go - 选择 channel 时陷入死锁

Question

package main

import (
    "fmt"
    "time"
)

func Server1(ch chan string) {
    time.Sleep(2*time.Second)
    ch <- "from Server-1"
}

func Server2(ch chan string) {
    time.Sleep(3*time.Second)
    ch <- "from Server-2"
}

func main() {
    input1 := make(chan string)
    input2 := make(chan string)
    
    go Server1(input1)
    go Server2(input2)
    
    for {
        select {
            case s1 := <- input1:
                fmt.Printf(s1 + "\n")
            case s2 := <- input2:
                fmt.Printf(s2 + "\n")   
        }
    }
}

运行上面的代码，得到如下错误

from Server-1
from Server-2
fatal error: all goroutines are asleep - deadlock!

在这个 channel 选择中，我们有两个线程以不同的计时器间隔运行，为什么会出现死锁？

Answer 1

您启动了 2 个 goroutine，它们仅在其 channel 上发送单个值，然后它们将终止，因此 main 函数将单独卡在那里。这就是僵局的原因。

如果修改 Server1() 和 Server2() 以无休止地发送值，则不会出现死锁:

func Server1(ch chan string) {
    for {
        time.Sleep(2 * time.Second)
        ch <- "from Server-1"
    }
}

func Server2(ch chan string) {
    for {
        time.Sleep(3 * time.Second)
        ch <- "from Server-2"
    }
}