🐛 Sub fiber's error handlers not working

[h4ckedneko]

Fiber version
v2.30.0

Issue description
In v2.24.0, custom error handler in fiber sub apps is captured by the main fiber app when you mounted them (#1560). But this doesn't work anymore with the recent version.

Code snippet

package main

import "github.com/gofiber/fiber/v2"

// Custom error handler.
func apiErrorHandler = func(c *fiber.Ctx, err error) error {
    // Custom error handling logic here.
    // ...

    // Return from handler
    return nil
}

func main() {
  app := fiber.New()

  // Steps to reproduce  
  api := fiber.New(fiber.Config{
    ErrorHandler: apiErrorHandler,
  })
  
  app.Mount("/api", api)

  log.Fatal(app.Listen(":3000"))
}

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[ReneWerner87]

package main

import (
	"fmt"
	"log"

	"github.com/gofiber/fiber/v2"
)

// Custom error handler.
func apiErrorHandler(c *fiber.Ctx, err error) error {
	// Custom error handling logic here.
	// ...
	fmt.Println("apiErrorHandler", err)
	// Return from handler
	return fiber.DefaultErrorHandler(c, err)
}

func main() {
	app := fiber.New()

	// Steps to reproduce
	api := fiber.New(fiber.Config{
		ErrorHandler: apiErrorHandler,
	})

	api.Get("/test", func(ctx *fiber.Ctx) error {
		return fiber.ErrBadRequest
	})

	app.Mount("/api", api)

	log.Fatal(app.Listen(":3000"))
}

curl -i http://localhost:3000/api/test

can not reproduce, need more information on this

for me it works as seen in the screenshot

can you( @h4ckedneko ) refine your example or describe the error in more detail ?

[h4ckedneko]

Hello, I created a git repo to reproduce this bug:
https://github.com/h4ckedneko/fiber-bug

[h4ckedneko]

Both v2.25.0 and v2.31.0 have the same fiber setup. But if you run them, the recent version doesn't print "custom error handler called" meaning the custom error handler is not being registered.

[h4ckedneko]

[ReneWerner87]

okay found the error, it only occurs when you root mounted apps from other fiber instances

we will fix it, thanks for the info

related to the refactoring from :
https://github.com/gofiber/fiber/pull/1749/files

[h4ckedneko]

Thanks Fiber team! 🥳