channel

channel和goroutine是Go语言的核心命脉。这篇文章来简单介绍一下Go chan的原理，源码并不好读，应结合gmp调度模型来理解，后续补充吧。

由上图可见，chan的底层结构是一个hchan结构体，其中buf字段指向了一个环形的数组缓冲区，若channel是非缓冲类型的，则没有该底层结构。

当channel无缓冲区时，只根据recvq和sendq双线链表对数据的收发进行管理，每次收发数据，都直接在两个队列的队首进行操作，保证先进先出。例如把sendq和recvq队首的g出队，将前者的值拷贝进后者，即完成了一次通道的收发操作。

当channel有缓冲区时，recvx和sendx分别指向环形数组中下一个读取和写入的位置，他们被qcount所管理。当qcount等于dataqsiz时，再次执行写入操作的goroutine将被挂到sendq双向链表后边，等待发送。同理，当qcount 等于 0 时，若有goroutine想要接收数据，则该goroutine会被挂到recvq双向链表的后边，等待接收。保证先进先出。若缓冲区有位置，则直接写入或直接取走值。

const (
	maxAlign  = 8   //最大对其方式
	hchanSize = unsafe.Sizeof(hchan{}) + uintptr(-int(unsafe.Sizeof(hchan{}))&(maxAlign-1)) //hchan的大小
	debugChan = false  //debug标记
)

chan 底层结构hchan

type hchan struct {
	qcount   uint           // total data in the queue        底层环形数组，当前数据个数
	dataqsiz uint           // size of the circular queue     底层环形数组的大小
	buf      unsafe.Pointer // points to an array of dataqsiz elements 底层环形数组指针
	elemsize uint16        // 元素大小
	closed   uint32        // 标识通道是否关闭，0：非关闭状态
	elemtype *_type // element type 元素类型
	sendx    uint   // send index  环形数组发送的索引位置
	recvx    uint   // receive index 环形数组接收的索引位置
	recvq    waitq  // list of recv waiters 负责接收的goroutine的队列
	sendq    waitq  // list of send waiters 负责发送的goroutine的队列

	// lock protects all fields in hchan, as well as several
	// fields in sudogs blocked on this channel.
	//
	// Do not change another G's status while holding this lock
	// (in particular, do not ready a G), as this can deadlock
	// with stack shrinking.
	lock mutex     //锁，负责保护以上的字段
}

waitq的结构,双向链表

type waitq struct {
	first *sudog //链表头
	last  *sudog //链表尾
}

makechan 相当于make(chan, len)

func makechan(t *chantype, size int) *hchan {
	elem := t.elem // 获取欲创建channel的成员类型

	// compiler checks this but be safe.
	if elem.size >= 1<<16 { //成员的size过大 panic
		throw("makechan: invalid channel element type")
	}
	if hchanSize%maxAlign != 0 || elem.align > maxAlign { //非8字节内存对齐，或对齐方式不正确，panic
		throw("makechan: bad alignment")
	}

	mem, overflow := math.MulUintptr(elem.size, uintptr(size)) //创建底层连续的内存区域，并检查是否溢出
	if overflow || mem > maxAlloc-hchanSize || size < 0 { //若溢出 或 内存过大，或chan的长度是负数，panic
		panic(plainError("makechan: size out of range"))
	}

	// Hchan does not contain pointers interesting for GC when elements stored in buf do not contain pointers.
	// buf points into the same allocation, elemtype is persistent.
	// SudoG's are referenced from their owning thread so they can't be collected.
	// TODO(dvyukov,rlh): Rethink when collector can move allocated objects.
	var c *hchan
	switch {
	case mem == 0:  //chan占用内存为0
		// Queue or element size is zero.
		c = (*hchan)(mallocgc(hchanSize, nil, true))
		// Race detector uses this location for synchronization.
		c.buf = c.raceaddr()
	case elem.ptrdata == 0: //chan的elem中不包含指针
		// Elements do not contain pointers.
		// Allocate hchan and buf in one call.
		c = (*hchan)(mallocgc(hchanSize+mem, nil, true))
		c.buf = add(unsafe.Pointer(c), hchanSize) // 给buf设置内存区域
	default: //成员包含指针
		// Elements contain pointers.
		c = new(hchan)
		c.buf = mallocgc(mem, elem, true)
	}

