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<ol class="chapter"><li class="chapter-item expanded affix "><a href="../index.html">Go语言圣经</a></li><li class="chapter-item expanded affix "><a href="../preface.html">前言</a></li><li class="chapter-item expanded "><a href="../ch1/ch1.html"><strong aria-hidden="true">1.</strong> 入门</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../ch1/ch1-01.html"><strong aria-hidden="true">1.1.</strong> Hello, World</a></li><li class="chapter-item expanded "><a href="../ch1/ch1-02.html"><strong aria-hidden="true">1.2.</strong> 命令行参数</a></li><li class="chapter-item expanded "><a href="../ch1/ch1-03.html"><strong aria-hidden="true">1.3.</strong> 查找重复的行</a></li><li class="chapter-item expanded "><a href="../ch1/ch1-04.html"><strong aria-hidden="true">1.4.</strong> GIF动画</a></li><li class="chapter-item expanded "><a href="../ch1/ch1-05.html"><strong aria-hidden="true">1.5.</strong> 获取URL</a></li><li class="chapter-item expanded "><a href="../ch1/ch1-06.html"><strong aria-hidden="true">1.6.</strong> 并发获取多个URL</a></li><li class="chapter-item expanded "><a href="../ch1/ch1-07.html"><strong aria-hidden="true">1.7.</strong> Web服务</a></li><li class="chapter-item expanded "><a href="../ch1/ch1-08.html"><strong aria-hidden="true">1.8.</strong> 本章要点</a></li></ol></li><li class="chapter-item expanded "><a href="../ch2/ch2.html"><strong aria-hidden="true">2.</strong> 程序结构</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../ch2/ch2-01.html"><strong aria-hidden="true">2.1.</strong> 命名</a></li><li class="chapter-item expanded "><a href="../ch2/ch2-02.html"><strong aria-hidden="true">2.2.</strong> 声明</a></li><li class="chapter-item expanded "><a href="../ch2/ch2-03.html"><strong aria-hidden="true">2.3.</strong> 变量</a></li><li class="chapter-item expanded "><a href="../ch2/ch2-04.html"><strong aria-hidden="true">2.4.</strong> 赋值</a></li><li class="chapter-item expanded "><a href="../ch2/ch2-05.html"><strong aria-hidden="true">2.5.</strong> 类型</a></li><li class="chapter-item expanded "><a href="../ch2/ch2-06.html"><strong aria-hidden="true">2.6.</strong> 包和文件</a></li><li class="chapter-item expanded "><a href="../ch2/ch2-07.html"><strong aria-hidden="true">2.7.</strong> 作用域</a></li></ol></li><li class="chapter-item expanded "><a href="../ch3/ch3.html"><strong aria-hidden="true">3.</strong> 基础数据类型</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../ch3/ch3-01.html"><strong aria-hidden="true">3.1.</strong> 整型</a></li><li class="chapter-item expanded "><a href="../ch3/ch3-02.html"><strong aria-hidden="true">3.2.</strong> 浮点数</a></li><li class="chapter-item expanded "><a href="../ch3/ch3-03.html"><strong aria-hidden="true">3.3.</strong> 复数</a></li><li class="chapter-item expanded "><a href="../ch3/ch3-04.html"><strong aria-hidden="true">3.4.</strong> 布尔型</a></li><li class="chapter-item expanded "><a href="../ch3/ch3-05.html"><strong aria-hidden="true">3.5.</strong> 字符串</a></li><li class="chapter-item expanded "><a href="../ch3/ch3-06.html"><strong aria-hidden="true">3.6.</strong> 常量</a></li></ol></li><li class="chapter-item expanded "><a href="../ch4/ch4.html"><strong aria-hidden="true">4.</strong> 复合数据类型</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../ch4/ch4-01.html"><strong aria-hidden="true">4.1.</strong> 数组</a></li><li class="chapter-item expanded "><a href="../ch4/ch4-02.html"><strong aria-hidden="true">4.2.</strong> Slice</a></li><li class="chapter-item expanded "><a href="../ch4/ch4-03.html"><strong aria-hidden="true">4.3.</strong> Map</a></li><li class="chapter-item expanded "><a href="../ch4/ch4-04.html"><strong aria-hidden="true">4.4.</strong> 结构体</a></li><li class="chapter-item expanded "><a href="../ch4/ch4-05.html"><strong aria-hidden="true">4.5.</strong> JSON</a></li><li class="chapter-item expanded "><a href="../ch4/ch4-06.html"><strong aria-hidd
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<main>
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<!-- 头部 -->
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<ul dir="auto">
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<li><em>KusonStack一站式可编程配置技术栈(Go): <a href="https://github.com/KusionStack/kusion">https://github.com/KusionStack/kusion</a></em></li>
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<li><em>KCL 配置编程语言(Rust): <a href="https://github.com/KusionStack/KCLVM">https://github.com/KusionStack/KCLVM</a></em></li>
|
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<li><em>凹语言™: <a href="https://github.com/wa-lang/wa">https://github.com/wa-lang/wa</a></em></li>
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</ul>
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<hr>
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<h2 id="88-示例-并发的目录遍历"><a class="header" href="#88-示例-并发的目录遍历">8.8. 示例: 并发的目录遍历</a></h2>
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<p>在本小节中，我们会创建一个程序来生成指定目录的硬盘使用情况报告，这个程序和Unix里的du工具比较相似。大多数工作用下面这个walkDir函数来完成，这个函数使用dirents函数来枚举一个目录下的所有入口。</p>
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<p><u><i>gopl.io/ch8/du1</i></u></p>
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<pre><code class="language-go">// walkDir recursively walks the file tree rooted at dir
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// and sends the size of each found file on fileSizes.
