multi: add htlcNotifier interface to switch and link

In this commit, a htlcNotifier interface is added to allow for easy
unit testing. Instances of the HtlcNotifier are added to the server,
switch and link.

htlcswitch/interfaces.go

@@ -180,3 +180,29 @@ type TowerClient interface {
 	// isTweakless should be true.
 	BackupState(*lnwire.ChannelID, *lnwallet.BreachRetribution, bool) error
 }
+
+// htlcNotifier is an interface which represents the input side of the
+// HtlcNotifier which htlc events are piped through. This interface is intended
+// to allow for mocking of the htlcNotifier in tests, so is unexported because
+// it is not needed outside of the htlcSwitch package.
+type htlcNotifier interface {
+	// NotifyForwardingEvent notifies the HtlcNotifier than a htlc has been
+	// forwarded.
+	NotifyForwardingEvent(key HtlcKey, info HtlcInfo,
+		eventType HtlcEventType)
+
+	// NotifyIncomingLinkFailEvent notifies that a htlc has failed on our
+	// incoming link. It takes an isReceive bool to differentiate between
+	// our node's receives and forwards.
+	NotifyLinkFailEvent(key HtlcKey, info HtlcInfo,
+		eventType HtlcEventType, linkErr *LinkError, incoming bool)
+
+	// NotifyForwardingFailEvent notifies the HtlcNotifier that a htlc we
+	// forwarded has failed down the line.
+	NotifyForwardingFailEvent(key HtlcKey, eventType HtlcEventType)
+
+	// NotifySettleEvent notifies the HtlcNotifier that a htlc that we
+	// committed to as part of a forward or a receive to our node has been
+	// settled.
+	NotifySettleEvent(key HtlcKey, eventType HtlcEventType)
+}