Go Chan 源码解析

发表于

本篇文章内容基于go1.14.2分析

golang的chan是一个内置类型，作为csp编程的核心数据结构，其底层数据结构是一个叫hchan的struct：

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go复制代码type hchan struct {
	qcount   uint           // 队列中的元素数量
	dataqsiz uint           // （环形）队列的大小
	buf      unsafe.Pointer // 队列的指针
	elemsize uint16 // 元素大小
	closed   uint32 // 是否已close
	elemtype *_type // 元素类型
	sendx    uint   // 环形队列中，send的位置
	recvx    uint   // 环形队列中 recv的位置
	recvq    waitq  // 读取等待队列
	sendq    waitq  // 发送等待队列
	lock mutex // 互斥锁
}

如图所示，chan最核心的部分由一个环形队列和2个waitq组成，环形队列用于存放数据（带缓冲的情况下），waitq用于实现阻塞和恢复goroutine。

chan的相关操作

对chan的操作有：make、读、写、close，当然还有select，这里只讨论前面四个操作。

创建 chan

当在代码中使用make创建chan时，编译器会根据情况自动替换成makechan64 或者makechan，makechan64 其实还是调用了makechan函数。

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go复制代码func makechan(t *chantype, size int) *hchan {
	elem := t.elem
	
  // 确保元素类型的size < 2^16，
	if elem.size >= 1<<16 {
		throw("makechan: invalid channel element type")
	}
  // 检查内存对齐
	if hchanSize%maxAlign != 0 || elem.align > maxAlign {
		throw("makechan: bad alignment")
	}

  // 计算缓冲区所需分配内存大小
	mem, overflow := math.MulUintptr(elem.size, uintptr(size))
	if overflow || mem > maxAlloc-hchanSize || size < 0 {
		panic(plainError("makechan: size out of range"))
	}

	var c *hchan
	switch {
	case mem == 0:
		// 即不带缓冲区的情况，只需要调用mallocgc分配
		c = (*hchan)(mallocgc(hchanSize, nil, true))
		// 理解为空地址
		c.buf = c.raceaddr()
	case elem.ptrdata == 0:
		// 元素类型不包含指针的情况
		c = (*hchan)(mallocgc(hchanSize+mem, nil, true))
		c.buf = add(unsafe.Pointer(c), hchanSize)
	default:
		// 默认情况下：包含指针
		c = new(hchan)
		c.buf = mallocgc(mem, elem, true)
	}

	c.elemsize = uint16(elem.size)
	c.elemtype = elem
	c.dataqsiz = uint(size)

	if debugChan {
		print("makechan: chan=", c, "; elemsize=", elem.size, "; dataqsiz=", size, "\n")
	}
	return c
}

chan 写操作

当对chan进行写入“ch <- interface{}” 时，会被编译器替换成chansend1函数的调用,最终还是调用了chansend函数：

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go复制代码//elem 是待写入元素的地址
func chansend1(c *hchan, elem unsafe.Pointer) {
	chansend(c, elem, true, getcallerpc())
}

先看看chansend的函数签名，只需关注ep和block这个两个参数即可，ep是要写入数据的地址，block表示是否阻塞式的调用

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go复制代码func chansend(c *hchan, ep unsafe.Pointer, block bool, callerpc uintptr) bool

chansend有以下几种处理流程：

  1. 当对一个nil chan进行写操作时，如果是非阻塞调用，直接返回；否则将当前协程挂起
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go复制代码// chansend 对一个 nil chan发送数据时，如果是非阻塞则直接返回，否则将当前协程挂起
if c == nil {
		if !block {
			return false
		}
		gopark(nil, nil, waitReasonChanSendNilChan, traceEvGoStop, 2)
		throw("unreachable")
	}
  1. 非阻塞模式且chan未close，没有缓冲区且没有等待接收或者缓冲区满的情况下，直接return false。
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go复制代码// 1. 非阻塞模式且chan未close
  // 2. 没有缓冲区且没有等待接收 或者 缓冲区满的情况下
  // 满足以上条件直接return false
if !block && c.closed == 0 && ((c.dataqsiz == 0 && c.recvq.first == nil) ||
		(c.dataqsiz > 0 && c.qcount == c.dataqsiz)) {
		return false
	}
  1. c.recvq中有等待读的接收者，将其出队，将数据直接copy给接收者，并唤醒接收者。
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go复制代码// 有等待的接收的goroutine
	// 出队，传递数据
	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).
		send(c, sg, ep, func() { unlock(&c.lock) }, 3)
		return true
	}

