【我的區塊鏈之路】- golang源碼分析之channel的底層實現

【轉載請標明出處】https://blog.csdn.net/qq_25870633/article/details/83388952

接上篇文章 【我的區塊鏈之路】- golang源碼分析之select的底層實現 我這裏因爲面試的時候也有被問到過 channel的底層實現，所以就一併的去把 channel 啊，goroutine啊，go協程的調度器啊，interface啊，mutex啊，rwmutex啊，timer啊等等底層全部都去錄了一遍。沒辦法啊，曾經我以爲自己是個逆風尿三丈的耀眼王者，然而這段時間發現自己只是一隻順風尿溼鞋的不屈黑鐵。。。

參考：

https://ninokop.github.io/

https://blog.csdn.net/nobugtodebug/article/details/45396507

https://studygolang.com/articles/10575?fr=sidebar

那麼，我們這篇文章主要是講channel的底層實現的。

首先，我們先看channel的實現是在哪裏？在runtime包下面咯，路徑爲：./src/runtime/chan.go 文件中，其中主要的結構體爲：

/**
定義了 channel 的結構體
*/
type hchan struct {
	qcount   uint           // total data in the queue   隊列中的當前數據的個數
	dataqsiz uint           // size of the circular queue channel的大小
	buf      unsafe.Pointer // points to an array of dataqsiz elements  數據緩衝區，存放數據的環形數組
	elemsize uint16         // channel 中數據類型的大小 （單個元素的大小）
	closed   uint32         // 表示 channel 是否關閉的標識位
	elemtype *_type // element type  隊列中的元素類型
    // send 和 recieve 的索引，用於實現環形數組隊列
	sendx    uint   // send index    當前發送元素的索引
	recvx    uint   // receive index    當前接收元素的索引
	recvq    waitq  // list of recv waiters    接收等待隊列；由 recv 行爲（也就是 <-ch）阻塞在 channel 上的 goroutine 隊列
	sendq    waitq  // list of send waiters    發送等待隊列；由 send 行爲 (也就是 ch<-) 阻塞在 channel 上的 goroutine 隊列

	// lock protects all fields in hchan, as well as several
	// fields in sudogs blocked on this channel.
    // lock保護hchan中的所有字段，以及此通道上阻塞的sudoG中的幾個字段。  
	//
	// 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.
    // 保持此鎖定時不要更改另一個G的狀態（特別是，沒有準備好G），因爲這可能會因堆棧收縮而死鎖。
	lock mutex
}



/**
發送及接收隊列的結構體
等待隊列的鏈表實現
*/
type waitq struct {
	first *sudog
	last  *sudog
}

然後還有在 runtime包的 ./src/runtime/runtime2.go 中定義的 sudoG對應的結構體：

/**
對 G 的封裝
*/
type sudog struct {
	// The following fields are protected by the hchan.lock of the
	// channel this sudog is blocking on. shrinkstack depends on
	// this for sudogs involved in channel ops.

	g          *g
	selectdone *uint32 // CAS to 1 to win select race (may point to stack)
	next       *sudog
	prev       *sudog
	elem       unsafe.Pointer // data element (may point to stack)

	// The following fields are never accessed concurrently.
	// For channels, waitlink is only accessed by g.
	// For semaphores, all fields (including the ones above)
	// are only accessed when holding a semaRoot lock.

	acquiretime int64
	releasetime int64
	ticket      uint32
	parent      *sudog // semaRoot binary tree
	waitlink    *sudog // g.waiting list or semaRoot
	waittail    *sudog // semaRoot
	c           *hchan // channel
}

有上述的結構體我們大致可以看出channel其實就是由一個環形數組實現的隊列，用於存儲消息元素；兩個鏈表實現的 goroutine 等待隊列，用於存儲阻塞在 recv 和 send 操作上的 goroutine；一個互斥鎖，用於各個屬性變動的同步，只不過這個鎖是一個輕量級鎖。其中 recvq 是讀操作阻塞在 channel 的 goroutine 列表，sendq 是寫操作阻塞在 channel 的 goroutine 列表。列表的實現是 sudog，其實就是一個對 g 的結構的封裝。

和select類似，hchan其實只是channel的頭部。頭部後面的一段內存連續的數組將作爲channel的緩衝區，即用於存放channel數據的環形隊列。qcount 和 dataqsiz 分別描述了緩衝區當前使用量【len】和容量【cap】。若channel是無緩衝的，則size是0，就沒有這個環形隊列了。如圖：

