package main

import (
    "bufio"
    "flag"
    "fmt"
    "io"
    "log"
    "net/http"
    "os"
    "time"
)

func version() (string, error) {
    f, err := os.Open(versionFile)
    if err != nil {
        return "", fmt.Errorf("opening version file: %v", err)
    }
    defer f.Close()
    sc := bufio.NewScanner(f)
    if !sc.Scan() {
        return "", fmt.Errorf("reading version file: %v", err)
    }
    return sc.Text(), nil
}

var file, versionFile string

func main() {
    logOut := log.New(os.Stdout, "", log.Ldate|log.Ltime|log.Lmicroseconds)
    logErr := log.New(os.Stdout, "err: ", log.Ldate|log.Ltime|log.Lmicroseconds|log.Lmsgprefix)

    flag.StringVar(&file, "file", "", "path to file to serve")
    flag.StringVar(&versionFile, "version-file", "", "path to file containing version of file")
    listen := flag.String("listen", "", "listen address (eg :80)")
    flag.Parse()
    if *listen == "" {
        logErr.Fatalf("-listen must not be empty")
    }

    http.HandleFunc("/update", func(w http.ResponseWriter, r *http.Request) {
        defer r.Body.Close()
        if r.Method != "GET" {
            logErr.Printf("unexpected method: %v from %v", r.Method, r.RemoteAddr)
            w.WriteHeader(http.StatusBadRequest)
            return
        }
        remoteVersion := r.URL.Query().Get("version")
        localVersion, err := version()
        if err != nil {
            logErr.Printf("failed reading local version: %v", err)
            w.WriteHeader(http.StatusInternalServerError)
            return
        }
        logOut.Printf("%v: %q => %q", r.RemoteAddr, remoteVersion, localVersion)
        if localVersion == remoteVersion {
            w.WriteHeader(http.StatusNoContent)
            return
        }
        f, err := os.Open(file)
        if err != nil {
            logErr.Printf("failed opening local file: %v", err)
            w.WriteHeader(http.StatusInternalServerError)
            return
        }
        defer f.Close()
        io.Copy(w, f)
    })
    server := &http.Server{
        Addr:         *listen,
        ErrorLog:     logErr,
        ReadTimeout:  20 * time.Second,
        WriteTimeout: 5 * time.Minute,
    }
    if err := server.ListenAndServe(); err != nil && err != http.ErrServerClosed {
        logErr.Printf("failed serving on %q: %v", *listen, err)
    }
}