golang中的select原理解析

基本用法

检查 ch 中有没有数据

select {
    case d <- ch:
    default:
}
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读取已经被 close 掉的 ch 时会返回零值，不会报错。因此在使用for + select的时候要格外注意。

for {
    select {
        case d <- ch:
        default:
    }
}
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d会永远都有值，for会永远执行下去。需要做一些优化。

outer:
for {
    select {
        case d, ok := <-ch1:
            if !ok {
                break outer
            }
        case d, ok := <-ch2:
            if !ok {
                break outer
            }
    }
}
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对于监听单个 ch 的，可以使用for + range代替。因为一旦 ch 被关闭，它会退出循环。

for d := range ch {
    
}
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select 监听多个 ch 的时候，执行顺序时随机的。

源码部分

关于channel源码解析 golang的channel实现原理。

src/runtime/chan.go

// compiler implements
//
//	select {
//	case c <- v:
//		... foo
//	default:
//		... bar
//	}
//
// as
//
//	if selectnbsend(c, v) {
//		... foo
//	} else {
//		... bar
//	}
//
func selectnbsend(c *hchan, elem unsafe.Pointer) (selected bool) {
	return chansend(c, elem, false, getcallerpc())
}

// compiler implements
//
//	select {
//	case v, ok = <-c:
//		... foo
//	default:
//		... bar
//	}
//
// as
//
//	if selected, ok = selectnbrecv(&v, c); selected {
//		... foo
//	} else {
//		... bar
//	}
//
func selectnbrecv(elem unsafe.Pointer, c *hchan) (selected, received bool) {
	return chanrecv(c, elem, false)
}

func chansend(c *hchan, ep unsafe.Pointer, block bool, callerpc uintptr) bool
func chanrecv(c *hchan, ep unsafe.Pointer, block bool) (selected, received bool)
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src/runtime/select.go

type scase struct {
	c    *hchan         // chan
	elem unsafe.Pointer // data element
}

type selectDir int
const (
	_             selectDir = iota
	selectSend              // case Chan <- Send
	selectRecv              // case <-Chan:
	selectDefault           // default
)

func reflect_rselect(cases []runtimeSelect) (int, bool) {
	if len(cases) == 0 {
		block()
	}
	sel := make([]scase, len(cases))
	orig := make([]int, len(cases))
	nsends, nrecvs := 0, 0
	dflt := -1
	for i, rc := range cases {
		var j int
		switch rc.dir {
		case selectDefault:
			dflt = i
			continue
		case selectSend:
			j = nsends
			nsends++
		case selectRecv:
			nrecvs++
			j = len(cases) - nrecvs
		}

         // sel 中，将 send 排在前面，recv 排在后面
         // 因此需要通过 orig 来辅助定位
		sel[j] = scase{c: rc.ch, elem: rc.val}
		orig[j] = i
	}

	// 只有 default case.
	if nsends+nrecvs == 0 {
		return dflt, false
	}

	// 只保留 send 和 recv
	if nsends+nrecvs < len(cases) {
		copy(sel[nsends:], sel[len(cases)-nrecvs:])
		copy(orig[nsends:], orig[len(cases)-nrecvs:])
	}

     // order 为空切片，长度是 2*(nsends+nrecvs)
	order := make([]uint16, 2*(nsends+nrecvs))
    
     // 竞争探测用的
	var pc0 *uintptr
	if raceenabled {
		pcs := make([]uintptr, nsends+nrecvs)
		for i := range pcs {
			selectsetpc(&pcs[i])
		}
		pc0 = &pcs[0]
	}

	chosen, recvOK := selectgo(&sel[0], &order[0], pc0, nsends, nrecvs, dflt == -1)

	// Translate chosen back to caller's ordering.
	if chosen < 0 {
		chosen = dflt
	} else {
		chosen = orig[chosen]
	}
	return chosen, recvOK
}

// 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.
    // cas0 为指向切片[]scase的首地址，通过unsafe.Pointer将其转化为 *[65536]scase，超出部分被舍弃
	cas1 := (*[1 << 16]scase)(unsafe.Pointer(cas0))
    // 同样的道理，order1为 *[131072]uint16
	order1 := (*[1 << 17]uint16)(unsafe.Pointer(order0))

