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< h1 id = "panic-深入剖析" > < a class = "header" href = "#panic-深入剖析" > panic 深入剖析< / a > < / h1 >
< p > 在正式开始之前，先来思考一个问题：假设我们想要从文件读取数据，如果失败，你有没有好的办法通知调用者为何失败？如果成功，你有没有好的办法把读取的结果返还给调用者？< / p >
< h2 id = "panic-与不可恢复错误" > < a class = "header" href = "#panic-与不可恢复错误" > panic! 与不可恢复错误< / a > < / h2 >
< p > 上面的问题在真实场景会经常遇到，其实处理起来挺复杂的，让我们先做一个假设：文件读取操作发生在系统启动阶段。那么可以轻易得出一个结论，一旦文件读取失败，那么系统启动也将失败，这意味着该失败是不可恢复的错误，无论是因为文件不存在还是操作系统硬盘的问题，这些只是错误的原因不同，但是归根到底都是不可恢复的错误（梳理清楚当前场景的错误类型非常重要）。< / p >
< p > 对于这些严重到影响程序运行的错误，触发 < code > panic< / code > 是很好的解决方式。在 Rust 中触发 < code > panic< / code > 有两种方式：被动触发和主动调用，下面依次来看看。< / p >
< h3 id = "被动触发" > < a class = "header" href = "#被动触发" > 被动触发< / a > < / h3 >
< p > 先来看一段简单又熟悉的代码:< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > fn main() {
let v = vec![1, 2, 3];
v[99];
}< / code > < / pre >
< p > 心明眼亮的同学立马就能看出这里发生了严重的错误 —— 数组访问越界，在其它编程语言中无一例外，都会报出严重的异常，甚至导致程序直接崩溃关闭。< / p >
< p > 而 Rust 也不例外，运行后将看到如下报错：< / p >
< pre > < code class = "language-shell" > $ cargo run
Compiling panic v0.1.0 (file:///projects/panic)
Finished dev [unoptimized + debuginfo] target(s) in 0.27s
Running `target/debug/panic`
thread 'main' panicked at 'index out of bounds: the len is 3 but the index is 99', src/main.rs:4:5
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
< / code > < / pre >
< p > 上面给出了非常详细的报错信息，包含了具体的异常描述以及发生的位置，甚至你还可以加入额外的命令来看到异常发生时的堆栈信息，这个会在后面详细展开。< / p >
< p > 总之，类似的 < code > panic< / code > 还有很多，而被动触发的 < code > panic< / code > 是我们日常开发中最常遇到的，这也是 Rust 给我们的一种保护，毕竟错误只有抛出来，才有可能被处理，否则只会偷偷隐藏起来，寻觅时机给你致命一击。< / p >
< h3 id = "主动调用" > < a class = "header" href = "#主动调用" > 主动调用< / a > < / h3 >
< p > 在某些特殊场景中，开发者想要主动抛出一个异常，例如开头提到的在系统启动阶段读取文件失败。< / p >
< p > 对此， < code > panic!< / code > 宏，当调用执行该宏时，< strong > 程序会打印出一个错误信息，展开报错点往前的函数调用堆栈，最后退出程序< / strong > 。< / p >
< blockquote >
< p > 切记，一定是不可恢复的错误，才调用 < code > panic!< / code > 处理，你总不想系统仅仅因为用户随便传入一个非法参数就崩溃吧？所以，< strong > 只有当你不知道该如何处理时，再去调用 panic!< / strong > .< / p >
< / blockquote >
< p > 首先，来调用一下 < code > panic!< / code > ，这里使用了最简单的代码实现，实际上你在程序的任何地方都可以这样调用：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > fn main() {
panic!("crash and burn");
}< / code > < / pre >
< p > 运行后输出:< / p >
< pre > < code class = "language-console" > thread 'main' panicked at 'crash and burn', src/main.rs:2:5
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
< / code > < / pre >
< p > 以上信息包含了两条重要信息：< / p >
< ul >
< li > < code > main< / code > 函数所在的线程崩溃了，发生的代码位置是 < code > src/main.rs< / code > 中的第 2 行第 5 个字符（包含该行前面的空字符）< / li >
< li > 在使用时加上一个环境变量可以获取更详细的栈展开信息：
< ul >
