Support HTTP-Date in Retry-After header (#410)

RFC specifies Retry-After header can be integer value expressing seconds
or an HTTP-Date indicating when to try again.
Removed superfluous check for HTTP status code.

autorest/sender.go

@@ -248,16 +248,24 @@ func DoRetryForStatusCodes(attempts int, backoff time.Duration, codes ...int) Se
 	}
 }
 
-// DelayWithRetryAfter invokes time.After for the duration specified in the "Retry-After" header in
-// responses with status code 429
+// DelayWithRetryAfter invokes time.After for the duration specified in the "Retry-After" header.
+// The value of Retry-After can be either the number of seconds or a date in RFC1123 format.
+// The function returns true after successfully waiting for the specified duration.  If there is
+// no Retry-After header or the wait is cancelled the return value is false.
 func DelayWithRetryAfter(resp *http.Response, cancel <-chan struct{}) bool {
 	if resp == nil {
 		return false
 	}
-	retryAfter, _ := strconv.Atoi(resp.Header.Get("Retry-After"))
-	if resp.StatusCode == http.StatusTooManyRequests && retryAfter > 0 {
+	var dur time.Duration
+	ra := resp.Header.Get("Retry-After")
+	if retryAfter, _ := strconv.Atoi(ra); retryAfter > 0 {
+		dur = time.Duration(retryAfter) * time.Second
+	} else if t, err := time.Parse(time.RFC1123, ra); err == nil {
+		dur = t.Sub(time.Now())
+	}
+	if dur > 0 {
 		select {
-		case <-time.After(time.Duration(retryAfter) * time.Second):
+		case <-time.After(dur):
 			return true
 		case <-cancel:
 			return false

autorest/sender_test.go

@@ -818,6 +818,33 @@ func TestDelayWithRetryAfterWithSuccess(t *testing.T) {
 	}
 }
 
+func TestDelayWithRetryAfterWithSuccessDateTime(t *testing.T) {
+	resumeAt := time.Now().Add(2 * time.Second).Round(time.Second)
+
+	client := mocks.NewSender()
+	resp := mocks.NewResponseWithStatus("503 Service temporarily unavailable", http.StatusServiceUnavailable)
+	mocks.SetResponseHeader(resp, "Retry-After", resumeAt.Format(time.RFC1123))
+	client.AppendResponse(resp)
+	client.AppendResponse(mocks.NewResponseWithStatus("200 OK", http.StatusOK))
+
+	r, _ := SendWithSender(client, mocks.NewRequest(),
+		DoRetryForStatusCodes(1, time.Duration(time.Second), http.StatusServiceUnavailable),
+	)
+
+	if time.Now().Before(resumeAt) {
+		t.Fatal("autorest: DelayWithRetryAfter failed stopped too soon")
+	}
+
+	Respond(r,
+		ByDiscardingBody(),
+		ByClosing())
+
+	if client.Attempts() != 2 {
+		t.Fatalf("autorest: Sender#DelayWithRetryAfter -- Got: StatusCode %v in %v attempts; Want: StatusCode 200 OK in 2 attempts -- ",
+			r.Status, client.Attempts()-1)
+	}
+}
+
 type temporaryError struct {
 	message string
 }