Add backoff http.RoundTripper that that handles exponential backoff

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)
+	}
+}