下面我們來看看實例化一個channel的實現：

make：

make 的過程還比較簡單，需要注意一點的是當元素不含指針的時候，會將整個 hchan 分配成一個連續的空間。下面就是make創建channel的代碼實現：

//go:linkname reflect_makechan reflect.makechan
func reflect_makechan(t *chantype, size int64) *hchan {
	return makechan(t, size)
}


/**
創建 chan
*/
func makechan(t *chantype, size int64) *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")
	}
	if size < 0 || int64(uintptr(size)) != size || (elem.size > 0 && uintptr(size) > (_MaxMem-hchanSize)/elem.size) {
		panic(plainError("makechan: size out of range"))
	}

	var c *hchan
	if elem.kind&kindNoPointers != 0 || size == 0 {
		// Allocate memory in one call.
		// Hchan does not contain pointers interesting for GC in this case:
		// 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.
		c = (*hchan)(mallocgc(hchanSize+uintptr(size)*elem.size, nil, true))
		if size > 0 && elem.size != 0 {
			c.buf = add(unsafe.Pointer(c), hchanSize)
		} else {
			// race detector uses this location for synchronization
			// Also prevents us from pointing beyond the allocation (see issue 9401).
			c.buf = unsafe.Pointer(c)
		}
	} else {
		c = new(hchan)
		c.buf = newarray(elem, int(size))
	}
	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
}

可以看出來和之前說的select一樣用了 //go:linkname 技巧，把函數關聯到了 reflect包中的對應函數上了，這樣紙就使用反射區出發整個make的入口。騷微過一下reflect包中真正make(chan type, int) 的函數吧

// MakeChan creates a new channel with the specified type and buffer size.
func MakeChan(typ Type, buffer int) Value {
	if typ.Kind() != Chan {
		panic("reflect.MakeChan of non-chan type")
	}
	if buffer < 0 {
		panic("reflect.MakeChan: negative buffer size")
	}
	if typ.ChanDir() != BothDir {
		panic("reflect.MakeChan: unidirectional channel type")
	}
	ch := makechan(typ.(*rtype), uint64(buffer))
	return Value{typ.common(), ch, flag(Chan)}
}

// 這個纔是被runtime中 用 //go:linkname 鏈接的函數
func makechan(typ *rtype, size uint64) (ch unsafe.Pointer)

劫爭上面繼續說，make中做了什麼：首先兩個 if 主要是一些異常情況的判斷，第三個 if 也很明顯，判斷 size 大小是否小於 0 或者過大。int64(uintptr(size)) != size 這句也是判斷 size 是否爲負。

然後接着判斷，如果channel中元素類型不爲指針或者channel爲無緩衝通道那麼就將其分配在連續的內存區域。【使用 mallocgc 函數進行分配內存空間】順便看下 mallocgc 函數（這個函數在select那章其實也用到的）的代碼吧：

// Allocate an object of size bytes.
// Small objects are allocated from the per-P cache's free lists.
// Large objects (> 32 kB) are allocated straight from the heap.

/**
分配大小爲字節的對象。
從每個P緩存的空閒列表中分配小對象。
大型對象（> 32 kB）直接從堆中分配。
*/
func mallocgc(size uintptr, typ *_type, needzero bool) unsafe.Pointer {
	if gcphase == _GCmarktermination {
		throw("mallocgc called with gcphase == _GCmarktermination")
	}

	if size == 0 {
		return unsafe.Pointer(&zerobase)
	}

	if debug.sbrk != 0 {
		align := uintptr(16)
		if typ != nil {
			align = uintptr(typ.align)
		}
		return persistentalloc(size, align, &memstats.other_sys)
	}

	// assistG is the G to charge for this allocation, or nil if
	// GC is not currently active.
	var assistG *g
	if gcBlackenEnabled != 0 {
		// Charge the current user G for this allocation.
		assistG = getg()
		if assistG.m.curg != nil {
			assistG = assistG.m.curg
		}
		// Charge the allocation against the G. We'll account
		// for internal fragmentation at the end of mallocgc.
		assistG.gcAssistBytes -= int64(size)

		if assistG.gcAssistBytes < 0 {
			// This G is in debt. Assist the GC to correct
			// this before allocating. This must happen
			// before disabling preemption.
			gcAssistAlloc(assistG)
		}
	}

