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Pythons subprocess module has a race condition when stdin is used. The
problem can be reproduced with the following script (based on the script
in issue "#1731717"/"msg32210" (slightly changed to use stdin):
----
import sys, os, threading, subprocess, time
class X(threading.Thread):
 def __init__(self, *args, **kwargs):
 super(X, self).__init__(*args, **kwargs)
 self.start()
def tt():
 s = subprocess.Popen(("cat"), stdin=subprocess.PIPE)
 s.communicate(input = '#')
for i in xrange(20):
 X(target = tt)
----
On multi-processor (or multi-core) machines the script hangs fairly
reliably.
Protecting the Popen call with a lock solves the problem. I searched the
documentation if using stdin with subprocess.Popen was not thread-safe,
but found no indication.
Tested with Python 2.5.1, 2.5.2 and 2.6a1. The problem can be reproduced
with all mentioned versions.