关于golang的for select case的坑

package main

import (
	"fmt"
	"time"
)

func main() {
	a := make(chan int)
	go func() {
		time.Sleep(5 * time.Second)
		a <- 1
	}()
	for {
		time.Sleep(1 * time.Second)
		select {
		case <-a:
			fmt.Println(1)
		case <-time.After(1 * time.Second):
			fmt.Println(2)
		default:
			fmt.Println(3)
		}
	}
}

执行上面一段代码，最终的结果只会出现1和3，永远都不会出现2