    // 实际长度
	ncases := nsends + nrecvs
    // 截取实际的长度，从0开始，截取ncases个，并且cap为ncases
	scases := cas1[:ncases:ncases]
    // 截取实际的长度
	pollorder := order1[:ncases:ncases]
    // 首先取 order1[ncases:]，也就是 order1 的后半部分，得到新切片后，再来取 [: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.
        // 如果 cas 中没有 ch，就过滤掉
		if cas.c == nil {
			cas.elem = nil // allow GC
			continue
		}
		// 随机打乱顺序
		j := fastrandn(uint32(norder + 1))
		pollorder[norder] = pollorder[j]
        
        // 给 pollorder 赋值
		pollorder[j] = uint16(i)
        
		norder++
	}
    // 重新截取切片
	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.
    // 使用 Hchan 的地址来对 cases 进行排序
    // 简单的堆排序，确保查找时间为 logn，和恒定的堆栈占用量
	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")
			}
		}
	}

	// lock all the channels involved in the select
    // 锁住所有的 chan
	sellock(scases, lockorder)

	var (
		gp     *g
		sg     *sudog
		c      *hchan
		k      *scase
		sglist *sudog
		sgnext *sudog
		qp     unsafe.Pointer
		nextp  **sudog
	)

	// pass 1 - look for something already waiting
    // 看是否有就绪的读或写chan
	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()
			if sg != nil {
				goto recv
			}
			if c.qcount > 0 {
				goto bufrecv
			}
			if c.closed != 0 {
				goto rclose
			}
		} else {
			if raceenabled {
				racereadpc(c.raceaddr(), casePC(casi), chansendpc)
			}
			if c.closed != 0 {
				goto sclose
			}
			sg = c.recvq.dequeue()
			if sg != nil {
				goto send
			}
			if c.qcount < c.dataqsiz {
				goto bufsend
			}
		}
	}

	if !block {
		selunlock(scases, lockorder)
		casi = -1
		goto retc
	}

	// pass 2 - enqueue on all chans
    // 没有就绪的chan，那么就需要将当前G加入所有chan的sendq或recvq中去，并挂起
	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()
		sg.g = gp
		sg.isSelect = true
		// 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
		// Construct waiting list in lock order.
		*nextp = sg
		nextp = &sg.waitlink

		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

	sellock(scases, lockorder)

	gp.selectDone = 0
	sg = (*sudog)(gp.param)
	gp.param = nil

	// 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
	sglist = gp.waiting
	// Clear all elem before unlinking from gp.waiting.
	for sg1 := gp.waiting; sg1 != nil; sg1 = sg1.waitlink {
		sg1.isSelect = false
		sg1.elem = nil
		sg1.c = nil
	}
	gp.waiting = nil

	for _, casei := range lockorder {
		k = &scases[casei]
		if sg == sglist {
			// 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 {
			c = k.c
			if int(casei) < nsends {
				c.sendq.dequeueSudoG(sglist)
			} else {
				c.recvq.dequeueSudoG(sglist)
			}
		}
		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"))
}
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select执行过程就是输入cases数组，输出选中的case索引，然后程序流程转到选中的case块。

如果cases为空，将会阻塞。因为Golang自带死锁检测机制，当发现当前协程再也没有机会被唤醒时，则会panic。

cas0为scase数组的首地址。

order0为一个两倍cas0数组长度的buffer，pollorder和lockorder
1、pollorder：每次selectgo执行都会把pollorder序列打乱，以达到随机检测case的目的。
2、lockorder：所有case语句中channel的地址来排序，构成一个简单的堆排序，也可以达到去重防止对channel加锁时重复加锁的目的，所以 sellock() 函数要带上 lockorder 参数。

函数返回值：

1、int：选中case的编号，这个case编号跟代码一致。

2、bool：如果是读操作，则表示是否成功从channle中读取了数据。

大致原理：

从语法层面，将所有的case存到一个数组中，然后抽象出两个轻量级的数组pollorder和lockorder，用来遍历，然后就是判断读操作是否能读（缓冲队列是否有值，sendq是否有sudog），判断写操作是否可写（缓冲队列是否有值，recvq是否有sudog），如果都没有，那就将当前G加入到所有通道的sendq或者recvq；然后gopack挂起，直到被唤醒了，则说明，有一条case被激活了，然后去找到是哪条case，返回对于的ID。