< li > Linux/macOS 等 UNIX 系统： < code > RUST_BACKTRACE=1 cargo run< / code > < / li >
< li > Windows 系统（ ） ： < code > $env:RUST_BACKTRACE=1 ; cargo run< / code > < / li >
< / ul >
< / li >
< / ul >
< p > 下面让我们针对第二点进行详细展开讲解。< / p >
< h2 id = "backtrace-栈展开" > < a class = "header" href = "#backtrace-栈展开" > backtrace 栈展开< / a > < / h2 >
< p > 在真实场景中，错误往往涉及到很长的调用链甚至会深入第三方库，如果没有栈展开技术，错误将难以跟踪处理，下面我们来看一个真实的崩溃例子：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > fn main() {
let v = vec![1, 2, 3];
v[99];
}< / code > < / pre >
< p > 上面的代码很简单，数组只有 < code > 3< / code > 个元素，我们却尝试去访问它的第 < code > 100< / code > 号元素（数组索引从 < code > 0< / code > 开始），那自然会崩溃。< / p >
< p > 我们的读者里不乏正义之士，此时肯定要质疑，一个简单的数组越界访问，为何要直接让程序崩溃？是不是有些小题大作了？< / p >
< p > 如果有过 C 语言的经验，即使你越界了，问题不大，我依然尝试去访问，至于这个值是不是你想要的（< code > 100< / code > 号内存地址也有可能有值，只不过是其它变量或者程序的！），抱歉，不归我管，我只负责取，你要负责管理好自己的索引访问范围。上面这种情况被称为< strong > 缓冲区溢出< / strong > ，并可能会导致安全漏洞，例如攻击者可以通过索引来访问到数组后面不被允许的数据。< / p >
< p > 说实话，我宁愿程序崩溃，为什么？当你取到了一个不属于你的值，这在很多时候会导致程序上的逻辑 BUG！ < / p >
< pre > < code class = "language-console" > thread 'main' panicked at 'index out of bounds: the len is 3 but the index is 99', src/main.rs:4:5
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
< / code > < / pre >
< p > 好的，现在成功知道问题发生的位置，但是如果我们想知道该问题之前经过了哪些调用环节，该怎么办？那就按照提示使用 < code > RUST_BACKTRACE=1 cargo run< / code > 或 < code > $env:RUST_BACKTRACE=1 ; cargo run< / code > 来再一次运行程序：< / p >
< pre > < code class = "language-console" > thread 'main' panicked at 'index out of bounds: the len is 3 but the index is 99', src/main.rs:4:5
stack backtrace:
0: rust_begin_unwind
at /rustc/59eed8a2aac0230a8b53e89d4e99d55912ba6b35/library/std/src/panicking.rs:517:5
1: core::panicking::panic_fmt
at /rustc/59eed8a2aac0230a8b53e89d4e99d55912ba6b35/library/core/src/panicking.rs:101:14
2: core::panicking::panic_bounds_check
at /rustc/59eed8a2aac0230a8b53e89d4e99d55912ba6b35/library/core/src/panicking.rs:77:5
3: < usize as core::slice::index::SliceIndex< [T]> > ::index
at /rustc/59eed8a2aac0230a8b53e89d4e99d55912ba6b35/library/core/src/slice/index.rs:184:10
4: core::slice::index::< impl core::ops::index::Index< I> for [T]> ::index
at /rustc/59eed8a2aac0230a8b53e89d4e99d55912ba6b35/library/core/src/slice/index.rs:15:9
5: < alloc::vec::Vec< T,A> as core::ops::index::Index< I> > ::index
at /rustc/59eed8a2aac0230a8b53e89d4e99d55912ba6b35/library/alloc/src/vec/mod.rs:2465:9
6: world_hello::main
at ./src/main.rs:4:5
7: core::ops::function::FnOnce::call_once
at /rustc/59eed8a2aac0230a8b53e89d4e99d55912ba6b35/library/core/src/ops/function.rs:227:5
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.
