A notable example in the experimental package that uses context is one we've already looked at – semaphore. Now that we have a better understanding of what context is for, it should be pretty obvious why the acquire operation also takes a context in as an argument.

When creating our application, we can provide a context with a timeout or cancellation and act accordingly:

func main() {
    s := semaphore.NewWeighted(int64(5))
    ctx, canc := context.WithTimeout(context.Background(), time.Second)
    defer canc()
    wg := sync.WaitGroup{}
    wg.Add(20)
    for i := 0; i < 20; i++ {
        go func(i int) {
            defer wg.Done()
            if err := s.Acquire(ctx, 1); err != nil {
                fmt.Println(i, err)
                return
            }
            go func(i int) {
                fmt.Println(i)
                time.Sleep(time.Second / 2)
                s.Release(1)
            }(i)
        }(i)
    }
    wg.Wait()
}

Running this application will show that the semaphore is acquired for the first second, but after that the context expires and all the remaining operations fail.