Golang channel 初探
Goroutine和channel是Golang实现高并发的基础。深入理解其背后的实现，写起代码来才不慌-_-
首先我们定义如下代码，来看看Golang底层是如何实现channel的。
package main
import (
"fmt"
)
func main() {
c := make(chan int, 5)
c <- 1
g := <-c
fmt.Println(g)
}编译后我们看下相关函数
go build -gcflags "-N -l" -o chan chan.go
go tool objdump -s "main\.main" chan
可以看到初始化调用了runtime.makechan，
写channel调用了runtime.chansend1，
读channel调用了runtime.chanrecv1.
在runtime/chan.go中我们找到对应函数
func makechan(t *chantype, size int) *hchan 返回一个hchan结构，我们先看下channel的结构，我们为一眼可以理解的加点注释。
channel结构
type hchan struct {
qcount uint // total data in the queue 队列中存在的个数
dataqsiz uint // size of the circular queue buffer大小 实现看起来是个循环数组
buf unsafe.Pointer // points to an array of dataqsiz elements 数组指针
elemsize uint16 //channel类型的大小
closed uint32 //channel是否关闭
elemtype *_type // element type //channel 类型
sendx uint // send index //发送index
recvx uint // receive index //接收index
recvq waitq // list of recv waiters //接收链表 即读channel的goroutine
sendq waitq // list of send waiters //发送链表 即写channel的goroutine
// 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
}接下来来看下到底如何初始化的。
channel初始化
func makechan(t *chantype, size int) *hchan {
elem := t.elem
...
//一些合法判断
// 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 {
//channel buffer大小为0 或者类型大小为0
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 = unsafe.Pointer(c)
//channel非指针
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)
if debugChan {
print("makechan: chan=", c, "; elemsize=", elem.size, "; elemalg=", elem.alg, "; dataqsiz=", size, "\n")
}
return c
}可以看出主要是根据channel类型以及buffer大小申请hcan.buf的内存，同时设置对应的datasiz、elemsize等，比较简单。
那么写channel是怎么实现的呢
写channel
c<-1这种形式的写channel会调用chansend1
// entry point for c <- x from compiled code
//go:nosplit
func chansend1(c *hchan, elem unsafe.Pointer) {
chansend(c, elem, true, getcallerpc())
}
看源码还有select的selectnbsend，reflect的reflect_chansend，
它们最后都会调用chansend，所以只用看chansend的实现，它们只是参数不一样而已。
/*
* 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 {
//如果设置了block为true 且channel为nil，goroutine将会死在这
if c == nil {
if !block {
return false
}
gopark(nil, nil, "chan send (nil chan)", traceEvGoStop, 2)
throw("unreachable")
}
//忽略一些无用代码
.....
//block false channel没buffer或者已满 直接返回
if !block && c.closed == 0 && ((c.dataqsiz == 0 && c.recvq.first == nil) ||
(c.dataqsiz > 0 && c.qcount == c.dataqsiz)) {
return false
}
...
lock(&c.lock)
//写已经关闭的channel 将会panic
if c.closed != 0 {
unlock(&c.lock)
panic(plainError("send on closed channel"))
}
//优先检查读队列是否有等待的goroutine，有的话直接调用并返回
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
}
//channel有buffer可以写
if c.qcount < c.dataqsiz {
// Space is available in the channel buffer. Enqueue the element to send.
//根据sendx计算该数据在数组的位置
qp := chanbuf(c, c.sendx)
if raceenabled {
raceacquire(qp)
racerelease(qp)
}
//拷贝过去
typedmemmove(c.elemtype, qp, ep)
c.sendx++
if c.sendx == c.dataqsiz {
c.sendx = 0
}
//计数
c.qcount++
unlock(&c.lock)
return true
}
//没buffer 且block false直接返回
if !block {
unlock(&c.lock)
return false
}
//没buffer写 初始化一个sudog结构
// 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.
//将该数据放在sudog的elem里，所以没buffer的channel数据其实在调用的goroutine里
mysg.elem = ep
mysg.waitlink = nil
mysg.g = gp
mysg.isSelect = false
mysg.c = c
gp.waiting = mysg
gp.param = nil
//放到channel的写队列，并阻塞
c.sendq.enqueue(mysg)
goparkunlock(&c.lock, "chan send", 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
整个写channel的逻辑还是很清晰的。 参照这个其实读channel差不多可以推断出是个对应的逻辑。
读channel
我们还是在runtime/chan.go中找到相关代码
// entry points for <- c from compiled code
//go:nosplit
func chanrecv1(c *hchan, elem unsafe.Pointer) {
chanrecv(c, elem, true)
}
读channel还有chanrecv2，selectnbrecv，selectnbrecv2，reflect_chanrecv。跟写channel一样，
它们之间也是参数的区别。
我们继续看chanrecv
// 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")
}
//block true的时候读nil的channel 将会永远阻塞
if c == nil {
if !block {
return
}
gopark(nil, nil, "chan receive (nil chan)", traceEvGoStop, 2)
throw("unreachable")
}
//block false的时候 判断channel buffer 已满或者没buffer 没有读的goroutine就直接返回
if !block && (c.dataqsiz == 0 && c.sendq.first == nil ||
c.dataqsiz > 0 && atomic.Loaduint(&c.qcount) == 0) &&
atomic.Load(&c.closed) == 0 {
return
}
lock(&c.lock)
//如果channel已空 且没有读的数据 清除并退出
if c.closed != 0 && c.qcount == 0 {
if raceenabled {
raceacquire(unsafe.Pointer(c))
}
unlock(&c.lock)
if ep != nil {
typedmemclr(c.elemtype, ep)
}
return true, false
}
//还是优先检查写队列 拿出阻塞的goroutine
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
}
//buffer中有数据 跟写对应处理
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
}
//没有数据读 初始化一个sudog并阻塞
// 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
//把自己放到读队列
c.recvq.enqueue(mysg)
goparkunlock(&c.lock, "chan receive", 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
}关channel
最后我们来看看close channel
func closechan(c *hchan) {
//关nil channel直接panic
if c == nil {
panic(plainError("close of nil channel"))
}
lock(&c.lock)
//重复关闭 panic
if c.closed != 0 {
unlock(&c.lock)
panic(plainError("close of closed channel"))
}
if raceenabled {
callerpc := getcallerpc()
racewritepc(unsafe.Pointer(c), callerpc, funcPC(closechan))
racerelease(unsafe.Pointer(c))
}
//设置channel标记
c.closed = 1
var glist *g
// release all readers
for {
//通知所有的读goroutine
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, unsafe.Pointer(c))
}
gp.schedlink.set(glist)
glist = 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, unsafe.Pointer(c))
}
gp.schedlink.set(glist)
glist = gp
}
unlock(&c.lock)
// 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)
}
}