server.go

package main

import (
	"bytes"
	"encoding/json"
	"fmt"
	"io"
	"io/ioutil"
	"log"
	"net/http"
	"os"
	"sync"
)

type comment struct {
	Author string `json:"author"`
	Text   string `json:"text"`
}

const dataFile = "./comments.json"

var commentMutex = new(sync.Mutex)

// Handle comments
func handleComments(w http.ResponseWriter, r *http.Request) {
	// Since multiple requests could come in at once, ensure we have a lock
	// around all file operations
	commentMutex.Lock()
	defer commentMutex.Unlock()

	// Stat the file, so we can find it's current permissions
	fi, err := os.Stat(dataFile)
	if err != nil {
		http.Error(w, fmt.Sprintf("Unable to stat the data file (%s): %s", dataFile, err), http.StatusInternalServerError)
		return
	}

	// Read the comments from the file.
	commentData, err := ioutil.ReadFile(dataFile)
	if err != nil {
		http.Error(w, fmt.Sprintf("Unable to read the data file (%s): %s", dataFile, err), http.StatusInternalServerError)
		return
	}

	switch r.Method {
	case "POST":
		// Decode the JSON data
		comments := make([]comment, 0)
		if err := json.Unmarshal(commentData, &comments); err != nil {
			http.Error(w, fmt.Sprintf("Unable to Unmarshal comments from data file (%s): %s", dataFile, err), http.StatusInternalServerError)
			return
		}

		// Add a new comment to the in memory slice of comments
		comments = append(comments, comment{Author: r.FormValue("author"), Text: r.FormValue("text")})

		// Marshal the comments to indented json.
		commentData, err = json.MarshalIndent(comments, "", "    ")
		if err != nil {
			http.Error(w, fmt.Sprintf("Unable to marshal comments to json: %s", err), http.StatusInternalServerError)
			return
		}

		// Write out the comments to the file, preserving permissions
		err := ioutil.WriteFile(dataFile, commentData, fi.Mode())
		if err != nil {
			http.Error(w, fmt.Sprintf("Unable to write comments to data file (%s): %s", dataFile, err), http.StatusInternalServerError)
			return
		}

		w.Header().Set("Content-Type", "application/json")
		w.Header().Set("Cache-Control", "no-cache")
		io.Copy(w, bytes.NewReader(commentData))

	case "GET":
		w.Header().Set("Content-Type", "application/json")
		w.Header().Set("Cache-Control", "no-cache")
		// stream the contents of the file to the response
		io.Copy(w, bytes.NewReader(commentData))

	default:
		// Don't know the method, so error
		http.Error(w, fmt.Sprintf("Unsupported method: %s", r.Method), http.StatusMethodNotAllowed)
	}
}

func main() {
	http.HandleFunc("/comments.json", handleComments)
	http.Handle("/", http.FileServer(http.Dir("./public")))
	log.Println("Server started: http://localhost:3000")
	log.Fatal(http.ListenAndServe(":3000", nil))
}