func main() {
    c1 := make(chan int)
    c2 := make(chan int)
    go func() {
        time.Sleep(time.Second)
        <-c2
        c1 <- 1
    }()
    select {
    case v := <-c1:
        fmt.Printf("%d <- c1", v)
    case c2 <- 1:
        fmt.Println("c2 <- 1")
    }
}

func main() {
    c1 := make(chan int)
    go func() {
        time.Sleep(time.Second)
        c1 <- 1
    }()
    select {
    case <-c1:
        fmt.Printf("c1 <- 1")
    default:
        fmt.Println("default")
    }
}

func main() {
    c1 := make(chan int)
    go func() {
        time.Sleep(time.Second)
        <- c1
    }()
    select {
    case c1 <- 1:
        fmt.Printf("c1 <- 1")
    default:
        fmt.Println("default")
    }
}

func reflect_rselect(cases []runtimeSelect) (int, bool) {
    // If there is no case select ion, dormant the current goroutine
    if len(cases) == 0 {
        block()
    }
    sel := make([]scase, len(cases))
    order := make([]uint16, 2*len(cases))
    for i := range cases {
        rc := &cases[i]
        switch rc.dir {
        case selectDefault:
            sel[i] = scase{kind: caseDefault}
        case selectSend:
            // If it is sent, C < - 1, rc. Val is the address of 1.
            sel[i] = scase{kind: caseSend, c: rc.ch, elem: rc.val}
        case selectRecv:
            // If it is received, v:= < - c, rc. Val is the address of V.
            sel[i] = scase{kind: caseRecv, c: rc.ch, elem: rc.val}
        }
    }
    return selectgo(&sel[0], &order[0], len(cases))
}

// selectgo implements the select statement.
//
// cas0 points to an array of type [ncases]scase, and order0 points to
// an array of type [2*ncases]uint16 where ncases must be <= 65536.
// Both reside on the goroutine's stack (regardless of any escaping in
// selectgo).
//
// For race detector builds, pc0 points to an array of type
// [ncases]uintptr (also on the stack); for other builds, it's set to
// nil.
//
// selectgo returns the index of the chosen scase, which matches the
// ordinal position of its respective select{recv,send,default} call.
// Also, if the chosen scase was a receive operation, it reports whether
// a value was received.
func selectgo(cas0 *scase, order0 *uint16, pc0 *uintptr, nsends, nrecvs int, block bool) (int, bool) {
	if debugSelect {
		print("select: cas0=", cas0, "\n")
	}

	// NOTE: In order to maintain a lean stack size, the number of scases
	// is capped at 65536.
	cas1 := (*[1 << 16]scase)(unsafe.Pointer(cas0))
	order1 := (*[1 << 17]uint16)(unsafe.Pointer(order0))

	ncases := nsends + nrecvs
	scases := cas1[:ncases:ncases]
	pollorder := order1[:ncases:ncases]
	lockorder := order1[ncases:][:ncases:ncases]
	// NOTE: pollorder/lockorder's underlying array was not zero-initialized by compiler.

	// Even when raceenabled is true, there might be select
	// statements in packages compiled without -race (e.g.,
	// ensureSigM in runtime/signal_unix.go).
	var pcs []uintptr
	if raceenabled && pc0 != nil {
		pc1 := (*[1 << 16]uintptr)(unsafe.Pointer(pc0))
		pcs = pc1[:ncases:ncases]
	}
	casePC := func(casi int) uintptr {
		if pcs == nil {
			return 0
		}
		return pcs[casi]
	}

	var t0 int64
	if blockprofilerate > 0 {
		t0 = cputicks()
	}

	// The compiler rewrites selects that statically have
	// only 0 or 1 cases plus default into simpler constructs.
	// The only way we can end up with such small sel.ncase
	// values here is for a larger select in which most channels
	// have been nilled out. The general code handles those
	// cases correctly, and they are rare enough not to bother
	// optimizing (and needing to test).

