go panic recover源码解析

panic和recover是golang的两个内置函数。
当函数F调用panic，函数F会停止运行，F包裹着的defer函数会全部正常运行，然后返回调用F的函数。如果没有recover，F对于调用方，就像panic。Process继续执行堆栈，直到发生panic的goroutine所有方法返回。panic可以被runtime errors,或者直接调用 panic()函数触发

recover只在defer函数中有效
这一部分内容来自Defer, Panic, and Recover翻译

panic例子

func main() {
    defer println("defer in main")
    go func() {
        defer println("defer in goroutine")
        panic("panic in goroutine")
    }()

}
//输出panic test， defer，程序crash
//main函数里面的defer不会执行，在发生panic时只会执行当前协程中的defer函数，从后续的源码讲解中也可以看到

panic，recover例子

func main() {
    defer println("in main")
    go func() {
        defer func() {
        if r := recover(); r != nil {
            fmt.Println("Recovered in f", r)
        }
        panic("panic test")
        }()
    }()

}
//输出panic test，Recovered in f， in main
//程序正常

panic的源码是在go源码 runtime/panic.go文件中，这篇讲解主要从panic和recover函数的源码讲解panic是怎么运行

Panic数据结构

type _panic struct {
    argp      unsafe.Pointer // pointer to arguments of deferred call run during panic; cannot move - known to liblink
    //panic的参数
    arg       interface{}    // argument to panic
    //指向更早的panic
    link      *_panic        // link to earlier panic
    //是否被recover的标识
    recovered bool           // whether this panic is over
    //panic是否被强制终止
    aborted   bool           // the panic was aborted
}  

panic的实现主要是两个函数：panic, recover, 他们分别对应两个实现：gopanic、gorecover，都是在runtime/panic.go文件中实现

gopanic函数

func gopanic(e interface{}) {
    gp := getg()

    var p _panic
    p.arg = e
    p.link = gp._panic //p指向更早的panic
    gp._panic = (*_panic)(noescape(unsafe.Pointer(&p)))

    atomic.Xadd(&runningPanicDefers, 1)
    //遍历defer链表
    for {
        d := gp._defer
        if d == nil {
            break
        }

        // 如果defer已经启动，跳过
        if d.started {
            gp._defer = d.link
            freedefer(d)  //释放defer
            continue
        }

        // 标识defer已经启动
        d.started = true

        // 记录是当前Panic运行这个defer。如果在defer运行期间，有新的Panic，将会标记这个Panic abort=true(强制终止)
        d._panic = (*_panic)(noescape(unsafe.Pointer(&p)))

        p.argp = unsafe.Pointer(getargp(0))
        // 调用 defer
        reflectcall(nil, unsafe.Pointer(d.fn), deferArgs(d), uint32(d.siz), uint32(d.siz))
        p.argp = nil

        // reflectcall did not panic. Remove d.
        if gp._defer != d {
            throw("bad defer entry in panic")
        }
        d._panic = nil
        d.fn = nil
        gp._defer = d.link //遍历到下一个defer
        pc := d.pc
        sp := unsafe.Pointer(d.sp) // must be pointer so it gets adjusted during stack copy
        freedefer(d)
        //已经有recover被调用
        if p.recovered {
            //调用recovery函数
            mcall(recovery)
            throw("recovery failed") // mcall should not return
        }
    }
    //defer遍历完，终止程序
    fatalpanic(gp._panic) // should not return
    *(*int)(nil) = 0      // not reached
}

//panic没有被recover，会运行fatalpanic
func fatalpanic(msgs *_panic) {

    systemstack(func() {
        if startpanic_m() && msgs != nil {
            //打印panic messages
            printpanics(msgs)
        }
        //打印panic messages
        docrash = dopanic_m(gp, pc, sp)
    })

    //终止整个程序，所以需要注意：如果goroutine的Panic没有 recover，会终止整个程序
    systemstack(func() {
        exit(2)
    })

    *(*int)(nil) = 0 // not reached
}

gorecover函数

//defer有recover时，调用；置p的recovered标识位为true
func gorecover(argp uintptr) interface{} {
    // 在panic期间，作为defer的一部分被运行
    gp := getg()
    p := gp._panic
    if p != nil && !p.recovered && argp ==  uintptr(p.argp) {
        p.recovered = true
        return p.arg
    }
    return nil
}

recovery函数

//安排defer函数的调用者正常返回
func recovery(gp *g) {
    //跳转到deferreturn
    gogo(&gp.sched)
}

总结：当发生panic时，会遍历G的defer链表，如发现defer函数包含recover, 则会运行recovery函数，recovery会跳转到deferreturn,否则会退出整个程序。

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