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上下文 - context
#
Context是由Golang官方开发的并发控制包，一方面可以用于当请求超时或者取消时候，相关的goroutine马上退出释放资源，另一方面Context本身含义就是上下文，其可以在多个goroutine或者多个处理函数之间传递共享的信息。
创建一个新的context，必须基于一个父context，新的context又可以作为其他context的父context。所有context在一起构造成一个context树。
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上下文 - context
#
Context是由Golang官方开发的并发控制包，一方面可以用于当请求超时或者取消时候，相关的goroutine马上退出释放资源，另一方面Context本身含义就是上下文，其可以在多个goroutine或者多个处理函数之间传递共享的信息。
创建一个新的context，必须基于一个父context，新的context又可以作为其他context的父context。所有context在一起构造成一个context树。
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<article class="markdown"><h1 id="上下文---context">
上下文 - context
<a class="anchor" href="#%e4%b8%8a%e4%b8%8b%e6%96%87---context">#</a>
</h1>
<p>Context是由Golang官方开发的并发控制包，一方面可以用于当请求超时或者取消时候，相关的goroutine马上退出释放资源，另一方面Context本身含义就是上下文，其可以在多个goroutine或者多个处理函数之间传递共享的信息。</p>
<p>创建一个新的context，必须基于一个父context，新的context又可以作为其他context的父context。所有context在一起构造成一个context树。</p>
<p>
<img src="https://static.cyub.vip/images/202008/context-tree.jpg" alt="context tree" /></p>
<h2 id="context使用示例">
Context使用示例
<a class="anchor" href="#context%e4%bd%bf%e7%94%a8%e7%a4%ba%e4%be%8b">#</a>
</h2>
<p>Context一大用处就是超时控制。我们先看一个简单用法。</p>
<div class="highlight"><pre tabindex="0" style="color:#f8f8f2;background-color:#272822;-moz-tab-size:4;-o-tab-size:4;tab-size:4;"><code class="language-go" data-lang="go"><span style="display:flex;"><span><span style="color:#66d9ef">func</span> <span style="color:#a6e22e">main</span>() {
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">ctx</span>, <span style="color:#a6e22e">cancel</span> <span style="color:#f92672">:=</span> <span style="color:#a6e22e">context</span>.<span style="color:#a6e22e">WithTimeout</span>(<span style="color:#a6e22e">context</span>.<span style="color:#a6e22e">Background</span>(), <span style="color:#ae81ff">3</span> <span style="color:#f92672">*</span> <span style="color:#a6e22e">time</span>.<span style="color:#a6e22e">Second</span>)
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">defer</span> <span style="color:#a6e22e">cancel</span>()
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">go</span> <span style="color:#a6e22e">SlowOperation</span>(<span style="color:#a6e22e">ctx</span>)
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">go</span> <span style="color:#66d9ef">func</span>() {
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">for</span> {
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">time</span>.<span style="color:#a6e22e">Sleep</span>(<span style="color:#ae81ff">300</span> <span style="color:#f92672">*</span> <span style="color:#a6e22e">time</span>.<span style="color:#a6e22e">Millisecond</span>)
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">fmt</span>.<span style="color:#a6e22e">Println</span>(<span style="color:#e6db74">&#34;goroutine:&#34;</span>, <span style="color:#a6e22e">runtime</span>.<span style="color:#a6e22e">NumGoroutine</span>())
</span></span><span style="display:flex;"><span> }
</span></span><span style="display:flex;"><span> }()
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">time</span>.<span style="color:#a6e22e">Sleep</span>(<span style="color:#ae81ff">4</span> <span style="color:#f92672">*</span> <span style="color:#a6e22e">time</span>.<span style="color:#a6e22e">Second</span>)
</span></span><span style="display:flex;"><span>
</span></span><span style="display:flex;"><span>}
</span></span><span style="display:flex;"><span>
</span></span><span style="display:flex;"><span><span style="color:#66d9ef">func</span> <span style="color:#a6e22e">SlowOperation</span>(<span style="color:#a6e22e">ctx</span> <span style="color:#a6e22e">context</span>.<span style="color:#a6e22e">Context</span>) {
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">done</span> <span style="color:#f92672">:=</span> make(<span style="color:#66d9ef">chan</span> <span style="color:#66d9ef">int</span>, <span style="color:#ae81ff">1</span>)
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">go</span> <span style="color:#66d9ef">func</span>() { <span style="color:#75715e">// 模拟慢操作
</span></span></span><span style="display:flex;"><span><span style="color:#75715e"></span> <span style="color:#a6e22e">dur</span> <span style="color:#f92672">:=</span> <span style="color:#a6e22e">time</span>.<span style="color:#a6e22e">Duration</span>(<span style="color:#a6e22e">rand</span>.<span style="color:#a6e22e">Intn</span>(<span style="color:#ae81ff">5</span>)<span style="color:#f92672">+</span><span style="color:#ae81ff">1</span>) <span style="color:#f92672">*</span> <span style="color:#a6e22e">time</span>.<span style="color:#a6e22e">Second</span>
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">time</span>.<span style="color:#a6e22e">Sleep</span>(<span style="color:#a6e22e">dur</span>)
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">done</span> <span style="color:#f92672">&lt;-</span> <span style="color:#ae81ff">1</span>
</span></span><span style="display:flex;"><span> }()
</span></span><span style="display:flex;"><span>
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">select</span> {
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">case</span> <span style="color:#f92672">&lt;-</span><span style="color:#a6e22e">ctx</span>.<span style="color:#a6e22e">Done</span>():
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">fmt</span>.<span style="color:#a6e22e">Println</span>(<span style="color:#e6db74">&#34;SlowOperation timeout:&#34;</span>, <span style="color:#a6e22e">ctx</span>.<span style="color:#a6e22e">Err</span>())
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">case</span> <span style="color:#f92672">&lt;-</span><span style="color:#a6e22e">done</span>:
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">fmt</span>.<span style="color:#a6e22e">Println</span>(<span style="color:#e6db74">&#34;Complete work&#34;</span>)
</span></span><span style="display:flex;"><span> }
</span></span><span style="display:flex;"><span>}
</span></span></code></pre></div><p>上面代码会不停打印当前groutine数量，可以观察到SlowOperation函数执行超时之后，goroutine数量由4个变成2个，相关goroutetine退出了。源码可以去
<a href="https://play.golang.org/p/fjGJgMwtIl3">go playground</a>查看。</p>
<p>再看一个关于超时处理的例子， 源码可以去
<a href="https://play.golang.org/p/DctV9268FTD">go playground</a>查看：</p>
<div class="highlight"><pre tabindex="0" style="color:#f8f8f2;background-color:#272822;-moz-tab-size:4;-o-tab-size:4;tab-size:4;"><code class="language-go" data-lang="go"><span style="display:flex;"><span><span style="color:#75715e">//
</span></span></span><span style="display:flex;"><span><span style="color:#75715e">// 根据github仓库统计信息接口查询某个仓库信息
</span></span></span><span style="display:flex;"><span><span style="color:#75715e"></span><span style="color:#66d9ef">func</span> <span style="color:#a6e22e">QueryFrameworkStats</span>(<span style="color:#a6e22e">ctx</span> <span style="color:#a6e22e">context</span>.<span style="color:#a6e22e">Context</span>, <span style="color:#a6e22e">framework</span> <span style="color:#66d9ef">string</span>) <span style="color:#f92672">&lt;-</span><span style="color:#66d9ef">chan</span> <span style="color:#66d9ef">string</span> {
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">stats</span> <span style="color:#f92672">:=</span> make(<span style="color:#66d9ef">chan</span> <span style="color:#66d9ef">string</span>)
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">go</span> <span style="color:#66d9ef">func</span>() {
