// 无缓冲区的 channel
// 无缓冲区的 channel 必须有协程在期待它才能够向 channel 发送数据
ch := make(chan string)
// 向 channel 发送数据
ch <- "hllo"
// 从 channel 承受数据并赋给 x
x = <-ch
// 从 channel 接收数据并抛弃
<-ch
// 有缓冲区的 cahnnel
c := make(chan string,10)
type hchan struct {
// 环形缓冲区
// 队列中的总数据
qcount uint // total data in the queue
// 循环队列的大小
dataqsiz uint // size of the circular queue
// 指向 dataqsiz 的数组的元素
buf unsafe.Pointer // points to an array of dataqsiz elements
// 元素的大小
elemsize uint16
// 是否 closed
closed uint32
// 元素类型
elemtype *_type // element type
// 发送索引
sendx uint // send index
// 接管索引
recvx uint // receive index
// 接管队列，由链表实现
recvq waitq // list of recv waiters
// 发送队列，由链表实现
sendq waitq // list of send waiters
// 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.
// 爱护 hchan 所有字段
// channel 并不是无锁的
// 只在产生数据和接收数据时加锁，其余期间不加锁
lock mutex
}
type waitq struct {
first *sudog
last *sudog
}
// sudog represents a g in a wait list, such as for sending/receiving
// on a channel.
//
// sudog is necessary because the g ↔ synchronization object relation
// is many-to-many. A g can be on many wait lists, so there may be
// many sudogs for one g; and many gs may be waiting on the same
// synchronization object, so there may be many sudogs for one object.
//
// sudogs are allocated from a special pool. Use acquireSudog and
// releaseSudog to allocate and free them.
type sudog struct {
// The following fields are protected by the hchan.lock of the
// channel this sudog is blocking on. shrinkstack depends on
// this for sudogs involved in channel ops.
g *g
next *sudog
prev *sudog
elem unsafe.Pointer // data element (may point to stack)
// The following fields are never accessed concurrently.
// For channels, waitlink is only accessed by g.
// For semaphores, all fields (including the ones above)
// are only accessed when holding a semaRoot lock.
acquiretime int64
releasetime int64
ticket uint32
// isSelect indicates g is participating in a select, so
// g.selectDone must be CAS'd to win the wake-up race.
isSelect bool
// success indicates whether communication over channel c
// succeeded. It is true if the goroutine was awoken because a
// value was delivered over channel c, and false if awoken
// because c was closed.
success bool
parent *sudog // semaRoot binary tree
waitlink *sudog // g.waiting list or semaRoot
waittail *sudog // semaRoot
c *hchan // channel
}
ch := make(chan int,10)
0x0018 00024 (E:\project\deom\main.go:6) LEAQ type.chan int(SB), AX
0x001f 00031 (E:\project\deom\main.go:6) MOVL $10, BX
0x0024 00036 (E:\project\deom\main.go:6) PCDATA $1, $0
0x0024 00036 (E:\project\deom\main.go:6) CALL runtime.makechan(SB)
ch := make(chan int)
0x0018 00024 (E:\project\deom\main.go:6) LEAQ type.chan int(SB), AX
0x001f 00031 (E:\project\deom\main.go:6) XORL BX, BX
0x0021 00033 (E:\project\deom\main.go:6) PCDATA $1, $0
0x0021 00033 (E:\project\deom\main.go:6) CALL runtime.makechan(SB)
func makechan(t *chantype, size int) *hchan {
elem := t.elem
// compiler checks this but be safe.
if elem.size >= 1<<16 {throw("makechan: invalid channel element type")
}
if hchanSize%maxAlign != 0 || elem.align > maxAlign {throw("makechan: bad alignment")
}
mem, overflow := math.MulUintptr(elem.size, uintptr(size))
if overflow || mem > maxAlloc-hchanSize || size < 0 {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 {
// 队列或元素大小为零。case mem == 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:
// Elements do not contain pointers.
// Allocate hchan and buf in one call.
// 元素不蕴含指针。// 一次调用调配 hchan 和 buf。c = (*hchan)(mallocgc(hchanSize+mem, nil, true))
c.buf = add(unsafe.Pointer(c), hchanSize)
default:
// Elements contain pointers.
