openTracingSetup.go

	//Opentracing setup
	var jaegerCloser io.Closer
	var zErr error
	var closeJaegerIfNeeded = func() {
		if jaegerCloser != nil {
			if err := jaegerCloser.Close(); err != nil {
				logging.Error("Failed to close tracing with error: %s", err.Error())
			}
			jaegerCloser = nil
		}
	}
	defer closeJaegerIfNeeded()

	if ff.Opentracing.IsEnabled() {
		if jaegerCloser, zErr = setupOpenTracing(mode); zErr != nil {
			logging.Errorf("openTracing setup failed with error: %s", zErr.Error())
		}
	}

	flags.OnUpdate(func(ff *flags.FlannelFlags) {
		if ff.Opentracing.IsEnabled() {
			logging.Info("Opentracing was enabled")
			if jaegerCloser, zErr = setupOpenTracing(mode); zErr != nil {
				logging.Errorf("openTracing setup failed with error: %s", zErr.Error())
			}
		} else {
			logging.Info("Opentracing was disabled")
			closeJaegerIfNeeded()
			opentracing.SetGlobalTracer(opentracing.NoopTracer{})
		}
	})

I wish that flags.OnUpdate vends an initial value here