close chan

• close由發送端調用，在收到最後一個發送值後關閉管道（It should be executed only by the sender, never the receiver, and has the effect of shutting down the channel after the last sent value is received）
• close管道後，所有從管道的讀取會獲得元素對應類型的零值（any receive from c will succeed without blocking, returning the zero value for the channel element.）

package main

import (
    "fmt"
    "time"
)

func main() {
    stop := make(chan struct{})

    go func() {
        select {
        case _, ok := <-stop:
            fmt.Println("stop 01: ", ok)
        }
    }()

    go func() {
        select {
        case _, ok := <-stop:
            fmt.Println("stop 02: ", ok)
        }
    }()

    stop <- struct{}{}
    close(stop)

    time.Sleep(time.Second * 2)

    _, ok := <-stop
    fmt.Println("stop chan: ", ok)
}

stop 02: true
stop 01: false
stop chan: false

結論：chan中發送的值被第二個協程讀取，主協程和第一個協程的讀取都返回了bool的零值（false）

• 不允許給已關閉的chan發送值，見下例：

  package main
  
  func main() {
    stop := make(chan struct{})
    close(stop)
  
    stop <- struct{}{}
  }

  panic: send on closed channel

• 給chan發送值，只能被讀取一次

  package main
  
  import (
    "fmt"
    "time"
  )
  
  func main() {
    stop := make(chan struct{})
  
    go func() {
        select {
        case _, ok := <-stop:
            //stop = nil
            fmt.Println("stop 01: ", ok)
        }
    }()
  
    go func() {
        select {
        case _, ok := <-stop:
            fmt.Println("stop 02: ", ok)
        }
    }()
  
    stop <- struct{}{}
  
    time.Sleep(time.Second * 2)
  }

  stop 02: true

這裏因為只給chan發送了一個值，所以只會退出一個協程（隨機性）。