Track Configured Values (#2221)

* WIP: Use Parent Error Handler on Mount

* Add suggested boolean guard

* Move flag to App

* Move to copy of config as configured

* Apply the same trick to customMethod

app.go

@@ -110,10 +110,10 @@ type App struct {
 	latestRoute *Route
 	// TLS handler
 	tlsHandler *TLSHandler
-	// custom method check
-	customMethod bool
 	// Mount fields
 	mountFields *mountFields
+	// Indicates if the value was explicitly configured
+	configured Config
 }
 
 // Config is a struct holding the server settings.
@@ -513,6 +513,9 @@ func New(config ...Config) *App {
 		app.config = config[0]
 	}
 
+	// Initialize configured before defaults are set
+	app.configured = app.config
+
 	if app.config.ETag {
 		if !IsChild() {
 			fmt.Println("[Warning] Config.ETag is deprecated since v2.0.6, please use 'middleware/etag'.")
@@ -557,8 +560,6 @@ func New(config ...Config) *App {
 	}
 	if len(app.config.RequestMethods) == 0 {
 		app.config.RequestMethods = DefaultMethods
-	} else {
-		app.customMethod = true
 	}
 
 	app.config.trustedProxiesMap = make(map[string]struct{}, len(app.config.TrustedProxies))
@@ -999,7 +1000,10 @@ func (app *App) ErrorHandler(ctx *Ctx, err error) error {
 		if prefix != "" && strings.HasPrefix(ctx.path, prefix) {
 			parts := len(strings.Split(prefix, "/"))
 			if mountedPrefixParts <= parts {
-				mountedErrHandler = subApp.config.ErrorHandler
+				if subApp.configured.ErrorHandler != nil {
+					mountedErrHandler = subApp.config.ErrorHandler
+				}
+
 				mountedPrefixParts = parts
 			}
 		}

helpers.go

@@ -334,7 +334,7 @@ var getBytesImmutable = func(s string) (b []byte) {
 // HTTP methods and their unique INTs
 func (app *App) methodInt(s string) int {
 	// For better performance
-	if !app.customMethod {
+	if len(app.configured.RequestMethods) == 0 {
 		switch s {
 		case MethodGet:
 			return 0

mount_test.go

@@ -139,6 +139,24 @@ func Test_App_Group_Mount(t *testing.T) {
 	utils.AssertEqual(t, uint32(2), app.handlersCount)
 }
 
+func Test_App_UseParentErrorHandler(t *testing.T) {
+	app := New(Config{
+		ErrorHandler: func(ctx *Ctx, err error) error {
+			return ctx.Status(500).SendString("hi, i'm a custom error")
+		},
+	})
+
+	fiber := New()
+	fiber.Get("/", func(c *Ctx) error {
+		return errors.New("something happened")
+	})
+
+	app.Mount("/api", fiber)
+
+	resp, err := app.Test(httptest.NewRequest(MethodGet, "/api", nil))
+	testErrorResponse(t, err, resp, "hi, i'm a custom error")
+}
+
 func Test_App_UseMountedErrorHandler(t *testing.T) {
 	app := New()