Go - context

 

func main() {
    ctx, cancel := context.WithTimeout(context.Background(), 5 * time.Second)
    defer cancel()

    go f1(ctx)

    for i := 0; i < 10; i++ {
        select {
        case <-ctx.Done():
            fmt.Println("timed out")
        default:
            fmt.Println("main sleep for 1 second")
            time.Sleep(time.Second)
        }
    }

    time.Sleep(30 * time.Second)
    fmt.Println("main slept for 30 seconds")
}

func f1(ctx context.Context) {
    go f2()

    time.Sleep(10 * time.Second)
    fmt.Println("f1 slept for 10 seconds")
}

func f2() {
    time.Sleep(20 * time.Second)
    fmt.Println("f2 slept for 20 seconds")
}

 

zzh@ZZHPC:/zdata/Github/ztest$ go run main.go
main sleep for 1 second
main sleep for 1 second
main sleep for 1 second
main sleep for 1 second
main sleep for 1 second
main sleep for 1 second
timed out
timed out
timed out
timed out
f1 slept for 10 seconds
f2 slept for 20 seconds
main slept for 30 seconds

From the output, we can see that context timing out cannot stop goroutines from executing.

 

context.Context can only relay the message that timeout or cancellation happened. It does not have the power to actually stop any goroutines (for details, see cancel a blocking operation in Go). The goroutine itself is responsible for checking the timeout and cancellation, and abort early.

 

func main() {
    ctx, cancel := context.WithTimeout(context.Background(), 5 * time.Second)
    defer cancel()

    go f1(ctx)

    for i := 0; i < 10; i++ {
        select {
        case <-ctx.Done():
            fmt.Println("timed out")
        default:
            fmt.Println("main sleep for 1 second")
            time.Sleep(time.Second)
        }
    }

    time.Sleep(30 * time.Second)
    fmt.Println("main slept for 30 seconds")
}

func f1(ctx context.Context) {
    go f2()

    select {
    case <-ctx.Done():
        fmt.Println(ctx.Err())
    case <-time.After(10 * time.Second):
        fmt.Println("f1 slept for 10 seconds")
    }
}

func f2() {
    time.Sleep(20 * time.Second)
    fmt.Println("f2 slept for 20 seconds")
}

 

zzh@ZZHPC:/zdata/Github/ztest$ go run main.go
main sleep for 1 second
main sleep for 1 second
main sleep for 1 second
main sleep for 1 second
main sleep for 1 second
context deadline exceeded
timed out
timed out
timed out
timed out
timed out
f2 slept for 20 seconds
main slept for 30 seconds