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Hana 9e216da9ef go.mod: add go.mod and move pygments to third_party 
After go1.16, go will use module mode by default,
even when the repository is checked out under GOPATH
or in a one-off directory. Add go.mod, go.sum to keep
this repo buildable without opting out of the module
mode.

> go mod init github.com/mmcgrana/gobyexample
> go mod tidy
> go mod vendor

In module mode, the 'vendor' directory is special
and its contents will be actively maintained by the
go command. pygments aren't the dependency the go will
know about, so it will delete the contents from vendor
directory. Move it to `third_party` directory now.

And, vendor the blackfriday package.

Note: the tutorial contents are not affected by the
change in go1.16 because all the examples in this
tutorial ask users to run the go command with the
explicit list of files to be compiled (e.g.
`go run hello-world.go` or `go build command-line-arguments.go`).
When the source list is provided, the go command does
not have to compute the build list and whether it's
running in GOPATH mode or module mode becomes irrelevant.
2021-02-15 16:45:26 -05:00

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<!DOCTYPE html>
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<meta charset="utf-8">
<title>Go by Example: Mutexes</title>
<link rel=stylesheet href="site.css">
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<script>
onkeydown = (e) => {
if (e.key == "ArrowLeft") {
window.location.href = 'atomic-counters';
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<body>
<div class="example" id="mutexes">
<h2><a href="./">Go by Example</a>: Mutexes</h2>
<table>
<tr>
<td class="docs">
<p>In the previous example we saw how to manage simple
counter state using <a href="atomic-counters">atomic operations</a>.
For more complex state we can use a <em><a href="http://en.wikipedia.org/wiki/Mutual_exclusion">mutex</a></em>
to safely access data across multiple goroutines.</p>
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<td class="docs">
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<td class="code leading">
<a href="http://play.golang.org/p/0WEmOOjoCjp"><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>
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<td class="docs">
</td>
<td class="code leading">
<div class="highlight"><pre><span class="kn">import</span> <span class="p">(</span>
<span class="s">&quot;fmt&quot;</span>
<span class="s">&quot;math/rand&quot;</span>
<span class="s">&quot;sync&quot;</span>
<span class="s">&quot;sync/atomic&quot;</span>
<span class="s">&quot;time&quot;</span>
<span class="p">)</span>
</pre></div>
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<td class="docs">
</td>
<td class="code leading">
<div class="highlight"><pre><span class="kd">func</span> <span class="nx">main</span><span class="p">()</span> <span class="p">{</span>
</pre></div>
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</tr>
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<td class="docs">
<p>For our example the <code>state</code> will be a map.</p>
</td>
<td class="code leading">
<div class="highlight"><pre> <span class="kd">var</span> <span class="nx">state</span> <span class="p">=</span> <span class="nb">make</span><span class="p">(</span><span class="kd">map</span><span class="p">[</span><span class="kt">int</span><span class="p">]</span><span class="kt">int</span><span class="p">)</span>
</pre></div>
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<td class="docs">
<p>This <code>mutex</code> will synchronize access to <code>state</code>.</p>
</td>
<td class="code leading">
<div class="highlight"><pre> <span class="kd">var</span> <span class="nx">mutex</span> <span class="p">=</span> <span class="o">&amp;</span><span class="nx">sync</span><span class="p">.</span><span class="nx">Mutex</span><span class="p">{}</span>
</pre></div>
</td>
</tr>
<tr>
<td class="docs">
<p>We&rsquo;ll keep track of how many read and write
operations we do.</p>
</td>
<td class="code leading">
<div class="highlight"><pre> <span class="kd">var</span> <span class="nx">readOps</span> <span class="kt">uint64</span>
<span class="kd">var</span> <span class="nx">writeOps</span> <span class="kt">uint64</span>
</pre></div>
</td>
</tr>
<tr>
<td class="docs">
<p>Here we start 100 goroutines to execute repeated
reads against the state, once per millisecond in
each goroutine.</p>
</td>
<td class="code leading">
<div class="highlight"><pre> <span class="k">for</span> <span class="nx">r</span> <span class="o">:=</span> <span class="mi">0</span><span class="p">;</span> <span class="nx">r</span> <span class="p">&lt;</span> <span class="mi">100</span><span class="p">;</span> <span class="nx">r</span><span class="o">++</span> <span class="p">{</span>
<span class="k">go</span> <span class="kd">func</span><span class="p">()</span> <span class="p">{</span>
<span class="nx">total</span> <span class="o">:=</span> <span class="mi">0</span>
<span class="k">for</span> <span class="p">{</span>
</pre></div>
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<td class="docs">
<p>For each read we pick a key to access,
<code>Lock()</code> the <code>mutex</code> to ensure
exclusive access to the <code>state</code>, read
the value at the chosen key,
<code>Unlock()</code> the mutex, and increment
the <code>readOps</code> count.</p>
</td>
<td class="code leading">
<div class="highlight"><pre> <span class="nx">key</span> <span class="o">:=</span> <span class="nx">rand</span><span class="p">.</span><span class="nx">Intn</span><span class="p">(</span><span class="mi">5</span><span class="p">)</span>
<span class="nx">mutex</span><span class="p">.</span><span class="nx">Lock</span><span class="p">()</span>
<span class="nx">total</span> <span class="o">+=</span> <span class="nx">state</span><span class="p">[</span><span class="nx">key</span><span class="p">]</span>
