FileCoin.Syncstate RPC occasionally returns empty data

[J1a-wei]

I have a logic that gets the latest block. But sometime I get zero number(The frequency is about three or four times a day).
I did a test, and the cycle request to get a high interface. But Other problems occur. Panic happened.

Test code.

Request endpoint every three seconds, the number of statistics, generate a report once a hour.

package main

import (
	"bytes"
	"encoding/json"
	"io/ioutil"
	"net/http"
	"os"
	"strconv"
	"time"

	"go.uber.org/zap"
)

const filecoinEndpoint = "TODO"

var l *zap.SugaredLogger

var exceptionCount int
var totalCount int


func main() {
	log, _ := zap.NewDevelopment()
	l = log.Sugar()
  go check()
  // generate report / 1h
  reportTicker := time.NewTicker(1 * time.Hour)
  for {
    select {
    case <-reportTicker.C:
    if totalCount > 0 {
      var percent float64 = float64(exceptionCount)/ float64(totalCount)
      l.Infof("1 hour duration -- totalCount: %d , exceptionCount: %d, percentage： %.2f%%",totalCount,exceptionCount,percent)
      // init
      exceptionCount = 0
      totalCount = 0
    }
  }
  }


}


func check(){
	sec := os.Getenv("FILECOIN_FREQ")
	secInt := 3
	if sec != "" {
		var err error
		secInt, err = strconv.Atoi(sec)
		if err != nil {
			l.Error("ENV FILECOIN_FREQ convert into int error: ", err.Error())
			return
		}
	}
	ticker := time.NewTicker(time.Duration(secInt) * time.Second)
	for {
		select {
		case <-ticker.C:
			// l.Info("exec filecoin cronJob")
			totalCount++
			resp, err := doHttp()
			if err != nil {
				l.Error(err.Error())
        continue
			}

			curHeight := resp.Result.ActiveSyncs[0].Height
			tarHeight := resp.Result.ActiveSyncs[0].Target.Height

			if curHeight == 0 && tarHeight == 0 {
				exceptionCount++
				prettyJson, _ := json.MarshalIndent(resp, "", "  ")
				l.Info(string(prettyJson))
			}

		}
	}
}

// height of struct
type heightResponse struct {
	Result SyncState `json:"result"`
}
type ActiveSync struct {
	WorkerID int
	Base     TipSet
	Target   TipSet

	Stage  int
	Height int

	Start   time.Time
	End     time.Time
	Message string
}
type SyncState struct {
	ActiveSyncs []ActiveSync
	VMApplied   int
}
type TipSet struct {
	Height int
}

var httpClient http.Client = http.Client{
	Timeout: time.Minute,
}

type RequestBody struct {
	Id      int64       `json:"id"`
	Method  string      `json:"method"`
	Params  interface{} `json:"params"`
	Jsonrpc string      `json:"jsonrpc"`
}

func doHttp() (result *heightResponse, err error) {
	result = &heightResponse{}
	reqBody := RequestBody{
		Id:      0,
		Method:  "Filecoin.SyncState",
		Params:  []string{},
		Jsonrpc: "2.0",
	}

	reqBytes, _ := json.Marshal(reqBody)

	req, err := http.NewRequest(http.MethodPost, filecoinEndpoint, bytes.NewReader(reqBytes))
	if err != nil {
		return nil, err
	}
	req.Header.Set("Content-Type", "application/json;charset=UTF-8")
	resp, err := httpClient.Do(req)
	if err != nil {
		return nil, err
	}
	defer resp.Body.Close()
	respBytes, err := ioutil.ReadAll(resp.Body)
	if err != nil {
		return nil, err
	}
	err = json.Unmarshal(respBytes, result)
	return
}

