Go 语言中 Channel 的实现

var ch chan int

// 定义一个无缓冲的队列
ch := make(chan int)

// 定义一个有缓冲的队列
ch := make(chan int, 10)

ch := make(chan int)
// 写入数据
ch <- 1

package main

import "fmt"

func main() {
	ch := make(chan int, 10)
	defer close(ch)
	ch <- 1

	// 读取数据
	i := <-ch
	fmt.Println(i)
}
// 1

func worker(ch chan int) {
	for n := range ch {
		fmt.Println("get data:", n)
	}
}

func producer(length int) chan int {
	return make(chan int, length)
}

// 限制只读
func worker(ch <-chan int) {
	for n := range ch {
		fmt.Println("get data:", n)
	}
}

// 限制只写
func writer(ch chan<- int) {
	for i := 0; i < 10; i++ {
		ch <- i
	}
}

ch := make(chan int, 1)
defer close(ch)

ticker := time.NewTicker(time.Minute * 5)
for range ticker.C {
	doSomeThing()
}

ctx, cancel := context.WithTimeout(context.Background(), 3*time.Second)
defer cancel()
ch := make(chan bool)
go func() {
  defer close(ch)
  //具体的任务，这里模拟做的任务需要1秒完成
  time.Sleep(time.Second * 1)
}()
select {
  case <- ch:
  fmt.Println("ch bool", <- ch)
  case <- ctx.Done():
  fmt.Println("ctx done")
}
// output:
// ch bool false

type hchan struct {
	qcount   uint           // total data in the queue
	dataqsiz uint           // size of the circular queue
	buf      unsafe.Pointer // points to an array of dataqsiz elements
	elemsize uint16
	closed   uint32
	elemtype *_type // element type
	sendx    uint   // send index
	recvx    uint   // receive index
	recvq    waitq  // list of recv waiters
	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.
	lock mutex
}

type waitq struct {
	first *sudog
	last  *sudog
}

func makechan(t *chantype, size int) *hchan {
	elem := t.elem
	mem, overflow := math.MulUintptr(elem.size, uintptr(size))
	// 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:
		// 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.
		c = (*hchan)(mallocgc(hchanSize+mem, nil, true))
		c.buf = add(unsafe.Pointer(c), hchanSize)
	default:
		// Elements contain pointers.
		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)

	return c
}

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

func chansend(c *hchan, ep unsafe.Pointer, block bool, callerpc uintptr) bool {
  ...
	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).
		send(c, sg, ep, func() { unlock(&c.lock) }, 3)
		return true
	}
  ...
}

func chansend(c *hchan, ep unsafe.Pointer, block bool, callerpc uintptr) bool {
...
	if c.qcount < c.dataqsiz {
		// 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)
		c.sendx++
		if c.sendx == c.dataqsiz {
			c.sendx = 0
		}
		c.qcount++
		unlock(&c.lock)
		return true
	}
...
}

func chansend(c *hchan, ep unsafe.Pointer, block bool, callerpc uintptr) bool {
  ...
	// Block on the channel. Some receiver will complete our operation for us.
	gp := getg()
	mysg := acquireSudog()
	mysg.releasetime = 0

	// 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)
	atomic.Store8(&gp.parkingOnChan, 1)
  
  // Puts the current goroutine into a waiting state and calls unlockf on the system stack.
	gopark(chanparkcommit, unsafe.Pointer(&c.lock), waitReasonChanSend, traceEvGoBlockSend, 2)

	gp.waiting = nil
	gp.activeStackChans = false
	closed := !mysg.success
	gp.param = nil
	mysg.c = nil
	releaseSudog(mysg)

	return true
}

// 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
}

func chanrecv(c *hchan, ep unsafe.Pointer, block bool) (selected, received bool) {
  // case 1
	if c == nil {
		if !block {
			return
		}
		gopark(nil, nil, waitReasonChanReceiveNilChan, traceEvGoStop, 2)
		throw("unreachable")
	}

	lock(&c.lock)
	// case 2
	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
	}
}

func chanrecv(c *hchan, ep unsafe.Pointer, block bool) (selected, received bool) {
	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
	}
}

func recv(c *hchan, sg *sudog, ep unsafe.Pointer, unlockf func(), skip int) {
	if c.dataqsiz == 0 {
		if ep != nil {
			// copy data from sender
			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.
		qp := chanbuf(c, c.recvx)
		// copy data from queue to receiver
		if ep != nil {
			typedmemmove(c.elemtype, ep, qp)
		}
		// copy data from sender to queue
		typedmemmove(c.elemtype, qp, sg.elem)
		c.recvx++
		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)
}

func chanrecv(c *hchan, ep unsafe.Pointer, block bool) (selected, received bool) {
	if c.qcount > 0 {
		// Receive directly from queue
		qp := chanbuf(c, c.recvx)
		if raceenabled {
			racenotify(c, c.recvx, nil)
		}
		if ep != nil {
			typedmemmove(c.elemtype, ep, qp)
		}
		typedmemclr(c.elemtype, qp)
		c.recvx++
		if c.recvx == c.dataqsiz {
			c.recvx = 0
		}
		c.qcount--
		unlock(&c.lock)
		return true, true
	}
}

func chanrecv(c *hchan, ep unsafe.Pointer, block bool) (selected, received bool) {
	// no sender available: block on this channel.
	gp := getg()
	mysg := acquireSudog()
	mysg.releasetime = 0

	// 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)
	// 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)

	gp.waiting = nil
	gp.activeStackChans = false

	success := mysg.success
	gp.param = nil
	mysg.c = nil
	releaseSudog(mysg)
	return true, success
}