derfer : 延迟调用，函数结束返回时执行，多个defer按照先进后出的顺序调用

原理：底层通过链表实现，每次新增的defer调用，通过头插法插入链表；defer执行时，从链表头开始遍历，相当于实现了后加入的defer先执行，先加的defer后执行

defer结构体

type _defer struct {started boolheap    bool// openDefer indicates that this _defer is for a frame with open-coded// defers. We have only one defer record for the entire frame (which may// currently have 0, 1, or more defers active).openDefer boolsp        uintptr // sp at time of deferpc        uintptr // pc at time of deferfn        func()  // can be nil for open-coded defers_panic    *_panic // panic that is running deferlink      *_defer // next defer on G; can point to either heap or stack!// If openDefer is true, the fields below record values about the stack// frame and associated function that has the open-coded defer(s). sp// above will be the sp for the frame, and pc will be address of the// deferreturn call in the function.fd   unsafe.Pointer // funcdata for the function associated with the framevarp uintptr        // value of varp for the stack frame// framepc is the current pc associated with the stack frame. Together,// with sp above (which is the sp associated with the stack frame),// framepc/sp can be used as pc/sp pair to continue a stack trace via// gentraceback().framepc uintptr
}

defer初始化

// Create a new deferred function fn, which has no arguments and results.
// The compiler turns a defer statement into a call to this.
func deferproc(fn func()) {gp := getg()if gp.m.curg != gp {// go code on the system stack can't deferthrow("defer on system stack")}d := newdefer()if d._panic != nil {throw("deferproc: d.panic != nil after newdefer")}// 这里使用头插法 插入链表d.link = gp._defergp._defer = dd.fn = fnd.pc = getcallerpc()// We must not be preempted between calling getcallersp and// storing it to d.sp because getcallersp's result is a// uintptr stack pointer.d.sp = getcallersp()// deferproc returns 0 normally.// a deferred func that stops a panic// makes the deferproc return 1.// the code the compiler generates always// checks the return value and jumps to the// end of the function if deferproc returns != 0.return0()// No code can go here - the C return register has// been set and must not be clobbered.
}

defer执行

func deferreturn() {gp := getg()for {d := gp._deferif d == nil {return}sp := getcallersp()if d.sp != sp {return}if d.openDefer {done := runOpenDeferFrame(gp, d)if !done {throw("unfinished open-coded defers in deferreturn")}gp._defer = d.linkfreedefer(d)// If this frame uses open defers, then this// must be the only defer record for the// frame, so we can just return.return}fn := d.fnd.fn = nil// 指向下一个defer节点gp._defer = d.linkfreedefer(d)fn()}
}