Problems with how we use poll.poll:

• It's not thread safe, yet we use a global poll object and we don't protect calls to poll() with a mutex. If called concurrently an exception is raised. See http://bugs.python.org/issue8865.
  • Also, since the poll object is global in the network module, poll() can be called concurrently by multiple connection pools in multiple instances of MongoClient.
• poll() can raise EINTR in versions of Python < 3.5.
• poll() can raise exceptions for various other reasons, which will cause the fd we're polling to never be unregistered.

Solutions:

• Instead of using a global poll object, create a poll object per Pool, or at the very least serialize use of the global object.
• Call poll() in a try / finally calling unregister() in the finally block
• Catch EINTR specifically and retry the call to poll()

While we're fixing poll, also handle EINTR being raised by select.select().