go - 为什么这个循环被认为是数据竞争[Golang]

Question

我有一个循环，我得到这个错误，我正在运行 go run -race main.go

这是有问题的循环:

var wg sync.WaitGroup

for _, member := range p.Members { << Line 254
    go func() {
        wg.Add(1)
        _, err := db.Exec("UPDATE party_members SET active = ? WHERE steamid = ?", false, member.SteamID) << Line 257
        if err != nil {
            log.Println(err)
        }
        _, err = db.Exec("INSERT INTO party_members SET belongs_to =?, active = ?, steamid = ?", partyUUID, true, member.SteamID)
        if err != nil {
            log.Println(err)
        }
        wg.Done()
    }() << Line 266
}

然后这是我得到的错误:

WARNING: DATA RACE
Read by goroutine 44:
  BLAH/controllers/partybot.func·001()
      /Users/Dev/gocode/src/BLAH/controllers/partybot/partybot.go:257 +0x136

Previous write by goroutine 43:
  BLAH/controllers/partybot.(*PartialParty).SendReadyCheck()
      /Users/Dev/gocode/src/BLAH/controllers/partybot/partybot.go:254 +0xda5

Goroutine 44 (running) created at:
  BLAH/controllers/partybot.(*PartialParty).SendReadyCheck()
      /Users/Dev/gocode/src/BLAH/controllers/partybot/partybot.go:266 +0xf13

Goroutine 43 (finished) created at:
  BLAH/controllers/partybot.(*PartyHub).SortMemberIntoParty()
      /Users/Dev/gocode/src/BLAH/controllers/partybot/partybot.go:173 +0xdb9
  BLAH/controllers/partybot.(*Connection).readPump()
      /Users/Dev/gocode/src/BLAH/controllers/partybot/partybot.go:359 +0x1872
  BLAH/controllers/partybot.WSPartyBot()
      /Users/Dev/gocode/src/BLAH/controllers/partybot/partybot.go:440 +0x4f0
  net/http.HandlerFunc.ServeHTTP()
      /usr/local/go/src/net/http/server.go:1265 +0x4e
  github.com/gorilla/mux.(*Router).ServeHTTP()
      /Users/Dev/gocode/src/github.com/gorilla/mux/mux.go:98 +0x377
  BLAH/controllers/common.func·001()
      /Users/Dev/gocode/src/BLAH/controllers/common/common.go:36 +0xc2
  net/http.HandlerFunc.ServeHTTP()
      /usr/local/go/src/net/http/server.go:1265 +0x4e
  net/http.(*ServeMux).ServeHTTP()
      /usr/local/go/src/net/http/server.go:1541 +0x20c
  net/http.serverHandler.ServeHTTP()
      /usr/local/go/src/net/http/server.go:1703 +0x1f6
  net/http.(*conn).serve()
      /usr/local/go/src/net/http/server.go:1204 +0x1087
==================

我不确定为什么这是一场数据竞赛，因为无论如何我都是 Go 的新手，如果有人能告诉我为什么这会很棒，我标记了有问题的行。

Answer 1

问题是 goroutine 正在通过闭包访问变量“member”，并且在 go 中这个引用在到达时被评估(即它看到 member 的值是 goroutine 执行时的值，而不是创建时的值通过 go func()... 调用。成员变量正在由循环更新，因此在循环更新和您开始阅读它们的 goroutine 之间存在竞争。(如果您进一步挖掘，您会发现db 看不到集合中的确切成员集。通常，您可以通过在循环中强制评估变量来解决此问题，方法是创建本地变量，或将其作为参数添加到函数中，例如

var wg sync.WaitGroup

for _, member := range p.Members {
    wg.Add(1)
    m := member
    go func() {
        _, err := db.Exec("UPDATE party_members SET active = ? WHERE steamid = ?", false, m.SteamID)
        ...
        wg.Done()
    }() 
}

或

var wg sync.WaitGroup

for _, member := range p.Members {
    wg.Add(1)
    go func(m memberType) {
        _, err := db.Exec("UPDATE party_members SET active = ? WHERE steamid = ?", false, m.SteamID)
        ...
        wg.Done()
    }(member) 
}

另请注意，您需要从循环中调用 wg.Add(1)，而不是从 goroutine 本身内部调用。