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func walkDir(dir string, fileSizes chan<- int64) {
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for _, entry := range dirents(dir) {
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if entry.IsDir() {
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subdir := filepath.Join(dir, entry.Name())
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walkDir(subdir, fileSizes)
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} else {
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fileSizes <- entry.Size()
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}
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}
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}
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// dirents returns the entries of directory dir.
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func dirents(dir string) []os.FileInfo {
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entries, err := ioutil.ReadDir(dir)
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if err != nil {
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fmt.Fprintf(os.Stderr, "du1: %v\n", err)
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return nil
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}
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return entries
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}
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</code></pre>
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<p>ioutil.ReadDir函数会返回一个os.FileInfo类型的slice，os.FileInfo类型也是os.Stat这个函数的返回值。对每一个子目录而言，walkDir会递归地调用其自身，同时也在递归里获取每一个文件的信息。walkDir函数会向fileSizes这个channel发送一条消息。这条消息包含了文件的字节大小。</p>
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<p>下面的主函数，用了两个goroutine。后台的goroutine调用walkDir来遍历命令行给出的每一个路径并最终关闭fileSizes这个channel。主goroutine会对其从channel中接收到的文件大小进行累加，并输出其和。</p>
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<pre><code class="language-go">package main
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import (
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"flag"
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"fmt"
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"io/ioutil"
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"os"
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"path/filepath"
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)
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func main() {
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// Determine the initial directories.
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flag.Parse()
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roots := flag.Args()
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if len(roots) == 0 {
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roots = []string{"."}
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}
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// Traverse the file tree.
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fileSizes := make(chan int64)
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go func() {
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for _, root := range roots {
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walkDir(root, fileSizes)
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}
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close(fileSizes)
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}()
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// Print the results.
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var nfiles, nbytes int64
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for size := range fileSizes {
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nfiles++
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nbytes += size
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}
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printDiskUsage(nfiles, nbytes)
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}
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func printDiskUsage(nfiles, nbytes int64) {
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fmt.Printf("%d files %.1f GB\n", nfiles, float64(nbytes)/1e9)
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}
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</code></pre>
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<p>这个程序会在打印其结果之前卡住很长时间。</p>
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<pre><code>$ go build gopl.io/ch8/du1
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$ ./du1 $HOME /usr /bin /etc
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213201 files 62.7 GB
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</code></pre>
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<p>如果在运行的时候能够让我们知道处理进度的话想必更好。但是，如果简单地把printDiskUsage函数调用移动到循环里会导致其打印出成百上千的输出。</p>
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<p>下面这个du的变种会间歇打印内容，不过只有在调用时提供了-v的flag才会显示程序进度信息。在roots目录上循环的后台goroutine在这里保持不变。主goroutine现在使用了计时器来每500ms生成事件，然后用select语句来等待文件大小的消息来更新总大小数据，或者一个计时器的事件来打印当前的总大小数据。如果-v的flag在运行时没有传入的话，tick这个channel会保持为nil，这样在select里的case也就相当于被禁用了。</p>
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<p><u><i>gopl.io/ch8/du2</i></u></p>
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|
|
<pre><code class="language-go">var verbose = flag.Bool("v", false, "show verbose progress messages")
|
|||
|
|
|
|||
|
|
func main() {
|
|||
|
|
// ...start background goroutine...
|
|||
|
|
|
|||
|
|
// Print the results periodically.