Terminal Log

2022-10-27T09:08:25.388Z	INFO	filecoin-cronjob/main.go:75	{
  "result": {
    "ActiveSyncs": [
      {
        "WorkerID": 101244,
        "Base": {
          "Height": 0
        },
        "Target": {
          "Height": 0
        },
        "Stage": 0,
        "Height": 0,
        "Start": "0001-01-01T00:00:00Z",
        "End": "0001-01-01T00:00:00Z",
        "Message": ""
      },
      {
        "WorkerID": 101245,
        "Base": {
          "Height": 2285176
        },
        "Target": {
          "Height": 2285176
        },
        "Stage": 4,
        "Height": 2285176,
        "Start": "2022-10-27T09:08:01.534489433Z",
        "End": "2022-10-27T09:08:01.728351906Z",
        "Message": ""
      },
      {
        "WorkerID": 101246,
        "Base": {
          "Height": 2285176
        },
        "Target": {
          "Height": 2285176
        },
        "Stage": 4,
        "Height": 2285176,
        "Start": "2022-10-27T09:08:01.71799741Z",
        "End": "2022-10-27T09:08:01.916167653Z",
        "Message": ""
      }
    ],
    "VMApplied": 0
  }
}
2022-10-27T09:08:28.249Z	INFO	filecoin-cronjob/main.go:75	{
  "result": {
    "ActiveSyncs": [
      {
        "WorkerID": 101244,
        "Base": {
          "Height": 0
        },
        "Target": {
          "Height": 0
        },
        "Stage": 0,
        "Height": 0,
        "Start": "0001-01-01T00:00:00Z",
        "End": "0001-01-01T00:00:00Z",
        "Message": ""
      },
      {
        "WorkerID": 101245,
        "Base": {
          "Height": 2285176
        },
        "Target": {
          "Height": 2285176
        },
        "Stage": 4,
        "Height": 2285176,
        "Start": "2022-10-27T09:08:01.534489433Z",
        "End": "2022-10-27T09:08:01.728351906Z",
        "Message": ""
      },
      {
        "WorkerID": 101246,
        "Base": {
          "Height": 2285176
        },
        "Target": {
          "Height": 2285176
        },
        "Stage": 4,
        "Height": 2285176,
        "Start": "2022-10-27T09:08:01.71799741Z",
        "End": "2022-10-27T09:08:01.916167653Z",
        "Message": ""
      }
    ],
    "VMApplied": 0
  }
}
2022-10-27T09:08:34.280Z	INFO	filecoin-cronjob/main.go:75	{
  "result": {
    "ActiveSyncs": [
      {
        "WorkerID": 101249,
        "Base": {
          "Height": 0
        },
        "Target": {
          "Height": 0
        },
        "Stage": 0,
        "Height": 0,
        "Start": "0001-01-01T00:00:00Z",
        "End": "0001-01-01T00:00:00Z",
        "Message": ""
      },
      {
        "WorkerID": 101250,
        "Base": {
          "Height": 2285177
        },
        "Target": {
          "Height": 2285177
        },
        "Stage": 4,
        "Height": 2285177,
        "Start": "2022-10-27T09:08:30.474224971Z",
        "End": "2022-10-27T09:08:30.953055824Z",
        "Message": ""
      },
      {
        "WorkerID": 101251,
        "Base": {
          "Height": 0
        },
        "Target": {
          "Height": 0
        },
        "Stage": 0,
        "Height": 0,
        "Start": "0001-01-01T00:00:00Z",
        "End": "0001-01-01T00:00:00Z",
        "Message": ""
      }
    ],
    "VMApplied": 0
  }
}
panic: runtime error: index out of range [0] with length 0

goroutine 6 [running]:
main.check()
	/home/skrbug/code/go/filecoin-cronjob/main.go:69 +0x255
created by main.main
	/home/skrbug/code/go/filecoin-cronjob/main.go:26 +0xdd

Version

lotus:v1.16.0

Two problems are occured.

1. PANIC is due to the non -returned data of the interface.
2. The logic of obtaining the latest block is incorrect.

Question

How to get the block height, do I need to traverse the maximum value? What is this panic reason?
Thanks in advance !