< / code > < / pre >
< p > 上面的代码就是一次栈展开（也称栈回溯），它包含了函数调用的顺序，当然按照逆序排列：最近调用的函数排在列表的最上方。因为咱们的 < code > main< / code > 函数基本是最先调用的函数了，所以排在了倒数第二位，还有一个关注点，排在最顶部最后一个调用的函数是 < code > rust_begin_unwind< / code > ，该函数的目的就是进行栈展开，呈现这些列表信息给我们。< / p >
< p > 要获取到栈回溯信息，你还需要开启 < code > debug< / code > 标志，该标志在使用 < code > cargo run< / code > 或者 < code > cargo build< / code > 时自动开启（这两个操作默认是 < code > Debug< / code > 运行方式）。同时，栈展开信息在不同操作系统或者 Rust 版本上也有所不同。< / p >
< h2 id = "panic-时的两种终止方式" > < a class = "header" href = "#panic-时的两种终止方式" > panic 时的两种终止方式< / a > < / h2 >
< p > 当出现 < code > panic!< / code > 时，程序提供了两种方式来处理终止流程：< strong > 栈展开< / strong > 和< strong > 直接终止< / strong > 。< / p >
< p > 其中，默认的方式就是 < code > 栈展开< / code > ，这意味着 Rust 会回溯栈上数据和函数调用，因此也意味着更多的善后工作，好处是可以给出充分的报错信息和栈调用信息，便于事后的问题复盘。< code > 直接终止< / code > ，顾名思义，不清理数据就直接退出程序，善后工作交与操作系统来负责。< / p >
< p > 对于绝大多数用户，使用默认选择是最好的，但是当你关心最终编译出的二进制可执行文件大小时，那么可以尝试去使用直接终止的方式，例如下面的配置修改 < code > Cargo.toml< / code > 文件，实现在 < a href = "https://course.rs/first-try/cargo.html#手动编译和运行项目" > < code > release< / code > < / a > 模式下遇到 < code > panic< / code > 直接终止：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > [profile.release]
panic = 'abort'
< span class = "boring" > }< / span > < / code > < / pre >
< h2 id = "线程-panic-后程序是否会终止" > < a class = "header" href = "#线程-panic-后程序是否会终止" > 线程 < code > panic< / code > 后，程序是否会终止？< / a > < / h2 >
< p > 长话短说，如果是 < code > main< / code > 线程，则程序会终止，如果是其它子线程，该线程会终止，但是不会影响 < code > main< / code > 线程。因此，尽量不要在 < code > main< / code > 线程中做太多任务，将这些任务交由子线程去做，就算子线程 < code > panic< / code > 也不会导致整个程序的结束。< / p >
< p > 具体解析见 < a href = "#panic-原理剖析" > panic 原理剖析< / a > 。< / p >
< h2 id = "何时该使用-panic" > < a class = "header" href = "#何时该使用-panic" > 何时该使用 panic!< / a > < / h2 >
< p > 下面让我们大概罗列下何时适合使用 < code > panic< / code > ，也许经过之前的学习，你已经能够对 < code > panic< / code > 的使用有了自己的看法，但是我们还是会罗列一些常见的用法来加深你的理解。< / p >
< p > 先来一点背景知识，在前面章节我们粗略讲过 < code > Result< T, E> < / code > 这个枚举类型，它是用来表示函数的返回结果：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > enum Result< T, E> {
Ok(T),
Err(E),
}
< span class = "boring" > }< / span > < / code > < / pre >
< p > 当没有错误发生时，函数返回一个用 < code > Result< / code > 类型包裹的值 < code > Ok(T)< / code > ，当错误时，返回一个 < code > Err(E)< / code > 。对于 < code > Result< / code > 返回我们有很多处理方法，最简单粗暴的就是 < code > unwrap< / code > 和 < code > expect< / code > ，这两个函数非常类似，我们以 < code > unwrap< / code > 举例：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > use std::net::IpAddr;
let home: IpAddr = "127.0.0.1".parse().unwrap();
< span class = "boring" > }< / span > < / code > < / pre >
< p > 上面的 < code > parse< / code > 方法试图将字符串 < code > "127.0.0.1" < / code > 解析为一个 IP 地址类型 < code > IpAddr< / code > ，它返回一个 < code > Result< IpAddr, E> < / code > 类型，如果解析成功，则把 < code > Ok(IpAddr)< / code > 中的值赋给 < code > home< / code > ，如果失败，则不处理 < code > Err(E)< / code > ，而是直接 < code > panic< / code > 。< / p >
< p > 因此 < code > unwrap< / code > 简而言之：成功则返回值，失败则 < code > panic< / code > ，总之不进行任何错误处理。< / p >
< h4 id = "示例原型测试" > < a class = "header" href = "#示例原型测试" > 示例、原型、测试< / a > < / h4 >
< p > 这几个场景下，需要快速地搭建代码，错误处理会拖慢编码的速度，也不是特别有必要，因此通过 < code > unwrap< / code > 、< code > expect< / code > 等方法来处理是最快的。< / p >
