Core: Fix startup error when `dns` exists but `fakedns` doesn't

Fixes https://github.com/XTLS/Xray-core/issues/4155

app/dns/nameserver.go

@@ -35,7 +35,7 @@ type Client struct {
 var errExpectedIPNonMatch = errors.New("expectIPs not match")
 
 // NewServer creates a name server object according to the network destination url.
-func NewServer(dest net.Destination, dispatcher routing.Dispatcher, queryStrategy QueryStrategy, fd dns.FakeDNSEngine) (Server, error) {
+func NewServer(ctx context.Context, dest net.Destination, dispatcher routing.Dispatcher, queryStrategy QueryStrategy) (Server, error) {
 	if address := dest.Address; address.Family().IsDomain() {
 		u, err := url.Parse(address.Domain())
 		if err != nil {
@@ -55,6 +55,10 @@ func NewServer(dest net.Destination, dispatcher routing.Dispatcher, queryStrateg
 		case strings.EqualFold(u.Scheme, "tcp+local"): // DNS-over-TCP Local mode
 			return NewTCPLocalNameServer(u, queryStrategy)
 		case strings.EqualFold(u.String(), "fakedns"):
+			var fd dns.FakeDNSEngine
+			core.RequireFeatures(ctx, func(fdns dns.FakeDNSEngine) { // FakeDNSEngine is optional
+				fd = fdns
+			})
 			return NewFakeDNSServer(fd), nil
 		}
 	}
@@ -78,13 +82,9 @@ func NewClient(
 ) (*Client, error) {
 	client := &Client{}
 
-	var fd dns.FakeDNSEngine
 	err := core.RequireFeatures(ctx, func(dispatcher routing.Dispatcher) error {
-		core.RequireFeatures(ctx, func(fdns dns.FakeDNSEngine) { // FakeDNSEngine is optional
-			fd = fdns
-		})
 		// Create a new server for each client for now
-		server, err := NewServer(ns.Address.AsDestination(), dispatcher, ns.GetQueryStrategy(), fd)
+		server, err := NewServer(ctx, ns.Address.AsDestination(), dispatcher, ns.GetQueryStrategy())
 		if err != nil {
 			return errors.New("failed to create nameserver").Base(err).AtWarning()
 		}