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">repos</span> <span style="color:#f92672">:=</span> <span style="color:#e6db74">&#34;https://api.github.com/repos/&#34;</span> <span style="color:#f92672">+</span> <span style="color:#a6e22e">framework</span>
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">req</span>, <span style="color:#a6e22e">err</span> <span style="color:#f92672">:=</span> <span style="color:#a6e22e">http</span>.<span style="color:#a6e22e">NewRequest</span>(<span style="color:#e6db74">&#34;GET&#34;</span>, <span style="color:#a6e22e">repos</span>, <span style="color:#66d9ef">nil</span>)
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">if</span> <span style="color:#a6e22e">err</span> <span style="color:#f92672">!=</span> <span style="color:#66d9ef">nil</span> {
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">return</span>
</span></span><span style="display:flex;"><span> }
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">req</span> = <span style="color:#a6e22e">req</span>.<span style="color:#a6e22e">WithContext</span>(<span style="color:#a6e22e">ctx</span>)
</span></span><span style="display:flex;"><span>
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">client</span> <span style="color:#f92672">:=</span> <span style="color:#f92672">&amp;</span><span style="color:#a6e22e">http</span>.<span style="color:#a6e22e">Client</span>{}
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">resp</span>, <span style="color:#a6e22e">err</span> <span style="color:#f92672">:=</span> <span style="color:#a6e22e">client</span>.<span style="color:#a6e22e">Do</span>(<span style="color:#a6e22e">req</span>)
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">if</span> <span style="color:#a6e22e">err</span> <span style="color:#f92672">!=</span> <span style="color:#66d9ef">nil</span> {
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">return</span>
</span></span><span style="display:flex;"><span> }
</span></span><span style="display:flex;"><span>
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">data</span>, <span style="color:#a6e22e">err</span> <span style="color:#f92672">:=</span> <span style="color:#a6e22e">ioutil</span>.<span style="color:#a6e22e">ReadAll</span>(<span style="color:#a6e22e">resp</span>.<span style="color:#a6e22e">Body</span>)
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">if</span> <span style="color:#a6e22e">err</span> <span style="color:#f92672">!=</span> <span style="color:#66d9ef">nil</span> {
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">return</span>
</span></span><span style="display:flex;"><span> }
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">defer</span> <span style="color:#a6e22e">resp</span>.<span style="color:#a6e22e">Body</span>.<span style="color:#a6e22e">Close</span>()
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">stats</span> <span style="color:#f92672">&lt;-</span> string(<span style="color:#a6e22e">data</span>)
</span></span><span style="display:flex;"><span> }()
</span></span><span style="display:flex;"><span>
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">return</span> <span style="color:#a6e22e">stats</span>
</span></span><span style="display:flex;"><span>}
</span></span><span style="display:flex;"><span>
</span></span><span style="display:flex;"><span><span style="color:#66d9ef">func</span> <span style="color:#a6e22e">main</span>() {
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">ctx</span>, <span style="color:#a6e22e">cancel</span> <span style="color:#f92672">:=</span> <span style="color:#a6e22e">context</span>.<span style="color:#a6e22e">WithTimeout</span>(<span style="color:#a6e22e">context</span>.<span style="color:#a6e22e">Background</span>(), <span style="color:#ae81ff">3</span><span style="color:#f92672">*</span><span style="color:#a6e22e">time</span>.<span style="color:#a6e22e">Second</span>)
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">defer</span> <span style="color:#a6e22e">cancel</span>()
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">framework</span> <span style="color:#f92672">:=</span> <span style="color:#e6db74">&#34;gin-gonic/gin&#34;</span>
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">select</span> {
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">case</span> <span style="color:#f92672">&lt;-</span><span style="color:#a6e22e">ctx</span>.<span style="color:#a6e22e">Done</span>():
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">fmt</span>.<span style="color:#a6e22e">Println</span>(<span style="color:#a6e22e">ctx</span>.<span style="color:#a6e22e">Err</span>())
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">case</span> <span style="color:#a6e22e">statsInfo</span> <span style="color:#f92672">:=</span> <span style="color:#f92672">&lt;-</span><span style="color:#a6e22e">QueryFrameworkStats</span>(<span style="color:#a6e22e">ctx</span>, <span style="color:#a6e22e">framework</span>):
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">fmt</span>.<span style="color:#a6e22e">Println</span>(<span style="color:#a6e22e">framework</span>, <span style="color:#e6db74">&#34; fork and start info : &#34;</span>, <span style="color:#a6e22e">statsInfo</span>)
</span></span><span style="display:flex;"><span> }
</span></span><span style="display:flex;"><span>}
</span></span></code></pre></div><p>Context另外一个用途就是传递上下文信息。从WithValue方法我们可以创建一个可以储存键值的context</p>
<h2 id="context源码分析">
Context源码分析
<a class="anchor" href="#context%e6%ba%90%e7%a0%81%e5%88%86%e6%9e%90">#</a>
</h2>
<h3 id="context接口">
Context接口
<a class="anchor" href="#context%e6%8e%a5%e5%8f%a3">#</a>
</h3>
<p>首先我们来看下Context接口</p>
<div class="highlight"><pre tabindex="0" style="color:#f8f8f2;background-color:#272822;-moz-tab-size:4;-o-tab-size:4;tab-size:4;"><code class="language-go" data-lang="go"><span style="display:flex;"><span><span style="color:#66d9ef">type</span> <span style="color:#a6e22e">Context</span> <span style="color:#66d9ef">interface</span> {
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">Deadline</span>() (<span style="color:#a6e22e">deadline</span> <span style="color:#a6e22e">time</span>.<span style="color:#a6e22e">Time</span>, <span style="color:#a6e22e">ok</span> <span style="color:#66d9ef">bool</span>)
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">Done</span>() <span style="color:#f92672">&lt;-</span><span style="color:#66d9ef">chan</span> <span style="color:#66d9ef">struct</span>{}
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">Err</span>() <span style="color:#66d9ef">error</span>
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">Value</span>(<span style="color:#a6e22e">key</span> <span style="color:#66d9ef">interface</span>{}) <span style="color:#66d9ef">interface</span>{}
</span></span><span style="display:flex;"><span>}
</span></span></code></pre></div><p>Context接口一共包含四个方法：</p>
<ul>
<li>Deadline：返回绑定该context任务的执行超时时间，若未设置，则ok等于false</li>
<li>Done：返回一个只读通道，当绑定该context的任务执行完成并调用cancel方法或者任务执行超时时候，该通道会被关闭</li>
<li>Err：返回一个错误，如果Done返回的通道未关闭则返回nil,如果context如果被取消，返回<code>Canceled</code>错误，如果超时则会返回<code>DeadlineExceeded</code>错误</li>
<li>Value：根据key返回，存储在context中k-v数据</li>
</ul>
<h3 id="实现context接口的类型">
实现Context接口的类型
<a class="anchor" href="#%e5%ae%9e%e7%8e%b0context%e6%8e%a5%e5%8f%a3%e7%9a%84%e7%b1%bb%e5%9e%8b">#</a>
</h3>
<p>Context一共有4个类型实现了Context接口, 分别是emptyCtx, cancelCtx,timerCtx,valueCtx。每个类型都关联一个创建方法。</p>
<p>
<img src="https://static.cyub.vip/images/202009/context_implement.jpg" alt="" /></p>
<h4 id="emptyctx">
emptyCtx
<a class="anchor" href="#emptyctx">#</a>
