[Golang1.20 source code reading] runtime/select.go

// Copyright 2009 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

package runtime

// This file contains the implementation of Go select statements.
//此文件包含Go select语句的实现。
import (
	"internal/abi"
	"unsafe"
)

const debugSelect = false

// Select case descriptor.
// Known to compiler.
// Changes here must also be made in src/cmd/compile/internal/walk/select.go's scasetype.
// Select case描述符。编译器已知。这里的更改也必须在src/cmd/compile/internal/walk/select.go的scasetype中进行。
type scase struct {
	c    *hchan         // chan 当前case所操作的channel指针
	elem unsafe.Pointer // data element 表示缓冲区地址
}

var (
	chansendpc = abi.FuncPCABIInternal(chansend)
	chanrecvpc = abi.FuncPCABIInternal(chanrecv)
)

func selectsetpc(pc *uintptr) {
	*pc = getcallerpc()
}

func sellock(scases []scase, lockorder []uint16) {
	var c *hchan
	for _, o := range lockorder {
		c0 := scases[o].c
		if c0 != c {
			c = c0
			lock(&c.lock)
		}
	}
}

func selunlock(scases []scase, lockorder []uint16) {
	// We must be very careful here to not touch sel after we have unlocked
	// the last lock, because sel can be freed right after the last unlock.
	// Consider the following situation.
	// First M calls runtime·park() in runtime·selectgo() passing the sel.
	// Once runtime·park() has unlocked the last lock, another M makes
	// the G that calls select runnable again and schedules it for execution.
	// When the G runs on another M, it locks all the locks and frees sel.
	// Now if the first M touches sel, it will access freed memory.
	// 我们必须非常小心，不要在解锁最后一把锁后触摸sel，因为sel可以在最后一次解锁后立即释放。
	// 考虑以下情况。第一个M调用runtime·park（）在runtime·selectgo（）中传递sel。
	// 一旦runtime·park（）解锁了最后一个锁，另一个M会使调用select的G再次可运行，并安排执行。
	// 当G在另一个M上运行时，它锁定所有锁并释放sel。现在，如果第一个M触摸sel，它将访问释放的内存。
	for i := len(lockorder) - 1; i >= 0; i-- {
		c := scases[lockorder[i]].c
		if i > 0 && c == scases[lockorder[i-1]].c {
			continue // will unlock it on the next iteration 将在下一次迭代中解锁
		}
		unlock(&c.lock)
	}
}

func selparkcommit(gp *g, _ unsafe.Pointer) bool {
	// There are unlocked sudogs that point into gp's stack. Stack
	// copying must lock the channels of those sudogs.
	// Set activeStackChans here instead of before we try parking
	// because we could self-deadlock in stack growth on a
	// channel lock.
	// 有一些未锁定的sudog指向gp的堆栈。堆栈复制必须锁定那些sudog的通道。
	// 在这里设置activeStackChans，而不是在我们尝试停车之前，因为我们可能会在通道锁上的堆栈增长中自死锁。
	gp.activeStackChans = true
	// Mark that it's safe for stack shrinking to occur now,
	// because any thread acquiring this G's stack for shrinking
	// is guaranteed to observe activeStackChans after this store.
	// 标记现在发生堆栈收缩是安全的，因为任何获取该G的堆栈进行收缩的线程都保证在该存储之后观察activeStackChans。
	gp.parkingOnChan.Store(false)
	// Make sure we unlock after setting activeStackChans and
	// unsetting parkingOnChan. The moment we unlock any of the
	// channel locks we risk gp getting readied by a channel operation
	// and so gp could continue running before everything before the
	// unlock is visible (even to gp itself).
	// //请确保我们在设置activeStackChans和取消设置parkingOnChan后解锁。当我们解锁任何通道锁时，我们就有可能让gp为通道操作做好准备，因此gp可以在解锁之前（甚至对gp本身）继续运行。
	// This must not access gp's stack (see gopark). In
	// particular, it must not access the *hselect. That's okay,
	// because by the time this is called, gp.waiting has all
	// channels in lock order.
	// 这不能访问gp的堆栈（请参阅gopark）。特别是，它不能访问*hselect。这没关系，因为当调用这个函数时，gp.waiting已经锁定了所有通道。
	var lastc *hchan
	for sg := gp.waiting; sg != nil; sg = sg.waitlink {
		if sg.c != lastc && lastc != nil {
			// As soon as we unlock the channel, fields in
			// any sudog with that channel may change,
			// including c and waitlink. Since multiple
			// sudogs may have the same channel, we unlock
			// only after we've passed the last instance
			// of a channel.
			//一旦我们解锁通道，任何具有该通道的sudog中的字段都可能更改，包括c和waitlink。由于多个sudog可能具有相同的通道，我们只有在通过通道的最后一个实例后才能解锁。
			unlock(&lastc.lock)
		}
		lastc = sg.c
	}
	if lastc != nil {
		unlock(&lastc.lock)
	}
	return true
}

func block() {
	gopark(nil, nil, waitReasonSelectNoCases, traceEvGoStop, 1) // forever
}

