Is this a race condition in Go?

php editor Apple will answer a common question for you in this article: "Is this a race condition in Go?" When writing concurrent programs, the race condition is A common problem that can lead to data inconsistencies and other unexpected results. In the Go language, we can use mechanisms such as mutex locks and channels to avoid race conditions. Let’s discuss it together!

Question content

func main() {
    m := map[string]int{
        "foo": 42,
        "bar": 1337,
    }

    go func() {
        time.Sleep(1 * time.Second)
        tmp := map[string]int{
            "foo": 44,
            "bar": 1339,
        }

        m = tmp
    }()

    for {
        val := m["foo"]
        fmt.Println(val)
    }
}

I see this in a lot of packages.

Why isn't this considered a race condition?

go run -race . No errors.

Solution

As @volker pointed out, this is a data race. And since it's only written once, it's difficult to detect. Here is a modified demo that can easily trigger data race errors:

package main

import (
    "fmt"
    "time"
)

func main() {
    m := map[string]int{
        "foo": 42,
        "bar": 1337,
    }

    done := make(chan any)

    go func() {
        for i := 0; i < 100; i++ {
            time.Sleep(time.Microsecond)
            tmp := map[string]int{
                "foo": 44,
                "bar": 1339,
            }

            m = tmp
        }

        close(done)
    }()

    for {
        select {
        case <-done:
            return
        default:
            val := m["foo"]
            fmt.Println(val)
        }
    }
}

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