</h4>
<p>emptyCtx是int类型，<strong>emptyCtx实现了Context接口，是一个空context，只能作为根context</strong>。</p>
<div class="highlight"><pre tabindex="0" style="color:#f8f8f2;background-color:#272822;-moz-tab-size:4;-o-tab-size:4;tab-size:4;"><code class="language-go" data-lang="go"><span style="display:flex;"><span><span style="color:#66d9ef">type</span> <span style="color:#a6e22e">emptyCtx</span> <span style="color:#66d9ef">int</span> <span style="color:#75715e">//
</span></span></span><span style="display:flex;"><span><span style="color:#75715e"></span>
</span></span><span style="display:flex;"><span><span style="color:#66d9ef">func</span> (<span style="color:#f92672">*</span><span style="color:#a6e22e">emptyCtx</span>) <span style="color:#a6e22e">Deadline</span>() (<span style="color:#a6e22e">deadline</span> <span style="color:#a6e22e">time</span>.<span style="color:#a6e22e">Time</span>, <span style="color:#a6e22e">ok</span> <span style="color:#66d9ef">bool</span>) {
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">return</span>
</span></span><span style="display:flex;"><span>}
</span></span><span style="display:flex;"><span>
</span></span><span style="display:flex;"><span><span style="color:#66d9ef">func</span> (<span style="color:#f92672">*</span><span style="color:#a6e22e">emptyCtx</span>) <span style="color:#a6e22e">Done</span>() <span style="color:#f92672">&lt;-</span><span style="color:#66d9ef">chan</span> <span style="color:#66d9ef">struct</span>{} {
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">return</span> <span style="color:#66d9ef">nil</span>
</span></span><span style="display:flex;"><span>}
</span></span><span style="display:flex;"><span>
</span></span><span style="display:flex;"><span><span style="color:#66d9ef">func</span> (<span style="color:#f92672">*</span><span style="color:#a6e22e">emptyCtx</span>) <span style="color:#a6e22e">Err</span>() <span style="color:#66d9ef">error</span> {
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">return</span> <span style="color:#66d9ef">nil</span>
</span></span><span style="display:flex;"><span>}
</span></span><span style="display:flex;"><span>
</span></span><span style="display:flex;"><span><span style="color:#66d9ef">func</span> (<span style="color:#f92672">*</span><span style="color:#a6e22e">emptyCtx</span>) <span style="color:#a6e22e">Value</span>(<span style="color:#a6e22e">key</span> <span style="color:#66d9ef">interface</span>{}) <span style="color:#66d9ef">interface</span>{} {
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">return</span> <span style="color:#66d9ef">nil</span>
</span></span><span style="display:flex;"><span>}
</span></span><span style="display:flex;"><span>
</span></span><span style="display:flex;"><span><span style="color:#66d9ef">func</span> (<span style="color:#a6e22e">e</span> <span style="color:#f92672">*</span><span style="color:#a6e22e">emptyCtx</span>) <span style="color:#a6e22e">String</span>() <span style="color:#66d9ef">string</span> {
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">switch</span> <span style="color:#a6e22e">e</span> {
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">case</span> <span style="color:#a6e22e">background</span>:
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">return</span> <span style="color:#e6db74">&#34;context.Background&#34;</span>
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">case</span> <span style="color:#a6e22e">todo</span>:
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">return</span> <span style="color:#e6db74">&#34;context.TODO&#34;</span>
</span></span><span style="display:flex;"><span> }
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">return</span> <span style="color:#e6db74">&#34;unknown empty Context&#34;</span>
</span></span><span style="display:flex;"><span>}
</span></span></code></pre></div><p><strong>Background/TODO</strong></p>
<p>context包还提供两个函数返回emptyCtx类型。</p>
<div class="highlight"><pre tabindex="0" style="color:#f8f8f2;background-color:#272822;-moz-tab-size:4;-o-tab-size:4;tab-size:4;"><code class="language-go" data-lang="go"><span style="display:flex;"><span><span style="color:#66d9ef">var</span> (
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">background</span> = new(<span style="color:#a6e22e">emptyCtx</span>)
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">todo</span> = new(<span style="color:#a6e22e">emptyCtx</span>)
</span></span><span style="display:flex;"><span>)
</span></span><span style="display:flex;"><span>
</span></span><span style="display:flex;"><span><span style="color:#66d9ef">func</span> <span style="color:#a6e22e">Background</span>() <span style="color:#a6e22e">Context</span> {
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">return</span> <span style="color:#a6e22e">background</span>
</span></span><span style="display:flex;"><span>}
</span></span><span style="display:flex;"><span>
</span></span><span style="display:flex;"><span><span style="color:#66d9ef">func</span> <span style="color:#a6e22e">TODO</span>() <span style="color:#a6e22e">Context</span> {
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">return</span> <span style="color:#a6e22e">todo</span>
</span></span><span style="display:flex;"><span>}
</span></span></code></pre></div><p>Background用于创建根context，一般用于主函数、初始化和测试中，<strong>我们创建的context一般都是基于Bacground创建的</strong>。<strong>TODO用于当我们不确定使用什么样的context的时候使用</strong>。</p>
<h4 id="cancelctx">
cancelCtx
<a class="anchor" href="#cancelctx">#</a>
</h4>
<p><code>cancelCtx</code>支持取消操作，取消同时也会对实现了<code>canceler</code>接口的子代进行取消操作。我们来看下<code>cancelCtx</code>结构体和<code>cancelceler</code>接口：</p>
<div class="highlight"><pre tabindex="0" style="color:#f8f8f2;background-color:#272822;-moz-tab-size:4;-o-tab-size:4;tab-size:4;"><code class="language-go" data-lang="go"><span style="display:flex;"><span><span style="color:#66d9ef">type</span> <span style="color:#a6e22e">cancelCtx</span> <span style="color:#66d9ef">struct</span> {
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">Context</span>
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">mu</span> <span style="color:#a6e22e">sync</span>.<span style="color:#a6e22e">Mutex</span>
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">done</span> <span style="color:#66d9ef">chan</span> <span style="color:#66d9ef">struct</span>{}
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">children</span> <span style="color:#66d9ef">map</span>[<span style="color:#a6e22e">canceler</span>]<span style="color:#66d9ef">struct</span>{}
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">err</span> <span style="color:#66d9ef">error</span>
</span></span><span style="display:flex;"><span>}
</span></span><span style="display:flex;"><span>
</span></span><span style="display:flex;"><span><span style="color:#66d9ef">type</span> <span style="color:#a6e22e">canceler</span> <span style="color:#66d9ef">interface</span> {
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">cancel</span>(<span style="color:#a6e22e">removeFromParent</span> <span style="color:#66d9ef">bool</span>, <span style="color:#a6e22e">err</span> <span style="color:#66d9ef">error</span>)
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">Done</span>() <span style="color:#f92672">&lt;-</span><span style="color:#66d9ef">chan</span> <span style="color:#66d9ef">struct</span>{}
</span></span><span style="display:flex;"><span>}
</span></span></code></pre></div><p>cancelCtx:</p>
<ul>
<li><code>Context</code>变量存储其父context</li>
