func (s *S) TestConnectCloseConcurrency(c *C) {
	restore := mgo.HackPingDelay(500 * time.Millisecond)
	defer restore()
	var wg sync.WaitGroup
	const n = 500
	wg.Add(n)
	for i := 0; i < n; i++ {
		go func() {
			defer wg.Done()
			session, err := mgo.Dial("localhost:40001")
			if err != nil {
				c.Fatal(err)
			}
			time.Sleep(1)
			session.Close()
		}()
	}
	wg.Wait()
}
func (s *S) TestNearestSecondary(c *C) {
	defer mgo.HackPingDelay(3 * time.Second)()

	rs1a := "127.0.0.1:40011"
	rs1b := "127.0.0.1:40012"
	rs1c := "127.0.0.1:40013"
	s.Freeze(rs1b)

	session, err := mgo.Dial(rs1a)
	c.Assert(err, IsNil)
	defer session.Close()

	// Wait for the sync up to run through the first couple of servers.
	for len(session.LiveServers()) != 2 {
		c.Log("Waiting for two servers to be alive...")
		time.Sleep(100 * time.Millisecond)
	}

	// Extra delay to ensure the third server gets penalized.
	time.Sleep(500 * time.Millisecond)

	// Release third server.
	s.Thaw(rs1b)

	// Wait for it to come up.
	for len(session.LiveServers()) != 3 {
		c.Log("Waiting for all servers to be alive...")
		time.Sleep(100 * time.Millisecond)
	}

	session.SetMode(mgo.Monotonic, true)
	var result struct{ Host string }

	// See which slave picks the line, several times to avoid chance.
	for i := 0; i < 10; i++ {
		session.Refresh()
		err = session.Run("serverStatus", &result)
		c.Assert(err, IsNil)
		c.Assert(hostPort(result.Host), Equals, hostPort(rs1c))
	}

	if *fast {
		// Don't hold back for several seconds.
		return
	}

	// Now hold the other server for long enough to penalize it.
	s.Freeze(rs1c)
	time.Sleep(5 * time.Second)
	s.Thaw(rs1c)

	// Wait for the ping to be processed.
	time.Sleep(500 * time.Millisecond)

	// Repeating the test should now pick the former server consistently.
	for i := 0; i < 10; i++ {
		session.Refresh()
		err = session.Run("serverStatus", &result)
		c.Assert(err, IsNil)
		c.Assert(hostPort(result.Host), Equals, hostPort(rs1b))
	}
}