channel是Go语言的一大特性，基于channel有很多值得探讨的问题，如
要了解本质，需要进源码查看，毕竟源码之下了无秘密。
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
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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
}
}
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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
}
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从这个函数可以看出，channel的数据结构为hchan
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
}
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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：
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。
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。
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。
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
读取空通道的流程
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
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
我们来看一下chan的具体实现
// 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
}
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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。
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。
/*
* 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
}
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向channel写入数据主要流程如下：
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)
}
}
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关闭的主要流程如下所示：
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。
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