280 lines
14 KiB
HTML
280 lines
14 KiB
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<div><a href="index.html">Index</a></div>
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<h1>
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<a
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id="user-content-another-day-another-incident-02"
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class="anchor"
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aria-hidden="true"
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href="#another-day-another-incident-02"
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><span aria-hidden="true" class="octicon octicon-link"></span></a
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>Another day another incident #02
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</h1>
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<p>Today's incident is all about Go context.</p>
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<p>TLDR: context got canceled, but it shouldn't.</p>
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<h2>
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<a
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id="user-content-the-problem"
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class="anchor"
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aria-hidden="true"
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href="#the-problem"
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><span aria-hidden="true" class="octicon octicon-link"></span></a
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>The problem
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</h2>
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<p>Imagine a chain of APIs:</p>
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<ul>
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<li>Calling API A</li>
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<li>Calling API B</li>
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</ul>
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<p>
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Normally, if API A fails, API B should not be called. But what if API A is
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<strong>optional</strong>, whether it successes or fails, API B should be
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called anyway.
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</p>
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<p>My buggy code is like this:</p>
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<div class="highlight highlight-source-go">
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<pre><span class="pl-k">if</span> <span class="pl-s1">err</span> <span class="pl-c1">:=</span> <span class="pl-en">doA</span>(<span class="pl-s1">ctx</span>); <span class="pl-s1">err</span> <span class="pl-c1">!=</span> <span class="pl-c1">nil</span> {
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<span class="pl-s1">log</span>.<span class="pl-en">Error</span>(<span class="pl-s1">err</span>)
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<span class="pl-c">// Skip error</span>
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}
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<span class="pl-en">doB</span>(<span class="pl-s1">ctx</span>)</pre>
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</div>
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<p>
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The problem is <code>doA</code> taking too long, so <code>ctx</code> is
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canceled, and the parent of <code>ctx</code> is canceled too. So when
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<code>doB</code> is called with <code>ctx</code>, it will be canceled too
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(not what we want but sadly that what we got).
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</p>
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<p>
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Example buggy code (<a
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href="https://go.dev/play/p/p4S27Su16VH"
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rel="nofollow"
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>The Go Playground</a
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>):
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</p>
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<div class="highlight highlight-source-go">
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<pre><span class="pl-k">func</span> <span class="pl-en">main</span>() {
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<span class="pl-s1">ctx</span>, <span class="pl-s1">cancel</span> <span class="pl-c1">:=</span> <span class="pl-s1">context</span>.<span class="pl-en">WithTimeout</span>(<span class="pl-s1">context</span>.<span class="pl-en">Background</span>(), <span class="pl-c1">2</span><span class="pl-c1">*</span><span class="pl-s1">time</span>.<span class="pl-c1">Second</span>)
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<span class="pl-k">defer</span> <span class="pl-en">cancel</span>()
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<span class="pl-en">doA</span>(<span class="pl-s1">ctx</span>)
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<span class="pl-en">doB</span>(<span class="pl-s1">ctx</span>)
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}
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<span class="pl-k">func</span> <span class="pl-en">doA</span>(<span class="pl-s1">ctx</span> context.<span class="pl-smi">Context</span>) {
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<span class="pl-s1">ctx</span>, <span class="pl-s1">ctxCancel</span> <span class="pl-c1">:=</span> <span class="pl-s1">context</span>.<span class="pl-en">WithTimeout</span>(<span class="pl-s1">ctx</span>, <span class="pl-c1">1</span><span class="pl-c1">*</span><span class="pl-s1">time</span>.<span class="pl-c1">Second</span>)
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<span class="pl-k">defer</span> <span class="pl-en">ctxCancel</span>()
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<span class="pl-k">select</span> {
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<span class="pl-k">case</span> <span class="pl-c1"><-</span><span class="pl-s1">time</span>.<span class="pl-en">After</span>(<span class="pl-c1">2</span> <span class="pl-c1">*</span> <span class="pl-s1">time</span>.<span class="pl-c1">Second</span>):
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<span class="pl-s1">fmt</span>.<span class="pl-en">Println</span>(<span class="pl-s">"doA"</span>)
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<span class="pl-k">case</span> <span class="pl-c1"><-</span><span class="pl-s1">ctx</span>.<span class="pl-en">Done</span>():
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<span class="pl-s1">fmt</span>.<span class="pl-en">Println</span>(<span class="pl-s">"doA"</span>, <span class="pl-s1">ctx</span>.<span class="pl-en">Err</span>())
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}
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}
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<span class="pl-k">func</span> <span class="pl-en">doB</span>(<span class="pl-s1">ctx</span> context.<span class="pl-smi">Context</span>) {
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<span class="pl-s1">ctx</span>, <span class="pl-s1">ctxCancel</span> <span class="pl-c1">:=</span> <span class="pl-s1">context</span>.<span class="pl-en">WithTimeout</span>(<span class="pl-s1">ctx</span>, <span class="pl-c1">3</span><span class="pl-c1">*</span><span class="pl-s1">time</span>.<span class="pl-c1">Second</span>)
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<span class="pl-k">defer</span> <span class="pl-en">ctxCancel</span>()
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<span class="pl-k">select</span> {
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<span class="pl-k">case</span> <span class="pl-c1"><-</span><span class="pl-s1">time</span>.<span class="pl-en">After</span>(<span class="pl-c1">2</span> <span class="pl-c1">*</span> <span class="pl-s1">time</span>.<span class="pl-c1">Second</span>):
