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feat: add incident #02
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<h1>
<a
id="user-content-another-day-another-incident-02"
class="anchor"
aria-hidden="true"
href="#another-day-another-incident-02"
><span aria-hidden="true" class="octicon octicon-link"></span></a
>Another day another incident #02
</h1>
<p>Today's incident is all about Go context.</p>
<p>TLDR: context got canceled, but it shouldn't.</p>
<h2>
<a
id="user-content-the-problem"
class="anchor"
aria-hidden="true"
href="#the-problem"
><span aria-hidden="true" class="octicon octicon-link"></span></a
>The problem
</h2>
<p>Imagine a chain of APIs:</p>
<ul>
<li>Calling API A</li>
<li>Calling API B</li>
</ul>
<p>
Normally, if API A fails, API B should not be called. But what if API A is
chore: fix grammar
2023-06-10 17:03:19 +00:00
<strong>optional</strong>, whether it successes or fails, API B should be
called anyway.
feat: add incident #02
2023-06-10 07:15:38 +00:00
</p>
<p>My buggy code is like this:</p>
<div class="highlight highlight-source-go">
<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> {
<span class="pl-s1">log</span>.<span class="pl-en">Error</span>(<span class="pl-s1">err</span>)
<span class="pl-c">// Skip error</span>
}
<span class="pl-en">doB</span>(<span class="pl-s1">ctx</span>)</pre>
</div>
<p>
The problem is <code>doA</code> taking too long, so <code>ctx</code> is
canceled, and the parent of <code>ctx</code> is canceled too. So when
<code>doB</code> is called with <code>ctx</code>, it will be canceled too
(not what we want but sadly that what we got).
</p>
<p>
Example buggy code (<a
href="https://go.dev/play/p/p4S27Su16VH"
rel="nofollow"
>The Go Playground</a
>):
</p>
<div class="highlight highlight-source-go">
<pre><span class="pl-k">func</span> <span class="pl-en">main</span>() {
<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>)
<span class="pl-k">defer</span> <span class="pl-en">cancel</span>()
<span class="pl-en">doA</span>(<span class="pl-s1">ctx</span>)
<span class="pl-en">doB</span>(<span class="pl-s1">ctx</span>)
}
<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>) {
<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>)
<span class="pl-k">defer</span> <span class="pl-en">ctxCancel</span>()
<span class="pl-k">select</span> {
<span class="pl-k">case</span> <span class="pl-c1">&lt;-</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>):
<span class="pl-s1">fmt</span>.<span class="pl-en">Println</span>(<span class="pl-s">"doA"</span>)
<span class="pl-k">case</span> <span class="pl-c1">&lt;-</span><span class="pl-s1">ctx</span>.<span class="pl-en">Done</span>():
<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>())
}
}
<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>) {
<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>)
<span class="pl-k">defer</span> <span class="pl-en">ctxCancel</span>()
<span class="pl-k">select</span> {
<span class="pl-k">case</span> <span class="pl-c1">&lt;-</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>):
<span class="pl-s1">fmt</span>.<span class="pl-en">Println</span>(<span class="pl-s">"doB"</span>)
<span class="pl-k">case</span> <span class="pl-c1">&lt;-</span><span class="pl-s1">ctx</span>.<span class="pl-en">Done</span>():
<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>())
}
}</pre>
</div>
<p>The output is:</p>
<div class="highlight highlight-text-adblock">
<pre>
doA context deadline exceeded
doB context deadline exceeded</pre
>
</div>
<p>As you see both <code>doA</code> and <code>doB</code> are canceled.</p>
<h2>
<a
id="user-content-the-temporary-solution"
class="anchor"
aria-hidden="true"
href="#the-temporary-solution"
><span aria-hidden="true" class="octicon octicon-link"></span></a
>The (temporary) solution
</h2>
<p>
Quick Google search leads me to
<a href="https://github.com/golang/go/issues/40221"
>context: add WithoutCancel #40221</a
>
and I quote:
</p>
<blockquote>
<p>
This is useful in multiple frequently recurring and important scenarios:
</p>
<ul>
<li>
Handling of rollback/cleanup operations in the context of an event
(e.g., HTTP request) that has to continue regardless of whether the
triggering event is canceled (e.g., due to timeout or the client going
away)
</li>
<li>
Handling of long-running operations triggered by an event (e.g., HTTP
request) that terminates before the termination of the long-running
