V2AS问路
goroutine 分析 协程的调度和执行顺序 并发写 run in the same address space 内存地址 闭包 存在两种并发 确定性 非确定性的 Go 的协程和通道理所当然的支持确定性的并发方式（
阅读原文时间：2023年07月08日阅读：6

package main

import (
"fmt"
"runtime"
"sync"
)

const N = 26

func main() {
const GOMAXPROCS = 1
runtime.GOMAXPROCS(GOMAXPROCS)
var wg sync.WaitGroup
wg.Add(N)
for i := 0; i < N; i++ {
go func(i int) {
defer wg.Done()
fmt.Println(i)
}(i)
}
wg.Wait()
}

25
0
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package main

import (
"fmt"
"runtime"
"sync"
)

const N = 26

func main() {
const GOMAXPROCS = 1
runtime.GOMAXPROCS(GOMAXPROCS)
var wg sync.WaitGroup
wg.Add(N)
for i := 0; i < N; i++ {
go func() {
defer wg.Done()
fmt.Println(i)
}()
}
wg.Wait()
}

26
26
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package main

import (
"fmt"
"runtime"
"sync"
)

const N = 26

func main() {
const GOMAXPROCS = 1
runtime.GOMAXPROCS(GOMAXPROCS)
var wg sync.WaitGroup
wg.Add(4)
for i := 0; i < N; i++ {
go func(i int) {
defer wg.Done()
fmt.Println(i)
}(i)
}
wg.Wait()
}

25
0
1
2

package main

import "fmt"

func main() {
for i:=0; i<10; i++ {
go func() {
fmt.Println(i)
}()
}
}

　　无任何打印

package main

import (
"fmt"
"runtime"
"sync"
)

const N = 26

func main() {
const GOMAXPROCS = 1
runtime.GOMAXPROCS(GOMAXPROCS)
var wg sync.WaitGroup
wg.Add(2 * N)
for i := 0; i < N; i++ {
go func(i int) {
defer wg.Done()
fmt.Printf("%c", 'a'+i)
}(i)

    go func(i int) {  
        defer wg.Done()  
        fmt.Printf("%c", 'A'+i)  
    }(i)  
}  
go func() {}()  
wg.Wait()

}

通过无缓冲的通道阻塞来实现控制goroutine的执行顺序

unbuffered channel
无缓冲的通道
在接收前没有能力保存任何值的通道
要求发送goroutine和接收goroutine同时准备好，才能完成发送和接收的操作
如果两个goroutine没有同时准备好，通道会导致先执行发送或接收操作的goroutine阻塞等待
这种对通道进行发送和接收的交互行为本身就是同步的
其中任意一个操作都无法离开另一个操作单独存在

Go基础系列：指定goroutine的执行顺序 - 骏马金龙 - 博客园 https://www.cnblogs.com/f-ck-need-u/p/9994652.html

package main

import (
"fmt"
"time"
)

func A(a, b chan struct{}) {
<-a
fmt.Println("A()!")
close(b)
}

func B(a, b chan struct{}) {
<-a
fmt.Println("B()!")
close(b)
}
func C(a chan struct{}) {
<-a
fmt.Println("C()!")
}

func main() {
/*
unbuffered channel
无缓冲的通道
在接收前没有能力保存任何值的通道
要求发送goroutine和接收goroutine同时准备好，才能完成发送和接收的操作
如果两个goroutine没有同时准备好，通道会导致先执行发送或接收操作的goroutine阻塞等待
这种对通道进行发送和接收的交互行为本身就是同步的
其中任意一个操作都无法离开另一个操作单独存在
*/
x := make(chan struct{})
y := make(chan struct{})
z := make(chan struct{})
go C(z)
go B(y, z)
go C(z)
go A(x, y)
go C(z)
close(x)
// 给打印留时间
time.Sleep(3 * time.Second)
}

A()!
B()!
C()!
C()!
C()!

goroutine并发写

package main

import (
"math/rand"
"sync"
)

const N = 10

func main() {
m := make(map[int]int)
wg := &sync.WaitGroup{}
wg.Add(N)
for i := 0; i < N; i++ {
go func() {
defer wg.Done()
m[rand.Int()] = rand.Int()
}()
}
wg.Wait()
println(len(m))
}

