add a callback for autonat status changes

autonat.go

@@ -61,6 +61,9 @@ type AmbientAutoNAT struct {
 	// If only a single autoNAT peer is known, then the confidence increases
 	// for each failure until it reaches 3.
 	confidence int
+
+	callbacks []func(NATStatus, int)
+	cbLk      sync.Mutex
 }
 
 // NewAutoNAT creates a new ambient NAT autodiscovery instance attached to a host
@@ -214,9 +217,24 @@ func (as *AmbientAutoNAT) autodetect() {
 		as.status = NATStatusUnknown
 		as.addr = nil
 	}
+	as.runCallbacks()
 	as.mx.Unlock()
 }
 
+func (as *AmbientAutoNAT) runCallbacks() {
+	as.cbLk.Lock()
+	defer as.cbLk.Unlock()
+	for _, cb := range as.callbacks {
+		cb(as.status, as.confidence)
+	}
+}
+
+func (as *AmbientAutoNAT) AddCallback(cb func(NATStatus, int)) {
+	as.cbLk.Lock()
+	defer as.cbLk.Unlock()
+	as.callbacks = append(as.callbacks, cb)
+}
+
 func (as *AmbientAutoNAT) getPeers() []peer.AddrInfo {
 	as.mx.Lock()
 	defer as.mx.Unlock()