Go面试：读写已经关闭的channel会怎样？

// 从已关闭的channel读取数据，chan容量为2
// 2
// 1
// 0 ，int的零值为0
func ReadFromClosedChanWithBuffer() {
	ch := make(chan int, 2)
	ch <- 1
	ch <- 2

	close(ch)

	fmt.Println(<-ch)
	fmt.Println(<-ch)
	fmt.Println(<-ch)
}

// 从已关闭的channel读取数据
// 0 ，int的零值为0
func ReadFromClosedChan() {
	ch := make(chan int, 2)
	close(ch)

    // channel关闭后，可以通过x, ok := <-c来判断channel的关闭状态；关闭后ok为false
	i,ok := <- ch
	if !ok{
		fmt.Println("channel closed")
	}
	fmt.Println(i)
}

// 向已关闭的channel写入数据，chan容量为2
// 执行后报错：panic: send on closed channel
func Write2ClosedChanWithBuffer() {
	// 带缓存的channel
	ch := make(chan int, 2)
	close(ch)

	ch <- 1
	ch <- 2
	ch <- 3
}

// 向已关闭的channel写入数据
// 执行后报错：panic: send on closed channel
func Write2ClosedChan() {
	// 带缓存的channel
	ch := make(chan int)
	close(ch)

	ch <- 1
}

func chansend(c *hchan, ep unsafe.Pointer, block bool, callerpc uintptr) bool {
	// 省略其它处理逻辑

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

	// 省略其它处理逻辑
}

func chanrecv(c *hchan, ep unsafe.Pointer, block bool) (selected, received bool) {
	// 省略其它处理逻辑

	lock(&c.lock)

    // channel关闭后
	if c.closed != 0 {
        // 且缓存为空
		if c.qcount == 0 {
			if raceenabled {
				raceacquire(c.raceaddr())
			}
			unlock(&c.lock)
			if ep != nil {
				typedmemclr(c.elemtype, ep)
			}
			return true, false
		}
		// The channel has been closed, but the channel's buffer have data.
	} else {
		// Just found waiting sender with not closed.
		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
		}
	}

	// 省略其它处理逻辑
}