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<meta name="description" content="等待组 - sync.WaitGroup # 源码分析 # type WaitGroup struct { noCopy noCopy // waitgroup是不能够拷贝复制的，是通过go vet来检测实现 /* waitgroup使用一个int64来计数：高32位，用来add计数，低32位用来记录waiter数量。 若要原子性更新int64就必须保证该int64对齐系数是8，即64位对齐。 对于64位系统，直接使用一个int64类型字段就能保证原子性要求，但对32位系统就不行了。 所以实现的时候并没有直接一个int64， 而是使用[3]int32数组，若[0]int32地址恰好是8对齐的，那就waitgroup int64 = [0]int32 &#43; [1]int32， 否则一定是4对齐的， 故[0]int32不用，恰好错开了4字节，此时[1]int32一定是8对齐的。此时waitgroup int64 = [1]int32 &#43; [2]int32 通过这个技巧恰好满足32位和64位系统下int64都能原子性操作 */ state1 [3]uint32 // waitgroup对齐系数是4 } func (wg *WaitGroup) state() (statep *uint64, semap *uint32) { // 当state1是8对齐的，则返回低8字节(statep)用来计数，即state1[0]是add计数，state1[1]是waiter计数 if uintptr(unsafe.Pointer(&amp;wg.state1))%8 == 0 { return (*uint64)(unsafe.Pointer(&amp;wg.state1)), &amp;wg.state1[2] } else { // 反之，则返回高8字节用来计数，即state1[1]是add计数，state1[2]是waiter计数 return (*uint64)(unsafe.Pointer(&amp;wg.state1[1])), &amp;wg.state1[0] } } // Add方法用来更新add计数器。即将原来计数值加上delta，delta可以为负值 // waitgroup的Done方法本质上就是Add(-1) // Add更新之后的计数器值不能小于0。当计数器值等于0时候，会释放信号，所有调用Wait方法而阻塞的Goroutine不再阻塞（释放的信号量=waiter计数） func (wg *WaitGroup) Add(delta int) { statep, semap := wg.">
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<meta property="og:description" content="等待组 - sync.WaitGroup # 源码分析 # type WaitGroup struct { noCopy noCopy // waitgroup是不能够拷贝复制的，是通过go vet来检测实现 /* waitgroup使用一个int64来计数：高32位，用来add计数，低32位用来记录waiter数量。 若要原子性更新int64就必须保证该int64对齐系数是8，即64位对齐。 对于64位系统，直接使用一个int64类型字段就能保证原子性要求，但对32位系统就不行了。 所以实现的时候并没有直接一个int64， 而是使用[3]int32数组，若[0]int32地址恰好是8对齐的，那就waitgroup int64 = [0]int32 &#43; [1]int32， 否则一定是4对齐的， 故[0]int32不用，恰好错开了4字节，此时[1]int32一定是8对齐的。此时waitgroup int64 = [1]int32 &#43; [2]int32 通过这个技巧恰好满足32位和64位系统下int64都能原子性操作 */ state1 [3]uint32 // waitgroup对齐系数是4 } func (wg *WaitGroup) state() (statep *uint64, semap *uint32) { // 当state1是8对齐的，则返回低8字节(statep)用来计数，即state1[0]是add计数，state1[1]是waiter计数 if uintptr(unsafe.Pointer(&amp;wg.state1))%8 == 0 { return (*uint64)(unsafe.Pointer(&amp;wg.state1)), &amp;wg.state1[2] } else { // 反之，则返回高8字节用来计数，即state1[1]是add计数，state1[2]是waiter计数 return (*uint64)(unsafe.Pointer(&amp;wg.state1[1])), &amp;wg.state1[0] } } // Add方法用来更新add计数器。即将原来计数值加上delta，delta可以为负值 // waitgroup的Done方法本质上就是Add(-1) // Add更新之后的计数器值不能小于0。当计数器值等于0时候，会释放信号，所有调用Wait方法而阻塞的Goroutine不再阻塞（释放的信号量=waiter计数） func (wg *WaitGroup) Add(delta int) { statep, semap := wg." />
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<article class="markdown"><h1 id="等待组---syncwaitgroup">
等待组 - sync.WaitGroup
<a class="anchor" href="#%e7%ad%89%e5%be%85%e7%bb%84---syncwaitgroup">#</a>
</h1>
<h2 id="源码分析">
源码分析
<a class="anchor" href="#%e6%ba%90%e7%a0%81%e5%88%86%e6%9e%90">#</a>
</h2>
<div class="highlight"><pre tabindex="0" style="color:#f8f8f2;background-color:#272822;-moz-tab-size:4;-o-tab-size:4;tab-size:4;"><code class="language-go" data-lang="go"><span style="display:flex;"><span><span style="color:#66d9ef">type</span> <span style="color:#a6e22e">WaitGroup</span> <span style="color:#66d9ef">struct</span> {
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">noCopy</span> <span style="color:#a6e22e">noCopy</span> <span style="color:#75715e">// waitgroup是不能够拷贝复制的，是通过go vet来检测实现
</span></span></span><span style="display:flex;"><span><span style="color:#75715e"></span>
</span></span><span style="display:flex;"><span> <span style="color:#75715e">/*
</span></span></span><span style="display:flex;"><span><span style="color:#75715e"> waitgroup使用一个int64来计数：高32位，用来add计数，低32位用来记录waiter数量。
</span></span></span><span style="display:flex;"><span><span style="color:#75715e"> 若要原子性更新int64就必须保证该int64对齐系数是8，即64位对齐。
</span></span></span><span style="display:flex;"><span><span style="color:#75715e"> 对于64位系统，直接使用一个int64类型字段就能保证原子性要求，但对32位系统就不行了。
</span></span></span><span style="display:flex;"><span><span style="color:#75715e">