	c.elemsize = uint16(elem.size) //初始化及赋值操作
	c.elemtype = elem
	c.dataqsiz = uint(size)
	lockInit(&c.lock, lockRankHchan) //初始化锁

	if debugChan { //调试模式则打印信息
		print("makechan: chan=", c, "; elemsize=", elem.size, "; dataqsiz=", size, "\n")
	}
	return c //返回hchan指针对象
}

reflect_makechan，makechan64 对makechan的封装

//go:linkname reflect_makechan reflect.makechan
func reflect_makechan(t *chantype, size int) *hchan {
	return makechan(t, size)
}

func makechan64(t *chantype, size int64) *hchan {
	if int64(int(size)) != size {
		panic(plainError("makechan: size out of range"))
	}

	return makechan(t, int(size))
}

chanbuf 返回chan的第i个位置的元素的unsafe指针

// chanbuf(c, i) is pointer to the i'th slot in the buffer.
func chanbuf(c *hchan, i uint) unsafe.Pointer {
	return add(c.buf, uintptr(i)*uintptr(c.elemsize))
}

full 检查channel是否已满，其报告通道发送时是否会阻塞

// full reports whether a send on c would block (that is, the channel is full).
// It uses a single word-sized read of mutable state, so although
// the answer is instantaneously true, the correct answer may have changed
// by the time the calling function receives the return value.
func full(c *hchan) bool {
	// c.dataqsiz is immutable (never written after the channel is created)
	// so it is safe to read at any time during channel operation.
	if c.dataqsiz == 0 { // chan的len为0时，检测接收队列是否为空
		// Assumes that a pointer read is relaxed-atomic.
		return c.recvq.first == nil
	}
	// Assumes that a uint read is relaxed-atomic.
	return c.qcount == c.dataqsiz //检测元素是否已满
}

chansend1对chansend的封装

// entry point for c <- x from compiled code
//
//go:nosplit
func chansend1(c *hchan, elem unsafe.Pointer) {
	chansend(c, elem, true, getcallerpc())
}

chansend chan <- x 向管道中发送数据

/*
 * generic single channel send/recv
 * If block is not nil,
 * then the protocol will not
 * sleep but return if it could
 * not complete.
 *
 * sleep can wake up with g.param == nil
 * when a channel involved in the sleep has
 * been closed.  it is easiest to loop and re-run
 * the operation; we'll see that it's now closed.
 */
func chansend(c *hchan, ep unsafe.Pointer, block bool, callerpc uintptr) bool {
	if c == nil { // 若管道为空
		if !block { // 若非阻塞类型，则返回false表示发送失败
			return false
		} // 否则发送的goroutine挂起，然后panic
		gopark(nil, nil, waitReasonChanSendNilChan, traceEvGoStop, 2)
		throw("unreachable")
	}

	if debugChan { //调试模式打印状态
		print("chansend: chan=", c, "\n")
	}

	if raceenabled { // 竞态检测
		racereadpc(c.raceaddr(), callerpc, abi.FuncPCABIInternal(chansend))
	}

	// Fast path: check for failed non-blocking operation without acquiring the lock.
	//
	// After observing that the channel is not closed, we observe that the channel is
	// not ready for sending. Each of these observations is a single word-sized read
	// (first c.closed and second full()).
	// Because a closed channel cannot transition from 'ready for sending' to
	// 'not ready for sending', even if the channel is closed between the two observations,
	// they imply a moment between the two when the channel was both not yet closed
	// and not ready for sending. We behave as if we observed the channel at that moment,
	// and report that the send cannot proceed.
	//
	// It is okay if the reads are reordered here: if we observe that the channel is not
	// ready for sending and then observe that it is not closed, that implies that the
	// channel wasn't closed during the first observation. However, nothing here
	// guarantees forward progress. We rely on the side effects of lock release in
	// chanrecv() and closechan() to update this thread's view of c.closed and full().
	if !block && c.closed == 0 && full(c) { // 非阻塞，未关闭，已满则返回false
		return false
	}