< p > 同时，当我们回头准备做错误处理时，可以全局搜索这些方法，不遗漏地进行替换。< / p >
< h4 id = "你确切的知道你的程序是正确时可以使用-panic" > < a class = "header" href = "#你确切的知道你的程序是正确时可以使用-panic" > 你确切的知道你的程序是正确时，可以使用 panic< / a > < / h4 >
< p > 因为 < code > panic< / code > 的触发方式比错误处理要简单，因此可以让代码更清晰，可读性也更加好，当我们的代码注定是正确时，你可以用 < code > unwrap< / code > 等方法直接进行处理，反正也不可能 < code > panic< / code > ： < / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > use std::net::IpAddr;
let home: IpAddr = "127.0.0.1".parse().unwrap();
< span class = "boring" > }< / span > < / code > < / pre >
< p > 例如上面的例子，< code > "127.0.0.1"< / code > 就是 < code > ip< / code > 地址，因此我们知道 < code > parse< / code > 方法一定会成功，那么就可以直接用 < code > unwrap< / code > 方法进行处理。< / p >
< p > 当然，如果该字符串是来自于用户输入，那在实际项目中，就必须用错误处理的方式，而不是 < code > unwrap< / code > ，否则你的程序一天要崩溃几十万次吧！< / p >
< h4 id = "可能导致全局有害状态时" > < a class = "header" href = "#可能导致全局有害状态时" > 可能导致全局有害状态时< / a > < / h4 >
< p > 有害状态大概分为几类：< / p >
< ul >
< li > 非预期的错误< / li >
< li > 后续代码的运行会受到显著影响< / li >
< li > 内存安全的问题< / li >
< / ul >
< p > 当错误预期会出现时， ， ， < strong > 因为错误是可预期的，因此也是可以处理的< / strong > 。< / p >
< p > 当启动时某个流程发生了错误，对后续代码的运行造成了影响，那么就应该使用 < code > panic< / code > ，而不是处理错误后继续运行，当然你可以通过重试的方式来继续。< / p >
< p > 上面提到过，数组访问越界，就要 < code > panic< / code > 的原因，这个就是属于内存安全的范畴，一旦内存访问不安全，那么我们就无法保证自己的程序会怎么运行下去，也无法保证逻辑和数据的正确性。< / p >
< h2 id = "panic-原理剖析" > < a class = "header" href = "#panic-原理剖析" > panic 原理剖析< / a > < / h2 >
< p > 本来不想写这块儿内容，因为真的难写，但是转念一想，既然号称圣经，那么本书就得与众不同，避重就轻显然不是该有的态度。< / p >
< p > 当调用 < code > panic!< / code > 宏时，它会< / p >
< ol >
< li > 格式化 < code > panic< / code > 信息，然后使用该信息作为参数，调用 < code > std::panic::panic_any()< / code > 函数< / li >
< li > < code > panic_any< / code > 会检查应用是否使用了 < a href = "https://doc.rust-lang.org/std/panic/fn.set_hook.html" > < code > panic hook< / code > < / a > ，如果使用了，该 < code > hook< / code > 函数就会被调用（< code > hook< / code > 是一个钩子函数，是外部代码设置的，用于在 < code > panic< / code > 触发时，执行外部代码所需的功能）< / li >
< li > 当 < code > hook< / code > 函数返回后，当前的线程就开始进行栈展开：从 < code > panic_any< / code > 开始，如果寄存器或者栈因为某些原因信息错乱了，那很可能该展开会发生异常，最终线程会直接停止，展开也无法继续进行< / li >
< li > 展开的过程是一帧一帧的去回溯整个栈，每个帧的数据都会随之被丢弃，但是在展开过程中，你可能会遇到被用户标记为 < code > catching< / code > 的帧（通过 < code > std::panic::catch_unwind()< / code > 函数标记），此时用户提供的 < code > catch< / code > 函数会被调用，展开也随之停止：当然，如果 < code > catch< / code > 选择在内部调用 < code > std::panic::resume_unwind()< / code > 函数，则展开还会继续。< / li >
< / ol >
< p > 还有一种情况，在展开过程中，如果展开本身 < code > panic< / code > 了，那展开线程会终止，展开也随之停止。< / p >
< p > 一旦线程展开被终止或者完成，最终的输出结果是取决于哪个线程 < code > panic< / code > ：对于 < code > main< / code > 线程，操作系统提供的终止功能 < code > core::intrinsics::abort()< / code > 会被调用，最终结束当前的 < code > panic< / code > 进程；如果是其它子线程，那么子线程就会简单的终止，同时信息会在稍后通过 < code > std::thread::join()< / code > 进行收集。< / p >
< h2 id = "课后练习" > < a class = "header" href = "#课后练习" > 课后练习< / a > < / h2 >
< blockquote >
< p > < a href = "https://practice-zh.course.rs/result-panic/panic.html" > Rust By Practice< / a > ，支持代码在线编辑和运行，并提供详细的< a href = "https://github.com/sunface/rust-by-practice/blob/master/solutions/result-panic/panic.md" > 习题解答< / a > 。< / p >
< / blockquote >
< / main >
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