</span></span></span><span style="display:flex;"><span><span style="color:#75715e"> 所以实现的时候并没有直接一个int64， 而是使用[3]int32数组，若[0]int32地址恰好是8对齐的，那就waitgroup int64 = [0]int32 + [1]int32，
</span></span></span><span style="display:flex;"><span><span style="color:#75715e"> 否则一定是4对齐的， 故[0]int32不用，恰好错开了4字节，此时[1]int32一定是8对齐的。此时waitgroup int64 = [1]int32 + [2]int32
</span></span></span><span style="display:flex;"><span><span style="color:#75715e"> 通过这个技巧恰好满足32位和64位系统下int64都能原子性操作
</span></span></span><span style="display:flex;"><span><span style="color:#75715e"> */</span>
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">state1</span> [<span style="color:#ae81ff">3</span>]<span style="color:#66d9ef">uint32</span> <span style="color:#75715e">// waitgroup对齐系数是4
</span></span></span><span style="display:flex;"><span><span style="color:#75715e"></span>}
</span></span><span style="display:flex;"><span>
</span></span><span style="display:flex;"><span><span style="color:#66d9ef">func</span> (<span style="color:#a6e22e">wg</span> <span style="color:#f92672">*</span><span style="color:#a6e22e">WaitGroup</span>) <span style="color:#a6e22e">state</span>() (<span style="color:#a6e22e">statep</span> <span style="color:#f92672">*</span><span style="color:#66d9ef">uint64</span>, <span style="color:#a6e22e">semap</span> <span style="color:#f92672">*</span><span style="color:#66d9ef">uint32</span>) {
</span></span><span style="display:flex;"><span> <span style="color:#75715e">// 当state1是8对齐的，则返回低8字节(statep)用来计数，即state1[0]是add计数，state1[1]是waiter计数
</span></span></span><span style="display:flex;"><span><span style="color:#75715e"></span> <span style="color:#66d9ef">if</span> uintptr(<span style="color:#a6e22e">unsafe</span>.<span style="color:#a6e22e">Pointer</span>(<span style="color:#f92672">&amp;</span><span style="color:#a6e22e">wg</span>.<span style="color:#a6e22e">state1</span>))<span style="color:#f92672">%</span><span style="color:#ae81ff">8</span> <span style="color:#f92672">==</span> <span style="color:#ae81ff">0</span> {
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">return</span> (<span style="color:#f92672">*</span><span style="color:#66d9ef">uint64</span>)(<span style="color:#a6e22e">unsafe</span>.<span style="color:#a6e22e">Pointer</span>(<span style="color:#f92672">&amp;</span><span style="color:#a6e22e">wg</span>.<span style="color:#a6e22e">state1</span>)), <span style="color:#f92672">&amp;</span><span style="color:#a6e22e">wg</span>.<span style="color:#a6e22e">state1</span>[<span style="color:#ae81ff">2</span>]
</span></span><span style="display:flex;"><span> } <span style="color:#66d9ef">else</span> {
</span></span><span style="display:flex;"><span> <span style="color:#75715e">// 反之，则返回高8字节用来计数，即state1[1]是add计数，state1[2]是waiter计数
</span></span></span><span style="display:flex;"><span><span style="color:#75715e"></span> <span style="color:#66d9ef">return</span> (<span style="color:#f92672">*</span><span style="color:#66d9ef">uint64</span>)(<span style="color:#a6e22e">unsafe</span>.<span style="color:#a6e22e">Pointer</span>(<span style="color:#f92672">&amp;</span><span style="color:#a6e22e">wg</span>.<span style="color:#a6e22e">state1</span>[<span style="color:#ae81ff">1</span>])), <span style="color:#f92672">&amp;</span><span style="color:#a6e22e">wg</span>.<span style="color:#a6e22e">state1</span>[<span style="color:#ae81ff">0</span>]
</span></span><span style="display:flex;"><span> }
</span></span><span style="display:flex;"><span>}
</span></span><span style="display:flex;"><span>
</span></span><span style="display:flex;"><span><span style="color:#75715e">// Add方法用来更新add计数器。即将原来计数值加上delta，delta可以为负值
</span></span></span><span style="display:flex;"><span><span style="color:#75715e">// waitgroup的Done方法本质上就是Add(-1)
