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golang源码解析之chan

岳时铭
2023-12-01

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。
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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