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deploy: d14d1d0a9cf38240bfa93c6f2a5d961f306145a6
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<li><em>Go语言圣经读书笔记(挖坑中...): <a href="https://github.com/chai2010/gopl-notes-zh">https://github.com/chai2010/gopl-notes-zh</a></em></li>
<li><em>凹语言(Go实现, 面向WASM设计): <a href="https://github.com/wa-lang/wa">https://github.com/wa-lang/wa</a></em></li>
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<hr>
<h2 id="510-recover捕获异常"><a class="header" href="#510-recover捕获异常">5.10. Recover捕获异常</a></h2>
<p>通常来说，不应该对panic异常做任何处理，但有时，也许我们可以从异常中恢复，至少我们可以在程序崩溃前，做一些操作。举个例子，当web服务器遇到不可预料的严重问题时，在崩溃前应该将所有的连接关闭；如果不做任何处理，会使得客户端一直处于等待状态。如果web服务器还在开发阶段，服务器甚至可以将异常信息反馈到客户端，帮助调试。</p>
<p>如果在deferred函数中调用了内置函数recover，并且定义该defer语句的函数发生了panic异常，recover会使程序从panic中恢复，并返回panic value。导致panic异常的函数不会继续运行，但能正常返回。在未发生panic时调用recover，recover会返回nil。</p>
<p>让我们以语言解析器为例，说明recover的使用场景。考虑到语言解析器的复杂性，即使某个语言解析器目前工作正常，也无法肯定它没有漏洞。因此，当某个异常出现时，我们不会选择让解析器崩溃，而是会将panic异常当作普通的解析错误，并附加额外信息提醒用户报告此错误。</p>
<pre><code class="language-Go">func Parse(input string) (s *Syntax, err error) {
defer func() {
if p := recover(); p != nil {
err = fmt.Errorf(&quot;internal error: %v&quot;, p)
}
}()
// ...parser...
}
</code></pre>
deploy: 1597ed7bcd228dd4c8bf769e72bd32eebb6d19ba
2022-10-06 07:02:50 +00:00
<p>recover函数帮助Parse从panic中恢复。在deferred函数内部，panic value被附加到错误信息中；并用err变量接收错误信息，返回给调用者。我们也可以通过调用runtime.Stack往错误信息中添加完整的堆栈调用信息。</p>
deploy: 06a1bdf7353d7ab131c932b22e155c41f763dde3
2022-08-04 06:59:33 +00:00
<p>不加区分的恢复所有的panic异常，不是可取的做法；因为在panic之后，无法保证包级变量的状态仍然和我们预期一致。比如，对数据结构的一次重要更新没有被完整完成、文件或者网络连接没有被关闭、获得的锁没有被释放。此外，如果写日志时产生的panic被不加区分的恢复，可能会导致漏洞被忽略。</p>
<p>虽然把对panic的处理都集中在一个包下，有助于简化对复杂和不可以预料问题的处理，但作为被广泛遵守的规范，你不应该试图去恢复其他包引起的panic。公有的API应该将函数的运行失败作为error返回，而不是panic。同样的，你也不应该恢复一个由他人开发的函数引起的panic，比如说调用者传入的回调函数，因为你无法确保这样做是安全的。</p>
<p>有时我们很难完全遵循规范，举个例子，net/http包中提供了一个web服务器，将收到的请求分发给用户提供的处理函数。很显然，我们不能因为某个处理函数引发的panic异常，杀掉整个进程；web服务器遇到处理函数导致的panic时会调用recover，输出堆栈信息，继续运行。这样的做法在实践中很便捷，但也会引起资源泄漏，或是因为recover操作，导致其他问题。</p>
<p>基于以上原因，安全的做法是有选择性的recover。换句话说，只恢复应该被恢复的panic异常，此外，这些异常所占的比例应该尽可能的低。为了标识某个panic是否应该被恢复，我们可以将panic value设置成特殊类型。在recover时对panic value进行检查，如果发现panic value是特殊类型，就将这个panic作为error处理，如果不是，则按照正常的panic进行处理（在下面的例子中，我们会看到这种方式）。</p>
<p>下面的例子是title函数的变形，如果HTML页面包含多个<code>&lt;title&gt;</code>，该函数会给调用者返回一个错误（error）。在soleTitle内部处理时，如果检测到有多个<code>&lt;title&gt;</code>，会调用panic，阻止函数继续递归，并将特殊类型bailout作为panic的参数。</p>
<pre><code class="language-Go">// soleTitle returns the text of the first non-empty title element
// in doc, and an error if there was not exactly one.
func soleTitle(doc *html.Node) (title string, err error) {
type bailout struct{}
defer func() {
switch p := recover(); p {
case nil: // no panic
case bailout{}: // &quot;expected&quot; panic
err = fmt.Errorf(&quot;multiple title elements&quot;)
default:
panic(p) // unexpected panic; carry on panicking
}
}()
// Bail out of recursion if we find more than one nonempty title.
forEachNode(doc, func(n *html.Node) {
if n.Type == html.ElementNode &amp;&amp; n.Data == &quot;title&quot; &amp;&amp;
n.FirstChild != nil {
if title != &quot;&quot; {
panic(bailout{}) // multiple titleelements
}
title = n.FirstChild.Data
}
}, nil)
if title == &quot;&quot; {
return &quot;&quot;, fmt.Errorf(&quot;no title element&quot;)
}
return title, nil
}
</code></pre>
<p>在上例中，deferred函数调用recover，并检查panic value。当panic value是bailout{}类型时，deferred函数生成一个error返回给调用者。当panic value是其他non-nil值时，表示发生了未知的panic异常，deferred函数将调用panic函数并将当前的panic value作为参数传入；此时，等同于recover没有做任何操作。（请注意：在例子中，对可预期的错误采用了panic，这违反了之前的建议，我们在此只是想向读者演示这种机制。）</p>
<p>有些情况下，我们无法恢复。某些致命错误会导致Go在运行时终止程序，如内存不足。</p>
<p><strong>练习5.19：</strong> 使用panic和recover编写一个不包含return语句但能返回一个非零值的函数。</p>
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