golang defer实现原理

defer是golang提供的关键字，在函数或者方法执行完成，返回之前调用。
每次defer都会将defer函数压入栈中，调用函数或者方法结束时，从栈中取出执行，所以多个defer的执行顺序是先入后出。

defer的触发时机

官网说的很清楚：
A “defer” statement invokes a function whose execution is deferred to the moment the surrounding function returns, either because the surrounding function executed a return statement, reached the end of its function body, or because the corresponding goroutine is panicking.

1. 包裹着defer语句的函数返回时
2. 包裹着defer语句的函数执行到最后时
3. 当前goroutine发生Panic时

defer，return，返回值的执行顺序

先来看3个例子

func f1() int { //匿名返回值
        var r int = 6
        defer func() {
                r *= 7
        }()
        return r
}

func f2() (r int) { //有名返回值
        defer func() {
                r *= 7
        }()
        return 6
}

func f3() (r int) { //有名返回值
    defer func(r int) {
        r *= 7
    }(r)
    return 6
}

f1的执行结果是6， f2的执行结果是42，f3的执行结果是6
在golang的官方文档里面介绍了，return，defer，返回值的执行顺序：
if the surrounding function returns through an explicit return statement, deferred functions are executed after any result parameters are set by that return statement but before the function returns to its caller.

1. 先给返回值赋值
2. 执行defer语句
3. 包裹函数返回

匿名返回值是在return执行时被声明，有名返回值则是在函数声明的同时被声明，因此在defer语句中只能访问有名返回值，而不能直接访问匿名返回值。所以f1的结果是6。f2是有名返回值，defer函数修改了f2的返回值。f3是有名返回值，但是因为r是作为defer的传参，所以defer对r的修改也不会影响调用函数的返回值。

defer源码解析
defer的实现源码是在runtime.deferproc
然后在函数返回之前的地方，运行函数runtime.deferreturn。
先了解defer结构体：

type _defer struct {
            siz     int32 
            started bool
            sp      uintptr // sp at time of defer
            pc      uintptr
            fn      *funcval
            _panic  *_panic // panic that is running defer
            link    *_defer
    }

sp 和 pc 分别指向了栈指针和调用方的程序计数器，fn是向 defer 关键字中传入的函数，Panic是导致运行defer的Panic。
每遇到一个defer关键字，defer函数都会被转换成runtime.deferproc
deferproc通过newdefer创建一个延迟函数，并将这个新建的延迟函数挂在当前goroutine的_defer的链表上

func deferproc(siz int32, fn *funcval) { // arguments of fn follow fn
            sp := getcallersp()
            argp := uintptr(unsafe.Pointer(&fn)) + unsafe.Sizeof(fn)
            callerpc := getcallerpc()

            d := newdefer(siz)
            if d._panic != nil {
                    throw("deferproc: d.panic != nil after newdefer")
            }
            d.fn = fn
            d.pc = callerpc
            d.sp = sp
            switch siz {
            case 0:
                    // Do nothing.
            case sys.PtrSize:
                    *(*uintptr)(deferArgs(d)) = *(*uintptr)(unsafe.Pointer(argp))
            default:
                    memmove(deferArgs(d), unsafe.Pointer(argp), uintptr(siz))
            }
            return0()
    }

newdefer会先从sched和当前p的deferpool取出一个_defer结构体，如果deferpool没有_defer，则初始化一个新的_defer。
_defer是关联到当前的g，所以defer只对当前g有效。
d.link = gp._defer
gp._defer = d //用链表连接当前g的所有defer

func newdefer(siz int32) *_defer {
            var d *_defer
            sc := deferclass(uintptr(siz))
            gp := getg()
            if sc < uintptr(len(p{}.deferpool)) {
                    pp := gp.m.p.ptr()
                    if len(pp.deferpool[sc]) == 0 && sched.deferpool[sc] != nil {
                            // Take the slow path on the system stack so
                            // we don't grow newdefer's stack.
                            systemstack(func() {
                                    lock(&sched.deferlock)
                                    for len(pp.deferpool[sc])  0 {
                            d = pp.deferpool[sc][n-1]
                            pp.deferpool[sc][n-1] = nil
                            pp.deferpool[sc] = pp.deferpool[sc][:n-1]
                    }
            }
            if d == nil {
                    // Allocate new defer+args.
                    systemstack(func() {
                            total := roundupsize(totaldefersize(uintptr(siz)))
                            d = (*_defer)(mallocgc(total, deferType, true))
                    })
                    if debugCachedWork {
                            // Duplicate the tail below so if there's a
                            // crash in checkPut we can tell if d was just
                            // allocated or came from the pool.
                            d.siz = siz
                            d.link = gp._defer
                            gp._defer = d
                            return d
                    }
            }
            d.siz = siz
            d.link = gp._defer
            gp._defer = d
            return d
    }

deferreturn 从当前g取出_defer链表执行，每个_defer调用freedefer释放_defer结构体，并将该_defer结构体放入当前p的deferpool中。