subprocess hangs on reading stdout

[Tim Arnold]

Hi, I'm querying a list of network servers for processes belonging to a 
specific user. The problem is that when I try to read the stdout from the 
subprocess it sometimes hangs. Not always though.

I thought maybe I needed to set unbufferered to true, so at the beginning of 
the code I set
os.environ['PYTHONUNBUFFERED'] = '1'
But I think it's more likely the subprocess that needs to be unbuffered.

Here's the bit of code:
---------------------------
        for machine_name in self.alive:# a list of servers that responded to 
ping already.
            cmd = ["/bin/remsh", machine_name, 'ps -flu %s' % uid]
            finish = time.time() + 4.0
            p = subprocess.Popen(cmd,stdout=subprocess.PIPE)
            while p.poll() is None:
                time.sleep(0.5)
                if finish < time.time():
                    p.kill()
                    print 'skipping' # this works ok
                    break

            s = ''
            if p:
                s = p.stdout.read() # trhis will hang occasionally
            if not s:
                continue
---------------------------

Any ideas? comments on code welcome also.
thanks,
--Tim Arnold