<span class="nx">mutex</span><span class="p">.</span><span class="nx">Unlock</span><span class="p">()</span>
<span class="nx">atomic</span><span class="p">.</span><span class="nx">AddUint64</span><span class="p">(</span><span class="o">&amp;</span><span class="nx">readOps</span><span class="p">,</span> <span class="mi">1</span><span class="p">)</span>
</pre></div>
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<td class="docs">
<p>Wait a bit between reads.</p>
</td>
<td class="code leading">
<div class="highlight"><pre> <span class="nx">time</span><span class="p">.</span><span class="nx">Sleep</span><span class="p">(</span><span class="nx">time</span><span class="p">.</span><span class="nx">Millisecond</span><span class="p">)</span>
<span class="p">}</span>
<span class="p">}()</span>
<span class="p">}</span>
</pre></div>
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<td class="docs">
<p>We&rsquo;ll also start 10 goroutines to simulate writes,
using the same pattern we did for reads.</p>
</td>
<td class="code leading">
<div class="highlight"><pre> <span class="k">for</span> <span class="nx">w</span> <span class="o">:=</span> <span class="mi">0</span><span class="p">;</span> <span class="nx">w</span> <span class="p">&lt;</span> <span class="mi">10</span><span class="p">;</span> <span class="nx">w</span><span class="o">++</span> <span class="p">{</span>
<span class="k">go</span> <span class="kd">func</span><span class="p">()</span> <span class="p">{</span>
<span class="k">for</span> <span class="p">{</span>
<span class="nx">key</span> <span class="o">:=</span> <span class="nx">rand</span><span class="p">.</span><span class="nx">Intn</span><span class="p">(</span><span class="mi">5</span><span class="p">)</span>
<span class="nx">val</span> <span class="o">:=</span> <span class="nx">rand</span><span class="p">.</span><span class="nx">Intn</span><span class="p">(</span><span class="mi">100</span><span class="p">)</span>
<span class="nx">mutex</span><span class="p">.</span><span class="nx">Lock</span><span class="p">()</span>
<span class="nx">state</span><span class="p">[</span><span class="nx">key</span><span class="p">]</span> <span class="p">=</span> <span class="nx">val</span>
<span class="nx">mutex</span><span class="p">.</span><span class="nx">Unlock</span><span class="p">()</span>
<span class="nx">atomic</span><span class="p">.</span><span class="nx">AddUint64</span><span class="p">(</span><span class="o">&amp;</span><span class="nx">writeOps</span><span class="p">,</span> <span class="mi">1</span><span class="p">)</span>
<span class="nx">time</span><span class="p">.</span><span class="nx">Sleep</span><span class="p">(</span><span class="nx">time</span><span class="p">.</span><span class="nx">Millisecond</span><span class="p">)</span>
<span class="p">}</span>
<span class="p">}()</span>
<span class="p">}</span>
</pre></div>
</td>
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<td class="docs">
<p>Let the 10 goroutines work on the <code>state</code> and
<code>mutex</code> for a second.</p>
</td>
<td class="code leading">
<div class="highlight"><pre> <span class="nx">time</span><span class="p">.</span><span class="nx">Sleep</span><span class="p">(</span><span class="nx">time</span><span class="p">.</span><span class="nx">Second</span><span class="p">)</span>
</pre></div>
</td>
</tr>
<tr>
<td class="docs">
<p>Take and report final operation counts.</p>
</td>
<td class="code leading">
<div class="highlight"><pre> <span class="nx">readOpsFinal</span> <span class="o">:=</span> <span class="nx">atomic</span><span class="p">.</span><span class="nx">LoadUint64</span><span class="p">(</span><span class="o">&amp;</span><span class="nx">readOps</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;readOps:&quot;</span><span class="p">,</span> <span class="nx">readOpsFinal</span><span class="p">)</span>
<span class="nx">writeOpsFinal</span> <span class="o">:=</span> <span class="nx">atomic</span><span class="p">.</span><span class="nx">LoadUint64</span><span class="p">(</span><span class="o">&amp;</span><span class="nx">writeOps</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;writeOps:&quot;</span><span class="p">,</span> <span class="nx">writeOpsFinal</span><span class="p">)</span>
</pre></div>
</td>
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<tr>
<td class="docs">
<p>With a final lock of <code>state</code>, show how it ended up.</p>
</td>
<td class="code">
<div class="highlight"><pre> <span class="nx">mutex</span><span class="p">.</span><span class="nx">Lock</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;state:&quot;</span><span class="p">,</span> <span class="nx">state</span><span class="p">)</span>
<span class="nx">mutex</span><span class="p">.</span><span class="nx">Unlock</span><span class="p">()</span>
<span class="p">}</span>
</pre></div>
</td>
</tr>
</table>
<table>
<tr>
<td class="docs">
<p>Running the program shows that we executed about
90,000 total operations against our <code>mutex</code>-synchronized
<code>state</code>.</p>
</td>
<td class="code leading">
<div class="highlight"><pre><span class="gp">$</span> go run mutexes.go
<span class="go">readOps: 83285</span>
<span class="go">writeOps: 8320</span>
<span class="go">state: map[1:97 4:53 0:33 2:15 3:2]</span>
</pre></div>
</td>
</tr>
<tr>
<td class="docs">
<p>Next we&rsquo;ll look at implementing this same state
management task using only goroutines and channels.</p>
</td>
<td class="code empty">
</td>
</tr>
</table>
<p class="next">
Next example: <a href="stateful-goroutines">Stateful Goroutines</a>.
</p>
<p class="footer">
by <a href="https://markmcgranaghan.com">Mark McGranaghan</a> | <a href="https://github.com/mmcgrana/gobyexample/blob/master/examples/mutexes">source</a> | <a href="https://github.com/mmcgrana/gobyexample#license">license</a>
</p>
</div>
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