　　当N相对大时，比如10e4报错

加锁

同步访问共享资源的方式之一

使用互斥锁mutex

互斥锁概念来自互斥(mutual excusion)概念

互斥锁用于在代码上创建一个临界区，保证同一时间只有一个goroutine可以执行这个临界区代码

《Go 语言实战》

package main

import (
"math/rand"
"sync"
)

const N = 100000

func main() {
m := make(map[int]int)
wg := &sync.WaitGroup{}
var mutex sync.Mutex
wg.Add(N)
for i := 0; i < N; i++ {
go func() {
defer wg.Done()
mutex.Lock()
m[rand.Int()] = rand.Int()
mutex.Unlock()
}()
}
wg.Wait()
println(len(m))
}

用无缓冲的通道来模拟2个goroutine间的网球比赛

package main

import (
"fmt"
"math/rand"
"sync"
"time"
)

// 用来等待程序结束
var wg sync.WaitGroup

func init() {
rand.Seed(time.Now().UnixNano())
}
func main() {
// 创建一个无缓冲的通道
court := make(chan int)
// 计数加2，表示要等待2个goroutine
wg.Add(2)
// 启动2个选手
go player("A", court)
go player("B", court)
// 发球
court <- 1
// 等待游戏结束
wg.Wait()
}

// player模拟一个选手在打网球
func player(name string, court chan int) {
// 在函数退出时调用Done来通知main函数工作已经完成
defer wg.Done()

for {  
    // 等待球被击打过来  
    ball, ok := <-court  
    if !ok {  
        // 如果通道关闭，我们就赢了  
        fmt.Printf("Player %s Won\\n", name)  
        return  
    }  
    // 选随机数，然后用这个数来判断我们是否丢球  
    n := rand.Intn(100)  
    if n%13 == 0 {  
        fmt.Printf("Player %s Missed\\n", name)  
        close(court)  
        return  
    }  
    // 显示击球数，并将击球数加1  
    fmt.Printf("Player %s Hit %d\\n", name, ball)  
    ball++

    //  将球打向对手  
    court <- ball

}

}

Player B Hit 1
Player A Hit 2
Player B Hit 3
Player A Hit 4
Player B Hit 5
Player A Hit 6
Player B Hit 7
Player A Hit 8
Player B Missed
Player A Won

接力比赛

package main

import (
"fmt"
"sync"
"time"
)

var wg sync.WaitGroup

func main() {
// 创建一个无缓冲的通道
baton := make(chan int)

// 为最后一位跑步者将计数加1  
wg.Add(1)

// 第一位跑步者持有接力棒  
go Runner(baton)

// 开始比赛  
baton <- 1

// 等待比赛结束  
wg.Wait()

}

// Runner 模拟接力比赛中的一位跑步者

func Runner(baton chan int) {
var newRunner int
// 等待接力棒
runner := <-baton

// 开始绕着跑道跑步  
fmt.Printf("Runner %d Running With Baton\\n", runner)

// 创建下一位跑步者  
if runner != 4 {  
    newRunner = runner + 1  
    fmt.Printf("Runner %d To The Line\\n", newRunner)  
    go Runner(baton)  
}

// 围绕跑道跑  
time.Sleep(100 \* time.Millisecond)

// 比赛结束了吗？  
if runner == 4 {  
    fmt.Printf("Runner %d Finished, Race over\\n", runner)  
    wg.Done()  
    return  
}

// 将接力棒交给下一位跑步者  
fmt.Printf("Runner %d Exchange With Runner %d\\n", runner, newRunner)

baton <- newRunner

}

Runner 1 Running With Baton
Runner 2 To The Line
Runner 1 Exchange With Runner 2
Runner 2 Running With Baton
Runner 3 To The Line
Runner 2 Exchange With Runner 3
Runner 3 Running With Baton
Runner 4 To The Line
Runner 3 Exchange With Runner 4
Runner 4 Running With Baton
Runner 4 Finished, Race over

package main

import (
"fmt"
"math/rand"
"sync"
"time"
)

const (
numberGorutines = 4 // 要使用的goroutine的数量
taskLoad = 10 // 要处理的工作的数量
)

var wg sync.WaitGroup

// init初始化包,Go语言运行时会在其他代码执行之前
// 优先执行这个函数
func init() {
// 初始化随机数种子
rand.Seed(time.Now().Unix())
}

func main() {
// 创建一个有缓冲的通道来管理工作
tasks := make(chan string, taskLoad)

// 启动goroutine来处理工作  
wg.Add(numberGorutines)  
for gr := 1; gr <= numberGorutines; gr++ {  
    go worker(tasks, gr)  
}

