main.go

package main

import (
	"context"
	"fmt"
	"io"
	"io/ioutil"
	"log"
	"net/http"
	"net/url"
	"time"
)

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

func checkSite(w http.ResponseWriter, r *http.Request) {
	q := r.RequestURI[1:]
	req, err := http.NewRequest(http.MethodGet, "http://"+q, nil)
	if err != nil {
		http.Error(w, err.Error(), http.StatusInternalServerError)
		return
	}

	ctx, cancel := context.WithTimeout(r.Context(), 5*time.Second)
	defer cancel()

	resp, err := http.DefaultClient.Do(req.WithContext(ctx))
	if urlErr, ok := err.(*url.Error); ok {
		if urlErr.Err == context.Canceled {
			log.Println("user cancel request")
			return
		}
		if urlErr.Err == context.DeadlineExceeded {
			fmt.Fprintf(w, "Site \"%s\" is down.", q)
			return
		}
	}
	if err != nil {
		http.Error(w, err.Error(), http.StatusInternalServerError)
		return
	}

	io.Copy(ioutil.Discard, resp.Body)
	resp.Body.Close()

	if resp.StatusCode != http.StatusOK {
		fmt.Fprintf(w, "Site \"%s\" is down.", q)
		return
	}
	fmt.Fprintf(w, "Site \"%s\" is up.", q)
}