<li><code>done</code>变量定义了一个通道，并且只在第一次取消调用才关闭此通道。该通道是惰性创建的</li>
<li><code>children</code>是一个映射类型，用来存储其子代context中实现的<code>canceler</code>，当该context取消时候，会遍历该映射来让子代context进行取消操作</li>
<li><code>err</code>记录错误信息，默认是nil，仅当第一次cancel调用时候，才会设置。</li>
</ul>
<p>我们分别来看下cancelCtx实现的Done,Err,cancel方法。</p>
<div class="highlight"><pre tabindex="0" style="color:#f8f8f2;background-color:#272822;-moz-tab-size:4;-o-tab-size:4;tab-size:4;"><code class="language-go" data-lang="go"><span style="display:flex;"><span><span style="color:#66d9ef">func</span> (<span style="color:#a6e22e">c</span> <span style="color:#f92672">*</span><span style="color:#a6e22e">cancelCtx</span>) <span style="color:#a6e22e">Done</span>() <span style="color:#f92672">&lt;-</span><span style="color:#66d9ef">chan</span> <span style="color:#66d9ef">struct</span>{} {
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">c</span>.<span style="color:#a6e22e">mu</span>.<span style="color:#a6e22e">Lock</span>() <span style="color:#75715e">// 加锁
</span></span></span><span style="display:flex;"><span><span style="color:#75715e"></span> <span style="color:#66d9ef">if</span> <span style="color:#a6e22e">c</span>.<span style="color:#a6e22e">done</span> <span style="color:#f92672">==</span> <span style="color:#66d9ef">nil</span> {
</span></span><span style="display:flex;"><span> <span style="color:#75715e">// done通道惰性创建，只有调用Done方法时候才会创建
</span></span></span><span style="display:flex;"><span><span style="color:#75715e"></span> <span style="color:#a6e22e">c</span>.<span style="color:#a6e22e">done</span> = make(<span style="color:#66d9ef">chan</span> <span style="color:#66d9ef">struct</span>{})
</span></span><span style="display:flex;"><span> }
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">d</span> <span style="color:#f92672">:=</span> <span style="color:#a6e22e">c</span>.<span style="color:#a6e22e">done</span>
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">c</span>.<span style="color:#a6e22e">mu</span>.<span style="color:#a6e22e">Unlock</span>()
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">return</span> <span style="color:#a6e22e">d</span>
</span></span><span style="display:flex;"><span>}
</span></span><span style="display:flex;"><span>
</span></span><span style="display:flex;"><span><span style="color:#66d9ef">func</span> (<span style="color:#a6e22e">c</span> <span style="color:#f92672">*</span><span style="color:#a6e22e">cancelCtx</span>) <span style="color:#a6e22e">Err</span>() <span style="color:#66d9ef">error</span> {
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">c</span>.<span style="color:#a6e22e">mu</span>.<span style="color:#a6e22e">Lock</span>()
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">err</span> <span style="color:#f92672">:=</span> <span style="color:#a6e22e">c</span>.<span style="color:#a6e22e">err</span>
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">c</span>.<span style="color:#a6e22e">mu</span>.<span style="color:#a6e22e">Unlock</span>()
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">return</span> <span style="color:#a6e22e">err</span>
</span></span><span style="display:flex;"><span>}
</span></span><span style="display:flex;"><span>
</span></span><span style="display:flex;"><span><span style="color:#66d9ef">func</span> (<span style="color:#a6e22e">c</span> <span style="color:#f92672">*</span><span style="color:#a6e22e">cancelCtx</span>) <span style="color:#a6e22e">cancel</span>(<span style="color:#a6e22e">removeFromParent</span> <span style="color:#66d9ef">bool</span>, <span style="color:#a6e22e">err</span> <span style="color:#66d9ef">error</span>) {
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">if</span> <span style="color:#a6e22e">err</span> <span style="color:#f92672">==</span> <span style="color:#66d9ef">nil</span> {
</span></span><span style="display:flex;"><span> <span style="color:#75715e">// 取消操作时候一定要传递err信息
</span></span></span><span style="display:flex;"><span><span style="color:#75715e"></span> panic(<span style="color:#e6db74">&#34;context: internal error: missing cancel error&#34;</span>)
</span></span><span style="display:flex;"><span> }
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">c</span>.<span style="color:#a6e22e">mu</span>.<span style="color:#a6e22e">Lock</span>()
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">if</span> <span style="color:#a6e22e">c</span>.<span style="color:#a6e22e">err</span> <span style="color:#f92672">!=</span> <span style="color:#66d9ef">nil</span> {
</span></span><span style="display:flex;"><span> <span style="color:#75715e">// 只允许第一次cancel调用操作，下一次进来直接返回
</span></span></span><span style="display:flex;"><span><span style="color:#75715e"></span> <span style="color:#a6e22e">c</span>.<span style="color:#a6e22e">mu</span>.<span style="color:#a6e22e">Unlock</span>()
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">return</span>
</span></span><span style="display:flex;"><span> }
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">c</span>.<span style="color:#a6e22e">err</span> = <span style="color:#a6e22e">err</span>
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">if</span> <span style="color:#a6e22e">c</span>.<span style="color:#a6e22e">done</span> <span style="color:#f92672">==</span> <span style="color:#66d9ef">nil</span> {
</span></span><span style="display:flex;"><span> <span style="color:#75715e">// 未先进行Done调用，而先行调用Cancel, 此时done是nil，
</span></span></span><span style="display:flex;"><span><span style="color:#75715e"></span> <span style="color:#75715e">// 这时候复用全局已关闭的通道
</span></span></span><span style="display:flex;"><span><span style="color:#75715e"></span> <span style="color:#a6e22e">c</span>.<span style="color:#a6e22e">done</span> = <span style="color:#a6e22e">closedchan</span>
</span></span><span style="display:flex;"><span> } <span style="color:#66d9ef">else</span> {
</span></span><span style="display:flex;"><span> <span style="color:#75715e">// 关闭Done返回的通道，发送关闭信号
</span></span></span><span style="display:flex;"><span><span style="color:#75715e"></span> close(<span style="color:#a6e22e">c</span>.<span style="color:#a6e22e">done</span>)
</span></span><span style="display:flex;"><span> }
</span></span><span style="display:flex;"><span> <span style="color:#75715e">// 子级context依次进行取消操作
</span></span></span><span style="display:flex;"><span><span style="color:#75715e"></span> <span style="color:#66d9ef">for</span> <span style="color:#a6e22e">child</span> <span style="color:#f92672">:=</span> <span style="color:#66d9ef">range</span> <span style="color:#a6e22e">c</span>.<span style="color:#a6e22e">children</span> {
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">child</span>.<span style="color:#a6e22e">cancel</span>(<span style="color:#66d9ef">false</span>, <span style="color:#a6e22e">err</span>)
</span></span><span style="display:flex;"><span> }
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">c</span>.<span style="color:#a6e22e">children</span> = <span style="color:#66d9ef">nil</span>
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">c</span>.<span style="color:#a6e22e">mu</span>.<span style="color:#a6e22e">Unlock</span>()
</span></span><span style="display:flex;"><span>
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">if</span> <span style="color:#a6e22e">removeFromParent</span> {
</span></span><span style="display:flex;"><span> <span style="color:#75715e">// 将当前context从其父级context中children map中移除掉，父级Context与该Context脱钩。
