$ go version
go version go1.21.4 windows/386

//cas0：case數(shù)組地址,按照往通道寫數(shù)據(jù)在前，從通道讀數(shù)據(jù)在后的排列順序(編譯時(shí)編譯器優(yōu)化行為操作的)
//nsends：往通道寫數(shù)據(jù)的case數(shù)量
//nrecvs：從通道讀數(shù)據(jù)的case數(shù)量
//block：是否阻塞
//返回值分別代表選中規(guī)定case位置和是否成功從通道接收數(shù)據(jù)，如果選中的是default，第一個(gè)返回值就返回-1
func selectgo(cas0 *scase, order0 *uint16, pc0 *uintptr, nsends, nrecvs int, block bool) (int, bool)

type scase struct {
	c    *hchan         //case對(duì)應(yīng)的讀或?qū)懲ǖ?
	elem unsafe.Pointer //指向要寫入元素或存放讀取元素的地址
}

func selectgo(cas0 *scase, order0 *uint16, pc0 *uintptr, nsends, nrecvs int, block bool) (int, bool) {
	...
	
	var casi int   //準(zhǔn)備好的case位置
	var cas *scase //case對(duì)象
	var caseSuccess bool
	var caseReleaseTime int64 = -1
	var recvOK bool //如果是從通道讀取數(shù)據(jù)，是否讀取成功
	for _, casei := range pollorder { //遍歷隨機(jī)順序的case
		casi = int(casei)   //case的位置
		cas = &scases[casi] //case對(duì)象
		c = cas.c //case通道

		if casi >= nsends { //前面講過，寫通道在前，讀通道在后，所以這里是讀通道case
			sg = c.sendq.dequeue() //取出往讀通道寫數(shù)據(jù)的協(xié)程隊(duì)列中的第一個(gè)協(xié)程
			if sg != nil { //如果存在往通道寫數(shù)據(jù)的協(xié)程
				goto recv  //從往通道寫數(shù)據(jù)的協(xié)程中讀取數(shù)據(jù)并返回case位置和讀取結(jié)果
			}
			if c.qcount > 0 { //如果緩沖區(qū)還有數(shù)據(jù)
				goto bufrecv  //從緩沖區(qū)讀取數(shù)據(jù)并返回case位置和讀取結(jié)果
			}
			if c.closed != 0 { //如果通道已關(guān)閉
				goto rclose    //釋放相關(guān)資源
			}
		} else { //寫通道的case
			if raceenabled {
				racereadpc(c.raceaddr(), casePC(casi), chansendpc)
			}
			if c.closed != 0 { //如果通道已經(jīng)關(guān)閉
				goto sclose    //直接panic
			}
			sg = c.recvq.dequeue() //從正在往通道讀數(shù)據(jù)的協(xié)程隊(duì)列中取得第一個(gè)
			if sg != nil { //如果往通道讀數(shù)據(jù)的協(xié)程存在
				goto send  //發(fā)送數(shù)據(jù)到讀通道的協(xié)程
			}
			if c.qcount < c.dataqsiz { //緩沖區(qū)還有位置
				goto bufsend
			}
		}
	}

	if !block { //如果不阻塞，也就是帶default分支
		selunlock(scases, lockorder)
		casi = -1 //case位置為-1
		goto retc //直接返回，不用進(jìn)入下一步
	}
	
	...
}

	bufrecv:
	recvOK = true  //返回讀數(shù)據(jù)成功
	qp = chanbuf(c, c.recvx) //緩沖區(qū)中要讀取數(shù)據(jù)的地址
	if cas.elem != nil {
		typedmemmove(c.elemtype, cas.elem, qp) //將讀取的緩沖區(qū)數(shù)據(jù)拷貝到case中的elem位置
	}
	typedmemclr(c.elemtype, qp) //清理緩沖區(qū)被讀的數(shù)據(jù)
	c.recvx++ //讀取緩沖區(qū)的位置+1
	if c.recvx == c.dataqsiz { //下一個(gè)要讀取緩沖區(qū)的位置如果等于緩沖區(qū)大小就將下次要讀取的緩沖區(qū)位置置為0
		c.recvx = 0
	}
	c.qcount-- //緩沖區(qū)中元素個(gè)數(shù)-1
	selunlock(scases, lockorder)
	goto retc

	bufsend:
	typedmemmove(c.elemtype, chanbuf(c, c.sendx), cas.elem) //將case中要寫入的元素寫到緩沖區(qū)
	c.sendx++ //寫入緩沖區(qū)的位置+1
	if c.sendx == c.dataqsiz { //如果下次要寫入緩沖區(qū)的位置等于緩沖區(qū)的大小就將緩沖區(qū)寫入位置置為開頭
		c.sendx = 0
	}
	c.qcount++ //緩沖區(qū)元素?cái)?shù)量+1
	selunlock(scases, lockorder)
	goto retc	

recv:
	recv(c, sg, cas.elem, func() { selunlock(scases, lockorder) }, 2) //從寫通道的協(xié)程讀取數(shù)據(jù)
	if debugSelect {
		print("syncrecv: cas0=", cas0, " c=", c, "\n")
	}
	recvOK = true //返回成功讀取
	goto retc