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

	lock(&c.lock) //上锁

	if c.closed != 0 { //chan已关闭，解锁，panic
		unlock(&c.lock)
		panic(plainError("send on closed channel"))
	}

	if sg := c.recvq.dequeue(); sg != nil { //接收队列取出一个g,绕过缓冲区，直接给接收者
		// Found a waiting receiver. We pass the value we want to send
		// directly to the receiver, bypassing the channel buffer (if any).
		send(c, sg, ep, func() { unlock(&c.lock) }, 3) //给这个g装配一个值
		return true //返回true表示发送成功
	}

	if c.qcount < c.dataqsiz { //buf中有空余的空间，且接收队列无g等待
		// Space is available in the channel buffer. Enqueue the element to send.
		qp := chanbuf(c, c.sendx)
		if raceenabled {
			racenotify(c, c.sendx, nil)
		}
		typedmemmove(c.elemtype, qp, ep) //将待发送值复制到buf的sendx位置
		c.sendx++  //sendx指针下移
		if c.sendx == c.dataqsiz { //指针越界归零，构成环状
			c.sendx = 0
		}
		c.qcount++ //hchan元素个数+1
		unlock(&c.lock) //解锁，返回true
		return true
	}

	if !block { //非阻塞
		unlock(&c.lock) //解锁，返回false
		return false
	}

 // 发送处于阻塞状态，则等待运行时调度当前g
	// Block on the channel. Some receiver will complete our operation for us.
	gp := getg()
	mysg := acquireSudog()
	mysg.releasetime = 0
	if t0 != 0 {
		mysg.releasetime = -1
	}
	// No stack splits between assigning elem and enqueuing mysg
	// on gp.waiting where copystack can find it.
	mysg.elem = ep
	mysg.waitlink = nil
	mysg.g = gp
	mysg.isSelect = false
	mysg.c = c
	gp.waiting = mysg
	gp.param = nil
	c.sendq.enqueue(mysg) //当前g放进发送队列
	// 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(chanparkcommit, unsafe.Pointer(&c.lock), waitReasonChanSend, traceEvGoBlockSend, 2) //g挂起
	// Ensure the value being sent is kept alive until the
	// receiver copies it out. The sudog has a pointer to the
	// stack object, but sudogs aren't considered as roots of the
	// stack tracer.
	KeepAlive(ep) //防止ep被回收

	// someone woke us up.
	if mysg != gp.waiting {
		throw("G waiting list is corrupted")
	}
	gp.waiting = nil
	gp.activeStackChans = false
	closed := !mysg.success
	gp.param = nil
	if mysg.releasetime > 0 {
		blockevent(mysg.releasetime-t0, 2)
	}
	mysg.c = nil
	releaseSudog(mysg)
	if closed {
		if c.closed == 0 {
			throw("chansend: spurious wakeup")
		}
		panic(plainError("send on closed channel"))
	}
	return true
}

send 向g中写入数据

// send processes a send operation on an empty channel c.
// The value ep sent by the sender is copied to the receiver sg.
// The receiver is then woken up to go on its merry way.
// Channel c must be empty and locked.  send unlocks c with unlockf.
// sg must already be dequeued from c.
// ep must be non-nil and point to the heap or the caller's stack.
func send(c *hchan, sg *sudog, ep unsafe.Pointer, unlockf func(), skip int) {
	if raceenabled { //竞态检测
		if c.dataqsiz == 0 { 
			racesync(c, sg)
		} else {
			// Pretend we go through the buffer, even though
			// we copy directly. Note that we need to increment
			// the head/tail locations only when raceenabled.
			racenotify(c, c.recvx, nil)
			racenotify(c, c.recvx, sg)
			c.recvx++
			if c.recvx == c.dataqsiz {
				c.recvx = 0
			}
			c.sendx = c.recvx // c.sendx = (c.sendx+1) % c.dataqsiz
		}
	}
	if sg.elem != nil {
		sendDirect(c.elemtype, sg, ep) //直接发送
		sg.elem = nil
	}
	gp := sg.g
	unlockf()
	gp.param = unsafe.Pointer(sg)
	sg.success = true
	if sg.releasetime != 0 {
		sg.releasetime = cputicks()
	}
	goready(gp, skip+1)  //就绪状态
}

sendDirect 从src复制值到g

// Sends and receives on unbuffered or empty-buffered channels are the
// only operations where one running goroutine writes to the stack of
// another running goroutine. The GC assumes that stack writes only
// happen when the goroutine is running and are only done by that
// goroutine. Using a write barrier is sufficient to make up for
// violating that assumption, but the write barrier has to work.
// typedmemmove will call bulkBarrierPreWrite, but the target bytes
// are not in the heap, so that will not help. We arrange to call
// memmove and typeBitsBulkBarrier instead.

func sendDirect(t *_type, sg *sudog, src unsafe.Pointer) {
	// src is on our stack, dst is a slot on another stack.