recvq是一个双向链表，每个sudog会关联上一个reader（被阻塞的g）

当sudog出队后，会调用send方法，通过sendDirect 实现数据在两个地址之间拷贝，最后调用goready唤醒reader（被阻塞的g）

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go复制代码func send(c *hchan, sg *sudog, ep unsafe.Pointer, unlockf func(), skip int) {
	// ... 剔除无关代码
	if sg.elem != nil {
		// 直接将数据拷贝到变量ep所在的地址
		sendDirect(c.elemtype, sg, ep)
		sg.elem = nil
	}
	gp := sg.g
	unlockf()
	gp.param = unsafe.Pointer(sg)
	if sg.releasetime != 0 {
		sg.releasetime = cputicks()
	}
	//将reader的goroutine唤起
	goready(gp, skip+1)
}
  1. 缓冲区未满的情况下，数据放入环形缓冲区即可。
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go复制代码	// 缓冲区未满
	// 将数据放到缓冲区
	if c.qcount < c.dataqsiz {
		// Space is available in the channel buffer. Enqueue the element to send.
		// 存放位置
		qp := chanbuf(c, c.sendx)
		if raceenabled {
			raceacquire(qp)
			racerelease(qp)
		}
		typedmemmove(c.elemtype, qp, ep)
		// 指针自增
		c.sendx++
		if c.sendx == c.dataqsiz {
			c.sendx = 0
		}
		c.qcount++
		unlock(&c.lock)
		return true
	}
  1. 缓冲区已满，阻塞模式下关联一个sudog数据结构并进入c.sendq队列，挂起当前协程。
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go复制代码	// 阻塞的情况
	gp := getg() //拿到当前g
	mysg := acquireSudog() // 获取一个sudog
	mysg.releasetime = 0
	if t0 != 0 {
		mysg.releasetime = -1
	
	mysg.elem = ep //关联ep，即待写入的数据地址
	mysg.waitlink = nil
	mysg.g = gp
	mysg.isSelect = false
	mysg.c = c
	gp.waiting = mysg
	gp.param = nil
	c.sendq.enqueue(mysg) // 入队
	// 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)
	// 将g休眠，让出cpu
  // gopark后，需等待reader来唤醒它
	gopark(chanparkcommit, unsafe.Pointer(&c.lock), waitReasonChanSend, traceEvGoBlockSend, 2)
	// 唤醒过后
	// 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.
	// 保持数据不被回收
	KeepAlive(ep)

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

chan 读操作

当对chan进行读操作时，编译器会替换成 chanrecv1或者chanrecv2函数，最终会调用chanrecv函数处理读取

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go复制代码// v := <- ch
func chanrecv1(c *hchan, elem unsafe.Pointer) {
	chanrecv(c, elem, true)
}
// v, ok := <- ch
func chanrecv2(c *hchan, elem unsafe.Pointer) (received bool) {
	_, received = chanrecv(c, elem, true)
	return
}

和chansend一样，chanrecv也是支持非阻塞式的调用

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go复制代码func chanrecv(c *hchan, ep unsafe.Pointer, block bool) (selected, received bool)

chanrecv有以下几种处理流程：

  1. 读nil chan，如果是非阻塞，直接返回；如果是阻塞式，将当前协程挂起。
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go复制代码	// 读阻塞
	if c == nil {
		if !block {
			return
		}
		gopark(nil, nil, waitReasonChanReceiveNilChan, traceEvGoStop, 2)
		throw("unreachable")
	}
  1. 非阻塞模式下，没有缓冲区且没有等待写的writer或者缓冲区没数据，直接返回。
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go复制代码	if !block && (c.dataqsiz == 0 && c.sendq.first == nil ||
		c.dataqsiz > 0 && atomic.Loaduint(&c.qcount) == 0) &&
		atomic.Load(&c.closed) == 0 {
		return
	}
  1. chan已经被close，并且队列中没有数据时，会将存放值的变量清零，然后返回。
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go复制代码	// c已经被close 并且 没有数据
	// 清除ep指针
	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
	}
  1. sendq中有等待的writer，writer出队，并调用recv函数
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go复制代码// 从sendq中取出sender
	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).
		// 从sender中读取数据
		recv(c, sg, ep, func() { unlock(&c.lock) }, 3)
		return true, true
	}

