在 goroutines 中使用 select 语句的通道通信行为

Question

这是 "A Tour of Go" 提供的版本的修改版本。

package main

import "fmt"

func fibonacci(c, quit chan int) {
    x, y := 0, 1
    for {
        select {
        case c <- x:
            x, y = y, x+y
            fmt.Println("GEN", x)
        case <-quit:
            fmt.Println("quit")
            return
        }
    }
}

func main() {
    c := make(chan int)
    quit := make(chan int)
    go func() {
        for i := 0; i < 10; i++ {
            fmt.Println("DISP", <-c)
        }
        quit <- 0
    }()
    fibonacci(c, quit)
}

以上代码的输出如下：

DISP 0
GEN 1
GEN 1
DISP 1
DISP 1
GEN 2
GEN 3
DISP 2
DISP 3
GEN 5
GEN 8
DISP 5
DISP 8
GEN 13
GEN 21
DISP 13
DISP 21
GEN 34
GEN 55
DISP 34
quit

我不明白代码的行为。为什么在显示它们之前生成了 2 个 Fibonacci 数？这是否取决于执行环境？

Answer 1

这与运行时如何调度 goroutines 有关。首先通过在每次推送到通道 c 后添加延迟，我们得到如下所示的正确结果：

package main

import (
    "fmt"
    "time"
)

func fibonacci(c, quit chan int) {
    x, y := 0, 1
    for {
        select {
        case c <- x:
            time.Sleep(1 * time.Millisecond)
            x, y = y, x+y
            fmt.Println("GEN", x)
        case <-quit:
            fmt.Println("quit")
            return
        }
    }
}

func main() {
    c := make(chan int)
    quit := make(chan int)
    go func() {
        for i := 0; i < 10; i++ {
            fmt.Println("DISP", <-c)
        }
        quit <- 0
    }()
    fibonacci(c, quit)
}

以上代码的结果是：

DISP 0
GEN 1
DISP 1
GEN 1
DISP 1
GEN 2
DISP 2
GEN 3
DISP 3
GEN 5
DISP 5
GEN 8
DISP 8
GEN 13
DISP 13
GEN 21
DISP 21
GEN 34
DISP 34
GEN 55
quit

去游乐场link：https://play.golang.org/p/QD5kyGXWoJk

通过在通道 c 上的每次发送后添加延迟，我们调用 goroutine 调度程序将当前 goroutine 放入等待队列，从而允许其他 goroutine 被调度并成功打印消息。 Golang 有协作调度，所以 goroutines 不会立即被抢占。

Answer 2

因为goroutine(go func() {...})中的接收者在从发送者(func fibonacci()中的select语句)接收到值时并发执行。

通过在发送值后添加延迟(c <- x:) 将避免同时发送多个，这样您可以看得更清楚。试试这个代码：(https://play.golang.org/p/9rOdS2YThKR)

package main

import (
    "fmt"
    "time"
)

func fibonacci(c, quit chan int) {
    x, y := 0, 1
    for {
        fmt.Println("-----------------------------------------")
        fmt.Println("current x:", x)
        select {
        //Send a value into a channel using the c <- x syntax, block until receiver is ready
        case c <- x:
            //When receiver gets x value, this code will executes
            //Delay, avoid mutilple sending at the same time
            time.Sleep(5 * time.Millisecond)
            //Increase
            x, y = y, x+y
            fmt.Println("increased x to", x)
        case <-quit:
            fmt.Println("quit")
            return
        }
    }
}

func main() {
    c := make(chan int)
    quit := make(chan int)
    go func() {
        //The <-c syntax receives a value from the channel, block until sender is ready
        for i := 0; i < 10; i++ {
            fmt.Println("received x:", <-c)
        }
        quit <- 0
    }()
    fibonacci(c, quit)
}

以上代码的结果是：

-----------------------------------------
current x: 0
received x: 0
increased x to 1
-----------------------------------------
current x: 1
received x: 1
increased x to 1
-----------------------------------------
current x: 1
received x: 1
increased x to 2
-----------------------------------------
current x: 2
received x: 2
increased x to 3
-----------------------------------------
current x: 3
received x: 3
increased x to 5
-----------------------------------------
current x: 5
received x: 5
increased x to 8
-----------------------------------------
current x: 8
received x: 8
increased x to 13
-----------------------------------------
current x: 13
received x: 13
increased x to 21
-----------------------------------------
current x: 21
received x: 21
increased x to 34
-----------------------------------------
current x: 34
received x: 34
increased x to 55
-----------------------------------------
current x: 55
quit

在 goroutines 中使用 select 语句的通道通信行为

Channel communication behaviour with select statement in goroutines

channel

go

goroutine