AppV2: ensure isolation of handler state per request

Implemented a new duplication mechanism for the handler to create
separate instances per request. This change addresses an issue where the
same handler instance was shared across multiple requests, leading to
state leakage and inconsistencies in AppV2.

Each request now uses a fresh handler instance, ensuring isolated and
consistent state management.

Updates goplus/builder#532

Signed-off-by: Aofei Sheng <aofei@aofeisheng.com>

classfile_v2.go

@@ -57,12 +57,22 @@ type iHandler interface {
 func Gopt_AppV2_Main(app AppType, handlers ...iHandler) {
 	app.InitYap()
 	for _, h := range handlers {
-		reflect.ValueOf(h).Elem().Field(1).Set(reflect.ValueOf(app)) // (*handler).AppV2 = app
+		hVal := reflect.ValueOf(h).Elem()
+		hVal.FieldByName("AppV2").Set(reflect.ValueOf(app))
+		hType := hVal.Type()
+		handle := func(ctx *Context) {
+			// We must duplicate the handler instance for each request
+			// to ensure state isolation.
+			h2Val := reflect.New(hType).Elem()
+			h2Val.Set(hVal)
+			h2 := h2Val.Addr().Interface().(iHandler)
+			h2.Main(ctx)
+		}
 		switch method, path := parseClassfname(h.Classfname()); method {
 		case "handle":
-			app.Handle(path, h.Main)
+			app.Handle(path, handle)
 		default:
-			app.Route(strings.ToUpper(method), path, h.Main)
+			app.Route(strings.ToUpper(method), path, handle)
 		}
 	}
 	if me, ok := app.(interface{ MainEntry() }); ok {