rclose:
	selunlock(scases, lockorder)
	recvOK = false //從通道中讀取數(shù)據(jù)失敗
	if cas.elem != nil {
		typedmemclr(c.elemtype, cas.elem) //釋放case中元素的空間
	}
	if raceenabled {
		raceacquire(c.raceaddr())
	}
	goto retc

send:
	send(c, sg, cas.elem, func() { selunlock(scases, lockorder) }, 2) //發(fā)送數(shù)據(jù)到往通道讀數(shù)據(jù)的協(xié)程
	if debugSelect {
		print("syncsend: cas0=", cas0, " c=", c, "\n")
	}
	goto retc

retc:
	if caseReleaseTime > 0 {
		blockevent(caseReleaseTime-t0, 1)
	}
	return casi, recvOK  //返回case位置和是否從通道成功讀取數(shù)據(jù)

sclose:
	selunlock(scases, lockorder)
	panic(plainError("send on closed channel"))

func selectgo(cas0 *scase, order0 *uint16, pc0 *uintptr, nsends, nrecvs int, block bool) (int, bool) {
	...

	gp = getg() //獲取當(dāng)前協(xié)程
	if gp.waiting != nil {
		throw("gp.waiting != nil")
	}
	nextp = &gp.waiting
	for _, casei := range lockorder { //按照對(duì)case加鎖的順序遍歷case
		casi = int(casei)   //case的位置
		cas = &scases[casi] //case對(duì)象
		c = cas.c  //case對(duì)象中的通道
		sg := acquireSudog() //初始化一個(gè)協(xié)程等待結(jié)構(gòu)
		sg.g = gp //協(xié)程等待結(jié)構(gòu)綁定協(xié)程
		sg.isSelect = true //表示該協(xié)程等待結(jié)構(gòu)與select操作相關(guān)
		sg.elem = cas.elem 
		sg.releasetime = 0
		if t0 != 0 {
			sg.releasetime = -1
		}
		sg.c = c
		*nextp = sg
		nextp = &sg.waitlink

		if casi < nsends { //如果case上是往通道寫數(shù)據(jù)，就將綁定當(dāng)前協(xié)程的等待對(duì)象插入當(dāng)前case通道的發(fā)送隊(duì)列中
			c.sendq.enqueue(sg) 
		} else { //如果case上是往通道讀數(shù)據(jù)，就將綁定當(dāng)前協(xié)程的等待對(duì)象插入當(dāng)前case通道的接收隊(duì)列中
			c.recvq.enqueue(sg)
		}
	}

	...
}

func selectgo(cas0 *scase, order0 *uint16, pc0 *uintptr, nsends, nrecvs int, block bool) (int, bool) {
	...

	sg = (*sudog)(gp.param) //被喚醒的協(xié)程等待結(jié)構(gòu)
	gp.param = nil
	
	casi = -1  //case位置
	cas = nil  //case對(duì)象
	caseSuccess = false
	sglist = gp.waiting //lockorder順序的協(xié)程等待結(jié)構(gòu)隊(duì)列，這里是隊(duì)列中的第一個(gè)協(xié)程等待結(jié)構(gòu)
	for sg1 := gp.waiting; sg1 != nil; sg1 = sg1.waitlink { //清空協(xié)程等待結(jié)構(gòu)隊(duì)列中元素便于進(jìn)行垃圾回收
		sg1.isSelect = false
		sg1.elem = nil
		sg1.c = nil
	}
	gp.waiting = nil

	for _, casei := range lockorder { //根據(jù)對(duì)case的加鎖順序進(jìn)行遍歷
		k = &scases[casei] //當(dāng)前case
		if sg == sglist {  //喚醒的協(xié)程等待結(jié)構(gòu)是當(dāng)前case的
			casi = int(casei) //喚醒的case位置
			cas = k //喚醒的case對(duì)象
			caseSuccess = sglist.success //往通道讀取或?qū)憯?shù)據(jù)結(jié)果
			if sglist.releasetime > 0 {
				caseReleaseTime = sglist.releasetime
			}
		} else { //喚醒的協(xié)程等待結(jié)構(gòu)不是當(dāng)前case的
			c = k.c
			if int(casei) < nsends { //case為發(fā)送通道，就是釋放當(dāng)前case通道里sendq隊(duì)列的協(xié)程等待結(jié)構(gòu)對(duì)象
				c.sendq.dequeueSudoG(sglist)
			} else {  //case為讀取通道，就是釋放當(dāng)前case通道里recvq隊(duì)列的協(xié)程等待結(jié)構(gòu)對(duì)象
				c.recvq.dequeueSudoG(sglist)
			}
		}
		sgnext = sglist.waitlink //下一個(gè)協(xié)程等待結(jié)構(gòu)
		sglist.waitlink = nil
		releaseSudog(sglist) //釋放上一個(gè)協(xié)程等待結(jié)構(gòu)
		sglist = sgnext
	}

	...
}