Go link checker

package main

// A tiny link checker: GET /check?url=https://example.com answers with the
// status code and how long the request took. The standard library carries
// the whole service; the binary built from it is a few megabytes and starts
// in milliseconds.

import (
	"encoding/json"
	"fmt"
	"log"
	"net/http"
	"os"
	"time"
)

func main() {
	port := os.Getenv("PORT")
	if port == "" {
		port = "8080"
	}

	http.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
		fmt.Fprintln(w, "GET /check?url=https://example.com")
	})

	http.HandleFunc("/check", func(w http.ResponseWriter, r *http.Request) {
		target := r.URL.Query().Get("url")
		if target == "" {
			http.Error(w, "missing ?url=", http.StatusUnprocessableEntity)
			return
		}
		client := &http.Client{Timeout: 10 * time.Second}
		start := time.Now()
		resp, err := client.Get(target)
		elapsed := time.Since(start)
		w.Header().Set("content-type", "application/json; charset=utf-8")
		if err != nil {
			w.WriteHeader(http.StatusBadGateway)
			json.NewEncoder(w).Encode(map[string]any{"url": target, "error": err.Error()})
			return
		}
		defer resp.Body.Close()
		json.NewEncoder(w).Encode(map[string]any{
			"url":    target,
			"status": resp.StatusCode,
			"ms":     elapsed.Milliseconds(),
		})
	})

	fmt.Println("Serving on localhost:" + port)
	log.Fatal(http.ListenAndServe(":"+port, nil))
}

module go-link-checker

go 1.26