func (pc *PasswordChanger) setMongoPassword(entity state.Entity, password string) error {
	type mongoPassworder interface {
		SetMongoPassword(password string) error
	}
	// We set the mongo password first on the grounds that
	// if it fails, the agent in question should still be able
	// to authenticate to another API server and ask it to change
	// its password.
	if entity0, ok := entity.(mongoPassworder); ok {
		if err := entity0.SetMongoPassword(password); err != nil {
			return err
		}
		logger.Infof("setting mongo password for %q", entity.Tag())
		return nil
	}
	return NotSupportedError(entity.Tag(), "mongo access")
}

func startPingerIfAgent(clock clock.Clock, root *apiHandler, entity state.Entity) error {
	// worker runs presence.Pingers -- absence of which will cause
	// embarrassing "agent is lost" messages to show up in status --
	// until it's stopped. It's stored in resources purely for the
	// side effects: we don't record its id, and nobody else
	// retrieves it -- we just expect it to be stopped when the
	// connection is shut down.
	agent, ok := entity.(statepresence.Agent)
	if !ok {
		return nil
	}
	worker, err := presence.New(presence.Config{
		Identity:   entity.Tag(),
		Start:      presenceShim{agent}.Start,
		Clock:      clock,
		RetryDelay: 3 * time.Second,
	})
	if err != nil {
		return err
	}
	root.getResources().Register(worker)

	// pingTimeout, by contrast, *is* used by the Pinger facade to
	// stave off the call to action() that will shut down the agent
	// connection if it gets lackadaisical about sending keepalive
	// Pings.
	//
	// Do not confuse those (apiserver) Pings with those made by
	// presence.Pinger (which *do* happen as a result of the former,
	// but only as a relatively distant consequence).
	//
	// We should have picked better names...
	action := func() {
		logger.Debugf("closing connection due to ping timout")
		if err := root.getRpcConn().Close(); err != nil {
			logger.Errorf("error closing the RPC connection: %v", err)
		}
	}
	pingTimeout := newPingTimeout(action, clock, maxClientPingInterval)
	return root.getResources().RegisterNamed("pingTimeout", pingTimeout)
}