Comment 3 for bug 613675

jwilk says in irc that calling 'cat' in the subprocesses exhibits the same hang:

<jwilk> barry: http://paste.debian.net/82314/ [13:17]

and the problem is:

<jwilk> http://paste.debian.net/82315/ - the 2nd process in keeping the first
        pipe open. [13:23]

A workaround is to add close_fds=True to the Popen constructors.