golang源码解析之chan
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golang源码解析之chan
导语：在go语言中，chan 和 goroutine 是其并发模型CSP最重要体现，本文将基于1.14版本，深入源码，尽可能详细分析其内部实现原理。
一、为什么要使用chan
在并发线程中通信一般来说有两种模型：共享内存和消息传递。
常见的共享内存方式涉及到数据竞争这些问题，引入到锁、原子操作来解决。而基于消息传递的方式保证了不会产生数据竞争状态。
其中，实现消息传递有两种常见的类型：基于channel的消息传递和基于actor的消息传递。
而golang，就是基于channel的代表语言。erlang则是基于actor的代表语言。
在CSP（communicating sequential process）中，它将channel列为第一类对象，它不关注发送消息的实体，而是关注发送消息时使用的channel。golang则是基于这篇论文中的部分理论诞生的，也就是理论中的Process/channel：process和channel没有从属关系，process可以消费任意个channel，而channel也不关心具体是哪个process在使用它进行通信；process之间依据channel进行消息传递，形成一套有序阻塞和可预测的并发模型。对应到golang中，process就是goroutine，channel就是chan。
备注：CSP理论模型电子版链接：http://www.usingcsp.com/cspbook.pdf ，作者Tony Hoare
二、chan是怎样实现的
敲黑板：chan的实质是一个队列
如果你创建的是一个带缓冲的chan，chan就是一个循环队列，如果不带缓冲就是一个普通的队列。
在src/runtime/chan.go中，定义了一个结构体：hchan，还实现了一些方法：makechan、chansend、chanrecv、closechan，我们所使用的chan的最主要的功能，包括chan的创建，向chan写数据，读chan中的数据，关闭chan，就是围绕这个结构体和这几个方法实现的。我们接下来的内容，也主要围绕它们展开。
hchan
源码如下：
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
}
qcount：buf数组中已经放入的元素个数
dataqsize：buf数组长度，创建时调用make指定
buf：buf 数组
elemsize：buf数组中每个元素的大小
closed：chan是否关闭， 0代表没有关闭
elemtype：chan中元素的类型
sendx：buf数组中以发送的索引位置，用以构造循环队列
recvx：buf数组中已接收的索引位置，用以构造循环队列
recvq：等待接收的goroutine，当chan中buf无数据并且无sendq时但有goroutine等待消费时会产生，实质是包含goroutine及有关信息的sudog，多个recvq会形成链表，依然是FIFO的标准队列
sendq：等待发送的goroutine，当chan中buf数据写满时但仍然有goroutine等待写入时会产生，实质是包含goroutine及有关信息的sudog，多个sendq会形成链表，依然是FIFO的标准队列。
lock：锁，用以保证chan中数据的顺序通信
makechan
源码如下：
func makechan(t *chantype, size int) *hchan {
elem := t.elem
// compiler checks this but be safe. 校验数据类型大小，大于1<<16（65536）异常
if elem.size >= 1<<16 {
throw("makechan: invalid channel element type")
}
//内存对齐（多平台兼容，降低维度提高速度，减少内存消耗），大于最大内次8字节时异常
if hchanSize%maxAlign != 0 || elem.align > maxAlign {
throw("makechan: bad alignment")
}
//判断所需空间是否大于堆可分配的最大内存
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 {
//size为0，分配hchan结构体空间
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()
//不包括指针，分配连续地址空间，包括hchan结构体+数据，将申请下来的地址首地址赋值给buf，便于GC回收，减小gc压力
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)
//包括指针，buf单独分配空间
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
}
注意：makechan 返回的是hchan指针，这也就是为什么chan是golang中的引用类型，传递的是指针而非值
chansend
源码如下：
func chansend(c *hchan, ep unsafe.Pointer, block bool, callerpc uintptr) bool {
//检测chan是否为空，为空报错，所以往一个nil的chan中写数据，程序会异常退出报错
if c == nil {
//如果是非阻塞的，返回false，不会触发
if !block {
return false
}
//如果是阻塞的goroutine停止
gopark(nil, nil, waitReasonChanSendNilChan, traceEvGoStop, 2)
throw("unreachable")
}
if debugChan {
print("chansend: chan=", c, "\n")
}
//开启竞争检测
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.
//如果size = 0 或者 缓冲满了，返回false，不会触发block传入时值为true
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()
}
//chan加锁
lock(&c.lock)
//往关闭了的chan写数据，直接panic
if c.closed != 0 {
unlock(&c.lock)
panic(plainError("send on closed channel"))
}
//看接收者是否为空，如果为空，说明buf一定为空，直接取接受者队列队首sudog，把数据发给它并且释放锁。
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
}
//如果buf还有空位，将数据写入buf数组中
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.
//获取当前goroutine
gp := getg()
//创建sudog
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.
//sudog赋值
mysg.elem = ep
mysg.waitlink = nil
mysg.g = gp
mysg.isSelect = false
mysg.c = c
gp.waiting = mysg
gp.param = nil
//将sudog加入sendq链表中
c.sendq.enqueue(mysg)
//将当前goroutine陷入沉睡
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)
//再次唤醒，说明数据已经发送出去了，写入buf，或者被接收者消费
// 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
}
send函数
func send(c *hchan, sg *sudog, ep unsafe.Pointer, unlockf func(), skip int) {
//不会触发，默认为false
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
}
}
//数据没问题，直接调用sendDirect，将数据拷贝到目标内存地址
if sg.elem != nil {
sendDirect(c.elemtype, sg, ep)
sg.elem = nil
}
//获取该goroutine
gp := sg.g
unlockf()
gp.param = unsafe.Pointer(sg)
if sg.releasetime != 0 {
sg.releasetime = cputicks()
}
//将该goroutine放入到p的runnext中，等待下次直接调度
goready(gp, skip+1)
}
chanrecv
源码如下：
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")
}
//如果从nil的chan中读数据，报错，程序退出
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.
//不会触发
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)
//向已经关闭的chan读数据，如果buf为空，返回false不会报错和panic，如果buf不为空，仍然能读取到数据
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
}
//如果发送者队列不为空，存在两种情况，第一种是不带buf，直接赋值，第二种是buf已满，这个时候需要取出buf中recvx位置数据，交给当前goroutine消费，并且把发送者队列队首数据写入buf中
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
}
//buf中有数据，从buf中拿数据
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)
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
}
recv
func recv(c *hchan, sg *sudog, ep unsafe.Pointer, unlockf func(), skip int) {
//不带buf情况
if c.dataqsiz == 0 {
if raceenabled {
racesync(c, sg)
}
if ep != nil {
// copy data from sender
recvDirect(c.elemtype, sg, ep)
}
} else {
//buf已满情况
// 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)
}
closechan
源码如下：
func closechan(c *hchan) {
//关闭一个nil的chan，直接panic
if c == nil {
panic(plainError("close of nil channel"))
}
lock(&c.lock)
//关闭一个已经关闭了的chan，直接panic
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())
}
//将closed置为非零
c.closed = 1
var glist gList
//清理所有的数据，包括recvq，sendq
// 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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