gotd: add fallback handlers

Cherry-picked from https://github.com/gotd/td/commit/3238f7e7d3623ecd3648e2124f962c4ea0d03134

pkg/gotd/gen/_template/handlers.tmpl

@@ -2,15 +2,16 @@
 {{ $pkg := $.Package }}
 {{ template "header" $ }}
 
-type handler = func(context.Context, Entities, UpdateClass) error
+type Handler = func(context.Context, Entities, UpdateClass) error
 
 type UpdateDispatcher struct {
-    handlers map[uint32]handler
+    handlers map[uint32]Handler
+	fallback Handler
 }
 
 func NewUpdateDispatcher() UpdateDispatcher {
     return UpdateDispatcher{
-        handlers: map[uint32]handler{},
+        handlers: map[uint32]Handler{},
     }
 }
 
@@ -71,10 +72,13 @@ func (u UpdateDispatcher) dispatch(ctx context.Context, e Entities, update Updat
     }
     typeID := update.TypeID()
     handler, ok := u.handlers[typeID]
-    if !ok {
-        return nil
-    }
-    return handler(ctx, e, update)
+	if ok {
+		return handler(ctx, e, update)
+	}
+	if u.fallback != nil {
+		return u.fallback(ctx, e, update)
+	}
+	return nil
 }
 
 {{- range $s := $.Structs }}{{ if eq $s.Interface "UpdateClass" }}
@@ -90,4 +94,9 @@ func (u UpdateDispatcher) On{{ $eventName }}(handler {{ $eventName }}Handler) {
 }
 {{- end }}{{ end }}
 
+// OnFallback sets fallback handler.
+func (u *UpdateDispatcher) OnFallback(handler Handler) {
+    u.fallback = handler
+}
+
 {{ end }}

pkg/gotd/tg/tl_handlers_gen.go

@@ -31,15 +31,16 @@ var (
 	_ = tdjson.Encoder{}
 )
 
-type handler = func(context.Context, Entities, UpdateClass) error
+type Handler = func(context.Context, Entities, UpdateClass) error
 
 type UpdateDispatcher struct {
-	handlers map[uint32]handler
+	handlers map[uint32]Handler
+	fallback Handler
 }
 
 func NewUpdateDispatcher() UpdateDispatcher {
 	return UpdateDispatcher{
-		handlers: map[uint32]handler{},
+		handlers: map[uint32]Handler{},
 	}
 }
 
@@ -100,10 +101,13 @@ func (u UpdateDispatcher) dispatch(ctx context.Context, e Entities, update Updat
 	}
 	typeID := update.TypeID()
 	handler, ok := u.handlers[typeID]
-	if !ok {
-		return nil
+	if ok {
+		return handler(ctx, e, update)
 	}
-	return handler(ctx, e, update)
+	if u.fallback != nil {
+		return u.fallback(ctx, e, update)
+	}
+	return nil
 }
 
 // NewMessageHandler is a NewMessage event handler.
@@ -1605,3 +1609,8 @@ func (u UpdateDispatcher) OnStarGiftAuctionUserState(handler StarGiftAuctionUser
 		return handler(ctx, e, update.(*UpdateStarGiftAuctionUserState))
 	}
 }
+
+// OnFallback sets fallback handler.
+func (u *UpdateDispatcher) OnFallback(handler Handler) {
+	u.fallback = handler
+}