	// Set mp.mallocing to keep from being preempted by GC.
	mp := acquirem()
	if mp.mallocing != 0 {
		throw("malloc deadlock")
	}
	if mp.gsignal == getg() {
		throw("malloc during signal")
	}
	mp.mallocing = 1

	shouldhelpgc := false
	dataSize := size
	c := gomcache()
	var x unsafe.Pointer
	noscan := typ == nil || typ.kind&kindNoPointers != 0
	if size <= maxSmallSize {
		if noscan && size < maxTinySize {
			// Tiny allocator.
			//
			// Tiny allocator combines several tiny allocation requests
			// into a single memory block. The resulting memory block
			// is freed when all subobjects are unreachable. The subobjects
			// must be noscan (don't have pointers), this ensures that
			// the amount of potentially wasted memory is bounded.
			//
			// Size of the memory block used for combining (maxTinySize) is tunable.
			// Current setting is 16 bytes, which relates to 2x worst case memory
			// wastage (when all but one subobjects are unreachable).
			// 8 bytes would result in no wastage at all, but provides less
			// opportunities for combining.
			// 32 bytes provides more opportunities for combining,
			// but can lead to 4x worst case wastage.
			// The best case winning is 8x regardless of block size.
			//
			// Objects obtained from tiny allocator must not be freed explicitly.
			// So when an object will be freed explicitly, we ensure that
			// its size >= maxTinySize.
			//
			// SetFinalizer has a special case for objects potentially coming
			// from tiny allocator, it such case it allows to set finalizers
			// for an inner byte of a memory block.
			//
			// The main targets of tiny allocator are small strings and
			// standalone escaping variables. On a json benchmark
			// the allocator reduces number of allocations by ~12% and
			// reduces heap size by ~20%.
			off := c.tinyoffset
			// Align tiny pointer for required (conservative) alignment.
			if size&7 == 0 {
				off = round(off, 8)
			} else if size&3 == 0 {
				off = round(off, 4)
			} else if size&1 == 0 {
				off = round(off, 2)
			}
			if off+size <= maxTinySize && c.tiny != 0 {
				// The object fits into existing tiny block.
				x = unsafe.Pointer(c.tiny + off)
				c.tinyoffset = off + size
				c.local_tinyallocs++
				mp.mallocing = 0
				releasem(mp)
				return x
			}
			// Allocate a new maxTinySize block.
			span := c.alloc[tinySpanClass]
			v := nextFreeFast(span)
			if v == 0 {
				v, _, shouldhelpgc = c.nextFree(tinySpanClass)
			}
			x = unsafe.Pointer(v)
			(*[2]uint64)(x)[0] = 0
			(*[2]uint64)(x)[1] = 0
			// See if we need to replace the existing tiny block with the new one
			// based on amount of remaining free space.
			if size < c.tinyoffset || c.tiny == 0 {
				c.tiny = uintptr(x)
				c.tinyoffset = size
			}
			size = maxTinySize
		} else {
			var sizeclass uint8
			if size <= smallSizeMax-8 {
				sizeclass = size_to_class8[(size+smallSizeDiv-1)/smallSizeDiv]
			} else {
				sizeclass = size_to_class128[(size-smallSizeMax+largeSizeDiv-1)/largeSizeDiv]
			}
			size = uintptr(class_to_size[sizeclass])
			spc := makeSpanClass(sizeclass, noscan)
			span := c.alloc[spc]
			v := nextFreeFast(span)
			if v == 0 {
				v, span, shouldhelpgc = c.nextFree(spc)
			}
			x = unsafe.Pointer(v)
			if needzero && span.needzero != 0 {
				memclrNoHeapPointers(unsafe.Pointer(v), size)
			}
		}
	} else {
		var s *mspan
		shouldhelpgc = true
		systemstack(func() {
			s = largeAlloc(size, needzero, noscan)
		})
		s.freeindex = 1
		s.allocCount = 1
		x = unsafe.Pointer(s.base())
		size = s.elemsize
	}

	var scanSize uintptr
	if !noscan {
		// If allocating a defer+arg block, now that we've picked a malloc size
		// large enough to hold everything, cut the "asked for" size down to
		// just the defer header, so that the GC bitmap will record the arg block
		// as containing nothing at all (as if it were unused space at the end of
		// a malloc block caused by size rounding).
		// The defer arg areas are scanned as part of scanstack.
		if typ == deferType {
			dataSize = unsafe.Sizeof(_defer{})
		}
		heapBitsSetType(uintptr(x), size, dataSize, typ)
		if dataSize > typ.size {
			// Array allocation. If there are any
			// pointers, GC has to scan to the last
			// element.
			if typ.ptrdata != 0 {
				scanSize = dataSize - typ.size + typ.ptrdata
			}
		} else {
			scanSize = typ.ptrdata
		}
		c.local_scan += scanSize
	}

