python – 是subprocess.Popen不线程安全吗？

import subprocess,threading,fcntl,os,time
end_time = time.time()+5
lock = threading.Lock()
use_lock = False
path_to_factorial = os.path.join(os.path.dirname(os.path.realpath(__file__)),'factorial.sh')

def testFunction():
    print threading.current_thread().name,'| pre-Popen'
    if use_lock: lock.acquire()
    p = subprocess.Popen([path_to_factorial],stdin=subprocess.PIPE,stdout=subprocess.PIPE,stderr=subprocess.PIPE)
    if use_lock: lock.release()
    print threading.current_thread().name,'| post-Popen'
    fcntl.fcntl(p.stdout,fcntl.F_SETFL,os.O_NONBLOCK)
    fcntl.fcntl(p.stderr,os.O_NONBLOCK)
    while time.time()<end_time:
        try: p.stdout.read()
        except: pass
        try: p.stderr.read()
        except: pass
    print threading.current_thread().name,'| DONE'

for i in range(3):
    threading.Thread(target=testFunction).start()

上面引用的shell脚本(factorial.sh)：

#!/bin/sh
echo "Calculating factorial (anything that's somewhat compute intensive,this script takes 3 sec on my machine"
ans=1
counter=0
fact=999
while [ $fact -ne $counter ]
do
    counter=`expr $counter + 1`
    ans=`expr $ans * $counter`
done
echo "Factorial calculation done"
read -p "Test input (this part is critical for bug to occur): " buf
echo "$buf"

系统信息：
Linux 2.6.32-358.123.2.openstack.el6.x86_64#1 SMP Thu Sep 26 17:14:58 EDT 2013 x86_64 x86_64 x86_64 GNU / Linux
Python 2.7.3(默认,2013年1月22日,11:34:30)
[GCC 4.4.6 20120305(Red Hat 4.4.6-4)]在linux2上