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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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<!-- 头部 -->
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<li><em>KusonStack一站式可编程配置技术栈: <a href="https://github.com/KusionStack/kusion">https://github.com/KusionStack/kusion</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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<h2 id="89-并发的退出"><a class="header" href="#89-并发的退出">8.9. 并发的退出</a></h2>
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<p>有时候我们需要通知goroutine停止它正在干的事情，比如一个正在执行计算的web服务，然而它的客户端已经断开了和服务端的连接。</p>
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<p>Go语言并没有提供在一个goroutine中终止另一个goroutine的方法，由于这样会导致goroutine之间的共享变量落在未定义的状态上。在8.7节中的rocket launch程序中，我们往名字叫abort的channel里发送了一个简单的值，在countdown的goroutine中会把这个值理解为自己的退出信号。但是如果我们想要退出两个或者任意多个goroutine怎么办呢？</p>
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<p>一种可能的手段是向abort的channel里发送和goroutine数目一样多的事件来退出它们。如果这些goroutine中已经有一些自己退出了，那么会导致我们的channel里的事件数比goroutine还多，这样导致我们的发送直接被阻塞。另一方面，如果这些goroutine又生成了其它的goroutine，我们的channel里的数目又太少了，所以有些goroutine可能会无法接收到退出消息。一般情况下我们是很难知道在某一个时刻具体有多少个goroutine在运行着的。另外，当一个goroutine从abort channel中接收到一个值的时候，他会消费掉这个值，这样其它的goroutine就没法看到这条信息。为了能够达到我们退出goroutine的目的，我们需要更靠谱的策略，来通过一个channel把消息广播出去，这样goroutine们能够看到这条事件消息，并且在事件完成之后，可以知道这件事已经发生过了。</p>
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<p>回忆一下我们关闭了一个channel并且被消费掉了所有已发送的值，操作channel之后的代码可以立即被执行，并且会产生零值。我们可以将这个机制扩展一下，来作为我们的广播机制：不要向channel发送值，而是用关闭一个channel来进行广播。</p>
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<p>只要一些小修改，我们就可以把退出逻辑加入到前一节的du程序。首先，我们创建一个退出的channel，不需要向这个channel发送任何值，但其所在的闭包内要写明程序需要退出。我们同时还定义了一个工具函数，cancelled，这个函数在被调用的时候会轮询退出状态。</p>
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<p><u><i>gopl.io/ch8/du4</i></u></p>
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<pre><code class="language-go">var done = make(chan struct{})
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func cancelled() bool {
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select {
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case <-done:
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return true
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default:
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return false
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}
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}
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</code></pre>
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<p>下面我们创建一个从标准输入流中读取内容的goroutine，这是一个比较典型的连接到终端的程序。每当有输入被读到（比如用户按了回车键），这个goroutine就会把取消消息通过关闭done的channel广播出去。</p>
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<pre><code class="language-go">// Cancel traversal when input is detected.
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go func() {
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os.Stdin.Read(make([]byte, 1)) // read a single byte
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close(done)
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}()
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</code></pre>
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<p>现在我们需要使我们的goroutine来对取消进行响应。在main goroutine中，我们添加了select的第三个case语句，尝试从done channel中接收内容。如果这个case被满足的话，在select到的时候即会返回，但在结束之前我们需要把fileSizes channel中的内容“排”空，在channel被关闭之前，舍弃掉所有值。这样可以保证对walkDir的调用不要被向fileSizes发送信息阻塞住，可以正确地完成。</p>
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<pre><code class="language-go">for {
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select {
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case <-done:
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// Drain fileSizes to allow existing goroutines to finish.
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for range fileSizes {
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// Do nothing.
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}
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return
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case size, ok := <-fileSizes:
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// ...
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}
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}
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</code></pre>
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<p>walkDir这个goroutine一启动就会轮询取消状态，如果取消状态被设置的话会直接返回，并且不做额外的事情。这样我们将所有在取消事件之后创建的goroutine改变为无操作。</p>
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<pre><code class="language-go">func walkDir(dir string, n *sync.WaitGroup, fileSizes chan<- int64) {
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defer n.Done()
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if cancelled() {
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return
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}
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for _, entry := range dirents(dir) {
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// ...
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}
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}
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</code></pre>
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<p>在walkDir函数的循环中我们对取消状态进行轮询可以带来明显的益处，可以避免在取消事件发生时还去创建goroutine。取消本身是有一些代价的；想要快速的响应需要对程序逻辑进行侵入式的修改。确保在取消发生之后不要有代价太大的操作可能会需要修改你代码里的很多地方，但是在一些重要的地方去检查取消事件也确实能带来很大的好处。</p>
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<p>对这个程序的一个简单的性能分析可以揭示瓶颈在dirents函数中获取一个信号量。下面的select可以让这种操作可以被取消，并且可以将取消时的延迟从几百毫秒降低到几十毫秒。</p>
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|
<pre><code class="language-go">func dirents(dir string) []os.FileInfo {
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|||
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|
select {
|
|||
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|
case sema <- struct{}{}: // acquire token
|
|||
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case <-done:
|
|||
|
|
return nil // cancelled
|
|||
|
|
}
|
|||
|
|
defer func() { <-sema }() // release token
|
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|
|
// ...read directory...
|
|||
|
|
}
|
|||
|
|
</code></pre>
|
|||
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<p>现在当取消发生时，所有后台的goroutine都会迅速停止并且主函数会返回。当然，当主函数返回时，一个程序会退出，而我们又无法在主函数退出的时候确认其已经释放了所有的资源（译注：因为程序都退出了，你的代码都没法执行了）。这里有一个方便的窍门我们可以一用：取代掉直接从主函数返回，我们调用一个panic，然后runtime会把每一个goroutine的栈dump下来。如果main goroutine是唯一一个剩下的goroutine的话，他会清理掉自己的一切资源。但是如果还有其它的goroutine没有退出，他们可能没办法被正确地取消掉，也有可能被取消但是取消操作会很花时间；所以这里的一个调研还是很有必要的。我们用panic来获取到足够的信息来验证我们上面的判断，看看最终到底是什么样的情况。</p>
|
|||
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|
<p><strong>练习 8.10：</strong> HTTP请求可能会因http.Request结构体中Cancel channel的关闭而取消。修改8.6节中的web crawler来支持取消http请求。（提示：http.Get并没有提供方便地定制一个请求的方法。你可以用http.NewRequest来取而代之，设置它的Cancel字段，然后用http.DefaultClient.Do(req)来进行这个http请求。）</p>
|
|||
|
|
<p><strong>练习 8.11：</strong> 紧接着8.4.4中的mirroredQuery流程，实现一个并发请求url的fetch的变种。当第一个请求返回时，直接取消其它的请求。</p>
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