</span></span></span><span style="display:flex;"><span><span style="color:#75715e">// Add更新之后的计数器值不能小于0。当计数器值等于0时候，会释放信号，所有调用Wait方法而阻塞的Goroutine不再阻塞（释放的信号量=waiter计数）
</span></span></span><span style="display:flex;"><span><span style="color:#75715e"></span><span style="color:#66d9ef">func</span> (<span style="color:#a6e22e">wg</span> <span style="color:#f92672">*</span><span style="color:#a6e22e">WaitGroup</span>) <span style="color:#a6e22e">Add</span>(<span style="color:#a6e22e">delta</span> <span style="color:#66d9ef">int</span>) {
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">statep</span>, <span style="color:#a6e22e">semap</span> <span style="color:#f92672">:=</span> <span style="color:#a6e22e">wg</span>.<span style="color:#a6e22e">state</span>()
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">if</span> <span style="color:#a6e22e">race</span>.<span style="color:#a6e22e">Enabled</span> { <span style="color:#75715e">// 竞态检查，忽略不看
</span></span></span><span style="display:flex;"><span><span style="color:#75715e"></span> <span style="color:#a6e22e">_</span> = <span style="color:#f92672">*</span><span style="color:#a6e22e">statep</span> <span style="color:#75715e">// trigger nil deref early
</span></span></span><span style="display:flex;"><span><span style="color:#75715e"></span> <span style="color:#66d9ef">if</span> <span style="color:#a6e22e">delta</span> &lt; <span style="color:#ae81ff">0</span> {
</span></span><span style="display:flex;"><span> <span style="color:#75715e">// Synchronize decrements with Wait.
</span></span></span><span style="display:flex;"><span><span style="color:#75715e"></span> <span style="color:#a6e22e">race</span>.<span style="color:#a6e22e">ReleaseMerge</span>(<span style="color:#a6e22e">unsafe</span>.<span style="color:#a6e22e">Pointer</span>(<span style="color:#a6e22e">wg</span>))
</span></span><span style="display:flex;"><span> }
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">race</span>.<span style="color:#a6e22e">Disable</span>()
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">defer</span> <span style="color:#a6e22e">race</span>.<span style="color:#a6e22e">Enable</span>()
</span></span><span style="display:flex;"><span> }
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">state</span> <span style="color:#f92672">:=</span> <span style="color:#a6e22e">atomic</span>.<span style="color:#a6e22e">AddUint64</span>(<span style="color:#a6e22e">statep</span>, uint64(<span style="color:#a6e22e">delta</span>)<span style="color:#f92672">&lt;&lt;</span><span style="color:#ae81ff">32</span>) <span style="color:#75715e">// delta左移32位，然后原子性更新statep值并返回更新后的statep值
</span></span></span><span style="display:flex;"><span><span style="color:#75715e"></span> <span style="color:#a6e22e">v</span> <span style="color:#f92672">:=</span> int32(<span style="color:#a6e22e">state</span> <span style="color:#f92672">&gt;&gt;</span> <span style="color:#ae81ff">32</span>) <span style="color:#75715e">// state高位的4字节是add计数，赋值给v
</span></span></span><span style="display:flex;"><span><span style="color:#75715e"></span> <span style="color:#a6e22e">w</span> <span style="color:#f92672">:=</span> uint32(<span style="color:#a6e22e">state</span>) <span style="color:#75715e">// state低位的4字节是waiter计数，赋值给w
</span></span></span><span style="display:flex;"><span><span style="color:#75715e"></span>
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">if</span> <span style="color:#a6e22e">v</span> &lt; <span style="color:#ae81ff">0</span> { <span style="color:#75715e">// add计数不能为负值。
</span></span></span><span style="display:flex;"><span><span style="color:#75715e"></span> panic(<span style="color:#e6db74">&#34;sync: negative WaitGroup counter&#34;</span>)
</span></span><span style="display:flex;"><span> }
</span></span><span style="display:flex;"><span>
</span></span><span style="display:flex;"><span> <span style="color:#75715e">// Add方法与Wait方法不能并发调用
