loopvar 改动不同版本的影响-并发

看一个关于并发的例子

复制代码

package main

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

func main() {
	fmt.Println("golang version:", runtime.Version())
	concurrencyDemo()
}

func concurrencyDemo() {
	isGold := func(num uint64) bool {
		return num%65536 == 0
	}
	var c = make(chan uint64)
	var m sync.Mutex
	for n, i := 0, uint64(0); n < 5; n++ {
		go func() {
			for {
				m.Lock()
				i++
				v := i
				m.Unlock()
				if isGold(v) {
					c <- v
				}
			}
		}()
	}
	for n := range c {
		fmt.Println("Found gold:", n)
	}
}

这个代码定义了一个isGold 的func，可以被65536整除的就是gold，然后定义了一个uint64的channel，接着开了5个go routine,每个go routine都是一个不会停止的协程，加锁i进行自增，并把自增后的i复制给v，如果v可以被65536整除，那么把v放到uint64的channel中，最后打印出channel的值。

如果代码写的不好，并发的时候是很容易出现data race的,运行的时候加上-race这个参数

golang 1.21 运行结果

复制代码

 go run -race demo/concurrency.go
golang version: go1.21.5
Found gold: 65536
Found gold: 131072
Found gold: 196608
Found gold: 262144
Found gold: 327680
Found gold: 393216
Found gold: 458752
Found gold: 524288
Found gold: 589824
Found gold: 655360
^Csignal: interrupt

golang 1.22运行结果

复制代码

go run -race demo/concurrency.go
golang version: go1.22.1
==================
WARNING: DATA RACE
Write at 0x00c000014148 by goroutine 7:
  main.concurrencyDemo.func2()
      /home/lfeng/GolandProjects/golang101/demo/concurrency.go:24 +0x65

Previous read at 0x00c000014148 by main goroutine:
  main.concurrencyDemo()
      /home/lfeng/GolandProjects/golang101/demo/concurrency.go:20 +0x87
  main.main()
      /home/lfeng/GolandProjects/golang101/demo/concurrency.go:11 +0xc4

Goroutine 7 (running) created at:
  main.concurrencyDemo()
      /home/lfeng/GolandProjects/golang101/demo/concurrency.go:21 +0x7a
  main.main()
      /home/lfeng/GolandProjects/golang101/demo/concurrency.go:11 +0xc4
==================
Found gold: 65536
Found gold: 65536
Found gold: 65536
Found gold: 65536
Found gold: 131072
Found gold: 131072
Found gold: 131072
Found gold: 65536
^Csignal: interrupt

可以看到1.21是没有data race的，但是1.22存在了data race

解决方法

把i的定义移到循环外面，修改后的代码

go 复制代码

package main

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

func main() {
	fmt.Println("golang version:", runtime.Version())
	concurrencyDemo()
}

func concurrencyDemo() {
	isGold := func(num uint64) bool {
		return num%65536 == 0
	}
	var c = make(chan uint64)
	var m sync.Mutex
	i := uint64(0)
	for n := 0; n < 5; n++ {
		go func() {
			for {
				m.Lock()
				i++
				v := i
				m.Unlock()
				if isGold(v) {
					c <- v
				}
			}
		}()
	}
	for n := range c {
		fmt.Println("Found gold:", n)
	}
}

修改后的代码不会存在data race了

再看个例子

复制代码

package main

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

func main() {
	fmt.Println("golang version:", runtime.Version())
	concurrencyDemo()
}

func concurrencyDemo() {
	var wg sync.WaitGroup
	wg.Add(5)
	for i := 0; i < 5; i++ {
		go func() {
			defer wg.Done()
			fmt.Println(i)
		}()
	}
}

这个例子其实很简单，使用waitGroup来控制协程的数量和协程的结束。协程中只是打印循环变量i

golang 1.21的运行结果

复制代码

go run -race demo/concurrency2.go
golang version: go1.21.5
5
5
==================
WARNING: DATA RACE
Read at 0x00c000120028 by goroutine 9:
  main.concurrencyDemo.func1()
      /home/lfeng/GolandProjects/golang101/demo/concurrency2.go:20 +0xaf

Previous write at 0x00c000120028 by main goroutine:
  main.concurrencyDemo()
      /home/lfeng/GolandProjects/golang101/demo/concurrency2.go:17 +0xa4
  main.main()
      /home/lfeng/GolandProjects/golang101/demo/concurrency2.go:11 +0xc4

Goroutine 9 (running) created at:
  main.concurrencyDemo()
      /home/lfeng/GolandProjects/golang101/demo/concurrency2.go:18 +0x85
  main.main()
      /home/lfeng/GolandProjects/golang101/demo/concurrency2.go:11 +0xc4
==================
5
5
5
Found 1 data race(s)
exit status 66

golang 1.22运行结果

复制代码

go run -race demo/concurrency2.go
golang version: go1.22.1
2
1
4
3
0

发现1.21存在data race,但是1.22是不存在data race的

解决方法

go func增加参数传递循环的i

修改后的代码

go 复制代码

package main

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

func main() {
	fmt.Println("golang version:", runtime.Version())
	concurrencyDemo()
}

func concurrencyDemo() {
	var wg sync.WaitGroup
	wg.Add(5)
	for i := 0; i < 5; i++ {
		go func(i int) {
			defer wg.Done()
			fmt.Println(i)
		}(i)
	}
}

修改后的代码也不会存在data race

结论

上面两个例子出现data race的原因都是1.21 loop 变量是初始化一次，但是1.22每次循环都会创建新变量