// 元素蕴含指针。c = new(hchan)
c.buf = mallocgc(mem, elem, true)
}
c.elemsize = uint16(elem.size)
c.elemtype = elem
c.dataqsiz = uint(size)
lockInit(&c.lock, lockRankHchan)
if debugChan {print("makechan: chan=", c, "; elemsize=", elem.size, "; dataqsiz=", size, "\n")
}
return c
}
func makechan64(t *chantype, size int64) *hchan {if int64(int(size)) != size {panic(plainError("makechan: size out of range"))
}
return makechan(t, int(size))
}
ch <- "data"
// ch <- 是一个语法糖
// entry point for c <- x from compiled code
//go:nosplit
// 编译阶段，会把 ch <- 转化为 runtime.chansend1()
// chansend1() 会调用 chansen()
func chansend1(c *hchan, elem unsafe.Pointer) {chansend(c, elem, true, getcallerpc())
}
/*
* 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 {
if !block {return false}
gopark(nil, nil, waitReasonChanSendNilChan, traceEvGoStop, 2)
throw("unreachable")
}
if debugChan {print("chansend: chan=", c, "\n")
}
if raceenabled {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) {return false}
var t0 int64
if blockprofilerate > 0 {t0 = cputicks()
}
// 加锁
// 缓冲区足够大，加锁工夫很短
lock(&c.lock)
// 判断 channel 是否敞开
if c.closed != 0 {unlock(&c.lock)
panic(plainError("send on closed channel"))
}
// 期待队列
// 接管队列出队。如果不为空（即有人在期待），进入 send
// 间接发送数据，if sg := c.recvq.dequeue(); sg != nil {
// Found a waiting receiver. We pass the value we want to send
// directly to the receiver, bypassing the channel buffer (if any).
// 找到一个正在期待的接收器。咱们将要发送的值间接传递给接收者，绕过通道缓冲区 //（如果有的话）。send(c, sg, ep, func() {unlock(&c.lock) }, 3)
return true
}
// 判断缓冲区是否还有空间
if c.qcount < c.dataqsiz {
// Space is available in the channel buffer. Enqueue the element to send.
//channel 缓冲区中有可用空间。将要发送的元素入队。// chanbuf(c, i) 是指向缓冲区中第 i 个槽的指针
// 找出缓冲区中要存入的地址
qp := chanbuf(c, c.sendx)
if raceenabled {racenotify(c, c.sendx, nil)
}
// func typedmemmove(typ *_type, dst unsafe.Pointer, src unsafe.Pointer)
// typedmemmove 将 t 类型的值从 src\ 复制到 dst。必须是 nosplit
// 存入缓冲区
typedmemmove(c.elemtype, qp, ep)
// 保护索引
c.sendx++
if c.sendx == c.dataqsiz {c.sendx = 0}
// 保护索引
c.qcount++
unlock(&c.lock)
return true
}
if !block {unlock(&c.lock)
return false
}
// Block on the channel. Some receiver will complete our operation for us.
// 休眠期待
// 拿到本人的协程构造体
gp := getg()
// 把本人包装为 Sudog
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.
// 记录要发送的数据
mysg.elem = ep
mysg.waitlink = nil
// 记录协程的指针
mysg.g = gp
mysg.isSelect = false
mysg.c = c
gp.waiting = mysg
gp.param = nil
// 入队
c.sendq.enqueue(mysg)
// 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)
// 休眠
gopark(chanparkcommit, unsafe.Pointer(&c.lock), waitReasonChanSend, traceEvGoBlockSend, 2)
// 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)
// 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 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 {
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
}
}
// 将数据间接拷贝到接管变量中
if sg.elem != nil {sendDirect(c.elemtype, sg, ep)
sg.elem = nil
}
gp := sg.g
unlockf()
gp.param = unsafe.Pointer(sg)
sg.success = true
if sg.releasetime != 0 {sg.releasetime = cputicks()
}
// 唤醒协程
goready(gp, skip+1)
}
// 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.
// 目的地的指针
dst := sg.elem
typeBitsBulkBarrier(t, uintptr(dst), uintptr(src), t.size)
// No need for cgo write barrier checks because dst is always
// Go memory.
