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Transcript to reproduce in Python 2.6.5:
>>> import subprocess, sys
>>> subprocess.call(('echo', 'foo'), stderr=sys.stdout)
echo: write: Bad file descriptor
1
>>> 
Expected behavior:
>>> import subprocess, sys
>>> subprocess.call(('echo', 'foo'), stderr=sys.stdout)
foo
0
>>> 
This happens because we've asked the child's stderr to be redirected, but not its stdout. So in _execute_child, errwrite is 1 while c2pwrite is None. So fd 1 (errwrite) correctly gets duped to 2. But then, since errwrite is not None and it's not in (p2cread, c2pwrite, 2), the child closes fd 1.
The equivalent thing happens if you supply stdout=sys.stderr and the child attempts to write to its stderr.
I've attached a patch to fix this. It simply adds 2 and 2 to the list of fds not to close for c2pwrite and errwrite, respectively.
This patch is against the 2.6.5 release.
There is also a workaround, in case anyone else is affected by this bug before a fix has been released:
>>> import os, subprocess, sys
>>> subprocess.call(('echo', 'foo'), stderr=os.dup(sys.stdout.fileno()))
foo
0
>>> 
(There could be a similar bug relating to the child's stdin, but I haven't investigated that.)

There was a typo in my description of the patch. It should read:
I've attached a patch to fix this. It simply adds 2 and *1* to the list of fds not to close for c2pwrite and errwrite, respectively.

#9905 has a similar patch and adds tests as well.