recv在这分两种处理：如果ch不带缓冲区的话，直接将writer的sg.elem数据拷贝到ep；如果带缓冲区的话，此时缓冲区肯定满了，那么就从缓冲区队列头部取出数据拷贝至ep，然后将writer的sg.elem数据拷贝到缓冲区中，最后唤醒writer（g）

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go复制代码func recv(c *hchan, sg *sudog, ep unsafe.Pointer, unlockf func(), skip int) {
   // 不带缓冲区的情况
   // 直接copy from sender
   if c.dataqsiz == 0 {
      if raceenabled {
         racesync(c, sg)
      }
      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.
      // 队列已满
      // 队列元素出队
      qp := chanbuf(c, c.recvx)
      if raceenabled {
         raceacquire(qp)
         racerelease(qp)
         raceacquireg(sg.g, qp)
         racereleaseg(sg.g, qp)
      }
      // copy data from queue to receiver
      // 数据拷贝给ep
      if ep != nil {
         typedmemmove(c.elemtype, ep, qp)
      }
      // copy data from sender to queue
      // 将sender的数据拷贝到这个槽中
      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
   }
   // 置空
   sg.elem = nil
   gp := sg.g
   unlockf()
   gp.param = unsafe.Pointer(sg)
   if sg.releasetime != 0 {
      sg.releasetime = cputicks()
   }
   // 唤醒sender协程
   goready(gp, skip+1)
}
  1. 直接从缓冲队列中读数。
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go复制代码	// 带缓冲区
	if c.qcount > 0 {
		// Receive directly from queue
		// 直接buf中取
		qp := chanbuf(c, c.recvx)
		if raceenabled {
			raceacquire(qp)
			racerelease(qp)
		}
		// 拷贝数据到ep指针
		if ep != nil {
			typedmemmove(c.elemtype, ep, qp)
		}
		// 清除qp
		typedmemclr(c.elemtype, qp)
		c.recvx++
		if c.recvx == c.dataqsiz {
			c.recvx = 0
		}
		c.qcount--
		unlock(&c.lock)
		return true, true
	}
  1. 阻塞的情况，缓冲区没有数据，且没有writer
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go复制代码
	// 阻塞
	gp := getg() //拿到当前的goroutine
	mysg := acquireSudog() // 获取一个sudog
	mysg.releasetime = 0
	if t0 != 0 {
		mysg.releasetime = -1
	}
	
	//sudog 关联
	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) //入队
	// 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)
  // 挂起当前goroutine，等待writer唤醒
	gopark(chanparkcommit, unsafe.Pointer(&c.lock), waitReasonChanReceive, traceEvGoBlockRecv, 2)

	// 唤醒后
	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)
	}
	closed := gp.param == nil
	gp.param = nil
	// sudog解除关联
	mysg.c = nil
  // 释放sudog
	releaseSudog(mysg)

close 关闭操作

当close一个chan时，编译器会替换成对closechan函数的调用，将closed字段置为1，并将recvq和sendq中的goroutine释放唤醒，对sendq中未写入的数据做清除，且writer会发生panic异常。

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go复制代码func closechan(c *hchan) {
	if c == nil {
		panic(plainError("close of nil channel"))
	}
	
