clientv3/balancer/picker/roundrobin_balanced.go
func (rb *rrBalanced) Pick(ctx context.Context, opts balancer.PickOptions) (balancer.SubConn, func(balancer.DoneInfo), error) {
	rb.mu.RLock()
	n := len(rb.scs)
	rb.mu.RUnlock()
	if n == 0 {
		return nil, nil, balancer.ErrNoSubConnAvailable
	}

	rb.mu.Lock()
	cur := rb.next
	sc := rb.scs[cur]
	picked := rb.scToAddr[sc].Addr
	rb.next = (rb.next + 1) % len(rb.scs)
	rb.mu.Unlock()

	rb.lg.Debug(
		"picked",
		zap.String("address", picked),
		zap.Int("subconn-index", cur),
		zap.Int("subconn-size", n),
	)

	doneFunc := func(info balancer.DoneInfo) {
		// TODO: error handling?
		fss := []zapcore.Field{
			zap.Error(info.Err),
			zap.String("address", picked),
			zap.Bool("success", info.Err == nil),
			zap.Bool("bytes-sent", info.BytesSent),
			zap.Bool("bytes-received", info.BytesReceived),
		}
		if info.Err == nil {
			rb.lg.Debug("balancer done", fss...)
		} else {
			rb.lg.Warn("balancer failed", fss...)
		}
	}
	return sc, doneFunc, nil
}
func (rb *rrBalanced) Pick(ctx context.Context, opts balancer.PickOptions) (balancer.SubConn, func(balancer.DoneInfo), error) {
	rb.mu.RLock()
	n := len(rb.scs)
	rb.mu.RUnlock()
	if n == 0 {
		return nil, nil, balancer.ErrNoSubConnAvailable
	}

	rb.mu.Lock()
	cur := rb.next
	sc := rb.scs[cur]
	picked := rb.scToAddr[sc].Addr
	rb.next = (rb.next + 1) % len(rb.scs)
	rb.mu.Unlock()

	rb.lg.Debug(
		"picked",
		zap.String("address", picked),
		zap.Int("subconn-index", cur),
		zap.Int("subconn-size", n),
	)

	doneFunc := func(info balancer.DoneInfo) {
		// TODO: error handling?
		fss := []zapcore.Field{
			zap.Error(info.Err),
			zap.String("address", picked),
			zap.Bool("success", info.Err == nil),
			zap.Bool("bytes-sent", info.BytesSent),
			zap.Bool("bytes-received", info.BytesReceived),
		}
		if info.Err == nil {
			rb.lg.Debug("balancer done", fss...)
		} else {
			rb.lg.Warn("balancer failed", fss...)
		}
	}
	return sc, doneFunc, nil
}
// Pick is called for every client request.