	// Once we read sg.elem out of sg, it will no longer
	// be updated if the destination's stack gets copied (shrunk).
	// So make sure that no preemption points can happen between read & use.
	dst := sg.elem
	typeBitsBulkBarrier(t, uintptr(dst), uintptr(src), t.size)
	// No need for cgo write barrier checks because dst is always
	// Go memory.
	memmove(dst, src, t.size)
}

recvDirect 从g复制值到dst

func recvDirect(t *_type, sg *sudog, dst unsafe.Pointer) {
	// dst is on our stack or the heap, src is on another stack.
	// The channel is locked, so src will not move during this
	// operation.
	src := sg.elem
	typeBitsBulkBarrier(t, uintptr(dst), uintptr(src), t.size)
	memmove(dst, src, t.size)
}

closechan 相当于close(chan) 关闭channel，该操作会唤醒所有监听该chan的goroutine，若向关闭的chan中发送数据，则会panic，负责读取的goroutine会接收到通道的零值

func closechan(c *hchan) {
	if c == nil { //若c本身就是空，二话不说，直接panic
		panic(plainError("close of nil channel"))
	}

	lock(&c.lock) //上锁
	if c.closed != 0 { // c已经被关闭，二次关闭会panic
		unlock(&c.lock) // 解锁
		panic(plainError("close of closed channel")) // panic
	}

	if raceenabled { // 竞态检测
		callerpc := getcallerpc()
		racewritepc(c.raceaddr(), callerpc, abi.FuncPCABIInternal(closechan))
		racerelease(c.raceaddr())
	}

	c.closed = 1 // closed 赋值1 代表channel已关闭

	var glist gList

	// release all readers   释放所有等待读取的goroutine
	for {
		sg := c.recvq.dequeue() //从接收队列出队
		if sg == nil {
			break
		}
		if sg.elem != nil { //清空成员
			typedmemclr(c.elemtype, sg.elem)
			sg.elem = nil
		}
		if sg.releasetime != 0 {
			sg.releasetime = cputicks()
		}
		gp := sg.g
		gp.param = unsafe.Pointer(sg)
		sg.success = false
		if raceenabled {
			raceacquireg(gp, c.raceaddr())
		}
		glist.push(gp) // 放入glist队列准备通知
	}

	// release all writers (they will panic) 释放所有等待写入的goroutine，向已关闭的通道发送数据，会panic
	for {
		sg := c.sendq.dequeue()
		if sg == nil {
			break
		}
		sg.elem = nil
		if sg.releasetime != 0 {
			sg.releasetime = cputicks()
		}
		gp := sg.g
		gp.param = unsafe.Pointer(sg)
		sg.success = false
		if raceenabled {
			raceacquireg(gp, c.raceaddr())
		}
		glist.push(gp)
	}
	unlock(&c.lock)

	// Ready all Gs now that we've dropped the channel lock.
	for !glist.empty() {
		gp := glist.pop()
		gp.schedlink = 0
		goready(gp, 3)  //就绪状态，准备被调度
	}
}

empty 检测通道中是否没有值了，其报告通道读取时是否会阻塞

// empty reports whether a read from c would block (that is, the channel is
// empty).  It uses a single atomic read of mutable state.
func empty(c *hchan) bool {
	// c.dataqsiz is immutable.
	if c.dataqsiz == 0 { //若非缓冲通道，则检测发送队列是否为空
		return atomic.Loadp(unsafe.Pointer(&c.sendq.first)) == nil
	}
	return atomic.Loaduint(&c.qcount) == 0 //否则检测通道中元素个数是否为0
}

