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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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<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="92-syncmutex互斥锁"><a class="header" href="#92-syncmutex互斥锁">9.2. sync.Mutex互斥锁</a></h2>
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<p>在8.6节中，我们使用了一个buffered channel作为一个计数信号量，来保证最多只有20个goroutine会同时执行HTTP请求。同理，我们可以用一个容量只有1的channel来保证最多只有一个goroutine在同一时刻访问一个共享变量。一个只能为1和0的信号量叫做二元信号量（binary semaphore）。</p>
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<p><u><i>gopl.io/ch9/bank2</i></u></p>
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<pre><code class="language-go">var (
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sema = make(chan struct{}, 1) // a binary semaphore guarding balance
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balance int
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)
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func Deposit(amount int) {
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sema <- struct{}{} // acquire token
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balance = balance + amount
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<-sema // release token
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}
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func Balance() int {
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sema <- struct{}{} // acquire token
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b := balance
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<-sema // release token
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return b
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}
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</code></pre>
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<p>这种互斥很实用，而且被sync包里的Mutex类型直接支持。它的Lock方法能够获取到token(这里叫锁)，并且Unlock方法会释放这个token：</p>
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<p><u><i>gopl.io/ch9/bank3</i></u></p>
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<pre><code class="language-go">import "sync"
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var (
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mu sync.Mutex // guards balance
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balance int
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)
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func Deposit(amount int) {
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mu.Lock()
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balance = balance + amount
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mu.Unlock()
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}
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func Balance() int {
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mu.Lock()
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b := balance
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mu.Unlock()
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return b
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}
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</code></pre>
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<p>每次一个goroutine访问bank变量时（这里只有balance余额变量），它都会调用mutex的Lock方法来获取一个互斥锁。如果其它的goroutine已经获得了这个锁的话，这个操作会被阻塞直到其它goroutine调用了Unlock使该锁变回可用状态。mutex会保护共享变量。惯例来说，被mutex所保护的变量是在mutex变量声明之后立刻声明的。如果你的做法和惯例不符，确保在文档里对你的做法进行说明。</p>
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<p>在Lock和Unlock之间的代码段中的内容goroutine可以随便读取或者修改，这个代码段叫做临界区。锁的持有者在其他goroutine获取该锁之前需要调用Unlock。goroutine在结束后释放锁是必要的，无论以哪条路径通过函数都需要释放，即使是在错误路径中，也要记得释放。</p>
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<p>上面的bank程序例证了一种通用的并发模式。一系列的导出函数封装了一个或多个变量，那么访问这些变量唯一的方式就是通过这些函数来做（或者方法，对于一个对象的变量来说）。每一个函数在一开始就获取互斥锁并在最后释放锁，从而保证共享变量不会被并发访问。这种函数、互斥锁和变量的编排叫作监控monitor（这种老式单词的monitor是受“monitor goroutine”的术语启发而来的。两种用法都是一个代理人保证变量被顺序访问）。</p>
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<p>由于在存款和查询余额函数中的临界区代码这么短——只有一行，没有分支调用——在代码最后去调用Unlock就显得更为直截了当。在更复杂的临界区的应用中，尤其是必须要尽早处理错误并返回的情况下，就很难去（靠人）判断对Lock和Unlock的调用是在所有路径中都能够严格配对的了。Go语言里的defer简直就是这种情况下的救星：我们用defer来调用Unlock，临界区会隐式地延伸到函数作用域的最后，这样我们就从“总要记得在函数返回之后或者发生错误返回时要记得调用一次Unlock”这种状态中获得了解放。Go会自动帮我们完成这些事情。</p>
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<pre><code class="language-go">func Balance() int {
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mu.Lock()
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defer mu.Unlock()
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return balance
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}
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</code></pre>
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<p>上面的例子里Unlock会在return语句读取完balance的值之后执行，所以Balance函数是并发安全的。这带来的另一点好处是，我们再也不需要一个本地变量b了。</p>
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<p>此外，一个deferred Unlock即使在临界区发生panic时依然会执行，这对于用recover（§5.10）来恢复的程序来说是很重要的。defer调用只会比显式地调用Unlock成本高那么一点点，不过却在很大程度上保证了代码的整洁性。大多数情况下对于并发程序来说，代码的整洁性比过度的优化更重要。如果可能的话尽量使用defer来将临界区扩展到函数的结束。</p>
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<p>考虑一下下面的Withdraw函数。成功的时候，它会正确地减掉余额并返回true。但如果银行记录资金对交易来说不足，那么取款就会恢复余额，并返回false。</p>
|
|||
|
|
<pre><code class="language-go">// NOTE: not atomic!
