Add backoff http.RoundTripper that that handles exponential backoff

backoff.go

@@ -0,0 +1,47 @@
+package maps
+
+import (
+	"math/rand"
+	"net/http"
+	"time"
+)
+
+type backoff struct {
+	Transport *http.RoundTripper
+	MaxTries  int
+	tries     int
+	sleep     func(time.Duration)
+}
+
+func (bt *backoff) RoundTrip(req *http.Request) (*http.Response, error) {
+	if bt.Transport == nil {
+		bt.Transport = &http.DefaultTransport
+	}
+	if bt.MaxTries == 0 {
+		bt.MaxTries = 5
+	}
+	if bt.sleep == nil {
+		bt.sleep = time.Sleep
+	}
+	wait := time.Second
+	var err error
+	for ; bt.tries < bt.MaxTries; bt.tries++ {
+		var resp *http.Response
+		resp, err = (*bt.Transport).RoundTrip(req)
+		if err != nil {
+			// fallthrough, retry
+		} else if resp.StatusCode >= http.StatusInternalServerError {
+			err = HTTPError{resp}
+		} else {
+			return resp, nil
+		}
+		if bt.tries == bt.MaxTries-1 {
+			// last try failed, just give up
+			continue
+		}
+		bt.sleep(wait)
+		jitter := time.Duration(rand.Intn(1000))*time.Millisecond - 500*time.Millisecond // jitter is +/- .5s
+		wait = wait*2 + jitter
+	}
+	return nil, err
+}

backoff_test.go

@@ -0,0 +1,35 @@
+package maps
+
+import (
+	"net/http"
+	"net/http/httptest"
+	"testing"
+	"time"
+)
+
+func TestBackoff(t *testing.T) {
+	s := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		http.Error(w, "fail", http.StatusInternalServerError)
+	}))
+	defer s.Close()
+	tries := 10
+	sleeps := 0
+	bt := &backoff{
+		MaxTries: tries,
+		sleep: func(d time.Duration) {
+			sleeps += 1
+			t.Logf("sleeping %s", d)
+		},
+	}
+	c := &http.Client{Transport: bt}
+	r, _ := http.NewRequest("GET", s.URL, nil)
+	if _, err := c.Do(r); err == nil {
+		t.Errorf("expected error")
+	}
+	if sleeps != tries-1 {
+		t.Errorf("unexpected # of calls to sleep, got %d, want %d", tries-1, sleeps)
+	}
+	if bt.tries != tries {
+		t.Errorf("got %d tries, want %d", bt.tries, tries)
+	}
+}

maps.go

@@ -23,7 +23,9 @@ func NewWorkClient(clientID, signature string) Client {
 }
 
 func (c Client) do(url string) (*http.Response, error) {
-	cl := &http.Client{Transport: c.Transport}
+	cl := &http.Client{Transport: &backoff{
+		Transport: &c.Transport,
+	}}
 	req, err := http.NewRequest("GET", url, nil)
 	if err != nil {
 		return nil, err