1.简介

channel是Go语言的一大特性，基于channel有很多值得探讨的问题，如

1. channel为什么是并发安全的？
2. 同步通道和异步通道有啥区别？
3. 通道为何会阻塞协程？
4. 使用通道导致阻塞的协程是如何解除阻塞的？

要了解本质，需要进源码查看，毕竟源码之下了无秘密。

2.原理

2.1创建

channel理论上有三种，带缓冲\不带缓冲\nil，写法如下：

// buffered ch := make(chan Task, 3) // unbuffered ch := make(chan int) // nil var ch chan int 复制代码

追踪make函数，会发现在builtin/builtin.go中仅有一个声明func make(t Type, size ...IntegerType) Type。真正的实现可以参考go内置函数make，简单来说在cmd/compile/internal/gc/typecheck.go中有函数typecheck1

// The result of typecheck1 MUST be assigned back to n, e.g. // 	n.Left = typecheck1(n.Left, top) func typecheck1(n *Node, top int) (res *Node) { 	if enableTrace && trace { 		defer tracePrint("typecheck1", n)(&res) 	}  	switch n.Op { 	case OMAKE: 		ok |= ctxExpr 		args := n.List.Slice() 		if len(args) == 0 { 			yyerror("missing argument to make") 			n.Type = nil 			return n 		}  		n.List.Set(nil) 		l := args[0] 		l = typecheck(l, Etype) 		t := l.Type 		if t == nil { 			n.Type = nil 			return n 		}  		i := 1 		switch t.Etype { 		default: 			yyerror("cannot make type %v", t) 			n.Type = nil 			return n  		case TCHAN: 			l = nil 			if i < len(args) { 				l = args[i] 				i++ 				l = typecheck(l, ctxExpr) 				l = defaultlit(l, types.Types[TINT]) 				if l.Type == nil { 					n.Type = nil 					return n 				} 				if !checkmake(t, "buffer", l) { 					n.Type = nil 					return n 				} 				n.Left = l 			} else { 				n.Left = nodintconst(0) 			} 			n.Op = OMAKECHAN //对应的函数位置 		}  		if i < len(args) { 			yyerror("too many arguments to make(%v)", t) 			n.Op = OMAKE 			n.Type = nil 			return n 		}  		n.Type = t  		if (top&ctxStmt != 0) && top&(ctxCallee|ctxExpr|Etype) == 0 && ok&ctxStmt == 0 { 			if !n.Diag() { 				yyerror("%v evaluated but not used", n) 				n.SetDiag(true) 			}  			n.Type = nil 			return n 		}  		return n 	} } 复制代码

最终真正实现位置为runtime/chan.go

func makechan(t *chantype, size int) *hchan {    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")    }     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:       // 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+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)     if debugChan {       print("makechan: chan=", c, "; elemsize=", elem.size, "; elemalg=", elem.alg, "; dataqsiz=", size, "\n")    }    return c } 复制代码

从这个函数可以看出，channel的数据结构为hchan

2.2结构

接下来我们看一下channel的数据结构，基于数据结构，可以推测出具体实现。

runtime/chan.go

type hchan struct { 	//channel队列里面总的数据量 	qcount   uint           // total data in the queue 	// 循环队列的容量，如果是非缓冲的channel就是0 	dataqsiz uint           // size of the circular queue 	// 缓冲队列，数组类型。 	buf      unsafe.Pointer // points to an array of dataqsiz elements 	// 元素占用字节的size 	elemsize uint16 	// 当前队列关闭标志位，非零表示关闭 	closed   uint32 	// 队列里面元素类型 	elemtype *_type // element type 	// 队列send索引 	sendx    uint   // send index 	// 队列索引 	recvx    uint   // receive index 	// 等待channel的G队列。 	recvq    waitq  // list of recv waiters 	// 向channel发送数据的G队列。 	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. 	// 全局锁 	lock mutex } 复制代码

