使用Go并发下载图片资源

package img

import (
"database/sql"
"fmt"
_ "github.com/go-sql-driver/mysql"
"io"
"net/http"
"os"
"runtime"
"strconv"
"sync"
"time"
)

const (
DataRoot     = "./tmp/" // 存放封面图的根目录
TimeoutLimit = 10       // 设置超时时间
)

// 表示章节ID和封面URL的对应关系
type VolumeCover struct {
VolumeID int
Url      string
Lock     sync.Mutex
Msg      chan string
Flag     bool    "timeout_flag"
}

// 将图片下载并保存到本地
func SaveImage(vc *VolumeCover) {
res, err := http.Get(vc.Url)
defer res.Body.Close()
if err != nil {
vc.Msg <- (strconv.Itoa(vc.VolumeID) + " HTTP_ERROR")
}
// 创建文件
dst, err := os.Create(DataRoot + strconv.Itoa(vc.VolumeID) + ".jpg")
if err != nil {
vc.Msg <- (strconv.Itoa(vc.VolumeID) + " OS_ERROR")
}
// 生成文件
io.Copy(dst, res.Body)
// goroutine通信
vc.Lock.Lock()
vc.Msg <- "in"
vc.Flag = true
vc.Lock.Unlock()
}

func Start(name string, password string, limit int) error {
runtime.GOMAXPROCS(4)
sl, err := sql.Open("mysql", name+":"+password+"@/xxx?charset=utf8")
defer sl.Close()
if err != nil {
return err
}
// 构造SELECT语句并检索
queryStr := "SELECT VolumeID, ImageUrl FROM volume "
if limit > 0 {
queryStr += "limit " + strconv.Itoa(limit)
}
rows, err := sl.Query(queryStr)
defer rows.Close()
if err != nil {
return err
}

// 构建列表
i := 0
vclist := make([]*VolumeCover, limit)
for rows.Next() {
vc := &VolumeCover{}
if err := rows.Scan(&vc.VolumeID, &vc.Url); err != nil {
return err
}
vc.Msg = make(chan string, 1)
//vc.To = make(chan bool, 1)
vc.Lock = *new(sync.Mutex)
vclist[i] = vc
i++
}

// start goroutines
for i := 0; i < len(vclist); i++ {
go SaveImage(vclist[i])
go func(i int) {
time.Sleep(TimeoutLimit * time.Second)
if vclist[i].Flag == false {
vclist[i].Msg <- "out"
}
}(i)
}

// 阻塞地获取结果
for i := 0; i < len(vclist); i++ {
func(c *VolumeCover) {
select {
case m := <-c.Msg:
fmt.Println(m)
}

}(vclist[i])
}
return nil
}