Go goroutine什么时候用channel，什么时候用锁或者waitgroup
什么是channel
简介
Go语言的一大核心思想就是“以通信的手段来共享内存”，channel就是其最佳的体现。
channel提供一种机制，可以同步两个并发执行的函数，还可以让两个函数通过互相传递特定类型的值来通信
channel有两种初始化方式，分别是带缓存的和不带缓存的
make(chan int, 0)
make(chan int, 10)
使用方法：
c := make(int)
defer close(c)
go func() {
c<-1
}()
n := <- c
fmt.Println(n)
channel的内部结构
chan的实现在runtime/chan.go，是一个hchan的结构体
type hchan struct {
qcount uint // 队列中的数据个数
dataqsiz uint // 唤醒队列的大小，channel本身是一个环形队列
buf unsafe.Pointer // 存放实际数据的指针，用unsafe.Pointer存放地址，为了避免gc
elemsize uint16
closed uint32 // 标识channel是否关闭
elemtype *_type // 数据元素类型
sendx uint // send的index
recvx uint // recv的index
recvq waitq // 阻塞在recv的队列
sendq waitq // 阻塞在send的队列
lock mutex // 锁
}
可以看出，channel本身就是一个环形缓冲区，数据存放到堆上面，channel的同步是通过锁实现的，并不是想象中的lock-free的方式，channel中有两个队列，一个是发送阻塞队列，一个是接收阻塞队列。当向一个已满的channel发送数据会被阻塞，此时发送协程会被添加到sendq中。同理，当向一个空的channel接收数据时，接收协程也会被阻塞，被置入到recvq中。
waitq是一个链表，里面对g结构做了一下简单的封装
创建channel
当我们在代码里通过make创建一个channel时，实际调用的是下面这个函数
CALL runtime.makechan(SB)
makechan的实现如下
func makechan(t *chantype, size int) *hchan {
elem := t.elem
// compiler checks this but be safe.
// 判断元素类型大小
if elem.size >= 1<<16 {
throw("makechan: invalid channel element type")
}
// 判断对齐限制
if hchanSize%maxAlign != 0 || elem.align > maxAlign {
throw("makechan: bad alignment")
}
// 判断size非负和是否大于maxAlloc限制
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.ptrdata == 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, "; dataqsiz=", size, "\n")
}
return c
}
根据上面代码，我们可以看到，创建channel分为三种情况：
- 缓冲区大小为0，此时只需要分配hchansize大小的内存就ok
- 缓冲区大小不为0，且channel的类型不包含指针，此时buf为hChanSize+元素大小*元素个数的连续内存
- 缓冲区大小不为0，且channel的类型包含指针，则不能简单的根据元素的大小去申请内存，需要通过mallocgc去分配内存
发送数据：
发送数据会调用chan.go中的如下接口：
CALL runtime.chansend1(SB)
chansend1会调用chansend接口，chansend实现如下：
/*
* 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)
// 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
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
}
c是具体的channel，ep是发送的数据，block为true表示阻塞的发送，一般向channel发送数据都是阻塞的，如果channel数据满了，会一直阻塞在这里。但是在select中如果有case监听某个channel的发送，那么此时的block参数为false，后续分析select实现会讲到。
select {
case <-c: // 这里为非阻塞发送
// do some thing
default:
// do some thing
}
chansend接口会对一些条件做判断
如果向一个为nil的channel发送数据，如果是阻塞发送会一直阻塞：
if c == nil {
if !block {
return false
}
gopark(nil, nil, waitReasonChanSendNilChan, traceEvGoStop, 2)
throw("unreachable")
}
首先会加锁，保证原子性，如果向一个已关闭的channel发送数据则会panic
lock(&c, lock)
if c.close != 0 {
unlock(&c, lock)
panic(plainError("send on closed channel"))
}
如果此时recvq中有等待协程，就直接调用send函数将数据复制给接收方，实现如下：
// 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)
}
如果此时没有等待协程，并且数据未满的情况下，就将数据copy到环形缓冲区中，将位置后移一位
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
}
如果此时环形缓冲区数据满了，如果是阻塞发送，此时会把发送方放到sendq队列中
接收数据：
接收数据会调用下面接口
CALL runtime.chanrecv1(SB)
chanrecv1会调用chanrecv接口，chanrecv方法如下
// 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)
// 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)
}
closed := gp.param == nil
gp.param = nil
mysg.c = nil
releaseSudog(mysg)
return true, !closed
}
c指需要操作的channel，接收的数据会写到ep中，block与send中的情况一样，表示是阻塞接收还是非阻塞接收，非阻塞i接收指在select中case接收一个channel值
select {
case a := <-c: // 这里为非阻塞接收，没有数据直接返回
// do some thing
default:
// do some thing
}
首先chanrecv也会做一些参数校验
如果channel为nil并且是非阻塞模式，直接返回，如果是阻塞模式，永远等待
if c == nil {
if !block {
return
}
gopark(nil,nil,waitReasonChanReceiveNilChan, traceEvGoStop, 2)
throw("unreachable")
}
随后会加锁，防止竞争读写
lock(&c.lock)
如果向一个已关闭的channel接收数据，此时channel里面还有数据，那么依然可以接收数据，属于正常接收数据情况。
如果向一个已关闭的channel接收数据，此时channel里面没有数据，那么此时返回的是（true, false），表示有值返回，但不是我们需要的值
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
}
接收也分为三种情况：
如果此时sendq中有发送方在阻塞，则会调用recv函数
// 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 {
raceacquire(qp)
racerelease(qp)
raceacquireg(sg.g, qp)
racereleaseg(sg.g, qp)
}
// 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)
if sg.releasetime != 0 {
sg.releasetime = cputicks()
}
goready(gp, skip+1)
}
此时有发送方在等待，表示此时channel中数据已满，这个时候会将channel头部的数据copy到接收方，然后将发送方队列头部的发送者的数据copy到那个位置。这涉及到两次copy操作。
第二种情况是如果没有发送方等待，此时会把数据copy到channel中
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
}
关闭channel
关闭channel时会调用如下接口
func closechan(c *hchan)
首先会做一些数据校验
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 // 置关闭标记位
如果向一个为nil的channel或者向一个已关闭的channel发起close操作就会panic。
随后会唤醒所有在recvq或者sendq里面的协程
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)
如果存在接收者，将接收数据通过typedmemclr置0
如果存在发送者，将所有发送者panic
- 总结
综上分析，在使用channel有这么几点要注意
确保所有数据发送完之后再关闭channel，由发送方来关闭
不要重复关闭channel
不要向为nil的channel里面发送值
不要向为nil的channel里面接收值
接收数据时，可以通过返回值判断是否ok
n. ok := <- c
if ok {
// do some thing
}