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<span class="pl-s1">fmt</span>.<span class="pl-en">Println</span>(<span class="pl-s">"doB"</span>)
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<span class="pl-k">case</span> <span class="pl-c1"><-</span><span class="pl-s1">ctx</span>.<span class="pl-en">Done</span>():
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<span class="pl-s1">fmt</span>.<span class="pl-en">Println</span>(<span class="pl-s">"doB"</span>, <span class="pl-s1">ctx</span>.<span class="pl-en">Err</span>())
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}
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}</pre>
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</div>
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<p>The output is:</p>
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<div class="highlight highlight-text-adblock">
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<pre>
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doA context deadline exceeded
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doB context deadline exceeded</pre
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>
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</div>
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<p>As you see both <code>doA</code> and <code>doB</code> are canceled.</p>
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<h2>
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<a
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id="user-content-the-temporary-solution"
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class="anchor"
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aria-hidden="true"
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href="#the-temporary-solution"
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><span aria-hidden="true" class="octicon octicon-link"></span></a
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>The (temporary) solution
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</h2>
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<p>
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Quick Google search leads me to
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<a href="https://github.com/golang/go/issues/40221"
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>context: add WithoutCancel #40221</a
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>
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and I quote:
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</p>
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<blockquote>
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<p>
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This is useful in multiple frequently recurring and important scenarios:
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</p>
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<ul>
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<li>
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Handling of rollback/cleanup operations in the context of an event
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(e.g., HTTP request) that has to continue regardless of whether the
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triggering event is canceled (e.g., due to timeout or the client going
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away)
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</li>
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<li>
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Handling of long-running operations triggered by an event (e.g., HTTP
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request) that terminates before the termination of the long-running
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operation
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</li>
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</ul>
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</blockquote>
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<p>
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So beside waiting to upgrade to Go <code>1.21</code> to use
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<code>context.WithoutCancel</code>, you can use this
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<a href="https://pkg.go.dev/context@master#WithoutCancel" rel="nofollow"
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>workaround code</a
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>:
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</p>
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<div class="highlight highlight-source-go">
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<pre><span class="pl-k">func</span> <span class="pl-en">DisconnectContext</span>(<span class="pl-s1">parent</span> context.<span class="pl-smi">Context</span>) context.<span class="pl-smi">Context</span> {
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<span class="pl-k">if</span> <span class="pl-s1">parent</span> <span class="pl-c1">==</span> <span class="pl-c1">nil</span> {
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<span class="pl-k">return</span> <span class="pl-s1">context</span>.<span class="pl-en">Background</span>()
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}
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<span class="pl-k">return</span> <span class="pl-smi">disconnectedContext</span>{
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<span class="pl-c1">parent</span>: <span class="pl-s1">parent</span>,
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}
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}
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<span class="pl-k">type</span> <span class="pl-smi">disconnectedContext</span> <span class="pl-k">struct</span> {
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<span class="pl-c1">parent</span> context.<span class="pl-smi">Context</span>
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}
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<span class="pl-k">func</span> (<span class="pl-s1">ctx</span> <span class="pl-smi">disconnectedContext</span>) <span class="pl-en">Deadline</span>() (<span class="pl-s1">deadline</span> time.<span class="pl-smi">Time</span>, <span class="pl-s1">ok</span> <span class="pl-smi">bool</span>) {
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<span class="pl-k">return</span>
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}
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<span class="pl-k">func</span> (<span class="pl-s1">ctx</span> <span class="pl-smi">disconnectedContext</span>) <span class="pl-en">Done</span>() <span class="pl-c1"><-</span><span class="pl-k">chan</span> <span class="pl-k">struct</span>{} {
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<span class="pl-k">return</span> <span class="pl-c1">nil</span>
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}
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<span class="pl-k">func</span> (<span class="pl-s1">ctx</span> <span class="pl-smi">disconnectedContext</span>) <span class="pl-en">Err</span>() <span class="pl-smi">error</span> {
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<span class="pl-k">return</span> <span class="pl-c1">nil</span>
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}
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<span class="pl-k">func</span> (<span class="pl-s1">ctx</span> <span class="pl-smi">disconnectedContext</span>) <span class="pl-en">Value</span>(<span class="pl-s1">key</span> <span class="pl-smi">any</span>) <span class="pl-smi">any</span> {
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<span class="pl-k">return</span> <span class="pl-s1">ctx</span>.<span class="pl-c1">parent</span>.<span class="pl-en">Value</span>(<span class="pl-s1">key</span>)
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}</pre>
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</div>
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<p>
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So the buggy code becomes (<a