	// generate permuted order
	norder := 0
	for i := range scases {
		cas := &scases[i]

		// Omit cases without channels from the poll and lock orders.
		if cas.c == nil {
			cas.elem = nil // allow GC
			continue
		}

		j := fastrandn(uint32(norder + 1))
		pollorder[norder] = pollorder[j]
		pollorder[j] = uint16(i)
		norder++
	}
	pollorder = pollorder[:norder]
	lockorder = lockorder[:norder]

	// sort the cases by Hchan address to get the locking order.
	// simple heap sort, to guarantee n log n time and constant stack footprint.
	for i := range lockorder {
		j := i
		// Start with the pollorder to permute cases on the same channel.
		c := scases[pollorder[i]].c
		for j > 0 && scases[lockorder[(j-1)/2]].c.sortkey() < c.sortkey() {
			k := (j - 1) / 2
			lockorder[j] = lockorder[k]
			j = k
		}
		lockorder[j] = pollorder[i]
	}
	for i := len(lockorder) - 1; i >= 0; i-- {
		o := lockorder[i]
		c := scases[o].c
		lockorder[i] = lockorder[0]
		j := 0
		for {
			k := j*2 + 1
			if k >= i {
				break
			}
			if k+1 < i && scases[lockorder[k]].c.sortkey() < scases[lockorder[k+1]].c.sortkey() {
				k++
			}
			if c.sortkey() < scases[lockorder[k]].c.sortkey() {
				lockorder[j] = lockorder[k]
				j = k
				continue
			}
			break
		}
		lockorder[j] = o
	}

	if debugSelect {
		for i := 0; i+1 < len(lockorder); i++ {
			if scases[lockorder[i]].c.sortkey() > scases[lockorder[i+1]].c.sortkey() {
				print("i=", i, " x=", lockorder[i], " y=", lockorder[i+1], "\n")
				throw("select: broken sort")
			}
		}
	}

	go

	// 锁定所有参与选择的通道
	// lock all the channels involved in the select
	sellock(scases, lockorder)

	var (
		gp     *g
		sg     *sudog
		c      *hchan
		k      *scase
		sglist *sudog
		sgnext *sudog
		qp     unsafe.Pointer
		nextp  **sudog
	)
	// pass 1 - 寻找已经在等待的事情
	// pass 1 - look for something already waiting
	var casi int
	var cas *scase
	var caseSuccess bool
	var caseReleaseTime int64 = -1
	var recvOK bool
	for _, casei := range pollorder {
		casi = int(casei)
		cas = &scases[casi]
		c = cas.c

		if casi >= nsends {
			sg = c.sendq.dequeue() // 从通道的发送队列中出队一个发送操作
			if sg != nil {
				goto recv // 如果成功出队一个发送操作，则跳转到 "recv" 标签
			}
			if c.qcount > 0 { // 如果通道有缓冲数据，则跳转到 "bufrecv" 标签
				goto bufrecv
			}
			if c.closed != 0 { // 如果通道已关闭，则跳转到 "rclose" 标签
				goto rclose
			}
		} else {
			if raceenabled {
				racereadpc(c.raceaddr(), casePC(casi), chansendpc)
			}
			if c.closed != 0 { // 如果通道已关闭，则跳转到 "sclose" 标签
				goto sclose
			}
			sg = c.recvq.dequeue() // 从通道的接收队列中出队一个接收操作
			if sg != nil {         // 如果成功出队一个接收操作，则跳转到 "send" 标签
				goto send
			}
			if c.qcount < c.dataqsiz { // 如果通道的缓冲区有空间，则跳转到 "bufsend" 标签
				goto bufsend
			}
		}
	}