// selectgo implements the select statement.
// selectgo实现select语句。
// 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).
// cas0指向[ncases]scase类型的数组，order0指向[2*ncases]uint16类型的数组（其中ncases必须<=65536）。两者都驻留在goroutine的堆栈中（不管selectgo中是否有转义）。
// 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.
// 对于竞赛检测器构建，pc0指向一个[ncases]uintptr类型的数组（也在堆栈上）；对于其他构建，它被设置为零。
// 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.
// selectgo返回所选scase的索引，该索引与相应select｛recv，send，default｝调用的序号位置相匹配。此外，如果选择的scase是一个接收操作，它会报告是否接收到值。
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.
	// 注：为了保持精简堆栈大小，scases的数量上限为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.
	// 注意：编译器初始化的pollorder/lockorder的基础数组不是零。
	// Even when raceenabled is true, there might be select
	// statements in packages compiled without -race (e.g.,
	// ensureSigM in runtime/signal_unix.go).
	// 即使raceenabled为true，在不使用race编译的包中也可能存在select语句（例如，runtime/sign_unix.go中的ensureSigM）。
	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).
	// 编译器将静态地只有0或1个事例加上默认值的选择重写为更简单的构造。在这里，我们可以得到如此小的sel.ncase值的唯一方法是进行更大的选择，
	// 其中大多数通道都被幂零掉了。通用代码正确地处理了这些情况，而且它们非常罕见，不需要进行优化（也不需要进行测试）。
	// generate permuted order 生成排列顺序
	norder := 0
	for i := range scases {
		cas := &scases[i]

		// Omit cases without channels from the poll and lock orders. 从投票和锁定命令中省略没有通道的case
		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.
	// 根据Hchan地址对case进行排序以获得锁定顺序。简单的堆排序，以保证n log n时间和恒定的堆栈占用空间。
	for i := range lockorder {
		j := i
		// Start with the pollorder to permute cases on the same channel.
		// 从轮询顺序开始，在同一通道上排列case。
		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 锁定选择中涉及的所有通道
	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 寻找已经在等待的东西
	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 在所有通道排队
	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.
		// 在分配elem和在gp.waiting上排队sg之间没有堆栈分割。等待复制堆栈可以找到它。
		sg.elem = cas.elem
		sg.releasetime = 0
		if t0 != 0 {
			sg.releasetime = -1
		}
		sg.c = c
		// Construct waiting list in lock order. 按锁定顺序构建等候名单。
		*nextp = sg
		nextp = &sg.waitlink

		if casi < nsends {
			c.sendq.enqueue(sg)
		} else {
			c.recvq.enqueue(sg)
		}
	}

	// wait for someone to wake us up 等着有人唤醒我们
	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.
	//向任何试图缩小我们的堆栈的人发出信号，表明我们即将停在通道上。当这个G的状态改变时和我们设置gp.activeStackChans时之间的窗口对于堆栈收缩是不安全的。
	gp.parkingOnChan.Store(true)
	gopark(selparkcommit, nil, waitReasonSelect, traceEvGoBlockSelect, 1)
	gp.activeStackChans = false

	sellock(scases, lockorder)

	gp.selectDone.Store(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.
	// 从不成功的通道中退出队列，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）。我们把SudoG单独按锁定顺序连接起来。
	casi = -1
	cas = nil
	caseSuccess = false
	sglist = gp.waiting
	// Clear all elem before unlinking from gp.waiting. 在取消与gp.waiting的链接之前，请清除所有elem。
	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. sg已经被唤醒我们的G退出了队列。
			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 可以从缓冲区接收
	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 可以发送到缓冲区
	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) 可以从休眠发送者（sg）接收
	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 在关闭通道结束时读取
	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) 可以发送到睡眠接收器（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 在封闭通道上发送
	selunlock(scases, lockorder)
	panic(plainError("send on closed channel"))
}

func (c *hchan) sortkey() uintptr {
	return uintptr(unsafe.Pointer(c))
}

// A runtimeSelect is a single case passed to rselect.
// This must match ../reflect/value.go:/runtimeSelect
// untimeSelect是传递给rselect的单个事例。这必须匹配/reflect/value.go:/runtimeSelect
type runtimeSelect struct {
	dir selectDir
	typ unsafe.Pointer // channel type (not used here)
	ch  *hchan         // channel
	val unsafe.Pointer // ptr to data (SendDir) or ptr to receive buffer (RecvDir)
}

// These values must match ../reflect/value.go:/SelectDir.
type selectDir int

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

//go:linkname reflect_rselect reflect.rselect
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[j] = scase{c: rc.ch, elem: rc.val}
		orig[j] = i
	}

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

	// Compact sel and orig if necessary.
	if nsends+nrecvs < len(cases) {
		copy(sel[nsends:], sel[len(cases)-nrecvs:])
		copy(orig[nsends:], orig[len(cases)-nrecvs:])
	}

	order := make([]uint16, 2*(nsends+nrecvs))
	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
}

func (q *waitq) dequeueSudoG(sgp *sudog) {
	x := sgp.prev
	y := sgp.next
	if x != nil {
		if y != nil {
			// middle of queue 队列中间
			x.next = y
			y.prev = x
			sgp.next = nil
			sgp.prev = nil
			return
		}
		// end of queue 队列末尾
		x.next = nil
		q.last = x
		sgp.prev = nil
		return
	}
	if y != nil {
		// start of queue 队列开始
		y.prev = nil
		q.first = y
		sgp.next = nil
		return
	}

	// x==y==nil. Either sgp is the only element in the queue,
	// or it has already been removed. Use q.first to disambiguate.
	// x==y===nil。sgp是队列中唯一的元素，或者它已经被删除。使用q.first来消除歧义。
	if q.first == sgp {
		q.first = nil
		q.last = nil
	}
}