Golang脱坑指南: goroutine(不断更新)

源代码与测试代码 https://github.com/YJZhangChina/GNP/tree/master/basic 下 goroutine_traps.go 与 goroutine_traps_test.go

1. 不安全的指针传递

Goroutine的调度次序未知，如果执行多次有指针传递的函数，且函数内存在写操作，需要考虑脏读和原子性问题。定义了Message结构体存储当前值current，valueChange方法用于修改current为当前步骤数step，并用Message指导后续操作。多次运goroutine执行该方法，由于指针传递，Message实例仅有一份，goroutine无法保证函数流程的原子性，造成了写无效和脏读。适当增大循环次数size可更容易发现该问题。

Testing UnsafePointerPassing in GoroutineTraps // 模拟结果可能不同
After step 9 the value is 9.
After step 0 the value is 0.
After step 1 the value is 1.
After step 2 the value is 2.
After step 3 the value is 3.
After step 4 the value is 4.
After step 7 the value is 7.
After step 8 the value is 8.
After step 5 the value is 5.
After step 6 the value is 5. // Error

指针传递前对Message实例进行克隆，产生写时复制的效果，修改后的结果不受影响。

Testing SafePointerPassing in GoroutineTraps // 模拟结果可能不同
After step 0 the value is 0.
After step 9 the value is 9.
After step 5 the value is 5.
After step 6 the value is 6.
After step 7 the value is 7.
After step 8 the value is 8.
After step 2 the value is 2.
After step 1 the value is 1.
After step 3 the value is 3.
After step 4 the value is 4.

goroutine_traps.go

package basic

import (
	"fmt"
	"sync"
)

type Message struct {
	current int
}

func valueChange(step int, message *Message, waiter *sync.WaitGroup) {
	message.current = step
	fmt.Printf("After step %d the value is %d.
", step, message.current)
	waiter.Done()
}

goroutine_traps_test.go

package basic

import (
	"fmt"
	"sync"
	"testing"
)

func TestUnsafePointerPassing(t *testing.T) {
	fmt.Println("Testing UnsafePointerPassing in GoroutineTraps")
	size := 10
	var waiter sync.WaitGroup
	waiter.Add(size)
	message := &Message{current: 1}
	for i := 0; i < size; i++ {
		go valueChange(i+1, message, &waiter)
	}
	waiter.Wait()
}

func TestSafePointerPassing(t *testing.T) {
	fmt.Println("Testing SafePointerPassing in GoroutineTraps")
	size := 10
	var waiter sync.WaitGroup
	waiter.Add(size)
	message := &Message{current: 1}
	for i := 0; i < 10; i++ {
		var clone = new(Message)
		*clone = *message
		go valueChange(i+1, clone, &waiter)
	}
	waiter.Wait()
}