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<li><em>Go语言圣经读书笔记(挖坑中...): <a href="https://github.com/chai2010/gopl-notes-zh">https://github.com/chai2010/gopl-notes-zh</a></em></li>
<li><em>凹语言(Go实现, 面向WASM设计): <a href="https://github.com/wa-lang/wa">https://github.com/wa-lang/wa</a></em></li>
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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>
<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>
<p><u><i>gopl.io/ch8/du1</i></u></p>
<pre><code class="language-go">// walkDir recursively walks the file tree rooted at dir
// and sends the size of each found file on fileSizes.
func walkDir(dir string, fileSizes chan&lt;- int64) {
for _, entry := range dirents(dir) {
if entry.IsDir() {
subdir := filepath.Join(dir, entry.Name())
walkDir(subdir, fileSizes)
} else {
fileSizes &lt;- entry.Size()
}
}
}
// dirents returns the entries of directory dir.
func dirents(dir string) []os.FileInfo {
entries, err := ioutil.ReadDir(dir)
if err != nil {
fmt.Fprintf(os.Stderr, &quot;du1: %v\n&quot;, err)
return nil
}
return entries
}
</code></pre>
<p>ioutil.ReadDir函数会返回一个os.FileInfo类型的sliceos.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>
<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>
<pre><code class="language-go">package main
import (
&quot;flag&quot;
&quot;fmt&quot;
&quot;io/ioutil&quot;
&quot;os&quot;
&quot;path/filepath&quot;
)
func main() {
// Determine the initial directories.
flag.Parse()
roots := flag.Args()
if len(roots) == 0 {
roots = []string{&quot;.&quot;}
}
// Traverse the file tree.
fileSizes := make(chan int64)
go func() {
for _, root := range roots {
walkDir(root, fileSizes)
}
close(fileSizes)
}()
// Print the results.
var nfiles, nbytes int64
for size := range fileSizes {
nfiles++
nbytes += size
}
printDiskUsage(nfiles, nbytes)
}
func printDiskUsage(nfiles, nbytes int64) {
fmt.Printf(&quot;%d files %.1f GB\n&quot;, nfiles, float64(nbytes)/1e9)
}
</code></pre>
<p>这个程序会在打印其结果之前卡住很长时间。</p>
<pre><code>$ go build gopl.io/ch8/du1
$ ./du1 $HOME /usr /bin /etc
213201 files 62.7 GB
</code></pre>
<p>如果在运行的时候能够让我们知道处理进度的话想必更好。但是如果简单地把printDiskUsage函数调用移动到循环里会导致其打印出成百上千的输出。</p>
<p>下面这个du的变种会间歇打印内容不过只有在调用时提供了-v的flag才会显示程序进度信息。在roots目录上循环的后台goroutine在这里保持不变。主goroutine现在使用了计时器来每500ms生成事件然后用select语句来等待文件大小的消息来更新总大小数据或者一个计时器的事件来打印当前的总大小数据。如果-v的flag在运行时没有传入的话tick这个channel会保持为nil这样在select里的case也就相当于被禁用了。</p>
<p><u><i>gopl.io/ch8/du2</i></u></p>
<pre><code class="language-go">var verbose = flag.Bool(&quot;v&quot;, false, &quot;show verbose progress messages&quot;)
func main() {
// ...start background goroutine...
// Print the results periodically.
var tick &lt;-chan time.Time
if *verbose {
tick = time.Tick(500 * time.Millisecond)
}
var nfiles, nbytes int64
loop:
for {
select {
case size, ok := &lt;-fileSizes:
if !ok {
break loop // fileSizes was closed
}
nfiles++
nbytes += size
case &lt;-tick:
printDiskUsage(nfiles, nbytes)
}
}
printDiskUsage(nfiles, nbytes) // final totals
}
</code></pre>
<p>由于我们的程序不再使用range循环第一个select的case必须显式地判断fileSizes的channel是不是已经被关闭了这里可以用到channel接收的二值形式。如果channel已经被关闭了的话程序会直接退出循环。这里的break语句用到了标签break这样可以同时终结select和for两个循环如果没有用标签就break的话只会退出内层的select循环而外层的for循环会使之进入下一轮select循环。</p>
<p>现在程序会悠闲地为我们打印更新流:</p>
<pre><code>$ 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, &amp;n, fileSizes)
}
go func() {
n.Wait()
close(fileSizes)
}()
// ...select loop...
}
func walkDir(dir string, n *sync.WaitGroup, fileSizes chan&lt;- 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 &lt;- 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 &lt;- struct{}{} // acquire token
defer func() { &lt;-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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