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
+}