operation
</li>
</ul>
</blockquote>
<p>
So beside waiting to upgrade to Go <code>1.21</code> to use
<code>context.WithoutCancel</code>, you can use this
<a href="https://pkg.go.dev/context@master#WithoutCancel" rel="nofollow"
>workaround code</a
>:
</p>
<div class="highlight highlight-source-go">
<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> {
<span class="pl-k">if</span> <span class="pl-s1">parent</span> <span class="pl-c1">==</span> <span class="pl-c1">nil</span> {
<span class="pl-k">return</span> <span class="pl-s1">context</span>.<span class="pl-en">Background</span>()
}
<span class="pl-k">return</span> <span class="pl-smi">disconnectedContext</span>{
<span class="pl-c1">parent</span>: <span class="pl-s1">parent</span>,
}
}
<span class="pl-k">type</span> <span class="pl-smi">disconnectedContext</span> <span class="pl-k">struct</span> {
<span class="pl-c1">parent</span> context.<span class="pl-smi">Context</span>
}
<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>) {
<span class="pl-k">return</span>
}
<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">&lt;-</span><span class="pl-k">chan</span> <span class="pl-k">struct</span>{} {
<span class="pl-k">return</span> <span class="pl-c1">nil</span>
}
<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> {
<span class="pl-k">return</span> <span class="pl-c1">nil</span>
}
<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> {
<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>)
}</pre>
</div>
<p>
So the buggy code becomes (<a
href="https://go.dev/play/p/oIU-WxEJ_F3"
rel="nofollow"
>The Go Playground</a
>):
</p>
<div class="highlight highlight-source-go">
<pre><span class="pl-k">func</span> <span class="pl-en">main</span>() {
<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>)
<span class="pl-k">defer</span> <span class="pl-en">cancel</span>()
<span class="pl-en">doA</span>(<span class="pl-s1">ctx</span>)
<span class="pl-en">doB</span>(<span class="pl-s1">ctx</span>)
}
<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>) {
<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>)
<span class="pl-k">defer</span> <span class="pl-en">ctxCancel</span>()
<span class="pl-k">select</span> {
<span class="pl-k">case</span> <span class="pl-c1">&lt;-</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>):
<span class="pl-s1">fmt</span>.<span class="pl-en">Println</span>(<span class="pl-s">"doA"</span>)
<span class="pl-k">case</span> <span class="pl-c1">&lt;-</span><span class="pl-s1">ctx</span>.<span class="pl-en">Done</span>():
<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>())
}
}
<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>) {
<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>)
<span class="pl-k">defer</span> <span class="pl-en">ctxCancel</span>()
<span class="pl-k">select</span> {
<span class="pl-k">case</span> <span class="pl-c1">&lt;-</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>):
<span class="pl-s1">fmt</span>.<span class="pl-en">Println</span>(<span class="pl-s">"doB"</span>)
<span class="pl-k">case</span> <span class="pl-c1">&lt;-</span><span class="pl-s1">ctx</span>.<span class="pl-en">Done</span>():
<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>())
}
}</pre>
</div>
<p>The output is:</p>
<div class="highlight highlight-text-adblock">
<pre>
doA context deadline exceeded
doB</pre
>
</div>
<p>
As you see only <code>doA</code> is canceled, <code>doB</code> is done
perfectly. And that what we want in this case.
</p>
chore: add ref
2023-06-10 13:54:19 +00:00
<h2>
<a
id="user-content-thanks"
class="anchor"
aria-hidden="true"
href="#thanks"
><span aria-hidden="true" class="octicon octicon-link"></span></a
>Thanks
</h2>
<ul>
<li>
<a
href="https://www.sohamkamani.com/golang/context-cancellation-and-values/"
rel="nofollow"
>Using Context in Golang - Cancellation, Timeouts and Values (With
Examples)</a
>
</li>
<li>
<a
href="https://uptrace.dev/blog/golang-context-timeout.html"
rel="nofollow"
>Go Context timeouts can be harmful</a
>
</li>
</ul>
feat: add incident #02
2023-06-10 07:15:38 +00:00
<div>
Feel free to ask me via
<a href="mailto:hauvipapro+posts@gmail.com">email</a> or
<a rel="me" href="https://hachyderm.io/@haunguyen">Mastodon</a>. Source
code is available on
<a href="https://github.com/haunt98/posts-go">GitHub</a>
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