oauth2: parse RFC 6749 error response

Parse error response described in https://datatracker.ietf.org/doc/html/rfc6749#section-5.2

Handle unorthodox servers responding 200 in error case.

Implements API changes in accepted proposal golang/go#58125

Fixes #441
Fixes #274
Updates #173

Change-Id: If9399c3f952ac0501edbeefeb3a71ed057ca8d37
GitHub-Last-Rev: 0030e27
GitHub-Pull-Request: #610
Reviewed-on: https://go-review.googlesource.com/c/oauth2/+/451076
Run-TryBot: Matt Hickford <matt.hickford@gmail.com>
Run-TryBot: Damien Neil <dneil@google.com>
Reviewed-by: Matt Hickford <matt.hickford@gmail.com>
Reviewed-by: Damien Neil <dneil@google.com>
Reviewed-by: Cody Oss <codyoss@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

Author: hickford

internal/token.go

@@ -55,12 +55,18 @@ type Token struct {
 }
 
 // tokenJSON is the struct representing the HTTP response from OAuth2
-// providers returning a token in JSON form.
+// providers returning a token or error in JSON form.
+// https://datatracker.ietf.org/doc/html/rfc6749#section-5.1
 type tokenJSON struct {
 	AccessToken  string         `json:"access_token"`
 	TokenType    string         `json:"token_type"`
 	RefreshToken string         `json:"refresh_token"`
 	ExpiresIn    expirationTime `json:"expires_in"` // at least PayPal returns string, while most return number
+	// error fields
+	// https://datatracker.ietf.org/doc/html/rfc6749#section-5.2
+	ErrorCode        string `json:"error"`
+	ErrorDescription string `json:"error_description"`
+	ErrorURI         string `json:"error_uri"`
 }
 
 func (e *tokenJSON) expiry() (t time.Time) {
@@ -236,21 +242,29 @@ func doTokenRoundTrip(ctx context.Context, req *http.Request) (*Token, error) {
 	if err != nil {
 		return nil, fmt.Errorf("oauth2: cannot fetch token: %v", err)
 	}
-	if code := r.StatusCode; code < 200 || code > 299 {
-		return nil, &RetrieveError{
-			Response: r,
-			Body:     body,
-		}
+
+	failureStatus := r.StatusCode < 200 || r.StatusCode > 299
+	retrieveError := &RetrieveError{
+		Response: r,
+		Body:     body,
+		// attempt to populate error detail below
 	}
 
 	var token *Token
 	content, _, _ := mime.ParseMediaType(r.Header.Get("Content-Type"))
 	switch content {
 	case "application/x-www-form-urlencoded", "text/plain":
+		// some endpoints return a query string
 		vals, err := url.ParseQuery(string(body))
 		if err != nil {
-			return nil, err
+			if failureStatus {
+				return nil, retrieveError
+			}
+			return nil, fmt.Errorf("oauth2: cannot parse response: %v", err)
 		}
+		retrieveError.ErrorCode = vals.Get("error")
+		retrieveError.ErrorDescription = vals.Get("error_description")
+		retrieveError.ErrorURI = vals.Get("error_uri")
 		token = &Token{
 			AccessToken:  vals.Get("access_token"),
 			TokenType:    vals.Get("token_type"),
@@ -265,8 +279,14 @@ func doTokenRoundTrip(ctx context.Context, req *http.Request) (*Token, error) {
 	default:
 		var tj tokenJSON
 		if err = json.Unmarshal(body, &tj); err != nil {
-			return nil, err
+			if failureStatus {
+				return nil, retrieveError
+			}
+			return nil, fmt.Errorf("oauth2: cannot parse json: %v", err)
 		}
+		retrieveError.ErrorCode = tj.ErrorCode
+		retrieveError.ErrorDescription = tj.ErrorDescription
+		retrieveError.ErrorURI = tj.ErrorURI
 		token = &Token{
 			AccessToken:  tj.AccessToken,
 			TokenType:    tj.TokenType,
@@ -276,17 +296,37 @@ func doTokenRoundTrip(ctx context.Context, req *http.Request) (*Token, error) {
 		}
 		json.Unmarshal(body, &token.Raw) // no error checks for optional fields
 	}
+	// according to spec, servers should respond status 400 in error case
+	// https://www.rfc-editor.org/rfc/rfc6749#section-5.2
+	// but some unorthodox servers respond 200 in error case
+	if failureStatus || retrieveError.ErrorCode != "" {
+		return nil, retrieveError
+	}
 	if token.AccessToken == "" {
 		return nil, errors.New("oauth2: server response missing access_token")
 	}
 	return token, nil
 }
 