chanrecv1 对chanrecv的封装，<-chan 从通道中读取值

// entry points for <- c from compiled code
//
//go:nosplit
func chanrecv1(c *hchan, elem unsafe.Pointer) {
	chanrecv(c, elem, true)
}

chanrecv2 value, ok := <-chan，读取通道时两个返回值的封装

//go:nosplit
func chanrecv2(c *hchan, elem unsafe.Pointer) (received bool) {
	_, received = chanrecv(c, elem, true)
	return
}

chanrecv 通道读取操作

// chanrecv receives on channel c and writes the received data to ep.
// ep may be nil, in which case received data is ignored.
// If block == false and no elements are available, returns (false, false).
// Otherwise, if c is closed, zeros *ep and returns (true, false).
// Otherwise, fills in *ep with an element and returns (true, true).
// A non-nil ep must point to the heap or the caller's stack.
func chanrecv(c *hchan, ep unsafe.Pointer, block bool) (selected, received bool) {
	// raceenabled: don't need to check ep, as it is always on the stack
	// or is new memory allocated by reflect.

	if debugChan { // 调试模式，打印信息
		print("chanrecv: chan=", c, "\n")
	}

	if c == nil { // 若通道为空
		if !block { //非阻塞，直接返回，否则 g挂起，panic
			return
		}
		gopark(nil, nil, waitReasonChanReceiveNilChan, traceEvGoStop, 2)
		throw("unreachable")
	}

	// Fast path: check for failed non-blocking operation without acquiring the lock.
	if !block && empty(c) { //非阻塞，并且c为空
		// After observing that the channel is not ready for receiving, we observe whether the
		// channel is closed.
		//
		// Reordering of these checks could lead to incorrect behavior when racing with a close.
		// For example, if the channel was open and not empty, was closed, and then drained,
		// reordered reads could incorrectly indicate "open and empty". To prevent reordering,
		// we use atomic loads for both checks, and rely on emptying and closing to happen in
		// separate critical sections under the same lock.  This assumption fails when closing
		// an unbuffered channel with a blocked send, but that is an error condition anyway.
		if atomic.Load(&c.closed) == 0 { // 未关闭，直接返回
			// Because a channel cannot be reopened, the later observation of the channel
			// being not closed implies that it was also not closed at the moment of the
			// first observation. We behave as if we observed the channel at that moment
			// and report that the receive cannot proceed.
			return
		}
		// The channel is irreversibly closed. Re-check whether the channel has any pending data
		// to receive, which could have arrived between the empty and closed checks above.
		// Sequential consistency is also required here, when racing with such a send.
		if empty(c) { //通道为空
			// The channel is irreversibly closed and empty.
			if raceenabled { // 竞态检测
				raceacquire(c.raceaddr())
			}
			if ep != nil { // 清空内存，返回true，false 代表被选中，已关闭
				typedmemclr(c.elemtype, ep)
			}
			return true, false
		}
	}

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

	lock(&c.lock) //上锁

	if c.closed != 0 { // 若已关闭
		if c.qcount == 0 { //元素个数为0
			if raceenabled {
				raceacquire(c.raceaddr())
			}
			unlock(&c.lock) //解锁，清空内存
			if ep != nil {
				typedmemclr(c.elemtype, ep)
			}
			return true, false
		}
		// The channel has been closed, but the channel's buffer have data.
	} else { //未关闭
		// Just found waiting sender with not closed.
		if sg := c.sendq.dequeue(); sg != nil { //发送队列有g，则直接从g中拷贝值
			// Found a waiting sender. If buffer is size 0, receive value
			// directly from sender. Otherwise, receive from head of queue
			// and add sender's value to the tail of the queue (both map to
			// the same buffer slot because the queue is full).
			recv(c, sg, ep, func() { unlock(&c.lock) }, 3)
			return true, true
		}
	}

	if c.qcount > 0 { // 发送队列无g，且通道中有剩余的值
		// Receive directly from queue
		qp := chanbuf(c, c.recvx) //取一个值
		if raceenabled {
			racenotify(c, c.recvx, nil)
		}
		if ep != nil {
			typedmemmove(c.elemtype, ep, qp) //移动值
		}
		typedmemclr(c.elemtype, qp) //清空
		c.recvx++ //接收指针下移
		if c.recvx == c.dataqsiz { //指针归零，构成环
			c.recvx = 0
		}
		c.qcount-- //元素个数-1
		unlock(&c.lock) //解锁
		return true, true //返回被选中，且通道未关闭
	}

