Golang：实现断点续传（http单线程下载文件）

package breakpointContinuation

import (
	"fmt"
	"io"
	"io/ioutil"
	"net/http"
	"os"
	"strconv"
	"strings"
	"testing"
)

const (
	// 断点续传下载正常响应码
	NormalRespCode = "206 Partial Content"
	// 断点续传下载，偏移量值超文件最大值长度错误响应码
	ErrRespCode = "416 Requested Range Not Satisfiable"
	// 七牛文件
	SrcFileUrl = "http://qnota.xxxxxx"
	// （文件下载的进度)已下载长度记录文本地址
	FileName = "ac_ota"
	TempFile = "F:\\pic\\" + FileName + "_temp.txt"
	// 下载到本地的文件地址
	DestFile ="F:\\pic\\" + FileName + "_bak"
	BuffByte = 1024 * 20 -1
)

func TestDown(t *testing.T) {

	//file2,_:=os.OpenFile(destFile,os.O_CREATE|os.O_WRONLY,os.ModePerm)
	file2, err := os.Create(DestFile)
	defer file2.Close()
	if err != nil {
		panic(err)
	}

	file3,_:=os.OpenFile(TempFile,os.O_CREATE|os.O_RDWR,os.ModePerm)
	//1.读取临时文件中的数据，根据seek
	file3.Seek(0,io.SeekStart)
	bs:=make([]byte,100,100)
	n1,err:=file3.Read(bs)
	countStr:=string(bs[:n1])
	countArr := strings.Split(countStr, "/")
	var count, total int64 = 0, 0
	var bfb = "0%"
	var countf, totalf float64
	countf, totalf = 0.0001, 10000
	if len(countArr) >= 3 {
		count,_=strconv.ParseInt(countArr[0],10,64)
		total,_=strconv.ParseInt(countArr[1],10,64)
		bfb = countArr[2]
		//countf,_=strconv.ParseFloat(countArr[0], 64)
		//totalf,_=strconv.ParseFloat(countArr[1], 64)
	}
	//bfb := 	fmt.Sprintf("%.2f", countf/totalf*100)+"%"
	fmt.Println(fmt.Sprintf("开始下载，已下载：%d, 总共：%d, 进度：%s", count, total, bfb))

	for {
		req, err := http.NewRequest(http.MethodGet, SrcFileUrl, nil)
		if err != nil {
			return
		}

		// Range实现:  fmt.Sprintf("bytes=%d-%d", h.StartPos, h.EndPos)
		range01 := fmt.Sprintf("bytes=%d-%d", count , count + BuffByte)
		req.Header.Set("Range", range01)

		client := &http.Client{}
		resp, err := client.Do(req)
		if err != nil {
			fmt.Println("request fail:", err)
			return
		}
		defer resp.Body.Close()

		//fmt.Println("Accept-Ranges:", resp.Header.Get("Accept-Ranges"))
		Content_Range := resp.Header.Get("Content-Range")

		totalRange := strings.Split(Content_Range, "/")
		if len(totalRange) >= 2 {
			total,_ =strconv.ParseInt(totalRange[1], 10,64)
		}
		//fmt.Println("Content-Length:", resp.Header.Get("Content-Length"), "返回总Length：", total)



		// 206 Partial Content\416 Requested Range Not Satisfiable
		fmt.Println("resp.Status:", resp.Status, resp.Status == "206 Partial Content" )

		if resp.Status != NormalRespCode {
			if resp.Status == ErrRespCode  {
				fmt.Println("文件下载完毕。。")
				file3.Close()
			} else {
				fmt.Println(fmt.Sprintf("文件传输异常报错：err[%s]", resp.Status))
			}

			break
		}

		n3:=-1//写入的数据量

		file2.Seek(count,0) // 设置file2下一次读或者写的起点

		data, err := ioutil.ReadAll(resp.Body)
		//fmt.Println("data size:", len(data))
		//io.Copy(file2, res.Body)

		n3,_=file2.Write(data)
		count += int64(n3)
		countf = float64(count)
		totalf = float64(total)
		bfb = 	fmt.Sprintf("%.2f", countf/totalf*100)+"%"
		fmt.Println("本次读取了：", n3, "总共已下载:", count, "文件总大小：", total, "下载进度：", bfb)
		w := strconv.Itoa(int(count)) + "/" + strconv.Itoa(int(total)) + "/" + bfb

		file3.Seek(0,io.SeekStart) // 设置file3的下次读写起点为源点：0点，即覆盖重写。
		file3.WriteString(w)

		// 假装断电
		if count> 1024*1024*4{
		panic("假装断电了。。。，假装的。。。")
		}
	}
}

 异常打印：

正常打印： 

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