通过该hchan的数据结构和makechan函数，数据结构里有几个值得说明的数据：

1. dataqsiz表示channel的长度，如果为非缓冲队列，则值为0。通过dataqsiz实现环形队列。
2. buf存放真正的数据
3. sendx和recvx指在环形队列中数据入channel和出channel的位置
4. sendq存放向channel发送数据的goroutine队列
5. recvq存放等待获取channel数据的goroutine队列
6. lock为全局锁

2.3Anwser

通过追查到的代码，我们可以回答最开始提出的几个问题了。

2.3.1channel为什么是并发安全的？

因为做操作之前，都会先获取全局锁，只有获取成功的才能进行操作，保证了并发安全。

2.3.2同步通道和异步通道有啥区别？

使用的底层数据结构、操作代码都是一样的，只不过dataqsiz的值不一样，一个为0，一个为正数。

2.3.3通道为何会阻塞协程？

当通道已经满了，但协程继续往通道里写入，或者通道里没有数据，但是协程从通道里获取数据时，协程会被阻塞。

实现的原理与Golang并发调度的GMP模型强相关。

写入满通道的流程

1. 当前goroutine（G1）创建自身的一个引用（sudog），放置到hchan的sendq队列
2. 当前goroutine（G1）会调用gopark函数，将当前协程置为waiting状态；
3. 将M和G1绑定关系断开；
4. scheduler会调度另外一个就绪态的goroutine与M建立绑定关系，然后M 会运行另外一个G。

读取空通道的流程

1. 当前goroutine（G2）会创建自身的一个引用（sudog）
2. 将代表G2的sudog存入recvq等待队列
3. G2会调用gopark函数进入等待状态，让出OS thread，然后G2进入阻塞态

2.3.4使用通道导致阻塞的协程是如何解除阻塞的？

对于已经满的通道，当有协程G2做读操作时，会解除G1的阻塞，流程为

1. G2调用 t:=<-ch 获取一个元素A；
2. 从hchan的buf里面取出一个元素；
3. 从sendq等待队列里面pop一个sudog；
4. 将G1要写入的数据复制到buf中A的位置，然后更新buf的sendx和recvx索引值；
5. G2调用goready(G1)将G1置为Runable状态，表示G1可以恢复运行；

对于读取空的通道，当有协程G1做写操作时，会解除G2的阻塞，流程为

1. 将待写入的消息发送给接收的goroutine G2；
2. G1调用goready(G2) 将G2设置成就绪状态，等待调度；

2.4实现

我们来看一下chan的具体实现

2.4.1读取数据

// 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.    //    // After observing that the channel is not ready for receiving, we observe that the    // channel is not closed. Each of these observations is a single word-sized read    // (first c.sendq.first or c.qcount, and second c.closed).    // 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.    //    // The order of operations is important here: reversing the operations can lead to    // incorrect behavior when racing with a close.    if !block && (c.dataqsiz == 0 && c.sendq.first == nil ||       c.dataqsiz > 0 && atomic.Loaduint(&c.qcount) == 0) &&       atomic.Load(&c.closed) == 0 {       return    }     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 {          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)    goparkunlock(&c.lock, waitReasonChanReceive, traceEvGoBlockRecv, 3)     // someone woke us up    if mysg != gp.waiting {       throw("G waiting list is corrupted")    }    gp.waiting = nil    if mysg.releasetime > 0 {       blockevent(mysg.releasetime-t0, 2)    }    closed := gp.param == nil    gp.param = nil    mysg.c = nil    releaseSudog(mysg)    return true, !closed } 复制代码

接收channel的数据的流程如下：

CASE1：前置channel为nil的场景：

如果block为非阻塞，直接return；

如果block为阻塞，就调用gopark()阻塞当前goroutine，并抛出异常。

• 前置场景，block为非阻塞，且channel为非缓冲队列且sender等待队列为空 或者 channel为有缓冲队列但是队列里面元素数量为0，且channel未关闭，这个时候直接return；
• 调用 lock(&c.lock) 锁住channel的全局锁；

