fix discovery Registry w/ no expiration

discovery/registry.go

@@ -107,10 +107,13 @@ func (reg *Registry) Observe(dh *protocol.DiscoveryHeaders) (d device.D, isNew b
 	}
 
 	// Observe the entry and update its timeout and headers.
-	e.updateState(now, &discoveredDeviceState{
-		dh:      dh,
-		expires: now.Add(reg.Expiration),
-	})
+	st := discoveredDeviceState{
+		dh: dh,
+	}
+	if reg.Expiration > 0 {
+		st.expires = now.Add(reg.Expiration)
+	}
+	e.updateState(now, &st)
 
 	d = e.device
 	return
@@ -179,7 +182,9 @@ type registryEntry struct {
 
 func (e *registryEntry) updateState(now time.Time, st *discoveredDeviceState) {
 	e.device.UpdateHeaders(now, st.dh)
-	e.updateExpirationC <- st.expires
+	if !st.expires.IsZero() {
+		e.updateExpirationC <- st.expires
+	}
 }
 
 func (e *registryEntry) manageEntryLifecycle() {