A buggy multi-process hello world.

hello.go

package main

import (
	"bufio"
	"fmt"
	"net/http"
	"os"
	"sync"
)

const userFile = "users.txt"

type UserRegistry struct {
	mu    sync.Mutex
	users map[string]bool
}

func loadUsers(path string) map[string]bool {
	users := make(map[string]bool)
	f, err := os.Open(path)
	if err != nil {
		return users // no file yet, start fresh
	}
	defer f.Close()

	scanner := bufio.NewScanner(f)
	for scanner.Scan() {
		users[scanner.Text()] = true
	}
	return users
}

func (r *UserRegistry) sayHello(toWhom string) string {
	r.mu.Lock()
	defer r.mu.Unlock()

	if _, ok := r.users[toWhom]; ok {
		return "Hello registered user " + toWhom
	}

	r.users[toWhom] = true

	f, err := os.OpenFile(userFile, os.O_APPEND|os.O_CREATE|os.O_WRONLY, 0644)
	if err != nil {
		return "Hello new user " + toWhom + " (failed to persist: " + err.Error() + ")"
	}
	defer f.Close()
	fmt.Fprintln(f, toWhom)

	return "Hello new user " + toWhom
}

func main() {
	registry := &UserRegistry{users: loadUsers(userFile)}

	http.HandleFunc("/hello", func(w http.ResponseWriter, req *http.Request) {
		toWhom := req.URL.Query().Get("name")
		if toWhom == "" {
			http.Error(w, "missing name parameter", http.StatusBadRequest)
			return
		}
		fmt.Fprintln(w, registry.sayHello(toWhom))
	})

	fmt.Println("Listening on :8080")
	if err := http.ListenAndServe(":8080", nil); err != nil {
		fmt.Println("Server error:", err)
	}
}