// 增加一组要完成的工作  
for post := 1; post <= taskLoad; post++ {  
    tasks <- fmt.Sprintf("Task : %d", post)  
}

// 当所有工作都处理完时关闭通道  
// 以便所有goroutine退出  
close(tasks)

// 等待所有工作完成  
wg.Wait()

}

// worker作为goroutine启动来处理
// 从有缓冲的通道传入的工作

func worker(tasks chan string, worker int) {
// 通知函数已经返回
defer wg.Done()

for {  
    // 等待分配工作  
    task, ok := <-tasks  
    if !ok {  
        // 这意味着通道已经空了，并且已被关闭  
        fmt.Printf("Worker: %d : Shutting Down\\n", worker)  
        return  
    }  
    // 显示我们开始工作了  
    fmt.Printf("Worker: %d : Started %s\\n", worker, task)

    // 随机等一段时间来模拟工作  
    sleep := rand.Int63n(100)  
    time.Sleep(time.Duration(sleep) \* time.Millisecond)

    // 显示我们完成了工作  
    fmt.Printf("Worker: %d : Completed %s \\n", worker, task)  
}

}

Worker: 4 : Started Task : 1
Worker: 1 : Started Task : 2
Worker: 2 : Started Task : 3
Worker: 3 : Started Task : 4
Worker: 2 : Completed Task : 3
Worker: 2 : Started Task : 5
Worker: 2 : Completed Task : 5
Worker: 2 : Started Task : 6
Worker: 2 : Completed Task : 6
Worker: 2 : Started Task : 7
Worker: 4 : Completed Task : 1
Worker: 4 : Started Task : 8
Worker: 2 : Completed Task : 7
Worker: 2 : Started Task : 9
Worker: 4 : Completed Task : 8
Worker: 4 : Started Task : 10
Worker: 1 : Completed Task : 2
Worker: 1 : Shutting Down
Worker: 2 : Completed Task : 9
Worker: 2 : Shutting Down
Worker: 3 : Completed Task : 4
Worker: 3 : Shutting Down
Worker: 4 : Completed Task : 10
Worker: 4 : Shutting Down

能够从已经关闭的通道接收数据这一点非常重要，因为这允许通道关闭后
依旧能够取出其中缓冲的全部值，而不会有数据丢失。
从一个已经关闭且没有数据的通道里获取数据，总会立刻返回，并返回一个通道类型的零值。

goroutine

https://tour.golang.org/concurrency/1

Goroutines run in the same address space, so access to shared memory must be synchronized. The sync package provides useful primitives, although you won't need them much in Go as there are other primitives. (See the next slide.)

https://golang.org/doc/faq#closures_and_goroutines

What happens with closures running as goroutines?

Some confusion may arise when using closures with concurrency. Consider the following program:

func main() {
done := make(chan bool)

values := \[\]string{"a", "b", "c"}  
for \_, v := range values {  
    go func() {  
        fmt.Println(v)  
        done <- true  
    }()  
}

// wait for all goroutines to complete before exiting  
for \_ = range values {  
    <-done  
}

}

One might mistakenly expect to see a, b, c as the output. What you'll probably see instead is c, c, c. This is because each iteration of the loop uses the same instance of the variable v, so each closure shares that single variable. When the closure runs, it prints the value of v at the time fmt.Println is executed, but v may have been modified since the goroutine was launched. To help detect this and other problems before they happen, run go vet.

To bind the current value of v to each closure as it is launched, one must modify the inner loop to create a new variable each iteration. One way is to pass the variable as an argument to the closure:

for \_, v := range values {  
    go func(**u** string) {  
        fmt.Println(**u**)  
        done <- true  
    }(**v**)  
}

In this example, the value of v is passed as an argument to the anonymous function. That value is then accessible inside the function as the variable u.

Even easier is just to create a new variable, using a declaration style that may seem odd but works fine in Go:

for \_, v := range values {  
    **v := v** // create a new 'v'.  
    go func() {  
        fmt.Println(**v**)  
        done <- true  
    }()  
}

This behavior of the language, not defining a new variable for each iteration, may have been a mistake in retrospect. It may be addressed in a later version but, for compatibility, cannot change in Go version 1.

闭包

https://github.com/unknwon/the-way-to-go_ZH_CN/blob/master/eBook/14.1.md

存在两种并发方式：确定性的（明确定义排序）和非确定性的（加锁/互斥从而未定义排序）。Go 的协程和通道理所当然的支持确定性的并发方式（例如通道具有一个 sender 和一个 receiver）。

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