Support recurring NAT Type measurement

currentNATType will from now on be guarded by currentNATTypeAccess for any access.

NAT Type update rule is flattened into state transfer lookup table to assist reading.

proxy/lib/snowflake.go

@@ -73,6 +73,10 @@ const readLimit = 100000 //Maximum number of bytes to be read from an HTTP reque
 
 var broker *SignalingServer
 
+var currentNATTypeAccess = &sync.RWMutex{}
+
+// currentNATType describes local network environment.
+// Obtain currentNATTypeAccess before access.
 var currentNATType = NATUnknown
 
 const (
@@ -183,7 +187,10 @@ func (s *SignalingServer) pollOffer(sid string, shutdown chan struct{}) *webrtc.
 			return nil
 		default:
 			numClients := int((tokens.count() / 8) * 8) // Round down to 8
-			body, err := messages.EncodePollRequest(sid, "standalone", currentNATType, numClients)
+			currentNATTypeAccess.RLock()
+			currentNATTypeLoaded := currentNATType
+			currentNATTypeAccess.RUnlock()
+			body, err := messages.EncodePollRequest(sid, "standalone", currentNATTypeLoaded, numClients)
 			if err != nil {
 				log.Printf("Error encoding poll message: %s", err.Error())
 				return nil
@@ -530,7 +537,12 @@ func (sf *SnowflakeProxy) Start() error {
 
 	// use probetest to determine NAT compatability
 	sf.checkNATType(config, sf.NATProbeURL)
-	log.Printf("NAT type: %s", currentNATType)
+
+	currentNATTypeAccess.RLock()
+	currentNATTypeLoaded := currentNATType
+	currentNATTypeAccess.RUnlock()
+
+	log.Printf("NAT type: %s", currentNATTypeLoaded)
 
 	ticker := time.NewTicker(pollInterval)
 	defer ticker.Stop()
@@ -604,12 +616,54 @@ func (sf *SnowflakeProxy) checkNATType(config webrtc.Configuration, probeURL str
 		return
 	}
 
+	currentNATTypeAccess.RLock()
+	currentNATTypeLoaded := currentNATType
+	currentNATTypeAccess.RUnlock()
+
+	currentNATTypeTestResult := NATUnknown
 	select {
 	case <-dataChan:
-		currentNATType = NATUnrestricted
+		currentNATTypeTestResult = NATUnrestricted
 	case <-time.After(dataChannelTimeout):
-		currentNATType = NATRestricted
+		currentNATTypeTestResult = NATRestricted
 	}
+
+	currentNATTypeToStore := NATUnknown
+	switch currentNATTypeLoaded + "->" + currentNATTypeTestResult {
+	case NATUnknown + "->" + NATUnknown:
+		currentNATTypeToStore = NATUnknown
+
+	case NATUnknown + "->" + NATUnrestricted:
+		currentNATTypeToStore = NATUnrestricted
+
+	case NATUnknown + "->" + NATRestricted:
+		currentNATTypeToStore = NATRestricted
+
+	case NATUnrestricted + "->" + NATUnknown:
+		currentNATTypeToStore = NATUnrestricted
+
+	case NATUnrestricted + "->" + NATUnrestricted:
+		currentNATTypeToStore = NATUnrestricted
+
+	case NATUnrestricted + "->" + NATRestricted:
+		currentNATTypeToStore = NATRestricted
+
+	case NATRestricted + "->" + NATUnknown:
+		currentNATTypeToStore = NATRestricted
+
+	case NATRestricted + "->" + NATUnrestricted:
+		currentNATTypeToStore = NATUnrestricted
+
+	case NATRestricted + "->" + NATRestricted:
+		currentNATTypeToStore = NATRestricted
+	}
+
+	log.Printf("NAT Type measurement: %v -> %v = %v\n", currentNATTypeLoaded, currentNATTypeTestResult, currentNATTypeToStore)
+
+	currentNATTypeAccess.Lock()
+	currentNATType = currentNATTypeToStore
+	currentNATTypeAccess.Unlock()
+
 	if err := pc.Close(); err != nil {
 		log.Printf("error calling pc.Close: %v", err)
 	}