go 竞态检测

Go 工具套件在 Go 版本 1.1 引入了一个竞态检测工具(race detector)。这个竞态检测工具是在编译流程中内置到你程序的代码。一旦你的程序开始运行，它能够发现和报告任何他所检测到的竞态情况

package main

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

var Wait sync.WaitGroup
var Counter int = 0

func main() {
    for routine := 1; routine <= 2; routine++ {
        Wait.Add(1)
        go Work(routine)
    }

    Wait.Wait()
    fmt.Printf("Final Counter: %d\n", Counter)
}

func Work(id int) {
    for i := 0; i < 10; i++ {
        Counter++
        time.Sleep(1 * time.Nanosecond)
    }

    Wait.Done()
}

使用go build -race -o main.out main.go（也可以直接使用go run -race），然后运行：

[root@admin go-race]# go build -race -o main.out main.go
[root@admin go-race]# ls
[root@admin go-race]# ./main.out 
==================
WARNING: DATA RACE
Read at 0x000000609908 by goroutine 8:
  main.Work()
      /root/code/go_work/project/gotour/go-race/main.go:24 +0x47

Previous write at 0x000000609908 by goroutine 7:
  main.Work()
      /root/code/go_work/project/gotour/go-race/main.go:24 +0x64

Goroutine 8 (running) created at:
  main.main()
      /root/code/go_work/project/gotour/go-race/main.go:15 +0x75

Goroutine 7 (running) created at:
  main.main()
      /root/code/go_work/project/gotour/go-race/main.go:15 +0x75
==================
Final Counter: 20
Found 1 data race(s)

加锁，修改之：

package main

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

var Wait sync.WaitGroup
var Counter int = 0
var CounterLock sync.Mutex

func main() {
    for routine := 1; routine <= 2; routine++ {
        Wait.Add(1)
        go Work(routine)
    }

    Wait.Wait()
    fmt.Printf("Final Counter: %d\n", Counter)
}

func Work(id int) {
    for i := 0; i < 10; i++ {
        CounterLock.Lock()
        Counter++
        CounterLock.Unlock()
        time.Sleep(1 * time.Nanosecond)
    }

    Wait.Done()
}

再次运行，检测通过。

go 语言常见并发问题

• 并发访问 map
  map 并发访问是非安全的。

• 循环变量被捕获
  老生常谈的问题了。

  for _ , job := range jobs {
      go func () {
         ProcessJob ( job )
      }()
  }
  

• err 变量被捕获

  x , err := Foo ()
  if err != nil {
      ...
  }
  
  go func () {
      var y int
      y , err = Bar ()
      if err != nil {
          ...
      }
  }()
  
  var z string
  z , err = Baz ()
  if err != nil {
      ...
  }
  

• 命名返回值被捕获

  func NamedReturnCallee () ( result int) {
      result = 10
      if ... {
          return // this has the effect of " return 10"
      }
  
      go func () {
          ... = result // read result
      }()
  
      return 20 // this is equivalent to result =20
  }
  
  func Caller () {
      retVal := NamedReturnCallee ()
  }
  

  defer 同理

  func Redeem ( request Entity ) ( resp Response , err error ) {
      defer func () {
          resp , err = c.Foo (request , err)
      }()
  
      err = CheckRequest ( request )
      ... // err check but no return
      go func () {
          ProcessRequest (request, err != nil)
      }()
     
      return // the defer function runs after here
  }
  

• 传值和传引用的问题
  比如go的并发库中的mutex 等不允许被copy，所以传递时需要使用指针的方式