[日常] Go语言圣经-示例: 并发的目录遍历习题

package main

import (
        //      "filepath"
        "flag"
        "fmt"
        "io/ioutil"
        "os"
        "path"
        "sync"
        "time"
)
/*
练习 8.9： 编写一个du工具，每隔一段时间将root目录下的目录大小计算并显示出来。
*/

//接收命令行参数-v
var verbose = flag.Bool("v", false, "show verbose progress messages")

func main() {
        //接收命令行参数,多个路径
        flag.Parse()
        roots := flag.Args()
        //如果没传递任何路径,给默认值
        if len(roots) == 0 { 
                roots = []string{"/"}
        }   

        for {
                sumFileSize(roots)
                time.Sleep(20 * time.Second)
        }   
}

func sumFileSize(roots []string) {
        //发送和接收文件字节大小的channel
        fileSizes := make(chan int64)
        //goroutine的计数器
        var n sync.WaitGroup
        //循环命令行传递的路径
        for _, root := range roots {
                n.Add(1)
                //启动goroutine计算
                go walkDir(root, &n, fileSizes)
        }   
        //启动goroutine,等待所有计算目录的goroutine结束
        go func() {
                n.Wait()
                close(fileSizes)
        }() 
        //定时显示目录进度发送的channel
        var tick <-chan time.Time
        if *verbose {
                tick = time.Tick(500 * time.Millisecond)
        }   
        var nfiles, nbytes int64
        //select和loop循环,多路复用
loop:
        for {
                select {
                case size, ok := <-fileSizes:
                        if !ok {
                                break loop // fileSizes was closed
                        }
                        //计算目录数,计算字节大小
                        nfiles++
                        nbytes += size
                case <-tick:
                        //接收到定时channel打印进度
                        printDiskUsage(nfiles, nbytes)
                }
        }
        //最后打印总计
        printDiskUsage(nfiles, nbytes) // final totals
}

func walkDir(dir string, n *sync.WaitGroup, fileSizes chan<- int64) {
        defer n.Done()
        for _, entry := range dirents(dir) {
                if entry.IsDir() {
                        n.Add(1)
                        subdir := path.Join(dir, entry.Name())
                        //开启多个goroutine进行递归
                        go walkDir(subdir, n, fileSizes)
                } else {
                        fileSizes <- entry.Size()
                }
        }
}

var sema = make(chan struct{}, 20)

// dirents returns the entries of directory dir.
func dirents(dir string) []os.FileInfo {
        //使用计数信号量逻辑限制太多并发
        sema <- struct{}{}
        entries, err := ioutil.ReadDir(dir)
        <-sema
        if err != nil {
                fmt.Fprintf(os.Stderr, "du1: %v\n", err)
                return nil
        }
        return entries
}
func printDiskUsage(nfiles, nbytes int64) {
        fmt.Printf("%d files  %.1f GB\n", nfiles, float64(nbytes)/1e9)
}