defer的作用与底层原理

Golang defer

1、作用：

2、特点：

3、注意事项：

4、常见应用场景：

5、协程的多个defer是怎样的结构：

看一段测试代码：

底层代码

// Each P holds a pool for defers.

// Allocate a Defer, usually using per-P pool.
// Each defer must be released with freedefer.  The defer is not
// added to any defer chain yet.
func newdefer() *_defer {
	var d *_defer
	mp := acquirem()
	pp := mp.p.ptr()
	if len(pp.deferpool) == 0 && sched.deferpool != nil {
		lock(&sched.deferlock)
		for len(pp.deferpool) < cap(pp.deferpool)/2 && sched.deferpool != nil {
			d := sched.deferpool
			sched.deferpool = d.link
			d.link = nil
			pp.deferpool = append(pp.deferpool, d)
		}
		unlock(&sched.deferlock)
	}
	if n := len(pp.deferpool); n > 0 {
		d = pp.deferpool[n-1]
		pp.deferpool[n-1] = nil
		pp.deferpool = pp.deferpool[:n-1]
	}
	releasem(mp)
	mp, pp = nil, nil

	if d == nil {
		// Allocate new defer.
		d = new(_defer)
	}
	d.heap = true
	return d
}

那么到底defer怎么如第一个图那样连起来的，后进先出呢，再给出源码

// 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 defer
		throw("defer on system stack")
	}

	d := newdefer()
	if d._panic != nil {
		throw("deferproc: d.panic != nil after newdefer")
	}
	d.link = gp._defer
	gp._defer = d
	d.fn = fn
	d.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.
}

那我们在调用的时候，什么时候去触发我们协程的defer呢？

// deferreturn runs deferred functions for the caller's frame.
// The compiler inserts a call to this at the end of any
// function which calls defer.
func deferreturn() {
	gp := getg()
	for {
		d := gp._defer
		if d == nil {
			return
		}
		sp := getcallersp()
		if d.sp != sp {
			return
		}
		if d.openDefer {
			done := runOpenDeferFrame(d)
			if !done {
				throw("unfinished open-coded defers in deferreturn")
			}
			gp._defer = d.link
			freedefer(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.fn
		d.fn = nil
		gp._defer = d.link
		freedefer(d)
		fn()
	}
}