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href="https://go.dev/play/p/oIU-WxEJ_F3"
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rel="nofollow"
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>The Go Playground</a
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>):
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</p>
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<div class="highlight highlight-source-go">
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<pre><span class="pl-k">func</span> <span class="pl-en">main</span>() {
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<span class="pl-s1">ctx</span>, <span class="pl-s1">cancel</span> <span class="pl-c1">:=</span> <span class="pl-s1">context</span>.<span class="pl-en">WithTimeout</span>(<span class="pl-s1">context</span>.<span class="pl-en">Background</span>(), <span class="pl-c1">2</span><span class="pl-c1">*</span><span class="pl-s1">time</span>.<span class="pl-c1">Second</span>)
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<span class="pl-k">defer</span> <span class="pl-en">cancel</span>()
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<span class="pl-en">doA</span>(<span class="pl-s1">ctx</span>)
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<span class="pl-en">doB</span>(<span class="pl-s1">ctx</span>)
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}
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<span class="pl-k">func</span> <span class="pl-en">doA</span>(<span class="pl-s1">ctx</span> context.<span class="pl-smi">Context</span>) {
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<span class="pl-s1">ctx</span>, <span class="pl-s1">ctxCancel</span> <span class="pl-c1">:=</span> <span class="pl-s1">context</span>.<span class="pl-en">WithTimeout</span>(<span class="pl-s1">ctx</span>, <span class="pl-c1">1</span><span class="pl-c1">*</span><span class="pl-s1">time</span>.<span class="pl-c1">Second</span>)
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<span class="pl-k">defer</span> <span class="pl-en">ctxCancel</span>()
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<span class="pl-k">select</span> {
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<span class="pl-k">case</span> <span class="pl-c1"><-</span><span class="pl-s1">time</span>.<span class="pl-en">After</span>(<span class="pl-c1">2</span> <span class="pl-c1">*</span> <span class="pl-s1">time</span>.<span class="pl-c1">Second</span>):
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<span class="pl-s1">fmt</span>.<span class="pl-en">Println</span>(<span class="pl-s">"doA"</span>)
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<span class="pl-k">case</span> <span class="pl-c1"><-</span><span class="pl-s1">ctx</span>.<span class="pl-en">Done</span>():
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<span class="pl-s1">fmt</span>.<span class="pl-en">Println</span>(<span class="pl-s">"doA"</span>, <span class="pl-s1">ctx</span>.<span class="pl-en">Err</span>())
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}
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}
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<span class="pl-k">func</span> <span class="pl-en">doB</span>(<span class="pl-s1">ctx</span> context.<span class="pl-smi">Context</span>) {
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<span class="pl-s1">ctx</span>, <span class="pl-s1">ctxCancel</span> <span class="pl-c1">:=</span> <span class="pl-s1">context</span>.<span class="pl-en">WithTimeout</span>(<span class="pl-en">DisconnectContext</span>(<span class="pl-s1">ctx</span>), <span class="pl-c1">3</span><span class="pl-c1">*</span><span class="pl-s1">time</span>.<span class="pl-c1">Second</span>)
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<span class="pl-k">defer</span> <span class="pl-en">ctxCancel</span>()
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<span class="pl-k">select</span> {
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<span class="pl-k">case</span> <span class="pl-c1"><-</span><span class="pl-s1">time</span>.<span class="pl-en">After</span>(<span class="pl-c1">2</span> <span class="pl-c1">*</span> <span class="pl-s1">time</span>.<span class="pl-c1">Second</span>):
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<span class="pl-s1">fmt</span>.<span class="pl-en">Println</span>(<span class="pl-s">"doB"</span>)
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<span class="pl-k">case</span> <span class="pl-c1"><-</span><span class="pl-s1">ctx</span>.<span class="pl-en">Done</span>():
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<span class="pl-s1">fmt</span>.<span class="pl-en">Println</span>(<span class="pl-s">"doB"</span>, <span class="pl-s1">ctx</span>.<span class="pl-en">Err</span>())
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}
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}</pre>
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</div>
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<p>The output is:</p>
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<div class="highlight highlight-text-adblock">
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<pre>
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doA context deadline exceeded
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doB</pre
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>
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</div>
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<p>
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As you see only <code>doA</code> is canceled, <code>doB</code> is done
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perfectly. And that what we want in this case.
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</p>
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<h2>
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<a
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id="user-content-thanks"
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class="anchor"
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aria-hidden="true"
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href="#thanks"
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><span aria-hidden="true" class="octicon octicon-link"></span></a
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>Thanks
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</h2>
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<ul>
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<li>
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<a
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href="https://www.sohamkamani.com/golang/context-cancellation-and-values/"
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rel="nofollow"
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>Using Context in Golang - Cancellation, Timeouts and Values (With
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Examples)</a
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>
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</li>
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<li>
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<a
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href="https://uptrace.dev/blog/golang-context-timeout.html"
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rel="nofollow"
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>Go Context timeouts can be harmful</a
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>
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</li>
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</ul>
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<div>
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Feel free to ask me via
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<a href="mailto:hauvipapro+posts@gmail.com">email</a> or
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<a rel="me" href="https://hachyderm.io/@haunguyen">Mastodon</a>. Source
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code is available on
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<a href="https://github.com/haunt98/posts-go">GitHub</a>
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</div>
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