CASE2：channel已经被关闭且channel缓冲中没有数据了，这时直接返回success和空值；

CASE3：sender队列非空，调用 func recv(c *hchan, sg *sudog, ep unsafe.Pointer, unlockf func(), skip int)

函数处理：

1.先取channel缓冲队列的对头元素复制给receiver(也就是ep)；

2.将sender队列的对头元素里面的数据复制到channel缓冲队列刚刚弹出的元素的位置，这样缓冲队列就不用移动数据了。

channel是非缓冲channel，直接调用recvDirect函数直接从sender recv元素到ep对象，这样就只用复制一次；

对于sender队列非空情况下， 有缓冲的channel的缓冲队列一定是满的：

释放channel的全局锁；

调用goready函数标记当前goroutine处于ready，可以运行的状态；

CASE4：sender队列为空，缓冲队列非空，直接取队列元素，移动头索引；

CASE5：sender队列为空、缓冲队列也没有元素且不阻塞协程，直接return (false,false)；

CASE6：sender队列为空且channel的缓存队列为空，将goroutine加入recv队列，并阻塞。

2.4.2写入数据

/*  * generic single channel send/recv  * If block is not nil,  * then the protocol will not  * sleep but return if it could  * not complete.  *  * sleep can wake up with g.param == nil  * when a channel involved in the sleep has  * been closed.  it is easiest to loop and re-run  * the operation; we'll see that it's now closed.  */ 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, funcPC(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 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    }     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 {          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    }     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)    goparkunlock(&c.lock, waitReasonChanSend, traceEvGoBlockSend, 3)    // 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    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 } 复制代码

向channel写入数据主要流程如下：

• CASE1：当channel为空或者未初始化，如果block表示阻塞那么向其中发送数据将会永久阻塞；如果block表示非阻塞就会直接return；
• CASE2：前置场景，block为非阻塞，且channel没有关闭(已关闭的channel不能写入数据)且(channel为非缓冲队列且receiver等待队列为空)或则( channel为有缓冲队列但是队列已满)，这个时候直接return；
• 调用 lock(&c.lock) 锁住channel的全局锁；
• CASE3：不能向已经关闭的channel send数据，会导致panic。
• CASE4：如果channel上的recv队列非空，则跳过channel的缓存队列，直接向消息发送给接收的goroutine：
• 调用sendDirect方法，将待写入的消息发送给接收的goroutine；
• 释放channel的全局锁；
• 调用goready函数，将接收消息的goroutine设置成就绪状态，等待调度。
• CASE5：缓存队列未满，则将消息复制到缓存队列上，然后释放全局锁；
• CASE6：缓存队列已满且接收消息队列recv为空，则将当前的goroutine加入到send队列；
• 获取当前goroutine的sudog，然后入channel的send队列；
• 将当前goroutine休眠

关闭channel

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, funcPC(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 = nil       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 = nil       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)    } } 复制代码

关闭的主要流程如下所示：

• 获取全局锁；
• 设置channel数据结构chan的关闭标志位；
• 获取当前channel上面的读goroutine并链接成链表；
• 获取当前channel上面的写goroutine然后拼接到前面的读链表后面；
• 释放全局锁；
• 唤醒所有的读写goroutine。

总结

了解一下具体实现还是很好的，虽然在使用上不会带来变化，不过理解了内涵后，能够更加灵活地使用通道，可以更加容易的追查到问题，也能学习到高手的设计思想。

资料

1. Golang-Channel原理解析
2. golang对于 nil通道 close通道你所不知道的神器特性
3. Go语言make和new关键字的区别及实现原理
4. Go底层引用实现
5. 图解Golang的channel底层原理
6. go内置函数make
7. Golang并发调度的GMP模型

最后

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