追加不是线程安全的吗？

Question

我注意到，如果我尝试在for循环内使用goroutines将其追加到切片，那么在某些情况下我会丢失/空白数据：

destSlice := make([]myClass, 0)

var wg sync.WaitGroup
for _, myObject := range sourceSlice {
    wg.Add(1)
    go func(closureMyObject myClass) {
        defer wg.Done()
        var tmpObj myClass
        tmpObj.AttributeName = closureMyObject.AttributeName
        destSlice = append(destSlice, tmpObj)
    }(myObject)
}
wg.Wait()

有时，当我AttributeName从中打印all
时destSlice，某些元素是空字符串（""），而在另一些sourceSlice情况下，in中则不存在某些元素destSlice。

我的代码是否存在数据争用，是否意味着append对于多个goroutine并发使用来说不是线程安全的？

Answer 1

在Go中，没有值可以安全地进行并行读/写操作，切片（切片头）也不例外。

是的，您的代码存在数据竞争。运行带有-race选项的验证。

例：

type myClass struct {
    AttributeName string
}
sourceSlice := make([]myClass, 100)

destSlice := make([]myClass, 0)

var wg sync.WaitGroup
for _, myObject := range sourceSlice {
    wg.Add(1)
    go func(closureMyObject myClass) {
        defer wg.Done()
        var tmpObj myClass
        tmpObj.AttributeName = closureMyObject.AttributeName
        destSlice = append(destSlice, tmpObj)
    }(myObject)
}
wg.Wait()

运行它

go run -race play.go

输出为：

==================
WARNING: DATA RACE
Read at 0x00c420074000 by goroutine 6:
  main.main.func1()
      /home/icza/gows/src/play/play.go:20 +0x69

Previous write at 0x00c420074000 by goroutine 5:
  main.main.func1()
      /home/icza/gows/src/play/play.go:20 +0x106

Goroutine 6 (running) created at:
  main.main()
      /home/icza/gows/src/play/play.go:21 +0x1cb

Goroutine 5 (running) created at:
  main.main()
      /home/icza/gows/src/play/play.go:21 +0x1cb
==================
==================
WARNING: DATA RACE
Read at 0x00c42007e000 by goroutine 6:
  runtime.growslice()
      /usr/local/go/src/runtime/slice.go:82 +0x0
  main.main.func1()
      /home/icza/gows/src/play/play.go:20 +0x1a7

Previous write at 0x00c42007e000 by goroutine 5:
  main.main.func1()
      /home/icza/gows/src/play/play.go:20 +0xc4

Goroutine 6 (running) created at:
  main.main()
      /home/icza/gows/src/play/play.go:21 +0x1cb

Goroutine 5 (running) created at:
  main.main()
      /home/icza/gows/src/play/play.go:21 +0x1cb
==================
==================
WARNING: DATA RACE
Write at 0x00c420098120 by goroutine 80:
  main.main.func1()
      /home/icza/gows/src/play/play.go:20 +0xc4

Previous write at 0x00c420098120 by goroutine 70:
  main.main.func1()
      /home/icza/gows/src/play/play.go:20 +0xc4

Goroutine 80 (running) created at:
  main.main()
      /home/icza/gows/src/play/play.go:21 +0x1cb

Goroutine 70 (running) created at:
  main.main()
      /home/icza/gows/src/play/play.go:21 +0x1cb
==================
Found 3 data race(s)
exit status 66

解决方法很简单，使用一个sync.Mutex保护写destSlice值：

var (
    mu        = &sync.Mutex{}
    destSlice = make([]myClass, 0)
)

var wg sync.WaitGroup
for _, myObject := range sourceSlice {
    wg.Add(1)
    go func(closureMyObject myClass) {
        defer wg.Done()
        var tmpObj myClass
        tmpObj.AttributeName = closureMyObject.AttributeName
        mu.Lock()
        destSlice = append(destSlice, tmpObj)
        mu.Unlock()
    }(myObject)
}
wg.Wait()

您还可以通过其他方式解决它，例如，可以使用一个通道，在该通道上发送要附加的值，并从该通道接收指定的goroutine并执行附加操作。