// 间接拷贝
memmove(dst, src, t.size)
}
//go:nosplit
func acquireSudog() *sudog {
// Delicate dance: the semaphore implementation calls
// acquireSudog, acquireSudog calls new(sudog),
// new calls malloc, malloc can call the garbage collector,
// and the garbage collector calls the semaphore implementation
// in stopTheWorld.
// Break the cycle by doing acquirem/releasem around new(sudog).
// The acquirem/releasem increments m.locks during new(sudog),
// which keeps the garbage collector from being invoked.
mp := acquirem()
pp := mp.p.ptr()
if len(pp.sudogcache) == 0 {lock(&sched.sudoglock)
// First, try to grab a batch from central cache.
for len(pp.sudogcache) < cap(pp.sudogcache)/2 && sched.sudogcache != nil {
s := sched.sudogcache
sched.sudogcache = s.next
s.next = nil
pp.sudogcache = append(pp.sudogcache, s)
}
unlock(&sched.sudoglock)
// If the central cache is empty, allocate a new one.
if len(pp.sudogcache) == 0 {pp.sudogcache = append(pp.sudogcache, new(sudog))
}
}
n := len(pp.sudogcache)
s := pp.sudogcache[n-1]
pp.sudogcache[n-1] = nil
pp.sudogcache = pp.sudogcache[:n-1]
if s.elem != nil {throw("acquireSudog: found s.elem != nil in cache")
}
releasem(mp)
return s
}
- 发送数据前，曾经有 G 在休眠期待接管
- 此时缓冲区为空，不必思考缓冲区
- 将数据间接拷贝给 G 的接管变量，唤醒 G
实现
- 从接管期待队列中取出一个期待接管的 G
- 将数据间接拷贝到接管变量中
实现
- 没有 G 在休眠期待，而且没有缓冲或缓冲区满了
- 进入发送期待队列，休眠期待
实现
- 把本人包装成 sudog
- 把 sudog 放入 sendq 队列
- 休眠并解锁
- 被唤醒后，数据曾经被取走，保护其余数据
// <-ch 是一个语法糖
<- ch
// 编译阶段，转化为 runtime.chanrecv1()
data <- ch
// 编译阶段，转化为 runtime.chanrecv2()
value,ok <- ch
// 最初调用 chanrecv()
// entry points for <- c from compiled code
//go:nosplit
func chanrecv1(c *hchan, elem unsafe.Pointer) {chanrecv(c, elem, true)
}
//go:nosplit
func chanrecv2(c *hchan, elem unsafe.Pointer) (received bool) {_, received = chanrecv(c, elem, true)
return
}
- 接收数据前，曾经有 G 在休眠期待发送
- 而且这个 channel 没有缓冲区
- 将数据间接从 G 拷贝过去，唤醒 G
实现
- 判断有 G 在产生期待队列，进入`recv()`
- 判断此 channel 无缓冲区
- 间接从期待的 G 中取走数据，唤醒 G
- 接收数据前，曾经有 G 在休眠期待发送（缓冲区中的数据比发送队列的数据更晚，因为数据先发送到缓冲区，缓冲区满了当前再发送到发送队列）
- 而且 channel 有缓冲区
- 从缓冲区取走一个数据
- 将休眠 G 的数据放入缓冲区，唤醒 G
实现
- 判断有 G 在发送队列期待，进入 recv()
- 判断此 channel 有缓冲区 i
- 从缓冲区取走一个数据
- 将 G 的数据放入缓冲区，唤醒 G
- 没有 G 在休眠期待，然而缓冲区有内容
- 间接从缓冲区取走数据
实现
- 判断没有 G 在发送队列期待
- 判断 channel 有无缓冲区
- 从缓冲区取走一个数据
- 没有发送 G 在休眠期待而且没有缓冲区或者缓冲区为空
- 进入接管队列，休眠期待
实现
- 判断没有 G 在发送期待
- 判断 channel 有无缓冲区
- 把本人包装为 sudog
- sudog 放入接管期待队列，休眠
- 唤醒时，发送的 G 曾经把数据拷贝到位
// 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 {print("chanrecv: chan=", c, "\n")
}
if c == nil {
if !block {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) {
// 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 {
// 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) {
// The channel is irreversibly closed and empty.