	// Ensure that the stores above that initialize x to
	// type-safe memory and set the heap bits occur before
	// the caller can make x observable to the garbage
	// collector. Otherwise, on weakly ordered machines,
	// the garbage collector could follow a pointer to x,
	// but see uninitialized memory or stale heap bits.
	publicationBarrier()

	// Allocate black during GC.
	// All slots hold nil so no scanning is needed.
	// This may be racing with GC so do it atomically if there can be
	// a race marking the bit.
	if gcphase != _GCoff {
		gcmarknewobject(uintptr(x), size, scanSize)
	}

	if raceenabled {
		racemalloc(x, size)
	}

	if msanenabled {
		msanmalloc(x, size)
	}

	mp.mallocing = 0
	releasem(mp)

	if debug.allocfreetrace != 0 {
		tracealloc(x, size, typ)
	}

	if rate := MemProfileRate; rate > 0 {
		if size < uintptr(rate) && int32(size) < c.next_sample {
			c.next_sample -= int32(size)
		} else {
			mp := acquirem()
			profilealloc(mp, x, size)
			releasem(mp)
		}
	}

	if assistG != nil {
		// Account for internal fragmentation in the assist
		// debt now that we know it.
		assistG.gcAssistBytes -= int64(size - dataSize)
	}

	if shouldhelpgc {
		if t := (gcTrigger{kind: gcTriggerHeap}); t.test() {
			gcStart(gcBackgroundMode, t)
		}
	}

	return x
}

否則，在創建chan需要知道數據類型和緩衝區大小。channel 和 channel.buf 是分別進行分配的。對應上面的結構圖 newarray 將生成這個環形隊列。之所以要分開指針類型緩衝區主要是爲了區分gc操作，需要將它設置爲flagNoScan。並且指針大小固定，可以跟hchan頭部一起分配內存，不需要先new(hchan)再newarry。

我們再看下 newarray函數：

func newarray(typ *_type, n int) unsafe.Pointer {
	if n < 0 || uintptr(n) > maxSliceCap(typ.size) {
		panic(plainError("runtime: allocation size out of range"))
	}
	return mallocgc(typ.size*uintptr(n), typ, true)
}

可以看出來，其實newarray函數底層也是調了 mallocgc 函數來分配內存空間的。

總結：make chan 的過程是在堆上進行分配，返回是一個 hchan 的指針。

聲明但不make初始化的chan是nil chan。讀寫nil chan會阻塞，關閉nil chan會panic。

chan的讀寫：

從實現中可見讀寫chan都要lock，這跟讀寫共享內存一樣都有lock的開銷。

數據在chan中的傳遞方向從chansend開始從入參最終寫入recvq中的goroutine的數據域，這中間如果發生阻塞可能先寫入sendq中goroutine的數據域等待中轉。

從gopark返回後sudog對象可重用。

首先，我們來看下對應chan的讀寫函數的定義：

sned：

// entry point for c <- x from compiled code
//go:nosplit
func chansend1(c *hchan, elem unsafe.Pointer) {
	chansend(c, elem, true, getcallerpc(unsafe.Pointer(&c)))
}

/*
 * 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.
 * 通用單通道發送/接收
 * 如果阻塞不是nil，則將不會休眠，但如果無法完成則返回。
 * 當睡眠中涉及的通道關閉時，睡眠可以通過g.param == nil喚醒。 最簡單的循環和重新運行操作; 我們會 
 * 看到它現在已經關閉了。   
 */
func chansend(c *hchan, ep unsafe.Pointer, block bool, callerpc uintptr) bool {

    // 當 channel 未初始化或爲 nil 時，向其中發送數據將會永久阻塞
	if c == nil {
		if !block {
			return false
		}
        
