🩹 Fix: static server in sub app with mount (#3104)

mount.go

@@ -23,6 +23,8 @@ type mountFields struct {
 	subAppsProcessed sync.Once
 	// Prefix of app if it was mounted
 	mountPath string
+	// Parent app of the current app
+	parentApp *App
 }
 
 // Create empty mountFields instance
@@ -50,6 +52,7 @@ func (app *App) Mount(prefix string, subApp *App) Router {
 
 		subApp.mountFields.mountPath = path
 		app.mountFields.appList[path] = subApp
+		subApp.mountFields.parentApp = app
 	}
 
 	// register mounted group
@@ -99,6 +102,14 @@ func (app *App) MountPath() string {
 	return app.mountFields.mountPath
 }
 
+// FullMountPath returns the full mount path of the app, including the parent app's mount path.
+func (app *App) FullMountPath() string {
+	if app.mountFields.parentApp == nil {
+		return app.mountFields.mountPath
+	}
+	return getGroupPath(app.mountFields.parentApp.FullMountPath(), app.mountFields.mountPath)
+}
+
 // hasMountedApps Checks if there are any mounted apps in the current application.
 func (app *App) hasMountedApps() bool {
 	return len(app.mountFields.appList) > 1

router.go

@@ -5,6 +5,7 @@
 package fiber
 
 import (
+	"bytes"
 	"fmt"
 	"html"
 	"sort"
@@ -357,6 +358,10 @@ func (app *App) registerStatic(prefix, root string, config ...Static) {
 		IndexNames:           []string{"index.html"},
 		PathRewrite: func(fctx *fasthttp.RequestCtx) []byte {
 			path := fctx.Path()
+			mountPath := app.FullMountPath()
+			if n := len(mountPath); n > 0 && bytes.Equal(path[:n], utils.UnsafeBytes(mountPath)) {
+				path = path[n:]
+			}
 			if len(path) >= prefixLen {
 				if isStar && app.getString(path[0:prefixLen]) == prefix {
 					path = append(path[0:0], '/')

router_test.go

@@ -2,7 +2,7 @@
 // 📃 Github Repository: https://github.com/gofiber/fiber
 // 📌 API Documentation: https://docs.gofiber.io
 
-//nolint:bodyclose // Much easier to just ignore memory leaks in tests
+//nolint:bodyclose,goconst // Much easier to just ignore memory leaks in tests and constant variables in tests
 package fiber
 
 import (
@@ -471,6 +471,53 @@ func Test_Route_Static_HasPrefix(t *testing.T) {
 	body, err = io.ReadAll(resp.Body)
 	utils.AssertEqual(t, nil, err, "app.Test(req)")
 	utils.AssertEqual(t, true, strings.Contains(app.getString(body), "color"))
+
+	app = New()
+	app.Static("/css", dir)
+
+	resp, err = app.Test(httptest.NewRequest(MethodGet, "/css/style.css", nil))
+	utils.AssertEqual(t, nil, err, "app.Test(req)")
+	utils.AssertEqual(t, 200, resp.StatusCode, "Status code")
+
+	body, err = io.ReadAll(resp.Body)
+	utils.AssertEqual(t, nil, err, "app.Test(req)")
+	utils.AssertEqual(t, true, strings.Contains(app.getString(body), "color"))
+}
+
+func Test_Route_Static_SubApp(t *testing.T) {
+	t.Parallel()
+
+	dir := "./.github/testdata/fs/css"
+	app := New()
+
+	// subapp
+	subApp := New()
+	subApp.Static("/css", dir)
+	app.Mount("/sub", subApp)
+
+	// nested subapp
+	nestApp := New()
+	nestApp.Static("/css", dir)
+	subApp.Mount("/nest", nestApp)
+
+	// test subapp
+	resp, err := app.Test(httptest.NewRequest(MethodGet, "/sub/css/style.css", nil))
+	utils.AssertEqual(t, nil, err, "app.Test(req)")
+	utils.AssertEqual(t, 200, resp.StatusCode, "Status code")
+
+	body, err := io.ReadAll(resp.Body)
+	utils.AssertEqual(t, nil, err, "app.Test(req)")
+	utils.AssertEqual(t, true, strings.Contains(app.getString(body), "color"))
+
+	// test nested subapp
+
+	resp, err = app.Test(httptest.NewRequest(MethodGet, "/sub/nest/css/style.css", nil))
+	utils.AssertEqual(t, nil, err, "app.Test(req)")
+	utils.AssertEqual(t, 200, resp.StatusCode, "Status code")
+
+	body, err = io.ReadAll(resp.Body)
+	utils.AssertEqual(t, nil, err, "app.Test(req)")
+	utils.AssertEqual(t, true, strings.Contains(app.getString(body), "color"))
 }
 
 func Test_Router_NotFound(t *testing.T) {