09:41:48) 
[GCC 4.4.4] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import subprocess
>>> proc1 = subprocess.Popen(['cat'], stdin=subprocess.PIPE)
>>> proc2 = subprocess.Popen(['cat'], stdin=subprocess.PIPE)
>>> proc2.stdin.close()
>>> proc2.wait()
0
>>> 
coelacanth ~ 11:12:13 $ python3.1
Python 3.1.3 (r313:86834, May 1 2011, 09:41:48) 
[GCC 4.4.4] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import subprocess
>>> proc1 = subprocess.Popen(['cat'], stdin=subprocess.PIPE)
>>> proc2 = subprocess.Popen(['cat'], stdin=subprocess.PIPE)
>>> proc1.stdin.close()
>>> proc1.wait()
[hangs]

Hello Idan,
> The following program hangs on Debian
Debian is a good choice :-)
Concerning your example, it hangs is because the file descriptor used
to communicate with proc1 is inherited when proc2 is created (FDs are
inherited upon fork by default): thus, when you call
proc1.stdin.close(), `cat` doesn't receive EOF on read(0), and remains
stuck.
If you close proc2's stdin and wait it first, then you don't have this problem:
- because it's been created after proc1, its stdin FD has not been inherited
- when you call proc2.stdin.close(), `cat` receives EOF from read(0), and exits
There are two reasons while it doesn't hang on Python 3.x:
1) it uses close_fds=True by default
2) it sets the pipe FDs CLOEXEC, so that they're closed when cat is `executed`
Try with
"""
proc1 = subprocess.Popen(['cat'], stdin=subprocess.PIPE)
proc2 = subprocess.Popen(['cat'], stdin=subprocess.PIPE, close_fds=True)
proc1.stdin.close()
proc1.wait()
"""
And you'll be fine.
We can't make the change 1) in 2.7, because that's a behavior change.
We could however set the FDs used to communicate with the children CLOEXEC.
I'll work on a patch for 2.7 (but it won't be thread-safe, because
pipe2 is not exposed and there's no _posixsubprocess.c like in 3.x).

Here's a patch + test for 2.7.
> Really? This is on gentoo, not debian, admittedly:
That's because the change of close_fds to True by default and the CLOEXEC flag were added in 3.2.
Since 3.1 is in security-fix mode only, this patch is only relevant to 2.7.

Thanks for getting on top of this so quickly Charles. Setting close_fds=True worked like a charm.

You might want to add a comment in the patch that the cloexec flag is removed from the child's pipes by calling dup2() before exec() :) I was a bit baffled at first when reading the patch.

With comment.
I'll add a similar comment to default.

subprocess_cloexec-1.diff: I'm too tired too review the test. The subprocess.py part looks correct, except the pipe2 name. Python 3 uses "_create_pipe = _posixsubprocess.cloexec_pipe". Pick one of those.

Patch looks good after Victor's comment.

New changeset 498b03a55297 by Charles-François Natali in branch '2.7':
Issue #12786: Set communication pipes used by subprocess.Popen CLOEXEC to avoid
http://hg.python.org/cpython/rev/498b03a55297 

Patch committed.
Idan, thanks for the report.