outyet/main.go

// Copyright 2023 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

// Outyet is a web server that announces whether or not a particular Go version
// has been tagged.
package main

import (
	"expvar"
	"flag"
	"fmt"
	"html/template"
	"log"
	"net/http"
	"sync"
	"time"
)

// Command-line flags.
var (
	httpAddr   = flag.String("http", "localhost:8080", "Listen address")
	pollPeriod = flag.Duration("poll", 5*time.Second, "Poll period")
	version    = flag.String("version", "1.4", "Go version")
)

const baseChangeURL = "https://go.googlesource.com/go/+/"

func main() {
	flag.Parse()
	changeURL := fmt.Sprintf("%sgo%s", baseChangeURL, *version)
	http.Handle("/", NewServer(*version, changeURL, *pollPeriod))
	log.Printf("serving http://%s", *httpAddr)
	log.Fatal(http.ListenAndServe(*httpAddr, nil))
}

// Exported variables for monitoring the server.
// These are exported via HTTP as a JSON object at /debug/vars.
var (
	hitCount       = expvar.NewInt("hitCount")
	pollCount      = expvar.NewInt("pollCount")
	pollError      = expvar.NewString("pollError")
	pollErrorCount = expvar.NewInt("pollErrorCount")
)

// Server implements the outyet server.
// It serves the user interface (it's an http.Handler)
// and polls the remote repository for changes.
type Server struct {
	version string
	url     string
	period  time.Duration

	mu  sync.RWMutex // protects the yes variable
	yes bool
}

// NewServer returns an initialized outyet server.
func NewServer(version, url string, period time.Duration) *Server {
	s := &Server{version: version, url: url, period: period}
	go s.poll()
	return s
}

// poll polls the change URL for the specified period until the tag exists.
// Then it sets the Server's yes field true and exits.
func (s *Server) poll() {
	for !isTagged(s.url) {
		pollSleep(s.period)
	}
	s.mu.Lock()
	s.yes = true
	s.mu.Unlock()
	pollDone()
}

// Hooks that may be overridden for integration tests.
var (
	pollSleep = time.Sleep
	pollDone  = func() {}
)

// isTagged makes an HTTP HEAD request to the given URL and reports whether it
// returned a 200 OK response.
func isTagged(url string) bool {
	pollCount.Add(1)
	r, err := http.Head(url)
	if err != nil {
		log.Print(err)
		pollError.Set(err.Error())
		pollErrorCount.Add(1)
		return false
	}
	return r.StatusCode == http.StatusOK
}

// ServeHTTP implements the HTTP user interface.
func (s *Server) ServeHTTP(w http.ResponseWriter, r *http.Request) {
	hitCount.Add(1)
	s.mu.RLock()
	data := struct {
		URL     string
		Version string
		Yes     bool
	}{
		s.url,
		s.version,
		s.yes,
	}
	s.mu.RUnlock()
	err := tmpl.Execute(w, data)
	if err != nil {
		log.Print(err)
	}
}

// tmpl is the HTML template that drives the user interface.
var tmpl = template.Must(template.New("tmpl").Parse(`
<!DOCTYPE html><html><body><center>
	<h2>Is Go {{.Version}} out yet?</h2>
	<h1>
	{{if .Yes}}
		<a href="{{.URL}}">YES!</a>
	{{else}}
		No. :-(
	{{end}}
	</h1>
</center></body></html>
`))