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<main>
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<h1 id="线程间传递消息导致主线程无法结束"><a class="header" href="#线程间传递消息导致主线程无法结束">线程间传递消息导致主线程无法结束</a></h1>
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<p>本篇陷阱较短，主要解决新手在多线程间传递消息时可能会遇到的一个问题：主线程会一直阻塞，无法结束。</p>
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<p>Rust 标准库中提供了一个消息通道，非常好用，也相当简单明了，但是但是在使用起来还是可能存在坑：</p>
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<pre><pre class="playground"><code class="language-rust edition2021">use std::sync::mpsc;
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fn main() {
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use std::thread;
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let (send, recv) = mpsc::channel();
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let num_threads = 3;
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for i in 0..num_threads {
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let thread_send = send.clone();
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thread::spawn(move || {
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thread_send.send(i).unwrap();
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println!("thread {:?} finished", i);
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});
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}
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for x in recv {
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println!("Got: {}", x);
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}
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println!("finished iterating");
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}</code></pre></pre>
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<p>以上代码看起来非常正常，运行下试试:</p>
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<pre><code class="language-console">thread 0 finished
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thread 1 finished
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Got: 0
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Got: 1
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thread 2 finished
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Got: 2
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</code></pre>
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<p>奇怪，主线程竟然卡死了，最后一行<code> println!("finished iterating");</code>一直没有被输出。</p>
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<p>其实，上面的描述有问题，主线程并不是卡死，而是<code>for</code>循环并没有结束，至于<code>for</code>循环不结束的原因是消息通道没有被关闭。</p>
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<p>回忆一下 Rust 消息通道关闭的两个条件：所有发送者全部被<code>drop</code>或接收者被<code>drop</code>，由于<code>for</code>循环还在使用接收者，因为后者条件无法被满足，那么只能发送者全部被<code>drop</code>，才能让例子中的消息通道关闭。</p>
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<p>来分析下代码，每一个子线程都从<code>send</code>获取了一个拷贝，然后该拷贝在子线程结束时自动被<code>drop</code>，看上去没问题啊。等等，好像<code>send</code>本身并没有被<code>drop</code>，因为<code>send</code>要等到<code>main</code>函数结束才会被<code>drop</code>，那么代码就陷入了一个尴尬的境地：<code>main</code>函数要结束需要<code>for</code>循环结束，<code>for</code>循环结束需要<code>send</code>被<code>drop</code>，而<code>send</code>要被<code>drop</code>需要<code>main</code>函数结束。。。</p>
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<p>破局点只有一个，那就是主动<code>drop</code>掉<code>send</code>，这个简单，使用<code>std::mem::drop</code>函数即可，得益于<code>prelude</code>，我们只需要使用<code>drop</code>:</p>
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<pre><pre class="playground"><code class="language-rust edition2021">use std::sync::mpsc;
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fn main() {
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use std::thread;
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let (send, recv) = mpsc::channel();
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let num_threads = 3;
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for i in 0..num_threads {
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let thread_send = send.clone();
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thread::spawn(move || {
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thread_send.send(i).unwrap();
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println!("thread {:?} finished", i);
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});
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}
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drop(send);
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for x in recv {
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println!("Got: {}", x);
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}
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println!("finished iterating");
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}</code></pre></pre>
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<p>此时再运行，主线程将顺利结束。</p>
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<h2 id="总结"><a class="header" href="#总结">总结</a></h2>
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<p>本文总结了一个新手在使用消息通道时常见的错误，那就是忘记处理创建通道时得到的发送者，最后由于该发送者的存活导致通道无法被关闭，最终主线程阻塞，造成程序错误。</p>
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