func (s *PresenceSuite) TestWatchPeriod(c *C) {
	presence.FakePeriod(1)
	presence.RealTimeSlot()

	w := presence.NewWatcher(s.presence)
	p := presence.NewPinger(s.presence, "a")
	defer w.Stop()
	defer p.Stop()

	ch := make(chan presence.Change)
	w.Watch("a", ch)
	assertChange(c, ch, presence.Change{"a", false})

	// A single ping.
	c.Assert(p.Start(), IsNil)
	c.Assert(p.Stop(), IsNil)

	// Wait for next periodic refresh.
	time.Sleep(1 * time.Second)
	assertChange(c, ch, presence.Change{"a", true})
}
func (s *PresenceSuite) TestPingerPeriodAndResilience(c *C) {
	// This test verifies both the periodic pinging,
	// and also a great property of the design: deaths
	// also expire, which means erroneous scenarios are
	// automatically recovered from.

	const period = 1
	presence.FakePeriod(period)
	presence.RealTimeSlot()

	w := presence.NewWatcher(s.presence)
	p1 := presence.NewPinger(s.presence, "a")
	p2 := presence.NewPinger(s.presence, "a")
	defer w.Stop()
	defer p1.Stop()
	defer p2.Stop()

	// Start p1 and let it go on.
	c.Assert(p1.Start(), IsNil)

	w.Sync()
	assertAlive(c, w, "a", true)

	// Start and kill p2, which will temporarily
	// invalidate p1 and set the key as dead.
	c.Assert(p2.Start(), IsNil)
	c.Assert(p2.Kill(), IsNil)

	w.Sync()
	assertAlive(c, w, "a", false)

	// Wait for two periods, and check again. Since
	// p1 is still alive, p2's death will expire and
	// the key will come back.
	time.Sleep(period * 2 * time.Second)

	w.Sync()
	assertAlive(c, w, "a", true)
}