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Provide some more background on Context

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And add a comment about the `Err()` method.

Followup on #301
This commit is contained in:
Eli Bendersky 2020-01-04 14:05:02 -08:00
parent d6b866d2a1
commit 5d8d89d246
3 changed files with 28 additions and 7 deletions
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7
examples/context/context.go
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@ -1,7 +1,9 @@
// In the previous example we looked at setting up a simple
// [HTTP server](http-servers). HTTP servers are useful for
// demonstrating the usage of `context.Context` for
// controlling cancellation.
// controlling cancellation. A `Context` carries deadlines,
// cancellation signals, and other request-scoped values
// across API boundaries and goroutines.
package main
import (
@ -28,6 +30,9 @@ func hello(w http.ResponseWriter, req *http.Request) {
case <-time.After(10 * time.Second):
fmt.Fprintf(w, "hello\n")
case <-ctx.Done():
// The context's `Err()` method returns an error
// that explains why the `Done()` channel was
// closed.
err := ctx.Err()
fmt.Println("server:", err)
internalError := http.StatusInternalServerError

4
examples/context/context.hash
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@ -1,2 +1,2 @@
5893bc5d4537db64f263a6be8efa9f2cda8ca7d8
Elok_mfA6GV
e338acce5d0f64d6478be4b886bd24a0fd1a3afa
Lun5aFco3pX

24
public/context generated
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@ -30,11 +30,13 @@
<p>In the previous example we looked at setting up a simple
<a href="http-servers">HTTP server</a>. HTTP servers are useful for
demonstrating the usage of <code>context.Context</code> for
controlling cancellation.</p>
controlling cancellation. A <code>Context</code> carries deadlines,
cancellation signals, and other request-scoped values
across API boundaries and goroutines.</p>
</td>
<td class="code leading">
<a href="http://play.golang.org/p/Elok_mfA6GV"><img title="Run code" src="play.png" class="run" /></a><img title="Copy code" src="clipboard.png" class="copy" />
<a href="http://play.golang.org/p/Lun5aFco3pX"><img title="Run code" src="play.png" class="run" /></a><img title="Copy code" src="clipboard.png" class="copy" />
<div class="highlight"><pre><span class="kn">package</span> <span class="nx">main</span>
</pre></div>
@ -101,7 +103,21 @@ the work and return as soon as possible.</p>
<span class="k">case</span> <span class="o">&lt;-</span><span class="nx">time</span><span class="p">.</span><span class="nx">After</span><span class="p">(</span><span class="mi">10</span> <span class="o">*</span> <span class="nx">time</span><span class="p">.</span><span class="nx">Second</span><span class="p">):</span>
<span class="nx">fmt</span><span class="p">.</span><span class="nx">Fprintf</span><span class="p">(</span><span class="nx">w</span><span class="p">,</span> <span class="s">&quot;hello\n&quot;</span><span class="p">)</span>
<span class="k">case</span> <span class="o">&lt;-</span><span class="nx">ctx</span><span class="p">.</span><span class="nx">Done</span><span class="p">():</span>
<span class="nx">err</span> <span class="o">:=</span> <span class="nx">ctx</span><span class="p">.</span><span class="nx">Err</span><span class="p">()</span>
</pre></div>
</td>
</tr>
<tr>
<td class="docs">
<p>The context&rsquo;s <code>Err()</code> method returns an error
that explains why the <code>Done()</code> channel was
closed.</p>
</td>
<td class="code leading">
<div class="highlight"><pre> <span class="nx">err</span> <span class="o">:=</span> <span class="nx">ctx</span><span class="p">.</span><span class="nx">Err</span><span class="p">()</span>
<span class="nx">fmt</span><span class="p">.</span><span class="nx">Println</span><span class="p">(</span><span class="s">&quot;server:&quot;</span><span class="p">,</span> <span class="nx">err</span><span class="p">)</span>
<span class="nx">internalError</span> <span class="o">:=</span> <span class="nx">http</span><span class="p">.</span><span class="nx">StatusInternalServerError</span>
<span class="nx">http</span><span class="p">.</span><span class="nx">Error</span><span class="p">(</span><span class="nx">w</span><span class="p">,</span> <span class="nx">err</span><span class="p">.</span><span class="nx">Error</span><span class="p">(),</span> <span class="nx">internalError</span><span class="p">)</span>
@ -189,7 +205,7 @@ cancellation.</p>
</div>
<script>
var codeLines = [];
codeLines.push('package main\u000A');codeLines.push('import (\u000A \"fmt\"\u000A \"net/http\"\u000A \"time\"\u000A)\u000A');codeLines.push('func hello(w http.ResponseWriter, req *http.Request) {\u000A');codeLines.push(' ctx := req.Context()\u000A fmt.Println(\"server: hello handler started\")\u000A defer fmt.Println(\"server: hello handler ended\")\u000A');codeLines.push(' select {\u000A case \x3C-time.After(10 * time.Second):\u000A fmt.Fprintf(w, \"hello\\n\")\u000A case \x3C-ctx.Done():\u000A err := ctx.Err()\u000A fmt.Println(\"server:\", err)\u000A internalError := http.StatusInternalServerError\u000A http.Error(w, err.Error(), internalError)\u000A }\u000A}\u000A');codeLines.push('func main() {\u000A');codeLines.push(' http.HandleFunc(\"/hello\", hello)\u000A http.ListenAndServe(\":8090\", nil)\u000A}\u000A');codeLines.push('');codeLines.push('');
codeLines.push('package main\u000A');codeLines.push('import (\u000A \"fmt\"\u000A \"net/http\"\u000A \"time\"\u000A)\u000A');codeLines.push('func hello(w http.ResponseWriter, req *http.Request) {\u000A');codeLines.push(' ctx := req.Context()\u000A fmt.Println(\"server: hello handler started\")\u000A defer fmt.Println(\"server: hello handler ended\")\u000A');codeLines.push(' select {\u000A case \x3C-time.After(10 * time.Second):\u000A fmt.Fprintf(w, \"hello\\n\")\u000A case \x3C-ctx.Done():\u000A');codeLines.push(' err := ctx.Err()\u000A fmt.Println(\"server:\", err)\u000A internalError := http.StatusInternalServerError\u000A http.Error(w, err.Error(), internalError)\u000A }\u000A}\u000A');codeLines.push('func main() {\u000A');codeLines.push(' http.HandleFunc(\"/hello\", hello)\u000A http.ListenAndServe(\":8090\", nil)\u000A}\u000A');codeLines.push('');codeLines.push('');
</script>
<script src="site.js" async></script>
</body>