	if !block {
		selunlock(scases, lockorder)
		casi = -1
		goto retc
	}
	// 第二阶段 - 将所有通道入队
	// pass 2 - enqueue on all chans
	gp = getg()
	if gp.waiting != nil {
		throw("gp.waiting != nil")
	}
	nextp = &gp.waiting
	for _, casei := range lockorder {
		casi = int(casei)
		cas = &scases[casi]
		c = cas.c
		sg := acquireSudog() // 获取sudog，将当前goroutine绑定到sudog上
		sg.g = gp
		sg.isSelect = true
		// 在赋值 elem 和将 sg 进行入队操作到 gp.waiting 之间不能进行堆栈分割
		// 在此处 copystack 可以找到它
		// 在给 elem 赋值和将 sg 入队到 gp.waiting 时不能进行栈分割，
		// 这样 copystack 才能找到它。
		// No stack splits between assigning elem and enqueuing
		// sg on gp.waiting where copystack can find it.
		sg.elem = cas.elem
		sg.releasetime = 0
		if t0 != 0 {
			sg.releasetime = -1
		}
		sg.c = c
		// 按锁定顺序构造等待列表

		// Construct waiting list in lock order.
		*nextp = sg
		nextp = &sg.waitlink
		// 加入相应等待队列
		if casi < nsends {
			c.sendq.enqueue(sg)
		} else {
			c.recvq.enqueue(sg)
		}
	}

	// wait for someone to wake us up
	gp.param = nil
	// Signal to anyone trying to shrink our stack that we're about
	// to park on a channel. The window between when this G's status
	// changes and when we set gp.activeStackChans is not safe for
	// stack shrinking.
	atomic.Store8(&gp.parkingOnChan, 1)
	gopark(selparkcommit, nil, waitReasonSelect, traceEvGoBlockSelect, 1)
	gp.activeStackChans = false

	// 加锁所有的channel
	sellock(scases, lockorder)

	gp.selectDone = 0
	sg = (*sudog)(gp.param) // param 存放唤醒 goroutine 的 sudog，如果是关闭操作唤醒的，那么就为 nil

	gp.param = nil

	// pass 3 - 从非成功的通道中出队
	// 否则它们会堆积在安静的通道上
	// 记录成功的案例，如果有的话。
	// 我们已经按锁定顺序单链上了 SudoGs。
	// pass 3 - dequeue from unsuccessful chans
	// otherwise they stack up on quiet channels
	// record the successful case, if any.
	// We singly-linked up the SudoGs in lock order.
	casi = -1
	cas = nil
	caseSuccess = false
	// 当前goroutine 的 waiting 链表按照lockorder顺序存放着case的sudog
	sglist = gp.waiting
	// Clear all elem before unlinking from gp.waiting.
	// 在从 gp.waiting 取消case的sudog链接之前清除所有元素，便于GC
	for sg1 := gp.waiting; sg1 != nil; sg1 = sg1.waitlink {
		sg1.isSelect = false
		sg1.elem = nil
		sg1.c = nil
	}
	// 清楚当前goroutine的waiting链表，因为被sg代表的协程唤醒了

	gp.waiting = nil

	for _, casei := range lockorder {
		k = &scases[casei]
		if sg == sglist { // 如果相等说明，goroutine是被当前case的channel收发操作唤醒的
			// sg唤醒了当前goroutine, 则当前G已经从sg的队列中出队，这里不需要再次出队

			// sg has already been dequeued by the G that woke us up.
			casi = int(casei)
			cas = k
			caseSuccess = sglist.success
			if sglist.releasetime > 0 {
				caseReleaseTime = sglist.releasetime
			}
		} else {
			// 不是此case唤醒当前goroutine, 将goroutine从此case的发送队列或接收队列出队
			c = k.c
			if int(casei) < nsends {
				c.sendq.dequeueSudoG(sglist)
			} else {
				c.recvq.dequeueSudoG(sglist)
			}
		} // 释放当前case的sudog，然后处理下一个case的sudog
		sgnext = sglist.waitlink
		sglist.waitlink = nil
		releaseSudog(sglist)
		sglist = sgnext
	}

	if cas == nil {
		throw("selectgo: bad wakeup")
	}

	c = cas.c

	if debugSelect {
		print("wait-return: cas0=", cas0, " c=", c, " cas=", cas, " send=", casi < nsends, "\n")
	}

