追加不是线程安全的?

问题描述

我注意到，如果我尝试在 for 循环中使用 goroutines 附加到一个切片，就会出现丢失/空白数据的情况:

I noticed that if I tried appending to a slice using goroutines inside a for loop, there would be instances where I would get missing/blank data:

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，当我从 destSlice 打印所有 AttributeName 时，有些元素是空字符串 ("")，而有时，有些元素是空字符串sourceSlice 中的元素不存在于 destSlice 中.

Sometimes, when I print all AttributeNames from destSlice, some elements are empty strings (""), and other times, some elements from sourceSlice are not present in destSlice.

我的代码是否存在数据竞争，这是否意味着 append 对于多个 goroutine 并发使用不是线程安全的?

Does my code have a data race, and does this mean that append is not thread-safe for concurrent use by multiple goroutines?

推荐答案

在 Go 中，并发读/写没有值是安全的，切片(切片标题) 也不例外.

In Go no value is safe for concurrent read/write, slices (which are slice headers) are no exception.

是的，您的代码存在数据竞争.使用 -race 选项运行以进行验证.

Yes, your code has data races. Run with the -race option to verify.

示例:

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

解决方案很简单，使用 sync.Mutex 来保护写入 destSlice 值:

Solution is simple, use a sync.Mutex to protect writing the destSlice value:

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()

您也可以通过其他方式解决它，例如您可以使用一个通道，在该通道上发送要附加的值，并指定一个从该通道接收并执行附加的 goroutine.

You could also solve it in other ways, e.g. you could use a channel on which you'd send the value to be appended, and have a designated goroutine receiving from this channel and do the append.

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