	if !block { //非阻塞，解锁
		unlock(&c.lock)
		return false, false
	}

	// no sender available: block on this channel.
	gp := getg()
	mysg := acquireSudog()
	mysg.releasetime = 0
	if t0 != 0 {
		mysg.releasetime = -1
	}
	// No stack splits between assigning elem and enqueuing mysg
	// on gp.waiting where copystack can find it.
	mysg.elem = ep
	mysg.waitlink = nil
	gp.waiting = mysg
	mysg.g = gp
	mysg.isSelect = false
	mysg.c = c
	gp.param = nil
	c.recvq.enqueue(mysg) //加入接收队列，阻塞
	// 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(chanparkcommit, unsafe.Pointer(&c.lock), waitReasonChanReceive, traceEvGoBlockRecv, 2) //挂起

	// someone woke us up
	if mysg != gp.waiting {
		throw("G waiting list is corrupted")
	}
	gp.waiting = nil
	gp.activeStackChans = false
	if mysg.releasetime > 0 {
		blockevent(mysg.releasetime-t0, 2)
	}
	success := mysg.success
	gp.param = nil
	mysg.c = nil
	releaseSudog(mysg) //释放当前g，证明当前g已被调度
	return true, success
}

recv 在通道c上处理接收操作

// recv processes a receive operation on a full channel c.
// There are 2 parts:
//  1. The value sent by the sender sg is put into the channel
//     and the sender is woken up to go on its merry way.
//  2. The value received by the receiver (the current G) is
//     written to ep.
//
// For synchronous channels, both values are the same.
// For asynchronous channels, the receiver gets its data from
// the channel buffer and the sender's data is put in the
// channel buffer.
// Channel c must be full and locked. recv unlocks c with unlockf.
// sg must already be dequeued from c.
// A non-nil ep must point to the heap or the caller's stack.
func recv(c *hchan, sg *sudog, ep unsafe.Pointer, unlockf func(), skip int) {
	if c.dataqsiz == 0 { //非缓冲
		if raceenabled {
			racesync(c, sg)
		}
		if ep != nil {
			// copy data from sender
			recvDirect(c.elemtype, sg, ep) //直接复制值从发送的g到接收者
		}
	} else { //缓冲
		// Queue is full. Take the item at the
		// head of the queue. Make the sender enqueue
		// its item at the tail of the queue. Since the
		// queue is full, those are both the same slot.
		qp := chanbuf(c, c.recvx) //取一个值
		if raceenabled {
			racenotify(c, c.recvx, nil)
			racenotify(c, c.recvx, sg)
		}
		// copy data from queue to receiver
		if ep != nil {
			typedmemmove(c.elemtype, ep, qp) //复制值到ep
		}
		// copy data from sender to queue
		typedmemmove(c.elemtype, qp, sg.elem) //复制值到qp
		c.recvx++ //拿走一个，又加入一个，游标下移
		if c.recvx == c.dataqsiz {
			c.recvx = 0
		}
		c.sendx = c.recvx // c.sendx = (c.sendx+1) % c.dataqsiz
	}
	sg.elem = nil
	gp := sg.g
	unlockf()
	gp.param = unsafe.Pointer(sg)
	sg.success = true
	if sg.releasetime != 0 {
		sg.releasetime = cputicks()
	}
	goready(gp, skip+1) //就绪状态
}

chanparkcommit 挂起

func chanparkcommit(gp *g, chanLock 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 the
	// channel lock.
	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.
	atomic.Store8(&gp.parkingOnChan, 0)
	// Make sure we unlock after setting activeStackChans and
	// unsetting parkingOnChan. The moment we unlock chanLock
	// 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).
	unlock((*mutex)(chanLock))
	return true
}

selectnbsend select 操作，编译器行为

// 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)
}

reflect相关

//go:linkname reflect_chansend reflect.chansend
func reflect_chansend(c *hchan, elem unsafe.Pointer, nb bool) (selected bool) {
	return chansend(c, elem, !nb, getcallerpc())
}