        // gopark 會使當前 goroutine 休眠，並通過 unlockf 喚醒，但是此時傳入的 unlockf 爲 nil, 因此，goroutine 會一直休眠
		gopark(nil, nil, "chan send (nil chan)", traceEvGoStop, 2)
		throw("unreachable")
	}

	if debugChan {
		print("chansend: chan=", c, "\n")
	}

	if raceenabled {
		racereadpc(unsafe.Pointer(c), 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()
	}
    
     // 獲取同步鎖
	lock(&c.lock)
    
    // 向已經關閉的 channel 發送消息會產生 panic
	if c.closed != 0 {
		unlock(&c.lock)
		panic(plainError("send on closed channel"))
	}
    
    // CASE1: 當有 goroutine 在 recv 隊列上等待時，跳過緩存隊列，將消息直接發給 reciever goroutine
	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).
        // 找到了等待receiver。 我們將要發送的值直接傳遞給receiver，繞過通道緩衝區（如果有的話）。
		send(c, sg, ep, func() { unlock(&c.lock) }, 3)
		return true
	}
    
    // CASE2: 緩存隊列未滿，則將消息複製到緩存隊列上
	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
	}
    
    // CASE3: 緩存隊列已滿，將goroutine 加入 send 隊列
    // 初始化 sudog
	// 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.selectdone = nil
	mysg.c = c
	gp.waiting = mysg
	gp.param = nil

    // 加入隊列
	c.sendq.enqueue(mysg)
    // 休眠
	goparkunlock(&c.lock, "chan send", traceEvGoBlockSend, 3)
    
    // 喚醒 goroutine
	// 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
}


// 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)
}

send 有以下四種情況：【都是對不爲nil的chan的情況】

• 向已經close的chan寫數據，拋panic。
• 有 goroutine 阻塞在 channel recv 隊列上，此時緩存隊列( hchan.buf)爲空(即緩衝區內無元素)，直接將消息發送給 reciever goroutine,只產生一次複製
• 當 channel 緩存隊列( hchan.buf )有剩餘空間時，將數據放到隊列裏，等待接收，接收後總共產生兩次複製
• 當 channel 緩存隊列( hchan.buf )已滿時，將當前 goroutine 加入 send 隊列並阻塞。

【第一種情況】：向已經close的chan寫數據，會拋panic

if c.closed != 0 {
		unlock(&c.lock)
		panic(plainError("send on closed channel"))
}

【第二種情況】：從當前 channel 的等待隊列中取出等待的 goroutine，然後調用 send。goready 負責喚醒 goroutine

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
}


// 看send 部分邏輯

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)
}

【第三種情況】：通過比較 qcount 和 dataqsiz 來判斷 hchan.buf 是否還有可用空間。除此之後還需要調整一下 sendx 和 qcount

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
}

【第四種情況】：當 channel 緩存隊列( hchan.buf )已滿時，將當前 goroutine 加入 send 隊列並阻塞。

// 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.
    // 一些初始化工作
	mysg.elem = ep
	mysg.waitlink = nil
	mysg.g = gp
	mysg.selectdone = nil
	mysg.c = c
	gp.waiting = mysg
	gp.param = nil
	c.sendq.enqueue(mysg)   // 當前 goroutine 如等待隊列
	goparkunlock(&c.lock, "chan send", traceEvGoBlockSend, 3)  //休眠

receive：

// entry points for <- c from compiled code
//go:nosplit
func chanrecv1(c *hchan, elem unsafe.Pointer) {
	chanrecv(c, elem, true)
}

//go:nosplit
func chanrecv2(c *hchan, elem unsafe.Pointer) (received bool) {
	_, received = chanrecv(c, elem, true)
	return
}

// 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")
	}
    
     // 從 nil 的 channel 中接收消息，永久阻塞
	if c == nil {
		if !block {
			return
		}
		gopark(nil, nil, "chan receive (nil chan)", 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)
    
    // CASE1: 從已經 close 且爲空的 channel recv 數據，返回空值
	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
	}
    
    // CASE2: send 隊列不爲空
    // CASE2.a: 緩存隊列爲空，直接從 sender recv 元素
    // CASE2.b: 緩存隊列不爲空，此時只有可能是緩存隊列已滿，從隊列頭取出元素，
    //並喚醒 sender 將元素寫入緩存隊列尾部。由於爲環形隊列，因此，隊列滿時只需要將隊列頭複製給 reciever，
    //同時將 sender 元素複製到該位置，並移動隊列頭尾索引，不需要移動隊列元素
	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
	}
    