</span></span></span><span style="display:flex;"><span><span style="color:#75715e"></span> <span style="color:#75715e">// 这样当父级Context进行Cancel操作时候，不会再改Context进行取消操作了。因为再取消也没有意义了，因为该Context已经取消过了
</span></span></span><span style="display:flex;"><span><span style="color:#75715e"></span> <span style="color:#a6e22e">removeChild</span>(<span style="color:#a6e22e">c</span>.<span style="color:#a6e22e">Context</span>, <span style="color:#a6e22e">c</span>)
</span></span><span style="display:flex;"><span> }
</span></span><span style="display:flex;"><span>}
</span></span><span style="display:flex;"><span>
</span></span><span style="display:flex;"><span><span style="color:#66d9ef">func</span> <span style="color:#a6e22e">removeChild</span>(<span style="color:#a6e22e">parent</span> <span style="color:#a6e22e">Context</span>, <span style="color:#a6e22e">child</span> <span style="color:#a6e22e">canceler</span>) {
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">p</span>, <span style="color:#a6e22e">ok</span> <span style="color:#f92672">:=</span> <span style="color:#a6e22e">parentCancelCtx</span>(<span style="color:#a6e22e">parent</span>)
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">if</span> !<span style="color:#a6e22e">ok</span> {
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">return</span>
</span></span><span style="display:flex;"><span> }
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">p</span>.<span style="color:#a6e22e">mu</span>.<span style="color:#a6e22e">Lock</span>()
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">if</span> <span style="color:#a6e22e">p</span>.<span style="color:#a6e22e">children</span> <span style="color:#f92672">!=</span> <span style="color:#66d9ef">nil</span> {
</span></span><span style="display:flex;"><span> delete(<span style="color:#a6e22e">p</span>.<span style="color:#a6e22e">children</span>, <span style="color:#a6e22e">child</span>)
</span></span><span style="display:flex;"><span> }
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">p</span>.<span style="color:#a6e22e">mu</span>.<span style="color:#a6e22e">Unlock</span>()
</span></span><span style="display:flex;"><span>}
</span></span><span style="display:flex;"><span>
</span></span><span style="display:flex;"><span><span style="color:#66d9ef">func</span> <span style="color:#a6e22e">parentCancelCtx</span>(<span style="color:#a6e22e">parent</span> <span style="color:#a6e22e">Context</span>) (<span style="color:#f92672">*</span><span style="color:#a6e22e">cancelCtx</span>, <span style="color:#66d9ef">bool</span>) {
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">for</span> {
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">switch</span> <span style="color:#a6e22e">c</span> <span style="color:#f92672">:=</span> <span style="color:#a6e22e">parent</span>.(<span style="color:#66d9ef">type</span>) {
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">case</span> <span style="color:#f92672">*</span><span style="color:#a6e22e">cancelCtx</span>:
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">return</span> <span style="color:#a6e22e">c</span>, <span style="color:#66d9ef">true</span>
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">case</span> <span style="color:#f92672">*</span><span style="color:#a6e22e">timerCtx</span>:
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">return</span> <span style="color:#f92672">&amp;</span><span style="color:#a6e22e">c</span>.<span style="color:#a6e22e">cancelCtx</span>, <span style="color:#66d9ef">true</span>
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">case</span> <span style="color:#f92672">*</span><span style="color:#a6e22e">valueCtx</span>: <span style="color:#75715e">// 当父级context是不支持cancel操作的ValueCtx类型时候，向上一直查找
</span></span></span><span style="display:flex;"><span><span style="color:#75715e"></span> <span style="color:#a6e22e">parent</span> = <span style="color:#a6e22e">c</span>.<span style="color:#a6e22e">Context</span>
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">default</span>:
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">return</span> <span style="color:#66d9ef">nil</span>, <span style="color:#66d9ef">false</span>
</span></span><span style="display:flex;"><span> }
</span></span><span style="display:flex;"><span> }
</span></span><span style="display:flex;"><span>}
</span></span></code></pre></div><p>注意<code>parentCancelCtx</code>找到的节点不一定是就是父context，有可能是其父辈的context。可以参考下面这种图:</p>
<p>
<img src="https://static.cyub.vip/images/202008/context-cancel.jpg" alt="" /></p>
<p><strong>WithCancel</strong></p>
<p>接下来看cancelCtx类型Context的创建。WithCancel会创一个cancelCtx，以及它关联的取消函数。</p>
<div class="highlight"><pre tabindex="0" style="color:#f8f8f2;background-color:#272822;-moz-tab-size:4;-o-tab-size:4;tab-size:4;"><code class="language-go" data-lang="go"><span style="display:flex;"><span><span style="color:#66d9ef">type</span> <span style="color:#a6e22e">CancelFunc</span> <span style="color:#66d9ef">func</span>()
</span></span><span style="display:flex;"><span>
</span></span><span style="display:flex;"><span><span style="color:#66d9ef">func</span> <span style="color:#a6e22e">WithCancel</span>(<span style="color:#a6e22e">parent</span> <span style="color:#a6e22e">Context</span>) (<span style="color:#a6e22e">ctx</span> <span style="color:#a6e22e">Context</span>, <span style="color:#a6e22e">cancel</span> <span style="color:#a6e22e">CancelFunc</span>) {
</span></span><span style="display:flex;"><span> <span style="color:#75715e">// 根据父context创建新的cancelCtx类型的context
</span></span></span><span style="display:flex;"><span><span style="color:#75715e"></span> <span style="color:#a6e22e">c</span> <span style="color:#f92672">:=</span> <span style="color:#a6e22e">newCancelCtx</span>(<span style="color:#a6e22e">parent</span>)
</span></span><span style="display:flex;"><span> <span style="color:#75715e">// 向上递归找到父辈，并将新context的canceler添加到父辈的映射中
</span></span></span><span style="display:flex;"><span><span style="color:#75715e"></span> <span style="color:#a6e22e">propagateCancel</span>(<span style="color:#a6e22e">parent</span>, <span style="color:#f92672">&amp;</span><span style="color:#a6e22e">c</span>)
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">return</span> <span style="color:#f92672">&amp;</span><span style="color:#a6e22e">c</span>, <span style="color:#66d9ef">func</span>() { <span style="color:#a6e22e">c</span>.<span style="color:#a6e22e">cancel</span>(<span style="color:#66d9ef">true</span>, <span style="color:#a6e22e">Canceled</span>) }
</span></span><span style="display:flex;"><span>}
</span></span><span style="display:flex;"><span>
</span></span><span style="display:flex;"><span><span style="color:#66d9ef">func</span> <span style="color:#a6e22e">newCancelCtx</span>(<span style="color:#a6e22e">parent</span> <span style="color:#a6e22e">Context</span>) <span style="color:#a6e22e">cancelCtx</span> {
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">return</span> <span style="color:#a6e22e">cancelCtx</span>{<span style="color:#a6e22e">Context</span>: <span style="color:#a6e22e">parent</span>}
</span></span><span style="display:flex;"><span>}
</span></span><span style="display:flex;"><span>
</span></span><span style="display:flex;"><span><span style="color:#66d9ef">func</span> <span style="color:#a6e22e">propagateCancel</span>(<span style="color:#a6e22e">parent</span> <span style="color:#a6e22e">Context</span>, <span style="color:#a6e22e">child</span> <span style="color:#a6e22e">canceler</span>) {