//go:linkname reflect_chanrecv reflect.chanrecv
func reflect_chanrecv(c *hchan, nb bool, elem unsafe.Pointer) (selected bool, received bool) {
	return chanrecv(c, elem, !nb)
}

//go:linkname reflect_chanlen reflect.chanlen
func reflect_chanlen(c *hchan) int {
	if c == nil {
		return 0
	}
	return int(c.qcount)
}

//go:linkname reflectlite_chanlen internal/reflectlite.chanlen
func reflectlite_chanlen(c *hchan) int {
	if c == nil {
		return 0
	}
	return int(c.qcount)
}

//go:linkname reflect_chancap reflect.chancap
func reflect_chancap(c *hchan) int {
	if c == nil {
		return 0
	}
	return int(c.dataqsiz)
}

//go:linkname reflect_chanclose reflect.chanclose
func reflect_chanclose(c *hchan) {
	closechan(c)
}

enqueue 入队

func (q *waitq) enqueue(sgp *sudog) {
	sgp.next = nil
	x := q.last
	if x == nil {
		sgp.prev = nil
		q.first = sgp
		q.last = sgp
		return
	}
	sgp.prev = x
	x.next = sgp
	q.last = sgp
}

dequeue出队

func (q *waitq) dequeue() *sudog {
	for {
		sgp := q.first
		if sgp == nil {
			return nil
		}
		y := sgp.next
		if y == nil {
			q.first = nil
			q.last = nil
		} else {
			y.prev = nil
			q.first = y
			sgp.next = nil // mark as removed (see dequeueSudoG)
		}

		// if a goroutine was put on this queue because of a
		// select, there is a small window between the goroutine
		// being woken up by a different case and it grabbing the
		// channel locks. Once it has the lock
		// it removes itself from the queue, so we won't see it after that.
		// We use a flag in the G struct to tell us when someone
		// else has won the race to signal this goroutine but the goroutine
		// hasn't removed itself from the queue yet.
		if sgp.isSelect && !atomic.Cas(&sgp.g.selectDone, 0, 1) {
			continue
		}

		return sgp
	}
}

race相关

func (c *hchan) raceaddr() unsafe.Pointer {
	// Treat read-like and write-like operations on the channel to
	// happen at this address. Avoid using the address of qcount
	// or dataqsiz, because the len() and cap() builtins read
	// those addresses, and we don't want them racing with
	// operations like close().
	return unsafe.Pointer(&c.buf)
}

func racesync(c *hchan, sg *sudog) {
	racerelease(chanbuf(c, 0))
	raceacquireg(sg.g, chanbuf(c, 0))
	racereleaseg(sg.g, chanbuf(c, 0))
	raceacquire(chanbuf(c, 0))
}

// Notify the race detector of a send or receive involving buffer entry idx
// and a channel c or its communicating partner sg.
// This function handles the special case of c.elemsize==0.
func racenotify(c *hchan, idx uint, sg *sudog) {
	// We could have passed the unsafe.Pointer corresponding to entry idx
	// instead of idx itself.  However, in a future version of this function,
	// we can use idx to better handle the case of elemsize==0.
	// A future improvement to the detector is to call TSan with c and idx:
	// this way, Go will continue to not allocating buffer entries for channels
	// of elemsize==0, yet the race detector can be made to handle multiple
	// sync objects underneath the hood (one sync object per idx)
	qp := chanbuf(c, idx)
	// When elemsize==0, we don't allocate a full buffer for the channel.
	// Instead of individual buffer entries, the race detector uses the
	// c.buf as the only buffer entry.  This simplification prevents us from
	// following the memory model's happens-before rules (rules that are
	// implemented in racereleaseacquire).  Instead, we accumulate happens-before
	// information in the synchronization object associated with c.buf.
	if c.elemsize == 0 {
		if sg == nil {
			raceacquire(qp)
			racerelease(qp)
		} else {
			raceacquireg(sg.g, qp)
			racereleaseg(sg.g, qp)
		}
	} else {
		if sg == nil {
			racereleaseacquire(qp)
		} else {
			racereleaseacquireg(sg.g, qp)
		}
	}
}