fix: flagsmith init fails with invalid_grant on an expired session

internal/auth/oauth.go

@@ -191,10 +191,7 @@ func postToken(ctx context.Context, httpClient *http.Client, endpoint string, fo
 			Description string `json:"error_description"`
 		}
 		if json.Unmarshal(body, &oauthErr) == nil && oauthErr.Error != "" {
-			if oauthErr.Description != "" {
-				return nil, bug.Mark(fmt.Errorf("%s: %s", oauthErr.Error, oauthErr.Description))
-			}
-			return nil, bug.Mark(errors.New(oauthErr.Error))
+			return nil, bug.Mark(&tokenError{Code: oauthErr.Error, Description: oauthErr.Description})
 		}
 		return nil, bug.Mark(fmt.Errorf("token endpoint returned %s", resp.Status))
 	}
@@ -205,6 +202,20 @@ func postToken(ctx context.Context, httpClient *http.Client, endpoint string, fo
 	return &tok, nil
 }
 
+// tokenError is an OAuth error response from the token endpoint (RFC 6749
+// §5.2).
+type tokenError struct {
+	Code        string
+	Description string
+}
+
+func (e *tokenError) Error() string {
+	if e.Description != "" {
+		return e.Code + ": " + e.Description
+	}
+	return e.Code
+}
+
 func randomURLSafe(n int) string {
 	b := make([]byte, n)
 	if _, err := rand.Read(b); err != nil {
@@ -331,6 +342,11 @@ func credentialsFromToken(apiURL string, tok *tokenResponse) *Credentials {
 // the stored session is unusable.
 var ErrRefreshFailed = errors.New("refreshing session failed")
 
+// ErrSessionExpired means the server rejected the stored session's refresh
+// token — it has expired or been revoked. The instance is then as good as
+// logged out, so this matches ErrNotLoggedIn.
+var ErrSessionExpired = fmt.Errorf("session expired: %w", ErrNotLoggedIn)
+
 // EnsureFresh refreshes the access token if it is expired or about to expire.
 // The server rotates refresh tokens (120s grace), so refreshed credentials
 // must be saved by the caller.
@@ -350,6 +366,10 @@ func EnsureFresh(ctx context.Context, httpClient *http.Client, c *Credentials) (
 		"refresh_token": {c.RefreshToken},
 		"client_id":     {ClientID},
 	})
+	var tokErr *tokenError
+	if errors.As(err, &tokErr) && tokErr.Code == "invalid_grant" {
+		return nil, false, ErrSessionExpired
+	}
 	if err != nil {
 		return nil, false, fmt.Errorf("%w: %w", ErrRefreshFailed, err)
 	}

internal/auth/oauth_test.go

@@ -660,7 +660,7 @@ func TestEnsureFresh(t *testing.T) {
 		}
 	})
 
-	t.Run("revoked refresh token is a recognisable refresh failure", func(t *testing.T) {
+	t.Run("rejected refresh token means logged out", func(t *testing.T) {
 		// Given
 		f := newFakeAuthServer(t)
 		f.tokenHandler = func(url.Values) (int, any) {
@@ -672,8 +672,28 @@ func TestEnsureFresh(t *testing.T) {
 		_, _, err := EnsureFresh(context.Background(), http.DefaultClient, c)
 
 		// Then
-		if !errors.Is(err, ErrRefreshFailed) {
-			t.Errorf("err = %v, want ErrRefreshFailed", err)
+		if !errors.Is(err, ErrSessionExpired) || !errors.Is(err, ErrNotLoggedIn) {
+			t.Errorf("err = %v, want ErrSessionExpired matching ErrNotLoggedIn", err)
+		}
+		if errors.Is(err, bug.ErrUnexpected) {
+			t.Errorf("err = %v, want an expired session not to be reported as a bug", err)
+		}
+	})
+
+	t.Run("failing token endpoint is a refresh failure", func(t *testing.T) {
+		// Given
+		f := newFakeAuthServer(t)
+		f.tokenHandler = func(url.Values) (int, any) {
+			return http.StatusInternalServerError, map[string]string{"error": "server_error"}
+		}
+		c := &Credentials{APIURL: f.srv.URL, RefreshToken: "refresh-1", ExpiresAt: time.Now().Add(-time.Minute)}
+
+		// When
+		_, _, err := EnsureFresh(context.Background(), http.DefaultClient, c)
+
+		// Then
+		if !errors.Is(err, ErrRefreshFailed) || errors.Is(err, ErrNotLoggedIn) {
+			t.Errorf("err = %v, want ErrRefreshFailed, not ErrNotLoggedIn", err)
 		}
 	})
 }

internal/cmd/sentinel_test.go

@@ -31,6 +31,7 @@ func TestEverySentinelHasAHintDecision(t *testing.T) {
 		"auth.ErrNoDiscovery":          auth.ErrNoDiscovery,
 		"auth.ErrKeychainUnavailable":  auth.ErrKeychainUnavailable,
 		"auth.ErrRefreshFailed":        auth.ErrRefreshFailed,
+		"auth.ErrSessionExpired":       auth.ErrSessionExpired,
 		"auth.ErrServerSideKey":        auth.ErrServerSideKey,
 		"auth.ErrLegacyAuthtoken":      auth.ErrLegacyAuthtoken,
 		"auth.ErrNotMasterKey":         auth.ErrNotMasterKey,