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">if</span> <span style="color:#a6e22e">parent</span>.<span style="color:#a6e22e">Done</span>() <span style="color:#f92672">==</span> <span style="color:#66d9ef">nil</span> {
</span></span><span style="display:flex;"><span> <span style="color:#75715e">// parent.Done()返回nil表明父Context不支持取消操作
</span></span></span><span style="display:flex;"><span><span style="color:#75715e"></span> <span style="color:#75715e">// 大部分情况下，该父context已是根context，
</span></span></span><span style="display:flex;"><span><span style="color:#75715e"></span> <span style="color:#75715e">// 该父context是通过context.Background()，或者context.ToDo()创建的
</span></span></span><span style="display:flex;"><span><span style="color:#75715e"></span> <span style="color:#66d9ef">return</span>
</span></span><span style="display:flex;"><span> }
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">if</span> <span style="color:#a6e22e">p</span>, <span style="color:#a6e22e">ok</span> <span style="color:#f92672">:=</span> <span style="color:#a6e22e">parentCancelCtx</span>(<span style="color:#a6e22e">parent</span>); <span style="color:#a6e22e">ok</span> {
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">p</span>.<span style="color:#a6e22e">mu</span>.<span style="color:#a6e22e">Lock</span>()
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">if</span> <span style="color:#a6e22e">p</span>.<span style="color:#a6e22e">err</span> <span style="color:#f92672">!=</span> <span style="color:#66d9ef">nil</span> {
</span></span><span style="display:flex;"><span> <span style="color:#75715e">// 父conext已经取消操作过，
</span></span></span><span style="display:flex;"><span><span style="color:#75715e"></span> <span style="color:#75715e">// 子context立即进行取消操作，并传递父级的错误信息
</span></span></span><span style="display:flex;"><span><span style="color:#75715e"></span> <span style="color:#a6e22e">child</span>.<span style="color:#a6e22e">cancel</span>(<span style="color:#66d9ef">false</span>, <span style="color:#a6e22e">p</span>.<span style="color:#a6e22e">err</span>)
</span></span><span style="display:flex;"><span> } <span style="color:#66d9ef">else</span> {
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">if</span> <span style="color:#a6e22e">p</span>.<span style="color:#a6e22e">children</span> <span style="color:#f92672">==</span> <span style="color:#66d9ef">nil</span> {
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">p</span>.<span style="color:#a6e22e">children</span> = make(<span style="color:#66d9ef">map</span>[<span style="color:#a6e22e">canceler</span>]<span style="color:#66d9ef">struct</span>{})
</span></span><span style="display:flex;"><span> }
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">p</span>.<span style="color:#a6e22e">children</span>[<span style="color:#a6e22e">child</span>] = <span style="color:#66d9ef">struct</span>{}{}
</span></span><span style="display:flex;"><span> <span style="color:#75715e">// 将当前context的取消添加到父context中
</span></span></span><span style="display:flex;"><span><span style="color:#75715e"></span> }
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">p</span>.<span style="color:#a6e22e">mu</span>.<span style="color:#a6e22e">Unlock</span>()
</span></span><span style="display:flex;"><span> } <span style="color:#66d9ef">else</span> {
</span></span><span style="display:flex;"><span> <span style="color:#75715e">// 如果parent是不可取消的，则监控parent和child的Done()通道
</span></span></span><span style="display:flex;"><span><span style="color:#75715e"></span> <span style="color:#66d9ef">go</span> <span style="color:#66d9ef">func</span>() {
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">select</span> {
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">case</span> <span style="color:#f92672">&lt;-</span><span style="color:#a6e22e">parent</span>.<span style="color:#a6e22e">Done</span>():
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">child</span>.<span style="color:#a6e22e">cancel</span>(<span style="color:#66d9ef">false</span>, <span style="color:#a6e22e">parent</span>.<span style="color:#a6e22e">Err</span>())
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">case</span> <span style="color:#f92672">&lt;-</span><span style="color:#a6e22e">child</span>.<span style="color:#a6e22e">Done</span>():
</span></span><span style="display:flex;"><span> }
</span></span><span style="display:flex;"><span> }()
</span></span><span style="display:flex;"><span> }
</span></span><span style="display:flex;"><span>}
</span></span></code></pre></div><h4 id="timerctx">
timerCtx
<a class="anchor" href="#timerctx">#</a>
</h4>
<p>timerCtx是基于cancelCtx的context类型，它支持过期取消。</p>
<div class="highlight"><pre tabindex="0" style="color:#f8f8f2;background-color:#272822;-moz-tab-size:4;-o-tab-size:4;tab-size:4;"><code class="language-go" data-lang="go"><span style="display:flex;"><span><span style="color:#66d9ef">type</span> <span style="color:#a6e22e">timerCtx</span> <span style="color:#66d9ef">struct</span> {
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">cancelCtx</span>
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">timer</span> <span style="color:#f92672">*</span><span style="color:#a6e22e">time</span>.<span style="color:#a6e22e">Timer</span>
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">deadline</span> <span style="color:#a6e22e">time</span>.<span style="color:#a6e22e">Time</span>
</span></span><span style="display:flex;"><span>}
</span></span><span style="display:flex;"><span>
</span></span><span style="display:flex;"><span><span style="color:#66d9ef">func</span> (<span style="color:#a6e22e">c</span> <span style="color:#f92672">*</span><span style="color:#a6e22e">timerCtx</span>) <span style="color:#a6e22e">Deadline</span>() (<span style="color:#a6e22e">deadline</span> <span style="color:#a6e22e">time</span>.<span style="color:#a6e22e">Time</span>, <span style="color:#a6e22e">ok</span> <span style="color:#66d9ef">bool</span>) {
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">return</span> <span style="color:#a6e22e">c</span>.<span style="color:#a6e22e">deadline</span>, <span style="color:#66d9ef">true</span>
</span></span><span style="display:flex;"><span>}
</span></span><span style="display:flex;"><span>
</span></span><span style="display:flex;"><span><span style="color:#66d9ef">func</span> (<span style="color:#a6e22e">c</span> <span style="color:#f92672">*</span><span style="color:#a6e22e">timerCtx</span>) <span style="color:#a6e22e">String</span>() <span style="color:#66d9ef">string</span> {
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">return</span> <span style="color:#a6e22e">contextName</span>(<span style="color:#a6e22e">c</span>.<span style="color:#a6e22e">cancelCtx</span>.<span style="color:#a6e22e">Context</span>) <span style="color:#f92672">+</span> <span style="color:#e6db74">&#34;.WithDeadline(&#34;</span> <span style="color:#f92672">+</span>
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">c</span>.<span style="color:#a6e22e">deadline</span>.<span style="color:#a6e22e">String</span>() <span style="color:#f92672">+</span> <span style="color:#e6db74">&#34; [&#34;</span> <span style="color:#f92672">+</span>
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">time</span>.<span style="color:#a6e22e">Until</span>(<span style="color:#a6e22e">c</span>.<span style="color:#a6e22e">deadline</span>).<span style="color:#a6e22e">String</span>() <span style="color:#f92672">+</span> <span style="color:#e6db74">&#34;])&#34;</span>
</span></span><span style="display:flex;"><span>}
</span></span><span style="display:flex;"><span>
