Failed on travis: https://travis-ci.org/mongodb/mongo-python-driver/jobs/309136359#L1716

FAIL: test_max_idle_time_reaper (test_client.TestClient)
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Traceback (most recent call last):
  File "/home/travis/build/mongodb/mongo-python-driver/test/test_client.py", line 293, in test_max_idle_time_reaper
    self.assertEqual(1, len(server._pool.sockets))
AssertionError: 1 != 2

There is a race in the Pool.remove_stale_sockets method. We use this code to ensure there are at least minPoolSize sockets in the Pool:

        while len(
                self.sockets) + self.active_sockets < self.opts.min_pool_size:
            sock_info = self.connect()
            with self.lock:
                self.sockets.add(sock_info)

However, if another thread is in the process of checking out a socket at the same time we may add more that minPoolSize sockets. Imagine minPoolSize=1 and maxPoolSize=1:
1) Topology calls remove_stale_sockets. Pool size = 0
2) No sockets are in the pool. remove_stale_sockets creates a connection but has not added it to the Pool. Pool size is still 0.
3) Another thread calls pool.get_socket({}) and creates a socket. Pool size = 1
4) remove_stale_sockets resumes execution adds the new socket to the pool. Pool size = 2

remove_stale_sockets must acquire the Pool's socket semaphore before creating a socket to ensure that maxPoolSize is respected.