Go源码学习: 从channel中接收数据的内部实现是什么样的?

v <- ch
v, ok <- ch

runtime.chanrecv1() /usr/local/Cellar/go/1.17.2/libexec/src/runtime/chan.go:439 (PC: 0x100448e)
Warning: debugging optimized function
   434: }
   435:
   436: // entry points for <- c from compiled code
   437: //go:nosplit
   438: func chanrecv1(c *hchan, elem unsafe.Pointer) {
=> 439:         chanrecv(c, elem, true)
   440: }

> runtime.chanrecv2() /usr/local/Cellar/go/1.17.2/libexec/src/runtime/chan.go:444 (PC: 0x1004e0e)
Warning: debugging optimized function
   439:         chanrecv(c, elem, true)
   440: }
   441:
   442: //go:nosplit
   443: func chanrecv2(c *hchan, elem unsafe.Pointer) (received bool) {
=> 444:         _, received = chanrecv(c, elem, true)
   445:         return
   446: }

 runtime.chanrecv() /usr/local/Cellar/go/1.17.2/libexec/src/runtime/chan.go:454 (PC: 0x1004e4f)
Warning: debugging optimized function
   449: // ep may be nil, in which case received data is ignored.
   450: // If block == false and no elements are available, returns (false, false).
   451: // Otherwise, if c is closed, zeros *ep and returns (true, false).
   452: // Otherwise, fills in *ep with an element and returns (true, true).
   453: // A non-nil ep must point to the heap or the caller's stack.
=> 454: func chanrecv(c *hchan, ep unsafe.Pointer, block bool) (selected, received bool) {

	if c == nil {
		if !block {
			return
		}
		gopark(nil, nil, waitReasonChanReceiveNilChan, traceEvGoStop, 2)
		throw("unreachable")
	}

	if !block && empty(c) {
		if atomic.Load(&c.closed) == 0 {
			return
		}

		if empty(c) {
			if raceenabled {
				raceacquire(c.raceaddr())
			}
			if ep != nil {
				typedmemclr(c.elemtype, ep)
			}
			return true, false
		}
	}

func empty(c *hchan) bool {
	// c.dataqsiz is immutable.
	if c.dataqsiz == 0 {
		return atomic.Loadp(unsafe.Pointer(&c.sendq.first)) == nil
	}
	return atomic.Loaduint(&c.qcount) == 0
}

	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
	}

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 {
			racenotify(c, c.recvx, nil)
			racenotify(c, c.recvx, sg)
		}
		// 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)
	sg.success = true
	if sg.releasetime != 0 {
		sg.releasetime = cputicks()
	}
	goready(gp, skip+1)
}

	if c.qcount > 0 {
		// Receive directly from queue
		qp := chanbuf(c, c.recvx)
		if raceenabled {
			racenotify(c, c.recvx, nil)
		}
		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)
	}
	success := mysg.success
	gp.param = nil
	mysg.c = nil
	releaseSudog(mysg)
	return true, success