Add mutex to injection reolution

- Turns out we already support async DI resolution regardless of feature ordering
Previous code contain a race condition causing some resolution is lost
- Note that the new mutex cover s.pendingResolutions and s.features
but must not cover callbackResolution() due to deadlock
- Refactor some method names and simplify code

core/xray.go

@@ -44,22 +44,13 @@ func getFeature(allFeatures []features.Feature, t reflect.Type) features.Feature
 	return nil
 }
 
-func (r *resolution) resolve(allFeatures []features.Feature) (bool, error) {
-	var fs []features.Feature
-	for _, d := range r.deps {
-		f := getFeature(allFeatures, d)
-		if f == nil {
-			return false, nil
-		}
-		fs = append(fs, f)
-	}
-
+func (r *resolution) callbackResolution(allFeatures []features.Feature) error {
 	callback := reflect.ValueOf(r.callback)
 	var input []reflect.Value
 	callbackType := callback.Type()
 	for i := 0; i < callbackType.NumIn(); i++ {
 		pt := callbackType.In(i)
-		for _, f := range fs {
+		for _, f := range allFeatures {
 			if reflect.TypeOf(f).AssignableTo(pt) {
 				input = append(input, reflect.ValueOf(f))
 				break
@@ -84,15 +75,16 @@ func (r *resolution) resolve(allFeatures []features.Feature) (bool, error) {
 		}
 	}
 
-	return true, err
+	return err
 }
 
 // Instance combines all Xray features.
 type Instance struct {
-	access             sync.Mutex
+	statusLock         sync.Mutex
 	features           []features.Feature
-	featureResolutions []resolution
+	pendingResolutions []resolution
 	running            bool
+	resolveLock        sync.Mutex
 
 	ctx context.Context
 }
@@ -227,9 +219,12 @@ func initInstanceWithConfig(config *Config, server *Instance) (bool, error) {
 		}(),
 	)
 
-	if server.featureResolutions != nil {
+	server.resolveLock.Lock()
+	if server.pendingResolutions != nil {
+		server.resolveLock.Unlock()
 		return true, errors.New("not all dependencies are resolved.")
 	}
+	server.resolveLock.Unlock()
 
 	if err := addInboundHandlers(server, config.Inbound); err != nil {
 		return true, err
@@ -248,8 +243,8 @@ func (s *Instance) Type() interface{} {
 
 // Close shutdown the Xray instance.
 func (s *Instance) Close() error {
-	s.access.Lock()
-	defer s.access.Unlock()
+	s.statusLock.Lock()
+	defer s.statusLock.Unlock()
 
 	s.running = false
 
@@ -283,17 +278,28 @@ func (s *Instance) RequireFeatures(callback interface{}) error {
 		deps:     featureTypes,
 		callback: callback,
 	}
-	if finished, err := r.resolve(s.features); finished {
-		return err
+
+	s.resolveLock.Lock()
+	foundAll := true
+	for _, d := range r.deps {
+		f := getFeature(s.features, d)
+		if f == nil {
+			foundAll = false
+			break
+		}
+	}
+	if foundAll {
+		s.resolveLock.Unlock()
+		return r.callbackResolution(s.features)
+	} else {
+		s.pendingResolutions = append(s.pendingResolutions, r)
+		s.resolveLock.Unlock()
+		return nil
 	}
-	s.featureResolutions = append(s.featureResolutions, r)
-	return nil
 }
 
 // AddFeature registers a feature into current Instance.
 func (s *Instance) AddFeature(feature features.Feature) error {
-	s.features = append(s.features, feature)
-
 	if s.running {
 		if err := feature.Start(); err != nil {
 			errors.LogInfoInner(s.ctx, err, "failed to start feature")
@@ -301,27 +307,37 @@ func (s *Instance) AddFeature(feature features.Feature) error {
 		return nil
 	}
 
-	if s.featureResolutions == nil {
+	s.resolveLock.Lock()
+	s.features = append(s.features, feature)
+	if s.pendingResolutions == nil {
+		s.resolveLock.Unlock()
 		return nil
 	}
-
-	var pendingResolutions []resolution
-	for _, r := range s.featureResolutions {
-		finished, err := r.resolve(s.features)
-		if finished && err != nil {
-			return err
+	var pending []resolution
+	var availableResolution []resolution
+	for _, r := range s.pendingResolutions {
+		foundAll := true
+		for _, d := range r.deps {
+			f := getFeature(s.features, d)
+			if f == nil {
+				foundAll = false
+				break
+			}
 		}
-		if !finished {
-			pendingResolutions = append(pendingResolutions, r)
+		if foundAll {
+			availableResolution = append(availableResolution, r)
+		} else {
+			pending = append(pending, r)
 		}
 	}
-	if len(pendingResolutions) == 0 {
-		s.featureResolutions = nil
-	} else if len(pendingResolutions) < len(s.featureResolutions) {
-		s.featureResolutions = pendingResolutions
+	s.pendingResolutions = pending
+	s.resolveLock.Unlock()
+	
+	var err error
+	for _, r := range availableResolution {
+		err = r.callbackResolution(s.features) // only return the last error for now
 	}
-
-	return nil
+	return err
 }
 
 // GetFeature returns a feature of the given type, or nil if such feature is not registered.
@@ -334,8 +350,8 @@ func (s *Instance) GetFeature(featureType interface{}) features.Feature {
 //
 // xray:api:stable
 func (s *Instance) Start() error {
-	s.access.Lock()
-	defer s.access.Unlock()
+	s.statusLock.Lock()
+	defer s.statusLock.Unlock()
 
 	s.running = true
 	for _, f := range s.features {