Golang long polling example with request cancellation

main.go

package main

import (
    "context"
    "fmt"
    "net/http"
    "time"
)

func longOperation(ctx context.Context, ch chan<- string) {
    // Simulate long operation.
    // Change it to more than 10 seconds to get server timeout.
    select {
    case <-time.After(time.Second * 3):
        ch <- "Successful result."
    case <-ctx.Done():
        close(ch)
    }
}

func handler(w http.ResponseWriter, _ *http.Request) {
    notifier, ok := w.(http.CloseNotifier)
    if !ok {
        panic("Expected http.ResponseWriter to be an http.CloseNotifier")
    }

    ctx, cancel := context.WithCancel(context.Background())
    ch := make(chan string)
    go longOperation(ctx, ch)

    select {
    case result := <-ch:
        fmt.Fprint(w, result)
        cancel()
        return
    case <-time.After(time.Second * 10):
        fmt.Fprint(w, "Server is busy.")
    case <-notifier.CloseNotify():
        fmt.Println("Client has disconnected.")
    }
    cancel()
    <-ch
}

func main() {
    http.HandleFunc("/", handler)
    http.ListenAndServe("localhost:8080", nil)
}