</span></span></span><span style="display:flex;"><span><span style="color:#75715e"></span> <span style="color:#66d9ef">if</span> <span style="color:#a6e22e">w</span> <span style="color:#f92672">!=</span> <span style="color:#ae81ff">0</span> <span style="color:#f92672">&amp;&amp;</span> <span style="color:#a6e22e">delta</span> &gt; <span style="color:#ae81ff">0</span> <span style="color:#f92672">&amp;&amp;</span> <span style="color:#a6e22e">v</span> <span style="color:#f92672">==</span> int32(<span style="color:#a6e22e">delta</span>) {
</span></span><span style="display:flex;"><span> panic(<span style="color:#e6db74">&#34;sync: WaitGroup misuse: Add called concurrently with Wait&#34;</span>)
</span></span><span style="display:flex;"><span> }
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">if</span> <span style="color:#a6e22e">v</span> &gt; <span style="color:#ae81ff">0</span> <span style="color:#f92672">||</span> <span style="color:#a6e22e">w</span> <span style="color:#f92672">==</span> <span style="color:#ae81ff">0</span> { <span style="color:#75715e">// add计数大于0，或者waiter计数等于0，直接返回不执行后面逻辑。
</span></span></span><span style="display:flex;"><span><span style="color:#75715e"></span> <span style="color:#66d9ef">return</span>
</span></span><span style="display:flex;"><span> }
</span></span><span style="display:flex;"><span>
</span></span><span style="display:flex;"><span> <span style="color:#75715e">// statep指向state1字段，其指向的值和state进行比较，如果不一样，说明存在并发调用了Add和Wait方法
</span></span></span><span style="display:flex;"><span><span style="color:#75715e"></span> <span style="color:#75715e">// 此时v = 0, w &gt; 0，这个时候waitgroup的add计数和waiter计数不能再更改了。
</span></span></span><span style="display:flex;"><span><span style="color:#75715e"></span> <span style="color:#75715e">// *statep != state情况举例：假定当前groutine是g1，执行到此处时,
</span></span></span><span style="display:flex;"><span><span style="color:#75715e"></span> <span style="color:#75715e">// 恰好另外一个groutine g2并发调用了Wait方法，
</span></span></span><span style="display:flex;"><span><span style="color:#75715e"></span> <span style="color:#75715e">// 那么waitgroup的state1字段会更新，而g1中w的值还是g2调用Wait方法之前的waiter数，
</span></span></span><span style="display:flex;"><span><span style="color:#75715e"></span> <span style="color:#75715e">// 这会导致总有一个g永远得不到释放信号，从而造成g泄漏。所以此处要进行panic判断
</span></span></span><span style="display:flex;"><span><span style="color:#75715e"></span> <span style="color:#66d9ef">if</span> <span style="color:#f92672">*</span><span style="color:#a6e22e">statep</span> <span style="color:#f92672">!=</span> <span style="color:#a6e22e">state</span> {
</span></span><span style="display:flex;"><span> panic(<span style="color:#e6db74">&#34;sync: WaitGroup misuse: Add called concurrently with Wait&#34;</span>)
</span></span><span style="display:flex;"><span> }
</span></span><span style="display:flex;"><span>
</span></span><span style="display:flex;"><span> <span style="color:#f92672">*</span><span style="color:#a6e22e">statep</span> = <span style="color:#ae81ff">0</span> <span style="color:#75715e">// 重置计数器为0
</span></span></span><span style="display:flex;"><span><span style="color:#75715e"></span> <span style="color:#66d9ef">for</span> ; <span style="color:#a6e22e">w</span> <span style="color:#f92672">!=</span> <span style="color:#ae81ff">0</span>; <span style="color:#a6e22e">w</span><span style="color:#f92672">--</span> { <span style="color:#75715e">// 有w个waiter，则释放出w个信号
</span></span></span><span style="display:flex;"><span><span style="color:#75715e"></span> <span style="color:#a6e22e">runtime_Semrelease</span>(<span style="color:#a6e22e">semap</span>, <span style="color:#66d9ef">false</span>, <span style="color:#ae81ff">0</span>)
</span></span><span style="display:flex;"><span> }
</span></span><span style="display:flex;"><span>}
</span></span><span style="display:flex;"><span>