</span></span><span style="display:flex;"><span><span style="color:#66d9ef">func</span> (<span style="color:#a6e22e">c</span> <span style="color:#f92672">*</span><span style="color:#a6e22e">timerCtx</span>) <span style="color:#a6e22e">cancel</span>(<span style="color:#a6e22e">removeFromParent</span> <span style="color:#66d9ef">bool</span>, <span style="color:#a6e22e">err</span> <span style="color:#66d9ef">error</span>) {
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">c</span>.<span style="color:#a6e22e">cancelCtx</span>.<span style="color:#a6e22e">cancel</span>(<span style="color:#66d9ef">false</span>, <span style="color:#a6e22e">err</span>)
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">if</span> <span style="color:#a6e22e">removeFromParent</span> {
</span></span><span style="display:flex;"><span> <span style="color:#75715e">// 删除与父辈context的关联
</span></span></span><span style="display:flex;"><span><span style="color:#75715e"></span> <span style="color:#a6e22e">removeChild</span>(<span style="color:#a6e22e">c</span>.<span style="color:#a6e22e">cancelCtx</span>.<span style="color:#a6e22e">Context</span>, <span style="color:#a6e22e">c</span>)
</span></span><span style="display:flex;"><span> }
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">c</span>.<span style="color:#a6e22e">mu</span>.<span style="color:#a6e22e">Lock</span>()
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">if</span> <span style="color:#a6e22e">c</span>.<span style="color:#a6e22e">timer</span> <span style="color:#f92672">!=</span> <span style="color:#66d9ef">nil</span> {
</span></span><span style="display:flex;"><span> <span style="color:#75715e">// 停止timer并回收
</span></span></span><span style="display:flex;"><span><span style="color:#75715e"></span> <span style="color:#a6e22e">c</span>.<span style="color:#a6e22e">timer</span>.<span style="color:#a6e22e">Stop</span>()
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">c</span>.<span style="color:#a6e22e">timer</span> = <span style="color:#66d9ef">nil</span>
</span></span><span style="display:flex;"><span> }
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">c</span>.<span style="color:#a6e22e">mu</span>.<span style="color:#a6e22e">Unlock</span>()
</span></span><span style="display:flex;"><span>}
</span></span></code></pre></div><p><strong>WithDeadline</strong></p>
<p>WithDeadline会创建一个timerCtx，以及它关联的取消函数</p>
<div class="highlight"><pre tabindex="0" style="color:#f8f8f2;background-color:#272822;-moz-tab-size:4;-o-tab-size:4;tab-size:4;"><code class="language-go" data-lang="go"><span style="display:flex;"><span><span style="color:#66d9ef">func</span> <span style="color:#a6e22e">WithDeadline</span>(<span style="color:#a6e22e">parent</span> <span style="color:#a6e22e">Context</span>, <span style="color:#a6e22e">d</span> <span style="color:#a6e22e">time</span>.<span style="color:#a6e22e">Time</span>) (<span style="color:#a6e22e">Context</span>, <span style="color:#a6e22e">CancelFunc</span>) {
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">if</span> <span style="color:#a6e22e">cur</span>, <span style="color:#a6e22e">ok</span> <span style="color:#f92672">:=</span> <span style="color:#a6e22e">parent</span>.<span style="color:#a6e22e">Deadline</span>(); <span style="color:#a6e22e">ok</span> <span style="color:#f92672">&amp;&amp;</span> <span style="color:#a6e22e">cur</span>.<span style="color:#a6e22e">Before</span>(<span style="color:#a6e22e">d</span>) {
</span></span><span style="display:flex;"><span> <span style="color:#75715e">// 如果父context过期时间早于当前context过期时间，则创建cancelCtx
</span></span></span><span style="display:flex;"><span><span style="color:#75715e"></span> <span style="color:#66d9ef">return</span> <span style="color:#a6e22e">WithCancel</span>(<span style="color:#a6e22e">parent</span>)
</span></span><span style="display:flex;"><span> }
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">c</span> <span style="color:#f92672">:=</span> <span style="color:#f92672">&amp;</span><span style="color:#a6e22e">timerCtx</span>{
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">cancelCtx</span>: <span style="color:#a6e22e">newCancelCtx</span>(<span style="color:#a6e22e">parent</span>),
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">deadline</span>: <span style="color:#a6e22e">d</span>,
</span></span><span style="display:flex;"><span> }
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">propagateCancel</span>(<span style="color:#a6e22e">parent</span>, <span style="color:#a6e22e">c</span>)
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">dur</span> <span style="color:#f92672">:=</span> <span style="color:#a6e22e">time</span>.<span style="color:#a6e22e">Until</span>(<span style="color:#a6e22e">d</span>)
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">if</span> <span style="color:#a6e22e">dur</span> <span style="color:#f92672">&lt;=</span> <span style="color:#ae81ff">0</span> {
</span></span><span style="display:flex;"><span> <span style="color:#75715e">// 如果新创建的timerCtx正好过期了，则取消操作并传递DeadlineExceeded
</span></span></span><span style="display:flex;"><span><span style="color:#75715e"></span> <span style="color:#a6e22e">c</span>.<span style="color:#a6e22e">cancel</span>(<span style="color:#66d9ef">true</span>, <span style="color:#a6e22e">DeadlineExceeded</span>)
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">return</span> <span style="color:#a6e22e">c</span>, <span style="color:#66d9ef">func</span>() { <span style="color:#a6e22e">c</span>.<span style="color:#a6e22e">cancel</span>(<span style="color:#66d9ef">false</span>, <span style="color:#a6e22e">Canceled</span>) }
</span></span><span style="display:flex;"><span> }
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">c</span>.<span style="color:#a6e22e">mu</span>.<span style="color:#a6e22e">Lock</span>()
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">defer</span> <span style="color:#a6e22e">c</span>.<span style="color:#a6e22e">mu</span>.<span style="color:#a6e22e">Unlock</span>()
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">if</span> <span style="color:#a6e22e">c</span>.<span style="color:#a6e22e">err</span> <span style="color:#f92672">==</span> <span style="color:#66d9ef">nil</span> {
</span></span><span style="display:flex;"><span> <span style="color:#75715e">// 创建定时器，时间一到执行context取消操作
</span></span></span><span style="display:flex;"><span><span style="color:#75715e"></span> <span style="color:#a6e22e">c</span>.<span style="color:#a6e22e">timer</span> = <span style="color:#a6e22e">time</span>.<span style="color:#a6e22e">AfterFunc</span>(<span style="color:#a6e22e">dur</span>, <span style="color:#66d9ef">func</span>() {
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">c</span>.<span style="color:#a6e22e">cancel</span>(<span style="color:#66d9ef">true</span>, <span style="color:#a6e22e">DeadlineExceeded</span>)
</span></span><span style="display:flex;"><span> })
</span></span><span style="display:flex;"><span> }
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">return</span> <span style="color:#a6e22e">c</span>, <span style="color:#66d9ef">func</span>() { <span style="color:#a6e22e">c</span>.<span style="color:#a6e22e">cancel</span>(<span style="color:#66d9ef">true</span>, <span style="color:#a6e22e">Canceled</span>) }
</span></span><span style="display:flex;"><span>}
</span></span></code></pre></div><p><strong>WithTimeout</strong></p>
<p>WithTimeout用来创建超时就会取消的context，内部实现就是WithDealine，传递给WithDealine的过期时间就是当前时间加上timeout时间</p>