    // CASE3: 緩存隊列不爲空，直接從隊列取元素，移動頭索引
	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
	}
    
    // CASE4: 緩存隊列爲空，將 goroutine 加入 recv 隊列，並阻塞
	// 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.selectdone = nil
	mysg.c = c
	gp.param = nil
	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
}

// 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)
}

receive 有以下四種情況：【都是對不爲nil的chan的情況】

•  從已經 close 且爲空的 channel recv 數據，返回空值
• 當 send 隊列不爲空，分兩種情況：【一】緩存隊列爲空，直接從 send 隊列的sender中接收數據 元素；【二】緩存隊列不爲空，此時只有可能是緩存隊列已滿，從隊列頭取出元素，並喚醒 sender 將元素寫入緩存隊列尾部。由於爲環形隊列，因此，隊列滿時只需要將隊列頭複製給 reciever，同時將 sender 元素複製到該位置，並移動隊列頭尾索引，不需要移動隊列元素。【這就是爲什麼使用環形隊列的原因】
• 緩存隊列不爲空，直接從隊列取隊頭元素，移動頭索引。
• 緩存隊列爲空，將 goroutine 加入 recv 隊列，並阻塞。

【第一種情況】：從 closed channel 接收數據，如果 channel 中還有數據，接着走下面的流程。如果已經沒有數據了，則返回默認值。使用 ok-idiom 方式讀取的時候，第二個參數返回 false。

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
}

【第二種情況】：當前有send goroutine 阻塞在 channel 上，直接調 recv函數【a】當緩存隊列尾空時，直接從 send 隊列的sender中接收數據 元素。【b】緩存隊列不爲空，此時只有可能是緩存隊列已滿，從隊列頭取出元素，並喚醒 sender 將元素寫入緩存隊列尾部。同時更改隊列頭索引。

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
}

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)
}

【第三種情況】：buf 中有可用數據。直接從隊列取隊頭元素，移動頭索引。

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
}

【第四種情況】：buf 爲空，將當前 goroutine 加入 recv 隊列並阻塞。

// 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.selectdone = nil
	mysg.c = c
	gp.param = nil
	c.recvq.enqueue(mysg)
	goparkunlock(&c.lock, "chan receive", traceEvGoBlockRecv, 3)

close：

下面，我們來看看關閉通道具體的實現：

//go:linkname reflect_chanclose reflect.chanclose
func reflect_chanclose(c *hchan) {
	closechan(c)
}

func closechan(c *hchan) {
	if c == nil {
		panic(plainError("close of nil channel"))
	}

	lock(&c.lock)

    // 重複 close，產生 panic
	if c.closed != 0 {
		unlock(&c.lock)
		panic(plainError("close of closed channel"))
	}

	if raceenabled {
		callerpc := getcallerpc(unsafe.Pointer(&c))
		racewritepc(unsafe.Pointer(c), callerpc, funcPC(closechan))
		racerelease(unsafe.Pointer(c))
	}

	c.closed = 1

	var glist *g
    
    // 喚醒所有 reciever
	// 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, unsafe.Pointer(c))
		}
		gp.schedlink.set(glist)
		glist = gp
	}
    
    // 喚醒所有 sender，併產生 panic
	// 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)
    
    // 喚醒所喲叜glist中的goroutine
	// 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)
	}
}

close channel 的工作

• 將 c.closed 設置爲 1。
• 喚醒 recvq 隊列裏面的阻塞 goroutine
• 喚醒 sendq 隊列裏面的阻塞 goroutine

處理方式是分別遍歷 recvq 和 sendq 隊列，將所有的 goroutine 放到 glist 隊列中，最後喚醒 glist 隊列中的 goroutine。

OK上述就是channel的源碼分析，我們下面通過幾張圖來看一下chan的工作原理：

send的流程：

send的流程：

close的流程：

以上就是對 chan的底層操作原理及講解。

問chan是否線程安全的呢？是線程安全的，因爲其hchan結構彙總內置了mutex，且send 及 recv 及close 的操作中均會去 加鎖/解鎖 等動作。

到這裏我們對chan的底層講解就結束了！大家手下留情了~