if raceenabled {raceacquire(c.raceaddr())
}
if ep != nil {typedmemclr(c.elemtype, ep)
}
return true, false
}
}
var t0 int64
if blockprofilerate > 0 {t0 = cputicks()
}
// 加锁
lock(&c.lock)
// channel 是否敞开
if c.closed != 0 && c.qcount == 0 {
if raceenabled {raceacquire(c.raceaddr())
}
unlock(&c.lock)
if ep != nil {typedmemclr(c.elemtype, ep)
}
return true, false
}
// 发送队列出队
// 发送队列不为空（即有G在休眠期待），进入 recv()
if sg := c.sendq.dequeue(); sg != nil {
// 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 {
// Receive directly from queue
// 取出数据
qp := chanbuf(c, c.recvx)
if raceenabled {racenotify(c, c.recvx, nil)
}
if ep != nil {typedmemmove(c.elemtype, ep, qp)
}
// 间接拷贝
typedmemclr(c.elemtype, qp)
// 保护索引
c.recvx++
if c.recvx == c.dataqsiz {c.recvx = 0}
c.qcount--
unlock(&c.lock)
return true, true
}
if !block {unlock(&c.lock)
return false, false
}
// no sender available: block on this channel.
// 阻塞
gp := getg()
// 包装 sudog
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
// 入队, 接管队列
c.recvq.enqueue(mysg)
// 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)
// 休眠
gopark(chanparkcommit, unsafe.Pointer(&c.lock), waitReasonChanReceive, traceEvGoBlockRecv, 2)
// someone woke us up
if mysg != gp.waiting {throw("G waiting list is corrupted")
}
// 唤醒当前保护本人的状态；间接返回
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)
return true, success
}
// 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) {
// 缓冲区为空
if c.dataqsiz == 0 {
if raceenabled {racesync(c, sg)
}
if ep != nil {
// copy data from sender
// 间接拷贝
recvDirect(c.elemtype, sg, ep)
}
} else {
// 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.
// 队列已满。获取队列头部的我的项目。使发送者将其我的项目排在队列的尾部。因为队列已满，因而它们都是同一个插槽。// 有缓冲区
// 取出数据
qp := chanbuf(c, c.recvx)
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)
}
// 间接拷贝
// copy data from sender to queue
typedmemmove(c.elemtype, qp, sg.elem)
// 保护索引
c.recvx++
if c.recvx == c.dataqsiz {c.recvx = 0}
// 保护索引
c.sendx = c.recvx // c.sendx = (c.sendx+1) % c.dataqsiz
}
sg.elem = nil
// 保护协程的状态
gp := sg.g
unlockf()
gp.param = unsafe.Pointer(sg)
sg.success = true
if sg.releasetime != 0 {sg.releasetime = cputicks()
}
// 唤醒发送协程
// 协程唤醒当前，数据曾经被取走，保护本人的状态，本人返回
goready(gp, skip+1)
}
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)
}
func closechan(c *hchan) {
if c == nil {panic(plainError("close of nil channel"))
}
// 加锁
lock(&c.lock)
//close of closed channel
// 敞开已敞开的 channel 会产生 panic
// 如果 channel 曾经敞开，解锁，产生一个 panic
if c.closed != 0 {unlock(&c.lock)
panic(plainError("close of closed channel"))
}
if raceenabled {callerpc := getcallerpc()
racewritepc(c.raceaddr(), callerpc, abi.FuncPCABIInternal(closechan))
racerelease(c.raceaddr())
}
// closed 置 1
c.closed = 1
var glist gList
// release all readers
// 开释所有的读者
for {
// 获取承受协程
// 从承受队列出队
sg := c.recvq.dequeue()
// 承受队列为空，跳出循环
if sg == nil {break}
// 协程数据不为空
if sg.elem != nil {typedmemclr(c.elemtype, sg.elem)
sg.elem = nil
}
if sg.releasetime != 0 {sg.releasetime = cputicks()
}
// 保护协程状态
gp := sg.g
gp.param = unsafe.Pointer(sg)
sg.success = false
if raceenabled {raceacquireg(gp, c.raceaddr())
}
glist.push(gp)
}
// release all writers (they will 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)
}
}
// len = 循环队列中的总数据
len = qcount
// cap = 循环队列的大小
cap = dataqsiz