【Go学习】一道简单Golang面试题中关于panic和defer的执行顺序引发的惨案

【Go学习】一道简单Golang面试题中关于panic和defer的执行顺序引发的惨案

题目有点夸张，标题党一把，哈哈，不过也确实是在一个小的面试中碰到这个题目，然后当时经过我反复斟酌之后，愉快的写下了一个错误的答案，回来之后，自己验证了一下，于是就有了这篇文章，大神请绕道。
废话不多说直接上题目，说有如下程序（main.go），写出运行之后的结果：

package mainimport "fmt"func main(){defer_call()fmt.Println("333 Helloworld")
}func defer_call()  {defer func(){fmt.Println("11111")}()defer func(){fmt.Println("22222")}()defer func() {if r := recover(); r!= nil {fmt.Println("Recover from r : ",r)}}()defer func(){fmt.Println("33333")}()fmt.Println("111 Helloworld")panic("Panic 1!")panic("Panic 2!")fmt.Println("222 Helloworld")
}

我直接贴出运行结果：

111 Helloworld
33333
Recover from r :  Panic 1!
22222
11111
333 Helloworld

如果你做对了，建议跳过。其实我也只是把自己的验证过程记录如下，以便以后查阅。

我们用上一篇文章所搭建的golang的gdb调试环境来具体分析下为什么会是这个结果。

编译源代码使用以下命令, 这里的-l参数的意思和上面一样, 如果有需要还可以加-N参数:

/home/james/workspace/go_src/bin/go build -gcflags "-l" main.go

对这个编译方法有疑问的可以参考上一篇文章。
编译后使用gdb运行:

go里面的函数符号名称的命名规则是包名称.函数名称, 例如主函数的符号名称是main.main, 运行时中的newobject的符号名称是runtime.newobject.
首先给主函数下一个断点，给我们第一个panic("Panic 1!")所在行下一个断点，然后运行:

单步运行之后，我们可以找到panic函数所对应的源码：

在上一篇文章中所准备的源码中找到对应的文件src/rumtime/panic.go:425，即panic函数具体实现如下：

// The implementation of the predeclared function panic.
func gopanic(e interface{}) {gp := getg()    // getg()返回当前协程的 g 结构体指针，g 结构体描述 goroutineif gp.m.curg != gp {print("panic: ")printany(e)print("\n")throw("panic on system stack")}// m.softfloat is set during software floating point.// It increments m.locks to avoid preemption.// We moved the memory loads out, so there shouldn't be// any reason for it to panic anymore.if gp.m.softfloat != 0 {gp.m.locks--gp.m.softfloat = 0throw("panic during softfloat")}if gp.m.mallocing != 0 {print("panic: ")printany(e)print("\n")throw("panic during malloc")}if gp.m.preemptoff != "" {print("panic: ")printany(e)print("\n")print("preempt off reason: ")print(gp.m.preemptoff)print("\n")throw("panic during preemptoff")}if gp.m.locks != 0 {print("panic: ")printany(e)print("\n")throw("panic holding locks")}var p _panicp.arg = ep.link = gp._panicgp._panic = (*_panic)(noescape(unsafe.Pointer(&p)))atomic.Xadd(&runningPanicDefers, 1)for {d := gp._defer    // 获取当前协程defer链表的头节点if d == nil {break    // 当前协程的defer都被执行后，defer链表为空，此时退出for循环}// If defer was started by earlier panic or Goexit (and, since we're back here, that triggered a new panic),// take defer off list. The earlier panic or Goexit will not continue running.if d.started {    // 发生panic后，在defer中又遇到panic(),则会进入这个代码块if d._panic != nil {d._panic.aborted = true}d._panic = nild.fn = nilgp._defer = d.linkfreedefer(d)  // defer 已经被执行过，则释放这个defer，继续for循环。continue}// Mark defer as started, but keep on list, so that traceback// can find and update the defer's argument frame if stack growth// or a garbage collection happens before reflectcall starts executing d.fn.d.started = true// Record the panic that is running the defer.// If there is a new panic during the deferred call, that panic// will find d in the list and will mark d._panic (this panic) aborted.d._panic = (*_panic)(noescape(unsafe.Pointer(&p)))p.argp = unsafe.Pointer(getargp(0))reflectcall(nil, unsafe.Pointer(d.fn), deferArgs(d), uint32(d.siz), uint32(d.siz))   // 执行当前协程defer链表头的deferp.argp = nil// reflectcall did not panic. Remove d.if gp._defer != d {throw("bad defer entry in panic")}d._panic = nild.fn = nilgp._defer = d.link  // 从defer链中移除刚刚执行过的defer// trigger shrinkage to test stack copy. See stack_test.go:TestStackPanic//GC()pc := d.pcsp := unsafe.Pointer(d.sp) // must be pointer so it gets adjusted during stack copyfreedefer(d)   // 释放刚刚执行过的deferif p.recovered {    // defer()中遇到recover后进入这个代码块atomic.Xadd(&runningPanicDefers, -1)gp._panic = p.link// Aborted panics are marked but remain on the g.panic list.// Remove them from the list.for gp._panic != nil && gp._panic.aborted {gp._panic = gp._panic.link}if gp._panic == nil { // must be done with signalgp.sig = 0}// Pass information about recovering frame to recovery.gp.sigcode0 = uintptr(sp)gp.sigcode1 = pcmcall(recovery)   // 跳转到recover()处，继续往下执行throw("recovery failed") // mcall should not return}}// ran out of deferred calls - old-school panic now// Because it is unsafe to call arbitrary user code after freezing// the world, we call preprintpanics to invoke all necessary Error// and String methods to prepare the panic strings before startpanic.preprintpanics(gp._panic)startpanic()// startpanic set panicking, which will block main from exiting,// so now OK to decrement runningPanicDefers.atomic.Xadd(&runningPanicDefers, -1)printpanics(gp._panic)   // 输出panic信息dopanic(0)       // should not return*(*int)(nil) = 0 // not reached
}

上面代码虽然有些没有看懂，但是其执行流程还是比较清楚，从代码上来看，协程遇到panic时，遍历本协程的defer链表，并执行defer。在执行defer过程中，遇到recover则停止panic，返回recover处继续往下执行。如果没有遇到recover，遍历完本协程的defer链表后，向stderr抛出panic信息。从执行顺序上来看，实际上是按照先进后出的顺序执行defer。这个时候应该会理解上面的面试题答案为什么是那样了。