<div class="highlight"><pre tabindex="0" style="color:#f8f8f2;background-color:#272822;-moz-tab-size:4;-o-tab-size:4;tab-size:4;"><code class="language-go" data-lang="go"><span style="display:flex;"><span><span style="color:#66d9ef">func</span> <span style="color:#a6e22e">WithTimeout</span>(<span style="color:#a6e22e">parent</span> <span style="color:#a6e22e">Context</span>, <span style="color:#a6e22e">timeout</span> <span style="color:#a6e22e">time</span>.<span style="color:#a6e22e">Duration</span>) (<span style="color:#a6e22e">Context</span>, <span style="color:#a6e22e">CancelFunc</span>) {
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">return</span> <span style="color:#a6e22e">WithDeadline</span>(<span style="color:#a6e22e">parent</span>, <span style="color:#a6e22e">time</span>.<span style="color:#a6e22e">Now</span>().<span style="color:#a6e22e">Add</span>(<span style="color:#a6e22e">timeout</span>))
</span></span><span style="display:flex;"><span>}
</span></span></code></pre></div><h4 id="valuectx">
valueCtx
<a class="anchor" href="#valuectx">#</a>
</h4>
<p>valueCtx是可以传递共享信息的context。</p>
<div class="highlight"><pre tabindex="0" style="color:#f8f8f2;background-color:#272822;-moz-tab-size:4;-o-tab-size:4;tab-size:4;"><code class="language-go" data-lang="go"><span style="display:flex;"><span><span style="color:#66d9ef">type</span> <span style="color:#a6e22e">valueCtx</span> <span style="color:#66d9ef">struct</span> {
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">Context</span>
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">key</span>, <span style="color:#a6e22e">val</span> <span style="color:#66d9ef">interface</span>{}
</span></span><span style="display:flex;"><span>}
</span></span><span style="display:flex;"><span>
</span></span><span style="display:flex;"><span><span style="color:#66d9ef">func</span> (<span style="color:#a6e22e">c</span> <span style="color:#f92672">*</span><span style="color:#a6e22e">valueCtx</span>) <span style="color:#a6e22e">Value</span>(<span style="color:#a6e22e">key</span> <span style="color:#66d9ef">interface</span>{}) <span style="color:#66d9ef">interface</span>{} {
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">if</span> <span style="color:#a6e22e">c</span>.<span style="color:#a6e22e">key</span> <span style="color:#f92672">==</span> <span style="color:#a6e22e">key</span> {
</span></span><span style="display:flex;"><span> <span style="color:#75715e">// 当前context存在当前的key
</span></span></span><span style="display:flex;"><span><span style="color:#75715e"></span> <span style="color:#66d9ef">return</span> <span style="color:#a6e22e">c</span>.<span style="color:#a6e22e">val</span>
</span></span><span style="display:flex;"><span> }
</span></span><span style="display:flex;"><span>
</span></span><span style="display:flex;"><span> <span style="color:#75715e">// 当前context不存在，则会沿着context树，向上递归查找，直到根context，如果一直未找到，则会返回nil
</span></span></span><span style="display:flex;"><span><span style="color:#75715e"></span> <span style="color:#66d9ef">return</span> <span style="color:#a6e22e">c</span>.<span style="color:#a6e22e">Context</span>.<span style="color:#a6e22e">Value</span>(<span style="color:#a6e22e">key</span>)
</span></span><span style="display:flex;"><span>}
</span></span></code></pre></div><p>如果当前context不存在该key，则会沿着context树，向上递归查找，直到查找到根context，最后返回nil
<img src="https://static.cyub.vip/images/202008/context-value.jpg" alt="" /></p>
<p><strong>WithValue</strong></p>
<p>WithValue用来创建valueCtx。如果key是不可以比较的时候，则会发生恐慌。可以比较类型，可以参考
<a href="https://golang.org/ref/spec#Comparison_operators">Comparison_operators</a>。<strong>key应该是不导出变量，防止冲突</strong>。</p>
<div class="highlight"><pre tabindex="0" style="color:#f8f8f2;background-color:#272822;-moz-tab-size:4;-o-tab-size:4;tab-size:4;"><code class="language-go" data-lang="go"><span style="display:flex;"><span><span style="color:#66d9ef">func</span> <span style="color:#a6e22e">WithValue</span>(<span style="color:#a6e22e">parent</span> <span style="color:#a6e22e">Context</span>, <span style="color:#a6e22e">key</span>, <span style="color:#a6e22e">val</span> <span style="color:#66d9ef">interface</span>{}) <span style="color:#a6e22e">Context</span> {
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">if</span> <span style="color:#a6e22e">key</span> <span style="color:#f92672">==</span> <span style="color:#66d9ef">nil</span> {
</span></span><span style="display:flex;"><span> panic(<span style="color:#e6db74">&#34;nil key&#34;</span>)
</span></span><span style="display:flex;"><span> }
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">if</span> !<span style="color:#a6e22e">reflectlite</span>.<span style="color:#a6e22e">TypeOf</span>(<span style="color:#a6e22e">key</span>).<span style="color:#a6e22e">Comparable</span>() {
</span></span><span style="display:flex;"><span> panic(<span style="color:#e6db74">&#34;key is not comparable&#34;</span>)
</span></span><span style="display:flex;"><span> }
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">return</span> <span style="color:#f92672">&amp;</span><span style="color:#a6e22e">valueCtx</span>{<span style="color:#a6e22e">parent</span>, <span style="color:#a6e22e">key</span>, <span style="color:#a6e22e">val</span>}
</span></span><span style="display:flex;"><span>}
</span></span></code></pre></div><h2 id="总结">
总结
<a class="anchor" href="#%e6%80%bb%e7%bb%93">#</a>
</h2>
<h3 id="实现context接口的类型-1">
实现Context接口的类型
<a class="anchor" href="#%e5%ae%9e%e7%8e%b0context%e6%8e%a5%e5%8f%a3%e7%9a%84%e7%b1%bb%e5%9e%8b-1">#</a>
</h3>
<p>Context一共有4个类型实现了Context接口, 分别是<code>emptyCtx</code>, <code>cancelCtx</code>,<code>timerCtx</code>,<code>valueCtx</code>。</p>
<p>它们的功能与创建方法如下：</p>
<table>
<thead>
<tr>
<th>类型</th>
<th>创建方法</th>
<th>功能</th>
</tr>
</thead>
<tbody>
<tr>
<td>emptyCtx</td>
<td>Background()/TODO()</td>
<td>用做context树的根节点</td>
</tr>
<tr>
<td>cancelCtx</td>
<td>WithCancel()</td>
<td>可取消的context</td>
</tr>
<tr>
<td>timerCtx</td>
<td>WithDeadline()/WithTimeout()</td>
<td>可取消的context，过期或超时会自动取消</td>
</tr>
<tr>
<td>valueCtx</td>
<td>WithValue()</td>
<td>可存储共享信息的context</td>
</tr>
</tbody>
</table>
<h3 id="context实现两种递归">
Context实现两种递归
<a class="anchor" href="#context%e5%ae%9e%e7%8e%b0%e4%b8%a4%e7%a7%8d%e9%80%92%e5%bd%92">#</a>
</h3>
<p>Context实现两种方向的递归操作。</p>
<table>
<thead>
<tr>
<th>递归方向</th>
<th>目的</th>
</tr>
</thead>
<tbody>
<tr>
<td>向下递归</td>
<td>当对父Context进去手动取消操作，或超时取消时候，向下递归处理对实现了canceler接口的后代进行取消操作</td>
</tr>
<tr>
<td>向上队规</td>
<td>当对Context查询Key信息时候，若当前Context没有当前K-V信息时候，则向父辈递归查询，一直到查询到跟节点的emptyCtx，返回nil为止</td>
</tr>
</tbody>
</table>
<h3 id="context使用规范">
Context使用规范
<a class="anchor" href="#context%e4%bd%bf%e7%94%a8%e8%a7%84%e8%8c%83">#</a>
</h3>
<p>使用Context的是应该准守以下原则来保证在不同包中使用时候的接口一致性，以及能让静态分析工具可以检查context的传播：</p>
<ol>
<li>不要将Context作为结构体的一个字段存储，相反而应该显示传递Context给每一个需要它的函数，Context应该作为函数的第一个参数，并命名为ctx</li>
<li>不要传递一个nil Context给一个函数，即使该函数能够接受它。如果你不确定使用哪一个Context，那你就传递context.TODO</li>
<li>context是并发安全的，相同的Context能够传递给运行在不同goroutine的函数</li>
</ol>
<h2 id="参考资料">
参考资料
<a class="anchor" href="#%e5%8f%82%e8%80%83%e8%b5%84%e6%96%99">#</a>
</h2>
<ul>
<li>
<a href="https://juejin.im/post/6844904070667321357">深入理解Golang之context</a></li>
<li>
<a href="https://blog.golang.org/context">Go Concurrency Patterns: Context</a></li>
<li>
<a href="https://medium.com/@ankur_a22/using-goroutines-channels-contexts-timers-waitgroups-and-errgroups-24b6062c1c93">Using Goroutines, Channels, Contexts, Timers, WaitGroups and Errgroups</a></li>
</ul></article>
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