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Golang channel 源码分析

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Golang中，在接受和发送数据的同时，channel决定一个Goroutine是执行还是阻塞。关于Golang的调度器，可以看这里。

channel结构

Golang中，channel结构体是用来进行在Goroutine中进行信息传递的结构体。

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ch := make(chan int, 8)

运行是，它是这样的:

hchan 结构体

当使用make(chan int,8)时，channel是从hchan创建的。hchan的代码.

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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
}
  • dataqsize是buffer的大小，也就是make(chan T, N)中的N。
  • elemsize是channel中单个元素的大小
  • buf是带缓冲的channel(buffered channel)中用来保存数据的循环队列
  • closed表示channel是否关闭。0->打开，1->关闭
  • sendx和recvx表示循环队列接受和发送数据的下标
  • recvq和sendq是保存阻塞的Goroutine的等待队列，recvq保存读取数据而阻塞的Goroutine,sendq保存写入数据而阻塞的Goroutine
  • lock是在每个读写操作对channel的锁

sudog 结构体

sudog表示Goroutine

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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

	// isSelect indicates g is participating in a select, so
	// g.selectDone must be CAS'd to win the wake-up race.
	isSelect bool
	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
	parent      *sudog // semaRoot binary tree
	waitlink    *sudog // g.waiting list or semaRoot
	waittail    *sudog // semaRoot
	c           *hchan // channel
}

看下面的例子

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func goRoutineA(a <-chan int) {
	val := <-a
	fmt.Println("goRoutineA received the data", val)
}

func goRoutineB(b <-chan int) {
	val := <-b
	fmt.Println("goRoutineB received the data", val)
}

func TestGoRoutine(t *testing.T) {
	ch := make(chan int)
	go goRoutineA(ch)
	go goRoutineB(ch)
	ch <- 3
	time.Sleep(time.Second * 1)
}

运行程序时，在ch <- 3加个断点，看一下这时channel的内部结构:
channeld send
可以看到，recvq保存着因为读取数据而阻塞的两个Goroutine

recvq和sendq是一个链表:
recvq_structure

channel的发送数据操作

  1. nil channel发送

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    if c == nil {
    		...
    		gopark(nil, nil, waitReasonChanSendNilChan, traceEvGoStop, 2)
    		throw("unreachable")
    	}

    向nil channel发送数据，当前的Goroutine会暂停操作。

  2. closed channel 发送

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    if c.closed != 0 {
    		unlock(&c.lock)
    		panic(plainError("send on closed channel"))
    	}

    如果channel已经关闭，还要发送数据，会引发panic

  3. 在channel上阻塞的Goroutine，会直接向其发送数据

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    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
    	}

    如果在recvq上有等待接收数据的Goroutine,当前的写操作会直接向其发送数据。

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    func send(c *hchan, sg *sudog, ep unsafe.Pointer, unlockf func(), skip int) {
    	...
    	if sg.elem != nil {
    		sendDirect(c.elemtype, sg, ep)
    		sg.elem = nil
    	}
    	gp := sg.g
    	unlockf()
    	gp.param = unsafe.Pointer(sg)
    	if sg.releasetime != 0 {
    		sg.releasetime = cputicks()
    	}
    	goready(gp, skip+1)
    }

    这里，通过调用goready(gp, skip+1)，使得阻塞的Goroutine变的可执行(runnable)。

  4. 带缓冲的channel，如果还有hchan.buf有空间，将数据放在缓冲区

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    if c.qcount < c.dataqsiz {
    	// Space is available in the channel buffer. Enqueue the element to send.
    	qp := chanbuf(c, c.sendx)
    	if raceenabled {
    		raceacquire(qp)
    		racerelease(qp)
    	}
    	typedmemmove(c.elemtype, qp, ep)
    	c.sendx++
    	if c.sendx == c.dataqsiz {
    		c.sendx = 0
    	}
    	c.qcount++
    	unlock(&c.lock)
    	return true
    }

    chanbuf(c, c.sendx)会操作对应的内存区域

  5. 当hchan.buf已经满了

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    // 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)
    	gopark(chanparkcommit, unsafe.Pointer(&c.lock), waitReasonChanSend, traceEvGoBlockSend, 2)

