title: poll method of subprocess.popen returning None for long process
body: <p>I am executing curl command through subprocess. This curl command starts video processing at another server, and waits for the response. Once the process is done, the remote server  returns json object. I am checking the status of the subprocess using poll() value, which is None - process not completed, 0- process completed successfully and 1- for error. </p>
  
  <p>I am getting the correct response if the processing takes around 30 mins/or less on remote server, but if the processing is taking more time, I am getting just None value , even though I can see that the remote server has finished processing and already returned the json object.</p>
  
  <p>Can anyone tell me, what could be the possible reason for poll() returning only None after certain time.  Thank you in advance.</p>
  
  <p>My Popen object is :</p>
  
  <pre><code>object = subprocess.Popen(str(curlCmd), shell=True,
                             stdout=subprocess.PIPE,
                             stderr=subprocess.PIPE)
  </code></pre>
  
  <p>and I am calling <code>object.poll()</code> after every 2 seconds to check if the process is successfully completed or not. </p>