原始碼路徑runtime/chan.go

CSP 模型

     CSP 模型全稱為 communicating sequential processes，CSP 模型由併發執行實體(程序，執行緒或協程)，和訊息通道組成，實體之間通過訊息通道傳送訊息

channel讀寫

ch := make(chan int)

// write
ch <- x

// read
x <- ch

// another read
x = <- ch

  注意： channel 初始化後才能進行讀寫操作，否則阻塞

關閉 channel

ch := make(chan int)

close(ch)

• 關閉未初始化 channel 會產生 panic
• 重複關閉 channel 產生 panic
• 向已關閉 channel 中傳送會產生 panic
• 從已關閉的 channel 讀取不會 panic，能讀出 channel 未被讀取的，若訊息均已讀出則會讀到型別的零值。從已關閉的 channel 中讀取不會阻塞，會返回一個為 false 的 ok-idiom（判斷 channel 是否關閉）
• 關閉 channel 產生廣播機制，所有讀取channel的 goroutine 都會收到訊息

結構體

  hchan結構體

    Channel 實際上是個環形佇列。

    recvq和sendq分別用來儲存對應的阻塞佇列

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
}

1. makechan函式

  在堆上分配地址空間

  1.1 驗證機制

• 資料型別大小，大於1<<16時異常
• 記憶體對齊（降低定址次數），大於最大的記憶體8位元組數時異常
• 傳入的size大小，大於堆可分配的最大記憶體時異常

	// 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")
	}

	if size < 0 || uintptr(size) > maxSliceCap(elem.size) || uintptr(size)*elem.size > maxAlloc-hchanSize {
		panic(plainError("makechan: size out of range"))
	}

  1.2 分配地址空間

• size為0，分配hchan結構體空間，即無快取channel
• 不包括指標，分配連續地址空間，包括hchan結構體+資料
• 包括指標，buf單獨分配資料地址空間

	// 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 size == 0 || elem.size == 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.kind&kindNoPointers != 0:
		// Elements do not contain pointers.
		// Allocate hchan and buf in one call.
		c = (*hchan)(mallocgc(hchanSize+uintptr(size)*elem.size, nil, true))
		c.buf = add(unsafe.Pointer(c), hchanSize)
	default:
		// Elements contain pointers.
		c = new(hchan)
		c.buf = mallocgc(uintptr(size)*elem.size, elem, true)
	}

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

2. chansend函式

  操作如： ch <- x

  2.1 未創始化為nil，向其中傳送資料將會阻塞

	if c == nil {
		if !block {
			return false
		}
		gopark(nil, nil, waitReasonChanSendNilChan, traceEvGoStop, 2)
		throw("unreachable")
	}

  2.2 這是佇列滿的情況

	// 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 c.recvq.first or c.qcount depending on kind of channel).
	// 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.
	if !block && c.closed == 0 && ((c.dataqsiz == 0 && c.recvq.first == nil) ||
		(c.dataqsiz > 0 && c.qcount == c.dataqsiz)) {
		return false
	}

  2.3 已經關閉channel，向其中傳送將panic

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

  2.4 從當前channel等待佇列取出gorouine然後呼叫 send（第3節講解）方法

    goroutine 阻塞在 channel 上，直接將資料傳送給該 goroutine，從當前 channel 的等待佇列中取出等待的 goroutine，然後呼叫 send。goready 負責喚醒 goroutine，傳說中的站茅坑不使用，被搶

	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
	}

  2.5 快取佇列還有坑位的情況

  sendx代表隊列index，直接加入

	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
	}

  2.6 快取佇列麼有坑位的情況

   那加入send佇列

	// 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)
	goparkunlock(&c.lock, waitReasonChanSend, traceEvGoBlockSend, 3)

	// someone woke us up.
	if mysg != gp.waiting {
		throw("G waiting list is corrupted")
	}
	gp.waiting = nil
	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

3. send函式

  傳送給goroutine，呼叫goready喚醒goroutine

// 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.
			qp := chanbuf(c, c.recvx)
			raceacquire(qp)
			racerelease(qp)
			raceacquireg(sg.g, qp)
			racereleaseg(sg.g, qp)
			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)
	if sg.releasetime != 0 {
		sg.releasetime = cputicks()
	}
	goready(gp, skip+1)
}

4. closechan函式

  4.1 close未初始化channel產生panic，關閉已經closed的channel將產生panic

	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"))
	}

  4.2 釋放所有讀取

	// 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 = nil
		if raceenabled {
			raceacquireg(gp, c.raceaddr())
		}
		gp.schedlink.set(glist)
		glist = gp
	}

  4.3 釋放所有寫

	// 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 = nil
		if raceenabled {
			raceacquireg(gp, c.raceaddr())
		}
		gp.schedlink.set(glist)
		glist = gp
	}

  4.4

	// Ready all Gs now that we've dropped the channel lock.
	for glist != nil {
		gp := glist
		glist = glist.schedlink.ptr()
		gp.schedlink = 0
		goready(gp, 3)
	}

5. chanrecv函式 

  x :=  <-ch

  5.1 未初始化channel，阻塞

	if c == nil {
		if !block {
			return
		}
		gopark(nil, nil, waitReasonChanReceiveNilChan, traceEvGoStop, 2)
		throw("unreachable")
	}

  5.2 從已經關閉的且書記已經都讀出，則返回資料型別的0值

	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
	}

  5.3 當前有傳送 goroutine 阻塞在 channel 上，說明buf 已經滿員

	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
	}