allow discovery registry to register

Allow the discovery registry to optionally register devices that it
discovers with the device registry. This reduces the work that a user
has to do to couple them, which they almost always will want to do.

discovery/registry.go

@@ -35,6 +35,10 @@ type Registry struct {
 	// If <= 0, a device will never expire once observed.
 	Expiration time.Duration
 
+	// DeviceRegistry, if not nil, is a device.Registry that will be updated when
+	// new devices are registered.
+	DeviceRegistry *device.Registry
+
 	// Protects the following data members.
 	mu sync.Mutex
 	// Map of active devices.
@@ -70,6 +74,16 @@ func (reg *Registry) Devices() []device.D {
 // it has not been observed before, or refresh its timeout and metadata if it
 // has.
 func (reg *Registry) Observe(dh *protocol.DiscoveryHeaders) (d device.D, isNew bool) {
+	d, isNew = reg.observeImpl(dh)
+
+	if reg.DeviceRegistry != nil {
+		reg.DeviceRegistry.Add(d)
+	}
+
+	return
+}
+
+func (reg *Registry) observeImpl(dh *protocol.DiscoveryHeaders) (d device.D, isNew bool) {
 	// Use hardware address as ID.
 	id := dh.HardwareAddr().String()
 	now := time.Now()