|
|||
|
|
var tick <-chan time.Time
|
|||
|
|
if *verbose {
|
|||
|
|
tick = time.Tick(500 * time.Millisecond)
|
|||
|
|
}
|
|||
|
|
var nfiles, nbytes int64
|
|||
|
|
loop:
|
|||
|
|
for {
|
|||
|
|
select {
|
|||
|
|
case size, ok := <-fileSizes:
|
|||
|
|
if !ok {
|
|||
|
|
break loop // fileSizes was closed
|
|||
|
|
}
|
|||
|
|
nfiles++
|
|||
|
|
nbytes += size
|
|||
|
|
case <-tick:
|
|||
|
|
printDiskUsage(nfiles, nbytes)
|
|||
|
|
}
|
|||
|
|
}
|
|||
|
|
printDiskUsage(nfiles, nbytes) // final totals
|
|||
|
|
}
|
|||
|
|
</code></pre>
|
|||
|
|
<p>由于我们的程序不再使用range循环，第一个select的case必须显式地判断fileSizes的channel是不是已经被关闭了，这里可以用到channel接收的二值形式。如果channel已经被关闭了的话，程序会直接退出循环。这里的break语句用到了标签break，这样可以同时终结select和for两个循环；如果没有用标签就break的话只会退出内层的select循环，而外层的for循环会使之进入下一轮select循环。</p>
|
|||
|
|
<p>现在程序会悠闲地为我们打印更新流：</p>
|
|||
|
|
<pre><code>$ go build gopl.io/ch8/du2
|
|||
|
|
$ ./du2 -v $HOME /usr /bin /etc
|
|||
|
|
28608 files 8.3 GB
|
|||
|
|
54147 files 10.3 GB
|
|||
|
|
93591 files 15.1 GB
|
|||
|
|
127169 files 52.9 GB
|
|||
|
|
175931 files 62.2 GB
|
|||
|
|
213201 files 62.7 GB
|
|||
|
|
</code></pre>
|
|||
|
|
<p>然而这个程序还是会花上很长时间才会结束。完全可以并发调用walkDir，从而发挥磁盘系统的并行性能。下面这个第三个版本的du，会对每一个walkDir的调用创建一个新的goroutine。它使用sync.WaitGroup（§8.5）来对仍旧活跃的walkDir调用进行计数，另一个goroutine会在计数器减为零的时候将fileSizes这个channel关闭。</p>
|
|||
|
|
<p><u><i>gopl.io/ch8/du3</i></u></p>
|
|||
|
|
<pre><code class="language-go">func main() {
|
|||
|
|
// ...determine roots...
|
|||
|
|
// Traverse each root of the file tree in parallel.
|
|||
|
|
fileSizes := make(chan int64)
|
|||
|
|
var n sync.WaitGroup
|
|||
|
|
for _, root := range roots {
|
|||
|
|
n.Add(1)
|
|||
|
|
go walkDir(root, &n, fileSizes)
|
|||
|
|
}
|
|||
|
|
go func() {
|
|||
|
|
n.Wait()
|
|||
|
|
close(fileSizes)
|
|||
|
|
}()
|
|||
|
|
// ...select loop...
|
|||
|
|
}
|
|||
|
|
|
|||
|
|
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 := filepath.Join(dir, entry.Name())
|
|||
|
|
go walkDir(subdir, n, fileSizes)
|
|||
|
|
} else {
|
|||
|
|
fileSizes <- entry.Size()
|
|||
|
|
}
|
|||
|
|
}
|
|||
|
|
}
|
|||
|
|
</code></pre>
|
|||
|
|
<p>由于这个程序在高峰期会创建成百上千的goroutine，我们需要修改dirents函数，用计数信号量来阻止他同时打开太多的文件，就像我们在8.7节中的并发爬虫一样：</p>
|
|||
|
|
<pre><code class="language-go">// sema is a counting semaphore for limiting concurrency in dirents.
|
|||
|
|
var sema = make(chan struct{}, 20)
|
|||
|
|
|
|||
|
|
// dirents returns the entries of directory dir.
|
|||
|
|
func dirents(dir string) []os.FileInfo {
|
|||
|
|
sema <- struct{}{} // acquire token
|
|||
|
|
defer func() { <-sema }() // release token
|
|||
|
|
// ...
|
|||
|
|
</code></pre>
|
|||
|
|
<p>这个版本比之前那个快了好几倍，尽管其具体效率还是和你的运行环境，机器配置相关。</p>
|
|||
|
|
<p><strong>练习 8.9：</strong> 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。</p>
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