  // 加锁
	lock(&c.lock)
  // 不可重复close
	if c.closed != 0 {
		unlock(&c.lock)
		panic(plainError("close of closed channel"))
	}

	if raceenabled {
		callerpc := getcallerpc()
		racewritepc(c.raceaddr(), callerpc, funcPC(closechan))
		racerelease(c.raceaddr())
	}

	c.closed = 1

	var glist gList

	// 释放所有的
	for {
		// 出队
		sg := c.recvq.dequeue()
		if sg == nil {
			break
		}
		// 清零
		if sg.elem != nil {
			typedmemclr(c.elemtype, sg.elem)
			sg.elem = nil
		}
		if sg.releasetime != 0 {
			sg.releasetime = cputicks()
		}
		gp := sg.g
		gp.param = nil
		if raceenabled {
			raceacquireg(gp, c.raceaddr())
		}
		glist.push(gp)
	}

	// 释放所有writer
	for {
		// 出队
		sg := c.sendq.dequeue()
		if sg == nil {
			break
		}
		// 丢弃数据
		sg.elem = nil
		if sg.releasetime != 0 {
			sg.releasetime = cputicks()
		}
		gp := sg.g
		gp.param = nil
		if raceenabled {
			raceacquireg(gp, c.raceaddr())
		}
		glist.push(gp)
	}
	unlock(&c.lock)

	// 唤醒所有g
	for !glist.empty() {
		gp := glist.pop()
		gp.schedlink = 0
		goready(gp, 3)
	}
}

chan使用小技巧

  1. 避免read、write一个nil chan
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go复制代码func main() {
	ch := make(chan int,1)

	go func() {
		time.Sleep(1*time.Second)
		ch = nil
	}()

	ch<-1 
	ch<-1 // 协程直接挂起
}
  1. 从chan中read时，使用带指示的访问方式，读取的时候无法感知到close的关闭
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go复制代码func main() {
	ch := make(chan int)

	go func() {
		ch <- 10
		close(ch)
	}()

	for {
		select {
      // case i, ok := <-ch:
      // if ok {
      //  break
      //}
			case i := <-ch:
				fmt.Println(i)
				time.Sleep(100 * time.Millisecond)
		}
	}
}
  1. 从chan中read时，不要使用已存在变量接收, chan close之后，缓冲区没有数据的话，使用存在变量读取时，会将变量清零
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go复制代码func main() {
	a := 10
	ch := make(chan int,1)

	fmt.Println("before close a is: ", a) // a is 10
	close(ch)
	a = <-ch 
	fmt.Println("after close a is: ", a) // a is 0
}
  1. 使用select+default可以实现 chan的无阻塞读取
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go复制代码// 使用select反射包实现无阻塞读写
func tryRead(ch chan int) (int, bool) {
	var cases []reflect.SelectCase
	caseRead := reflect.SelectCase{
		Dir:  reflect.SelectRecv,
		Chan: reflect.ValueOf(ch),
	}

	cases = append(cases, caseRead)
	cases = append(cases, reflect.SelectCase{
		Dir: reflect.SelectDefault,
	})

	_, v, ok := reflect.Select(cases)

	if ok {

		return (v.Interface()).(int), ok
	}

	return 0, ok
}

func tryWrite(ch chan int, data int) bool {
	var cases []reflect.SelectCase
	caseWrite := reflect.SelectCase{
		Dir:  reflect.SelectSend,
		Chan: reflect.ValueOf(ch),
		Send: reflect.ValueOf(data),
	}

	cases = append(cases, caseWrite)
	cases = append(cases, reflect.SelectCase{
		Dir: reflect.SelectDefault,
	})
	chosen, _, _ := reflect.Select(cases)

	return chosen == 0
}

// 使用select + default实现无阻塞读写
func tryRead2(ch chan int) (int, bool) {
	select {
	case v, ok := <-ch:
		return v, ok
	default:
		return 0, false
	}
}

func tryWrite2(ch chan int, data int) bool {
	select {
	case ch <- data:
		return true
	default:
		return false
	}
}

原因是如果select的case中存在default，对chan的读写会使用无阻塞的方法

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go复制代码func selectnbsend(c *hchan, elem unsafe.Pointer) (selected bool) {
	return chansend(c, elem, false, getcallerpc())
}

func selectnbrecv(elem unsafe.Pointer, c *hchan) (selected bool) {
	selected, _ = chanrecv(c, elem, false)
	return
}

本文转载自: 掘金

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