Expose RetryAfter value in error

spotify.go

@@ -148,6 +148,9 @@ type Error struct {
 	Message string `json:"message"`
 	// The HTTP status code.
 	Status int `json:"status"`
+	// RetryAfter contains the time before which client should not retry a
+	// rate-limited request, calculated from the Retry-After header, when present.
+	RetryAfter time.Time `json:"-"`
 }
 
 func (e Error) Error() string {
@@ -196,6 +199,9 @@ func decodeError(resp *http.Response) error {
 		e.E.Message = fmt.Sprintf("spotify: unexpected HTTP %d: %s (empty error)",
 			resp.StatusCode, http.StatusText(resp.StatusCode))
 	}
+	if retryAfter, _ := strconv.Atoi(resp.Header.Get("Retry-After")); retryAfter != 0 {
+		e.E.RetryAfter = time.Now().Add(time.Duration(retryAfter) * time.Second)
+	}
 
 	return e.E
 }

spotify_test.go

@@ -2,10 +2,12 @@
 
 import (
 	"context"
+	"errors"
 	"io"
 	"net/http"
 	"net/http/httptest"
 	"os"
+	"strconv"
 	"strings"
 	"testing"
 	"time"
@@ -105,6 +107,52 @@
 	}
 }
 
+func TestRateLimitExceededReportsRetryAfter(t *testing.T) {
+	t.Parallel()
+	const retryAfter = 2
+
+	handlers := []http.HandlerFunc{
+		// first attempt fails
+		http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+			w.Header().Set("Retry-After", strconv.Itoa(retryAfter))
+			w.WriteHeader(rateLimitExceededStatusCode)
+			_, _ = io.WriteString(w, `{ "error": { "message": "slow down", "status": 429 } }`)
+		}),
+		// next attempt succeeds
+		http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+			f, err := os.Open("test_data/new_releases.txt")
+			if err != nil {
+				t.Fatal(err)
+			}
+			defer f.Close()
+			_, err = io.Copy(w, f)
+			if err != nil {
+				t.Fatal(err)
+			}
+		}),
+	}
+
+	i := 0
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		handlers[i](w, r)
+		i++
+	}))
+	defer server.Close()
+
+	client := &Client{http: http.DefaultClient, baseURL: server.URL + "/"}
+	_, err := client.NewReleases(context.Background())
+	if err == nil {
+		t.Fatal("expected an error")
+	}
+	var spotifyError Error
+	if !errors.As(err, &spotifyError) {
+		t.Fatalf("expected a spotify error, got %T", err)
+	}
+	if retryAfter*time.Second-time.Until(spotifyError.RetryAfter) > time.Second {
+		t.Error("expected RetryAfter value")
+	}
+}
+
 func TestClient_Token(t *testing.T) {
 	// oauth setup for valid test token
 	config := oauth2.Config{