+// mirrors oauth2.RetrieveError
 type RetrieveError struct {
-	Response *http.Response
-	Body     []byte
+	Response         *http.Response
+	Body             []byte
+	ErrorCode        string
+	ErrorDescription string
+	ErrorURI         string
 }
 
 func (r *RetrieveError) Error() string {
+	if r.ErrorCode != "" {
+		s := fmt.Sprintf("oauth2: %q", r.ErrorCode)
+		if r.ErrorDescription != "" {
+			s += fmt.Sprintf(" %q", r.ErrorDescription)
+		}
+		if r.ErrorURI != "" {
+			s += fmt.Sprintf(" %q", r.ErrorURI)
+		}
+		return s
+	}
 	return fmt.Sprintf("oauth2: cannot fetch token: %v\nResponse: %s", r.Response.Status, r.Body)
 }

oauth2_test.go

@@ -484,6 +484,7 @@ func TestTokenRetrieveError(t *testing.T) {
 			t.Errorf("Unexpected token refresh request URL, %v is found.", r.URL)
 		}
 		w.Header().Set("Content-type", "application/json")
+		// "The authorization server responds with an HTTP 400 (Bad Request)" https://www.rfc-editor.org/rfc/rfc6749#section-5.2
 		w.WriteHeader(http.StatusBadRequest)
 		w.Write([]byte(`{"error": "invalid_grant"}`))
 	}))
@@ -493,15 +494,47 @@ func TestTokenRetrieveError(t *testing.T) {
 	if err == nil {
 		t.Fatalf("got no error, expected one")
 	}
-	_, ok := err.(*RetrieveError)
+	re, ok := err.(*RetrieveError)
 	if !ok {
 		t.Fatalf("got %T error, expected *RetrieveError; error was: %v", err, err)
 	}
-	// Test error string for backwards compatibility
-	expected := fmt.Sprintf("oauth2: cannot fetch token: %v\nResponse: %s", "400 Bad Request", `{"error": "invalid_grant"}`)
+	expected := `oauth2: "invalid_grant"`
 	if errStr := err.Error(); errStr != expected {
 		t.Fatalf("got %#v, expected %#v", errStr, expected)
 	}
+	expected = "invalid_grant"
+	if re.ErrorCode != expected {
+		t.Fatalf("got %#v, expected %#v", re.ErrorCode, expected)
+	}
+}
+
+// TestTokenRetrieveError200 tests handling of unorthodox server that returns 200 in error case
+func TestTokenRetrieveError200(t *testing.T) {
+	ts := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		if r.URL.String() != "/token" {
+			t.Errorf("Unexpected token refresh request URL, %v is found.", r.URL)
+		}
+		w.Header().Set("Content-type", "application/json")
+		w.Write([]byte(`{"error": "invalid_grant"}`))
+	}))
+	defer ts.Close()
+	conf := newConf(ts.URL)
+	_, err := conf.Exchange(context.Background(), "exchange-code")
+	if err == nil {
+		t.Fatalf("got no error, expected one")
+	}
+	re, ok := err.(*RetrieveError)
+	if !ok {
+		t.Fatalf("got %T error, expected *RetrieveError; error was: %v", err, err)
+	}
+	expected := `oauth2: "invalid_grant"`
+	if errStr := err.Error(); errStr != expected {
+		t.Fatalf("got %#v, expected %#v", errStr, expected)
+	}
+	expected = "invalid_grant"
+	if re.ErrorCode != expected {
+		t.Fatalf("got %#v, expected %#v", re.ErrorCode, expected)
+	}
 }
 
 func TestRefreshToken_RefreshTokenReplacement(t *testing.T) {

token.go

@@ -175,14 +175,31 @@ func retrieveToken(ctx context.Context, c *Config, v url.Values) (*Token, error)
 }
 
 // RetrieveError is the error returned when the token endpoint returns a
-// non-2XX HTTP status code.
+// non-2XX HTTP status code or populates RFC 6749's 'error' parameter.
+// https://datatracker.ietf.org/doc/html/rfc6749#section-5.2
 type RetrieveError struct {
 	Response *http.Response
 	// Body is the body that was consumed by reading Response.Body.
 	// It may be truncated.
 	Body []byte
+	// ErrorCode is RFC 6749's 'error' parameter.
+	ErrorCode string
+	// ErrorDescription is RFC 6749's 'error_description' parameter.
+	ErrorDescription string
+	// ErrorURI is RFC 6749's 'error_uri' parameter.
+	ErrorURI string
 }
 
 func (r *RetrieveError) Error() string {
+	if r.ErrorCode != "" {
+		s := fmt.Sprintf("oauth2: %q", r.ErrorCode)
+		if r.ErrorDescription != "" {
+			s += fmt.Sprintf(" %q", r.ErrorDescription)
+		}
+		if r.ErrorURI != "" {
+			s += fmt.Sprintf(" %q", r.ErrorURI)
+		}
+		return s
+	}
 	return fmt.Sprintf("oauth2: cannot fetch token: %v\nResponse: %s", r.Response.Status, r.Body)
 }