	if casi < nsends {
		if !caseSuccess {
			goto sclose
		}
	} else {
		recvOK = caseSuccess
	}

	if raceenabled {
		if casi < nsends {
			raceReadObjectPC(c.elemtype, cas.elem, casePC(casi), chansendpc)
		} else if cas.elem != nil {
			raceWriteObjectPC(c.elemtype, cas.elem, casePC(casi), chanrecvpc)
		}
	}
	if msanenabled {
		if casi < nsends {
			msanread(cas.elem, c.elemtype.size)
		} else if cas.elem != nil {
			msanwrite(cas.elem, c.elemtype.size)
		}
	}
	if asanenabled {
		if casi < nsends {
			asanread(cas.elem, c.elemtype.size)
		} else if cas.elem != nil {
			asanwrite(cas.elem, c.elemtype.size)
		}
	}

	selunlock(scases, lockorder)
	goto retc

bufrecv:
	// can receive from buffer
	if raceenabled {
		if cas.elem != nil {
			raceWriteObjectPC(c.elemtype, cas.elem, casePC(casi), chanrecvpc)
		}
		racenotify(c, c.recvx, nil)
	}
	if msanenabled && cas.elem != nil {
		msanwrite(cas.elem, c.elemtype.size)
	}
	if asanenabled && cas.elem != nil {
		asanwrite(cas.elem, c.elemtype.size)
	}
	recvOK = true
	qp = chanbuf(c, c.recvx)
	if cas.elem != nil {
		typedmemmove(c.elemtype, cas.elem, qp)
	}
	typedmemclr(c.elemtype, qp)
	c.recvx++
	if c.recvx == c.dataqsiz {
		c.recvx = 0
	}
	c.qcount--
	selunlock(scases, lockorder)
	goto retc

bufsend:
	// can send to buffer
	if raceenabled {
		racenotify(c, c.sendx, nil)
		raceReadObjectPC(c.elemtype, cas.elem, casePC(casi), chansendpc)
	}
	if msanenabled {
		msanread(cas.elem, c.elemtype.size)
	}
	if asanenabled {
		asanread(cas.elem, c.elemtype.size)
	}
	typedmemmove(c.elemtype, chanbuf(c, c.sendx), cas.elem)
	c.sendx++
	if c.sendx == c.dataqsiz {
		c.sendx = 0
	}
	c.qcount++
	selunlock(scases, lockorder)
	goto retc

recv:
	// can receive from sleeping sender (sg)
	recv(c, sg, cas.elem, func() { selunlock(scases, lockorder) }, 2)
	if debugSelect {
		print("syncrecv: cas0=", cas0, " c=", c, "\n")
	}
	recvOK = true
	goto retc

rclose:
	// read at end of closed channel
	selunlock(scases, lockorder)
	recvOK = false
	if cas.elem != nil {
		typedmemclr(c.elemtype, cas.elem)
	}
	if raceenabled {
		raceacquire(c.raceaddr())
	}
	goto retc

send:
	// can send to a sleeping receiver (sg)
	if raceenabled {
		raceReadObjectPC(c.elemtype, cas.elem, casePC(casi), chansendpc)
	}
	if msanenabled {
		msanread(cas.elem, c.elemtype.size)
	}
	if asanenabled {
		asanread(cas.elem, c.elemtype.size)
	}
	send(c, sg, cas.elem, func() { selunlock(scases, lockorder) }, 2)
	if debugSelect {
		print("syncsend: cas0=", cas0, " c=", c, "\n")
	}
	goto retc

retc:
	if caseReleaseTime > 0 {
		blockevent(caseReleaseTime-t0, 1)
	}
	return casi, recvOK

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

func selectnbrecv(elem unsafe.Pointer, c *hchan) (selected bool) {
    selected, _ = chanrecv(c, elem, false)
    return
}

func selectnbsend(c *hchan, elem unsafe.Pointer) (selected bool) {
    return chansend(c, elem, false, getcallerpc())
}