golang的channel实现原理

golang的channel实现原理

chan结构

src/runtime/chan.go

type hchan struct {
    qcount   uint           // 当前队列中剩余元素个数
    dataqsiz uint           // 环形队列长度，即可以存放的元素个数
    buf      unsafe.Pointer // 环形队列指针，也就是指向底层数组的指针
    elemsize uint16         // 每个元素的大小
    elemtype *_type         // 元素类型
    closed   uint32         // 标识关闭状态
    sendx    uint           // 队列下标，指示元素写入时存放到队列中的位置，环形队列的尾部下标
    recvx    uint           // 队列下标，指示元素从队列的该位置读出，环形队列的头部下标
    recvq    waitq          // 等待读消息的goroutine队列
    sendq    waitq          // 等待写消息的goroutine队列
    lock mutex              // 互斥锁，chan不允许并发读写
}

type waitq struct {
	first *sudog
	last  *sudog
}

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
}

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
}

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
	}
}
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关于 环形队列

waitq只记录了第一个和最后一个G的信息。

这个sudug非常重要，源码中到处可以看到有使用。此处通过sudug的 prev 和 next 构造了一个双向的链表。sudug 中包含了G，以及其他参数。

channel的一些特性

• 从channel读数据，如果channel缓冲区为空或者没有缓冲区，当前G会被阻塞。

• 向channel写数据，如果channel缓冲区已满或者没有缓冲区，当前G会被阻塞。

• 被阻塞的G将会挂在channel的等待队列中：

  因读阻塞的G会被向channel写入数据的G唤醒；

  因写阻塞的G会被从channel读数据的G唤醒；

• 对于buffered channel：

  c.qcount > 0 意味着c.recvq为空；

  c.qcount < c.dataqsiz 意味着c.sendq为空；

• 一般情况下recvq和sendq至少有一个为空。只有一个例外，那就是只有一个 G 使用 select 语句向一个无缓冲的channel一边写数据，一边读数据。

• elemtype代表类型，用于数据传递过程中的赋值。

• elemsize代表类型大小，用于在buf中定位元素位置。

• 一个channel同时仅允许被一个G读或写。

创建channel

创建channel的过程实际上是初始化hchan结构。其中类型信息和缓冲区长度由make语句传入，buf的大小则与元素大小和缓冲区长度共同决定。

func makechan(t *chantype, size int) *hchan {
	// 元素类型type
    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")
	}
	// 返回 elem.size * size，并判断结果是否超出了平台的上限
	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 {
	case mem == 0:
         // 有可能size为0，或者元素的类型大小为0（空结构体）
		// 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:
         // 元素的类型不包含指针
		// 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)
	default:
         // 元素的类型包含指针
		// Elements contain pointers.
		c = new(hchan)
		c.buf = mallocgc(mem, elem, true)
	}

	c.elemsize = uint16(elem.size)
	c.elemtype = elem
    
     // 环形队列大小
	c.dataqsiz = uint(size)
    
     // 初始化锁
	lockInit(&c.lock, lockRankHchan)

	if debugChan {
		print("makechan: chan=", c, "; elemsize=", elem.size, "; dataqsiz=", size, "\n")
	}
	return c
}
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// 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.
// 判断一个G向一个chan发送数据时是否应该被阻塞，也就是是否chan已满
// 对于无缓冲chan，如果已经存在读的G，那么写G就不用阻塞，这是一个优化的点
// 对于有缓冲chan，只需要判断缓冲区是否已满即可
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 {
		// 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
}

// 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 {
		return atomic.Loadp(unsafe.Pointer(&c.sendq.first)) == nil
	}
	return atomic.Loaduint(&c.qcount) == 0
}
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发送数据

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()
	}

	lock(&c.lock)

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

	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
	}

	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
	}

	if !block {
		unlock(&c.lock)
		return false
	}

	// 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)
	// 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)
	// 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
		}
	}
	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()
	}
	goready(gp, skip+1)
}
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向一个channel中写数据简单过程如下：

• 如果等待接收队列 recvq 不为空，说明缓冲区中没有数据或者没有缓冲区，此时直接从 recvq 取出G，并把数据直接写入到G中，最后把该G唤醒，结束发送过程；
• 如果缓冲区中有空余位置，将数据写入缓冲区，结束发送过程；
• 如果缓冲区中没有空余位置，将待发送数据写入G，将当前G加入 sendq，进入睡眠，等待被读G唤醒；

此处说的把数据直接写入到G中是个什么意思呢？这个还需要借助接收数据的源码来一起看，后面再说。

接收数据

// 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()
	}

	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 {
			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
	}

	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)
	// 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)

	// 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)
	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) {
	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.
		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)
		}
		// 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)
}
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• 如果等待发送队列 sendq 不为空，且没有缓冲区，直接从 sendq 中取出G，把G中数据读出，最后把G唤醒，结束读取过程；
• 如果等待发送队列 sendq 不为空，此时说明缓冲区已满，从缓冲区中首部读出数据，把G中数据写入缓冲区尾部，把G唤醒，结束读取过程；
• 如果缓冲区中有数据，则从缓冲区取出数据，结束读取过程；
• 如果缓冲区为空或没有缓冲区，然后 sendq 也为空，则将当前G加入 recvq，进入睡眠，等待被写G唤醒；

发送和接收过程

1、发送数据
c <- d

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

ep 指向要发送的数据 d

如果从recvq中取到了接收者，那么将 ep 赋值给 接收者sudug.elem，直接返回，接收者也被唤醒。
如果能进缓冲区，那么发送成功，直接返回。
如果要进入sendq，那么将 ep 赋值给自己的 sudug.elem，挂起。

2、接收数据
d <- c
d, ok <- c

ep 指向要接收的数据 d
received 返回给 ok

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

如果能从sendq取到发送者，将发送者sudug.elem赋值给 ep，返回，发送者也被唤醒。
如果能从缓冲区拿到数据，那么直接赋值给 ep，返回。
如果要进入recvq，将ep赋值给自己的sudug.elem，挂起。

chanrecv的参数block如果传入false，意味着如果没有数据是不会被挂起的，比如我们在使用select
语句读取多个chan的时候。
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关闭chan

func closechan(c *hchan) {
	if c == nil {
		panic(plainError("close of nil channel"))
	}

	lock(&c.lock)
	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())
	}

	c.closed = 1

	var glist gList

	// release all readers
	for {
		sg := c.recvq.dequeue()
		if sg == nil {
			break
		}
		if sg.elem != nil {
			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)
	}

	// 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)
	}
}
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• 关闭channel时会把 recvq 中的G全部唤醒，本该写入G的数据位置为nil，也就是零值。但是，关闭通道后，如果又有G来读取通道数据，如果 channel 中有值，这里特指带 buffer 的 channel，那么就从 channel 中取，如果没有值，那么会返回 channel 元素的零值。

• 关闭channel时会把 sendq 中的G全部唤醒，但这些G会panic，因此在不确定是否还有 goroutine 需要向 channel 发送数据时，请勿贸然关闭 channel。

• 已关闭的 channel，再次关闭会 panic。

• 已关闭的 channel 不能再向其中发送内容，否则会 panic。