Fix serialization errors in custom error handling

common.go

@@ -145,7 +145,14 @@ func handleCustomError(c *fiber.Ctx, customErr interface{}) error {
 	}
 
 	// Return the error as JSON
-	return c.Status(statusCode).JSON(customErr)
+	if err := c.Status(statusCode).JSON(customErr); err != nil {
+		if fallbackErr := c.Status(500).JSON(fiber.Map{"error": "Failed to serialize error response"}); fallbackErr != nil {
+			// Both serializations failed, return original error to Fiber
+			return err
+		}
+		return nil
+	}
+	return nil
 }
 
 // Utility to check if a value is zero

fiberoapi.go

@@ -861,7 +861,18 @@ func Method[TInput any, TOutput any, TError any](
 			return handleCustomError(c, customErr)
 		}
 
-		return c.JSON(output)
+		if err := c.JSON(output); err != nil {
+			if fallbackErr := c.Status(500).JSON(ErrorResponse{
+				Code:    500,
+				Details: "Failed to serialize response",
+				Type:    "serialization_error",
+			}); fallbackErr != nil {
+				// Both serializations failed, return original error to Fiber
+				return err
+			}
+			return nil
+		}
+		return nil
 	}
 
 	app.f.Add(m, fullPath, fiberHandler)

serialization_error_test.go

@@ -0,0 +1,107 @@
+package fiberoapi
+
+import (
+	"encoding/json"
+	"io"
+	"net/http/httptest"
+	"testing"
+
+	"github.com/gofiber/fiber/v2"
+)
+
+// UnserializableOutput contains a channel which cannot be serialized to JSON
+type UnserializableOutput struct {
+	Name    string      `json:"name"`
+	Channel chan string `json:"channel"` // Channels cannot be JSON serialized
+}
+
+type SerializationTestInput struct{}
+
+type SerializationTestError struct {
+	StatusCode int    `json:"statusCode"`
+	Message    string `json:"message"`
+}
+
+func TestResponseSerializationError(t *testing.T) {
+	app := fiber.New()
+	oapi := New(app)
+
+	// Handler that returns an unserializable output (contains a channel)
+	Get(oapi, "/unserializable", func(c *fiber.Ctx, input SerializationTestInput) (UnserializableOutput, *SerializationTestError) {
+		return UnserializableOutput{
+			Name:    "test",
+			Channel: make(chan string), // This will fail JSON marshaling
+		}, nil
+	}, OpenAPIOptions{
+		Summary: "Test endpoint with unserializable response",
+	})
+
+	req := httptest.NewRequest("GET", "/unserializable", nil)
+	resp, err := app.Test(req)
+	if err != nil {
+		t.Fatalf("Failed to make request: %v", err)
+	}
+
+	// Should return 500 status code
+	if resp.StatusCode != 500 {
+		t.Errorf("Expected status 500, got %d", resp.StatusCode)
+	}
+
+	// Should return a proper error response
+	body, _ := io.ReadAll(resp.Body)
+	var errorResp ErrorResponse
+	if err := json.Unmarshal(body, &errorResp); err != nil {
+		t.Fatalf("Failed to unmarshal error response: %v. Body: %s", err, string(body))
+	}
+
+	if errorResp.Code != 500 {
+		t.Errorf("Expected error code 500, got %d", errorResp.Code)
+	}
+
+	if errorResp.Type != "serialization_error" {
+		t.Errorf("Expected error type 'serialization_error', got '%s'", errorResp.Type)
+	}
+}
+
+// UnserializableError contains a channel which cannot be serialized
+type UnserializableError struct {
+	StatusCode int         `json:"statusCode"`
+	Channel    chan string `json:"channel"`
+}
+
+func TestErrorSerializationError(t *testing.T) {
+	app := fiber.New()
+	oapi := New(app)
+
+	// Handler that returns an unserializable error
+	Get(oapi, "/unserializable-error", func(c *fiber.Ctx, input SerializationTestInput) (map[string]string, *UnserializableError) {
+		return nil, &UnserializableError{
+			StatusCode: 400,
+			Channel:    make(chan string), // This will fail JSON marshaling
+		}
+	}, OpenAPIOptions{
+		Summary: "Test endpoint with unserializable error",
+	})
+
+	req := httptest.NewRequest("GET", "/unserializable-error", nil)
+	resp, err := app.Test(req)
+	if err != nil {
+		t.Fatalf("Failed to make request: %v", err)
+	}
+
+	// Should return 500 status code (fallback error)
+	if resp.StatusCode != 500 {
+		t.Errorf("Expected status 500, got %d", resp.StatusCode)
+	}
+
+	// Should return a proper error response
+	body, _ := io.ReadAll(resp.Body)
+	var errorResp map[string]string
+	if err := json.Unmarshal(body, &errorResp); err != nil {
+		t.Fatalf("Failed to unmarshal error response: %v. Body: %s", err, string(body))
+	}
+
+	if errorResp["error"] != "Failed to serialize error response" {
+		t.Errorf("Expected error message 'Failed to serialize error response', got '%s'", errorResp["error"])
+	}
+}