just fetch periodically

main.go

@@ -2,6 +2,7 @@ package main
 
 import (
 	"bytes"
+	"context"
 	"flag"
 	"fmt"
 	"image"
@@ -12,45 +13,55 @@ import (
 	"os"
 	"sync"
 	"time"
+
+	"github.com/pkg/errors"
 )
 
 var port = flag.Int("port", 8000, "port")
+var freq = flag.Duration("d", 15*time.Second, "refresh interval")
 
 type Now struct {
-	lastUpdated time.Time
-	cacheWindow time.Duration
-	buffer      *bytes.Buffer
-	lock        sync.RWMutex
+	sync.RWMutex
+
+	cache *bytes.Buffer
 }
 
-func NewNow(d time.Duration) *Now {
-	return &Now{
-		cacheWindow: d,
-		buffer:      &bytes.Buffer{},
+func NewNow(ctx context.Context, d time.Duration) *Now {
+	n := &Now{
+		cache: &bytes.Buffer{},
 	}
+
+	n.Update()
+	go func() {
+		t := time.NewTicker(d)
+		for {
+			select {
+			case <-t.C:
+				n.Update()
+			case <-ctx.Done():
+				return
+			}
+		}
+	}()
+
+	return n
 }
 
 func (n *Now) ServeHTTP(w http.ResponseWriter, req *http.Request) {
-	log.Printf("%+v", req.RemoteAddr)
-	if time.Now().After(n.lastUpdated.Add(n.cacheWindow)) {
-		log.Println("updating image")
-		n.Update()
-		n.lastUpdated = time.Now()
-	}
-	n.lock.RLock()
-	w.Write(n.buffer.Bytes())
-	n.lock.RUnlock()
+	n.RLock()
+	w.Write(n.cache.Bytes())
+	n.RUnlock()
 }
 
-func (n *Now) Update() {
+func (n *Now) Update() error {
 	resp, err := http.Get("http://imgs.xkcd.com/comics/now.png")
 	if err != nil {
-		log.Fatal(err)
+		return errors.Wrap(err, "get")
 	}
 	defer resp.Body.Close()
 	src, err := png.Decode(resp.Body)
 	if err != nil {
-		log.Fatal(err)
+		return errors.Wrap(err, "png decode")
 	}
 	bounds := src.Bounds()
 	dest := image.NewNRGBA(bounds)
@@ -64,9 +75,12 @@ func (n *Now) Update() {
 			}
 		}
 	}
-	n.lock.Lock()
-	png.Encode(n.buffer, dest)
-	n.lock.Unlock()
+
+	n.Lock()
+	png.Encode(n.cache, dest)
+	n.Unlock()
+
+	return nil
 }
 
 func main() {
@@ -79,7 +93,9 @@ func main() {
 	log.Printf("serving on: http://%s:%d/", hostname, *port)
 	addr := fmt.Sprintf(":%d", *port)
 
-	http.Handle("/", NewNow(15*time.Minute))
+	n := NewNow(context.Background(), *freq)
+
+	http.Handle("/", n)
 
 	err = http.ListenAndServe(addr, nil)
 	if err != nil {