Fix: error handler of app and its sub apps not saved when mounted in …

…group (#1649)

app_test.go

@@ -240,6 +240,30 @@ func Test_App_ErrorHandler_RouteStack(t *testing.T) {
 	utils.AssertEqual(t, "1: USE error", string(body))
 }
 
+func Test_App_ErrorHandler_GroupMount(t *testing.T) {
+	micro := New(Config{
+		ErrorHandler: func(c *Ctx, err error) error {
+			utils.AssertEqual(t, "0: GET error", err.Error())
+			return c.Status(500).SendString("1: custom error")
+		},
+	})
+	micro.Get("/doe", func(c *Ctx) error {
+		return errors.New("0: GET error")
+	})
+
+	app := New()
+	v1 := app.Group("/v1")
+	v1.Mount("/john", micro)
+
+	resp, err := app.Test(httptest.NewRequest(MethodGet, "/v1/john/doe", nil))
+	utils.AssertEqual(t, nil, err, "app.Test(req)")
+	utils.AssertEqual(t, 500, resp.StatusCode, "Status code")
+
+	body, err := ioutil.ReadAll(resp.Body)
+	utils.AssertEqual(t, nil, err)
+	utils.AssertEqual(t, "1: custom error", string(body))
+}
+
 func Test_App_Nested_Params(t *testing.T) {
 	app := New()
 

group.go

@@ -7,6 +7,7 @@ package fiber
 import (
 	"fmt"
 	"reflect"
+	"strings"
 	"sync/atomic"
 )
 
@@ -22,13 +23,24 @@ type Group struct {
 // compose them as a single service using Mount.
 func (grp *Group) Mount(prefix string, fiber *App) Router {
 	stack := fiber.Stack()
+	groupPath := getGroupPath(grp.prefix, prefix)
+
 	for m := range stack {
 		for r := range stack[m] {
 			route := grp.app.copyRoute(stack[m][r])
-			grp.app.addRoute(route.Method, grp.app.addPrefixToRoute(getGroupPath(grp.prefix, prefix), route))
+			grp.app.addRoute(route.Method, grp.app.addPrefixToRoute(groupPath, route))
 		}
 	}
 
+	// Save the fiber's error handler and its sub apps
+	groupPath = strings.TrimRight(groupPath, "/")
+	if fiber.config.ErrorHandler != nil {
+		grp.app.errorHandlers[groupPath] = fiber.config.ErrorHandler
+	}
+	for mountedPrefixes, errHandler := range fiber.errorHandlers {
+		grp.app.errorHandlers[groupPath+mountedPrefixes] = errHandler
+	}
+
 	atomic.AddUint32(&grp.app.handlerCount, fiber.handlerCount)
 
 	return grp