Demo autocert server in Go

## autocert-server.go
package main

import (
        "crypto/tls"
        "flag"
        "fmt"
        "log"
        "net/http"
        "os"
        "os/user"
        "path/filepath"

        "golang.org/x/crypto/acme/autocert"
)

func main() {
        // setup a simple handler which sends a HTHS header for six months (!)
        http.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
                w.Header().Set("Strict-Transport-Security", "max-age=15768000 ; includeSubDomains")
                fmt.Fprintf(w, "Hello, HTTPS world!")
        })

        // look for the domains to be served from command line args
        flag.Parse()
        domains := flag.Args()
        if len(domains) == 0 {
                log.Fatalf("fatal; specify domains as arguments")
        }

        // create the autocert.Manager with domains and path to the cache
        certManager := autocert.Manager{
                Prompt:     autocert.AcceptTOS,
                HostPolicy: autocert.HostWhitelist(domains...),
        }

        // optionally use a cache dir
        dir := cacheDir()
        if dir != "" {
                certManager.Cache = autocert.DirCache(dir)
        }

        // create the server itself
        server := &http.Server{
                Addr: ":https",
                TLSConfig: &tls.Config{
                        GetCertificate: certManager.GetCertificate,
                },
        }

        log.Printf("Serving http/https for domains: %+v", domains)
        go func() {
                // serve HTTP, which will redirect automatically to HTTPS
                h := certManager.HTTPHandler(nil)
                log.Fatal(http.ListenAndServe(":http", h))
        }()

        // serve HTTPS!
        log.Fatal(server.ListenAndServeTLS("", ""))
}

// cacheDir makes a consistent cache directory inside /tmp. Returns "" on error.
func cacheDir() (dir string) {
        if u, _ := user.Current(); u != nil {
                dir = filepath.Join(os.TempDir(), "cache-golang-autocert-"+u.Username)
                if err := os.MkdirAll(dir, 0700); err == nil {
                        return dir
                }
        }
        return ""
}