🐛 [Bug]: Wrong handlers execution order in some mount cases

[sadfun]

Bug Description

In some rare cases, fiber can wrongly build app.treeStack, breaking the execution chain. For example, it can call a handler from a mounted app before it calls a middleware specified at the top level earlier.

How to Reproduce

In attached code snippet, fiber thinks that middleware printing "i'm ignored!" should be executed after /methods/test handler.

I think there are many more cases when this happens, but for now I'm able to reproduce only this one.

Expected Behavior

Handlers execute in correct order.

Fiber Version

2.45.0

Code Snippet (optional)

package main

import (
	"fmt"
	"github.com/gofiber/fiber/v2"
	"net/http"
)

func getApi() *fiber.App {
	var API = fiber.New()

	methods := API.Group("/methods")

	methods.Get("/test", func(c *fiber.Ctx) error {
		if c.Locals("hello") != "world" {
			panic("hello is not world")
		}
		return c.SendStatus(http.StatusOK)
	})

	return API
}

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

	app.Use(func(c *fiber.Ctx) error {
		return c.Next()
	})

	app.Mount("/ws", fiber.New())

	app.Use(func(c *fiber.Ctx) error {
		return c.Next()
	})

	app.Get("/status", func(c *fiber.Ctx) error {
		return c.SendString("ok")
	})

	app.Use(func(c *fiber.Ctx) error {
		fmt.Println("i'm ignored!")
		c.Locals("hello", "world")
		return c.Next()
	})

	app.Mount("", getApi())

	test(app)
}

func test(app *fiber.App) {
	req, _ := http.NewRequest("GET", fmt.Sprintf("/methods/test"), nil)

	app.Test(req)
}

Checklist:

• I agree to follow Fiber's Code of Conduct.
• I have checked for existing issues that describe my problem prior to opening this one.
• I understand that improperly formatted bug reports may be closed without explanation.

[ReneWerner87]

@sadfun
big thanks for the good example

it really seems to be a bug

i assume it is a problem when i replace the mounted app

fiber/mount.go

Lines 214 to 218 in c56b4e6

	newStack := make([]*Route, len(app.stack[m])+len(subRoutes)-1)
	copy(newStack[:i], app.stack[m][:i])
	copy(newStack[i:i+len(subRoutes)], subRoutes)
	copy(newStack[i+len(subRoutes):], app.stack[m][i+1:])
	app.stack[m] = newStack

will fix it in the next days and release it on friday

[ReneWerner87]

the problem is the positions i created when replacing the mounted app

and after that they are sorted