Is the Go HTTP handler goroutine expected to exit immediately in this case?

Question

I have one Go HTTP handler like this:

mux.HandleFunc("/test", func(w http.ResponseWriter, r *http.Request) {
    ctx, cancel := context.WithCancel(context.Background())

    defer cancel()

    if cn, ok := w.(http.CloseNotifier); ok {
        go func(done <-chan struct{}, closed <-chan bool) {
            select {
            case <-done:
            case <-closed:
                fmt.Println("client cancelled....................!!!!!!!!!")
                cancel()
            }
        }(ctx.Done(), cn.CloseNotify())
    }

    time.Sleep(5 * time.Second)

    fmt.Println("I am still running...........")

    fmt.Fprint(w, "cancellation testing......")
})

The API works fine, then with curl before the request finish I terminate the curl command deliberately with Control-C , and on server side I do see the client cancelled....................!!!!!!!!! get logged out, but after a while the I am still running........... get logged out also, I thought this goroutine will be terminated immediately!

So, is this desired behaviour, or I did something wrong?

If it is expected, since whatever the goroutine will complete its work, then what is the point of the early cancellation?

If I did something wrong, please help to point me out the correct way.

Answer 1

You create a contex.Context that can be cancelled, which you do cancel when the client closes the connection, BUT you do not check the context and your handler does nothing differently if it is cancelled. The context only carries timeout and cancellation signals, it does not have the power nor the intent to kill / terminate goroutines. The goroutines themselves have to monitor such cancellation signals and act upon it.

So what you see is the expected output of your code.

What you want is to monitor the context, and if it is cancelled, return "immediately" from the handler.

Of course if you're "sleeping", you can't monitor the context meanwhile. So instead use time.After() , like in this example:

mux.HandleFunc("/test", func(w http.ResponseWriter, r *http.Request) {
    ctx, cancel := context.WithCancel(context.Background())
    defer cancel()

    if cn, ok := w.(http.CloseNotifier); ok {
        go func(done <-chan struct{}, closed <-chan bool) {
            select {
            case <-done:
            case <-closed:
                fmt.Println("client cancelled....................!!!!!!!!!")
                cancel()
            }
        }(ctx.Done(), cn.CloseNotify())
    }

    select {
    case <-time.After(5 * time.Second):
        fmt.Println("5 seconds elapsed, client didn't close")
    case <-ctx.Done():
        fmt.Println("Context closed, client closed connection?")
        return
    }

    fmt.Fprint(w, "cancellation testing......")
})