Add OptionalFeatures injection

For example OptionalFeatures() is useful for fakedns module

app/dispatcher/default.go

@@ -106,7 +106,7 @@ func init() {
 	common.Must(common.RegisterConfig((*Config)(nil), func(ctx context.Context, config interface{}) (interface{}, error) {
 		d := new(DefaultDispatcher)
 		if err := core.RequireFeatures(ctx, func(om outbound.Manager, router routing.Router, pm policy.Manager, sm stats.Manager, dc dns.Client) error {
-			core.RequireFeatures(ctx, func(fdns dns.FakeDNSEngine) { // FakeDNSEngine is optional
+			core.OptionalFeatures(ctx, func(fdns dns.FakeDNSEngine) {
 				d.fdns = fdns
 			})
 			return d.Init(config.(*Config), om, router, pm, sm, dc)

app/dns/nameserver.go

@@ -56,7 +56,7 @@ func NewServer(ctx context.Context, dest net.Destination, dispatcher routing.Dis
 			return NewTCPLocalNameServer(u, queryStrategy)
 		case strings.EqualFold(u.String(), "fakedns"):
 			var fd dns.FakeDNSEngine
-			core.RequireFeatures(ctx, func(fdns dns.FakeDNSEngine) { // FakeDNSEngine is optional
+			core.RequireFeatures(ctx, func(fdns dns.FakeDNSEngine) {
 				fd = fdns
 			})
 			return NewFakeDNSServer(fd), nil

app/observatory/command/command.go

@@ -38,7 +38,7 @@ func init() {
 		sv := &service{v: s}
 		err := s.RequireFeatures(func(Observatory extension.Observatory) {
 			sv.observatory = Observatory
-		})
+		}, false)
 		if err != nil {
 			return nil, err
 		}

app/proxyman/command/command.go

@@ -177,7 +177,7 @@ func (s *service) Register(server *grpc.Server) {
 	common.Must(s.v.RequireFeatures(func(im inbound.Manager, om outbound.Manager) {
 		hs.ihm = im
 		hs.ohm = om
-	}))
+	}, false))
 	RegisterHandlerServiceServer(server, hs)
 
 	// For compatibility purposes

app/router/command/command.go

@@ -135,7 +135,7 @@ func (s *service) Register(server *grpc.Server) {
 		vCoreDesc := RoutingService_ServiceDesc
 		vCoreDesc.ServiceName = "v2ray.core.app.router.command.RoutingService"
 		server.RegisterService(&vCoreDesc, rs)
-	}))
+	}, false))
 }
 
 func init() {

core/xray.go

@@ -80,11 +80,12 @@ func (r *resolution) callbackResolution(allFeatures []features.Feature) error {
 
 // Instance combines all Xray features.
 type Instance struct {
-	statusLock         sync.Mutex
-	features           []features.Feature
-	pendingResolutions []resolution
-	running            bool
-	resolveLock        sync.Mutex
+	statusLock                 sync.Mutex
+	features                   []features.Feature
+	pendingResolutions         []resolution
+	pendingOptionalResolutions []resolution
+	running                    bool
+	resolveLock                sync.Mutex
 
 	ctx context.Context
 }
@@ -145,7 +146,14 @@ func addOutboundHandlers(server *Instance, configs []*OutboundHandlerConfig) err
 // See Instance.RequireFeatures for more information.
 func RequireFeatures(ctx context.Context, callback interface{}) error {
 	v := MustFromContext(ctx)
-	return v.RequireFeatures(callback)
+	return v.RequireFeatures(callback, false)
+}
+
+// OptionalFeatures is a helper function to aquire features from Instance in context.
+// See Instance.RequireFeatures for more information.
+func OptionalFeatures(ctx context.Context, callback interface{}) error {
+	v := MustFromContext(ctx)
+	return v.RequireFeatures(callback, true)
 }
 
 // New returns a new Xray instance based on given configuration.
@@ -263,7 +271,7 @@ func (s *Instance) Close() error {
 
 // RequireFeatures registers a callback, which will be called when all dependent features are registered.
 // The callback must be a func(). All its parameters must be features.Feature.
-func (s *Instance) RequireFeatures(callback interface{}) error {
+func (s *Instance) RequireFeatures(callback interface{}, optional bool) error {
 	callbackType := reflect.TypeOf(callback)
 	if callbackType.Kind() != reflect.Func {
 		panic("not a function")
@@ -292,7 +300,11 @@ func (s *Instance) RequireFeatures(callback interface{}) error {
 		s.resolveLock.Unlock()
 		return r.callbackResolution(s.features)
 	} else {
-		s.pendingResolutions = append(s.pendingResolutions, r)
+		if optional {
+			s.pendingOptionalResolutions = append(s.pendingOptionalResolutions, r)
+		} else {
+			s.pendingResolutions = append(s.pendingResolutions, r)
+		}
 		s.resolveLock.Unlock()
 		return nil
 	}
@@ -309,12 +321,9 @@ func (s *Instance) AddFeature(feature features.Feature) error {
 
 	s.resolveLock.Lock()
 	s.features = append(s.features, feature)
-	if s.pendingResolutions == nil {
-		s.resolveLock.Unlock()
-		return nil
-	}
-	var pending []resolution
+
 	var availableResolution []resolution
+	var pending []resolution
 	for _, r := range s.pendingResolutions {
 		foundAll := true
 		for _, d := range r.deps {
@@ -331,6 +340,24 @@ func (s *Instance) AddFeature(feature features.Feature) error {
 		}
 	}
 	s.pendingResolutions = pending
+
+	var pendingOptional []resolution
+	for _, r := range s.pendingOptionalResolutions {
+		foundAll := true
+		for _, d := range r.deps {
+			f := getFeature(s.features, d)
+			if f == nil {
+				foundAll = false
+				break
+			}
+		}
+		if foundAll {
+			availableResolution = append(availableResolution, r)
+		} else {
+			pendingOptional = append(pendingOptional, r)
+		}
+	}
+	s.pendingOptionalResolutions = pendingOptional
 	s.resolveLock.Unlock()
 	
 	var err error

core/xray_test.go

@@ -30,7 +30,7 @@ func TestXrayDependency(t *testing.T) {
 			t.Error("expected dns client fulfilled, but actually nil")
 		}
 		wait <- true
-	})
+	}, false)
 	instance.AddFeature(localdns.New())
 	<-wait
 }

proxy/dns/dns.go

@@ -27,7 +27,7 @@ func init() {
 	common.Must(common.RegisterConfig((*Config)(nil), func(ctx context.Context, config interface{}) (interface{}, error) {
 		h := new(Handler)
 		if err := core.RequireFeatures(ctx, func(dnsClient dns.Client, policyManager policy.Manager) error {
-			core.RequireFeatures(ctx, func(fdns dns.FakeDNSEngine) { // FakeDNSEngine is optional
+			core.OptionalFeatures(ctx, func(fdns dns.FakeDNSEngine) {
 				h.fdns = fdns
 			})
 			return h.Init(config.(*Config), dnsClient, policyManager)