</span></span><span style="display:flex;"><span><span style="color:#75715e">// Done() == Add(-1)
</span></span></span><span style="display:flex;"><span><span style="color:#75715e"></span><span style="color:#66d9ef">func</span> (<span style="color:#a6e22e">wg</span> <span style="color:#f92672">*</span><span style="color:#a6e22e">WaitGroup</span>) <span style="color:#a6e22e">Done</span>() {
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">wg</span>.<span style="color:#a6e22e">Add</span>(<span style="color:#f92672">-</span><span style="color:#ae81ff">1</span>)
</span></span><span style="display:flex;"><span>}
</span></span><span style="display:flex;"><span>
</span></span><span style="display:flex;"><span><span style="color:#75715e">// Wait会阻塞当前goroutine，直到add计数器值为0
</span></span></span><span style="display:flex;"><span><span style="color:#75715e"></span><span style="color:#66d9ef">func</span> (<span style="color:#a6e22e">wg</span> <span style="color:#f92672">*</span><span style="color:#a6e22e">WaitGroup</span>) <span style="color:#a6e22e">Wait</span>() {
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">statep</span>, <span style="color:#a6e22e">semap</span> <span style="color:#f92672">:=</span> <span style="color:#a6e22e">wg</span>.<span style="color:#a6e22e">state</span>()
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">for</span> {
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">state</span> <span style="color:#f92672">:=</span> <span style="color:#a6e22e">atomic</span>.<span style="color:#a6e22e">LoadUint64</span>(<span style="color:#a6e22e">statep</span>)
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">v</span> <span style="color:#f92672">:=</span> int32(<span style="color:#a6e22e">state</span> <span style="color:#f92672">&gt;&gt;</span> <span style="color:#ae81ff">32</span>)
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">w</span> <span style="color:#f92672">:=</span> uint32(<span style="color:#a6e22e">state</span>)
</span></span><span style="display:flex;"><span> <span style="color:#75715e">// 使用for + cas进制，原子性更新waiter计数
</span></span></span><span style="display:flex;"><span><span style="color:#75715e"></span> <span style="color:#66d9ef">if</span> <span style="color:#a6e22e">atomic</span>.<span style="color:#a6e22e">CompareAndSwapUint64</span>(<span style="color:#a6e22e">statep</span>, <span style="color:#a6e22e">state</span>, <span style="color:#a6e22e">state</span><span style="color:#f92672">+</span><span style="color:#ae81ff">1</span>) {
</span></span><span style="display:flex;"><span> <span style="color:#75715e">// 更新成功后，开始获取信号，未获取到信号的话则当前g一直阻塞
</span></span></span><span style="display:flex;"><span><span style="color:#75715e"></span> <span style="color:#a6e22e">runtime_Semacquire</span>(<span style="color:#a6e22e">semap</span>)
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">if</span> <span style="color:#f92672">*</span><span style="color:#a6e22e">statep</span> <span style="color:#f92672">!=</span> <span style="color:#ae81ff">0</span> {
</span></span><span style="display:flex;"><span> panic(<span style="color:#e6db74">&#34;sync: WaitGroup is reused before previous Wait has returned&#34;</span>)
</span></span><span style="display:flex;"><span> }
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">return</span>
</span></span><span style="display:flex;"><span> }
</span></span><span style="display:flex;"><span> }
</span></span><span style="display:flex;"><span>}
</span></span></code></pre></div><h3 id="总结">
总结
<a class="anchor" href="#%e6%80%bb%e7%bb%93">#</a>
</h3>
<ul>
<li>waitgroup是不能值传递的</li>
<li>Add方法的传值可以是负数，但加上该传值之后的waitgroup计数器值不能是负值</li>
<li>Done方法实际上调用的是Add(-1)</li>
<li>Add方法和Wait方法不能并发调用</li>
<li>Wait方法可以多次调用，调用此方法的goroutine会阻塞，一直阻塞到waitgroup计数器值变为0。</li>
</ul>
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