|
|||
|
|
func Withdraw(amount int) bool {
|
|||
|
|
Deposit(-amount)
|
|||
|
|
if Balance() < 0 {
|
|||
|
|
Deposit(amount)
|
|||
|
|
return false // insufficient funds
|
|||
|
|
}
|
|||
|
|
return true
|
|||
|
|
}
|
|||
|
|
</code></pre>
|
|||
|
|
<p>函数终于给出了正确的结果，但是还有一点讨厌的副作用。当过多的取款操作同时执行时，balance可能会瞬时被减到0以下。这可能会引起一个并发的取款被不合逻辑地拒绝。所以如果Bob尝试买一辆sports car时，Alice可能就没办法为她的早咖啡付款了。这里的问题是取款不是一个原子操作：它包含了三个步骤，每一步都需要去获取并释放互斥锁，但任何一次锁都不会锁上整个取款流程。</p>
|
|||
|
|
<p>理想情况下，取款应该只在整个操作中获得一次互斥锁。下面这样的尝试是错误的：</p>
|
|||
|
|
<pre><code class="language-go">// NOTE: incorrect!
|
|||
|
|
func Withdraw(amount int) bool {
|
|||
|
|
mu.Lock()
|
|||
|
|
defer mu.Unlock()
|
|||
|
|
Deposit(-amount)
|
|||
|
|
if Balance() < 0 {
|
|||
|
|
Deposit(amount)
|
|||
|
|
return false // insufficient funds
|
|||
|
|
}
|
|||
|
|
return true
|
|||
|
|
}
|
|||
|
|
</code></pre>
|
|||
|
|
<p>上面这个例子中，Deposit会调用mu.Lock()第二次去获取互斥锁，但因为mutex已经锁上了，而无法被重入（译注：go里没有重入锁，关于重入锁的概念，请参考java）——也就是说没法对一个已经锁上的mutex来再次上锁——这会导致程序死锁，没法继续执行下去，Withdraw会永远阻塞下去。</p>
|
|||
|
|
<p>关于Go的mutex不能重入这一点我们有很充分的理由。mutex的目的是确保共享变量在程序执行时的关键点上能够保证不变性。不变性的一层含义是“没有goroutine访问共享变量”，但实际上这里对于mutex保护的变量来说，不变性还包含更深层含义：当一个goroutine获得了一个互斥锁时，它能断定被互斥锁保护的变量正处于不变状态（译注：即没有其他代码块正在读写共享变量），在其获取并保持锁期间，可能会去更新共享变量，这样不变性只是短暂地被破坏，然而当其释放锁之后，锁必须保证共享变量重获不变性并且多个goroutine按顺序访问共享变量。尽管一个可以重入的mutex也可以保证没有其它的goroutine在访问共享变量，但它不具备不变性更深层含义。（译注：<a href="https://stackoverflow.com/questions/14670979/recursive-locking-in-go/14671462#14671462">更详细的解释</a>，Russ Cox认为可重入锁是bug的温床，是一个失败的设计）</p>
|
|||
|
|
<p>一个通用的解决方案是将一个函数分离为多个函数，比如我们把Deposit分离成两个：一个不导出的函数deposit，这个函数假设锁总是会被保持并去做实际的操作，另一个是导出的函数Deposit，这个函数会调用deposit，但在调用前会先去获取锁。同理我们可以将Withdraw也表示成这种形式：</p>
|
|||
|
|
<pre><code class="language-go">func Withdraw(amount int) bool {
|
|||
|
|
mu.Lock()
|
|||
|
|
defer mu.Unlock()
|
|||
|
|
deposit(-amount)
|
|||
|
|
if balance < 0 {
|
|||
|
|
deposit(amount)
|
|||
|
|
return false // insufficient funds
|
|||
|
|
}
|
|||
|
|
return true
|
|||
|
|
}
|
|||
|
|
|
|||
|
|
func Deposit(amount int) {
|
|||
|
|
mu.Lock()
|
|||
|
|
defer mu.Unlock()
|
|||
|
|
deposit(amount)
|
|||
|
|
}
|
|||
|
|
|
|||
|
|
func Balance() int {
|
|||
|
|
mu.Lock()
|
|||
|
|
defer mu.Unlock()
|
|||
|
|
return balance
|
|||
|
|
}
|
|||
|
|
|
|||
|
|
// This function requires that the lock be held.
|
|||
|
|
func deposit(amount int) { balance += amount }
|
|||
|
|
</code></pre>
|
|||
|
|
<p>当然，这里的存款deposit函数很小，实际上取款Withdraw函数不需要理会对它的调用，尽管如此，这里的表达还是表明了规则。</p>
|
|||
|
|
<p>封装（§6.6），用限制一个程序中的意外交互的方式，可以使我们获得数据结构的不变性。因为某种原因，封装还帮我们获得了并发的不变性。当你使用mutex时，确保mutex和其保护的变量没有被导出（在go里也就是小写，且不要被大写字母开头的函数访问啦），无论这些变量是包级的变量还是一个struct的字段。</p>
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