    在当前的栈上创建一个Goroutine，acquireSudog将当前的goroutine设置成park状态，然后将它放在channel的sendq队列。

发送操作总结

  1. lock锁住整个channel结构体
  2. 确定写操作。尝试从recvq中取出一个正在等待的goroutine，然后直接将数据写在里面。
  3. 如果recvq为空，确定buffer是否有空间。如果有，将数据放在缓冲区。
  4. 如果缓冲区没有空间，将数据保存在当前执行的goroutine，然后将这个goroutine放在sendq队列中，然后这个goroutine
    的执行暂停。

在第4点中，缓冲区不足的channel，或者没有缓存的channel，数据会保存在sudog结构体的elem

channel的读取数据操作

与channel的发送数据操作类似

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

	...

	lock(&c.lock)

	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
	}

	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
		qp := chanbuf(c, c.recvx)
		if raceenabled {
			raceacquire(qp)
			racerelease(qp)
		}
		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
	}

	if !block {
		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)
	gopark(chanparkcommit, unsafe.Pointer(&c.lock), waitReasonChanReceive, traceEvGoBlockRecv, 2)

	// 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)
	}
	closed := gp.param == nil
	gp.param = nil
	mysg.c = nil
	releaseSudog(mysg)
	return true, !closed
}

select操作

select的源代码在runtime/select.go

select的使用例子:

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func TestSelect(t *testing.T) {
	cInt := make(chan int, 5)
	cString := make(chan string, 5)
	select {
	case msg := <-cInt:
		fmt.Println("receive msg", msg)
	case msg := <-cString:
		fmt.Println("receive msg", msg)
	default:
		fmt.Println("no msg received")
	}
}

  1. 操作是互斥的，使所以需要获得参与select的channel的锁，先按hchane的地址排序，然后顺序获得锁，所以并不是同时获得所有参与select的channel的锁。

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    sellock(scases, lockorder)

    在scases数组上的每一个scase结构体，包含着当前的操作类型，和正在操作的channel。

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    type scase struct {
    	c           *hchan         // chan
    	elem        unsafe.Pointer // data element
    	kind        uint16
    	pc          uintptr // race pc (for race detector / msan)
    	releasetime int64
    }

    kind是当前操作类型的case,可以是CaseRecv, CaseSend 或者 CaseDefault.

  2. select的选取顺序是，以伪随机数的方法，将参与select的channel顺序打乱。所以select的顺序和程序声明的顺序是不一样的。

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    // generate permuted order
    	for i := 1; i < ncases; i++ {
    		j := fastrandn(uint32(i + 1))
    		pollorder[i] = pollorder[j]
    		pollorder[j] = uint16(i)
    	}
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for i := 0; i < ncases; i++ {
		casi = int(pollorder[i])
		cas = &scases[casi]
		c = cas.c

		switch cas.kind {
		case caseNil:
			continue

		case caseRecv:
			sg = c.sendq.dequeue()
			if sg != nil {
				goto recv
			}
			if c.qcount > 0 {
				goto bufrecv
			}
			if c.closed != 0 {
				goto rclose
			}

		case caseSend:
			if raceenabled {
				racereadpc(c.raceaddr(), cas.pc, chansendpc)
			}
			if c.closed != 0 {
				goto sclose
			}
			sg = c.recvq.dequeue()
			if sg != nil {
				goto send
			}
			if c.qcount < c.dataqsiz {
				goto bufsend
			}

		case caseDefault:
			dfli = casi
			dfl = cas
		}
	}

  1. 只要有不阻塞的channel，select就会返回，不会等待每一个参与select的channel都才准备好。

  2. 如果当前没有channle回应，并且没有default语句，当前的g就会对应的等待队列悬停。

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    // pass 2 - enqueue on all chans
    gp = getg()
    ...
    nextp = &gp.waiting
    for _, casei := range lockorder {
    	casi = int(casei)
    	cas = &scases[casi]
    	if cas.kind == caseNil {
    		continue
    	}
    	c = cas.c
    	sg := acquireSudog()
    	sg.g = gp
    	sg.isSelect = true
    	...

    	switch cas.kind {
    	case caseRecv:
    		c.recvq.enqueue(sg)

    	case caseSend:
    		c.sendq.enqueue(sg)
    	}
    }

    sg.isSelect 表示goroutine在select语句中

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