multi: pass BuildBreachRetribution callback to tower client

In this commit, a new BuildBreachRetribution callback is added to the
tower client's Config struct. The main LND server provides the client
with an implementation of the callback.

server.go

@@ -1523,12 +1523,37 @@ func newServer(cfg *Config, listenAddrs []net.Addr,
 			)
 		}
 
+		// buildBreachRetribution is a call-back that can be used to
+		// query the BreachRetribution info and channel type given a
+		// channel ID and commitment height.
+		buildBreachRetribution := func(chanID lnwire.ChannelID,
+			commitHeight uint64) (*lnwallet.BreachRetribution,
+			channeldb.ChannelType, error) {
+
+			channel, err := s.chanStateDB.FetchChannelByID(
+				nil, chanID,
+			)
+			if err != nil {
+				return nil, 0, err
+			}
+
+			br, err := lnwallet.NewBreachRetribution(
+				channel, commitHeight, 0, nil,
+			)
+			if err != nil {
+				return nil, 0, err
+			}
+
+			return br, channel.ChanType, nil
+		}
+
 		fetchClosedChannel := s.chanStateDB.FetchClosedChannelForID
 
 		s.towerClient, err = wtclient.New(&wtclient.Config{
-			FetchClosedChannel: fetchClosedChannel,
+			FetchClosedChannel:     fetchClosedChannel,
-			SessionCloseRange:  sessionCloseRange,
+			BuildBreachRetribution: buildBreachRetribution,
-			ChainNotifier:      s.cc.ChainNotifier,
+			SessionCloseRange:      sessionCloseRange,
+			ChainNotifier:          s.cc.ChainNotifier,
 			SubscribeChannelEvents: func() (subscribe.Subscription,
 				error) {
 
@@ -1558,9 +1583,10 @@ func newServer(cfg *Config, listenAddrs []net.Addr,
 			blob.Type(blob.FlagAnchorChannel)
 
 		s.anchorTowerClient, err = wtclient.New(&wtclient.Config{
-			FetchClosedChannel: fetchClosedChannel,
+			FetchClosedChannel:     fetchClosedChannel,
-			SessionCloseRange:  sessionCloseRange,
+			BuildBreachRetribution: buildBreachRetribution,
-			ChainNotifier:      s.cc.ChainNotifier,
+			SessionCloseRange:      sessionCloseRange,
+			ChainNotifier:          s.cc.ChainNotifier,
 			SubscribeChannelEvents: func() (subscribe.Subscription,
 				error) {
 

watchtower/wtclient/client.go

@@ -178,6 +178,11 @@ type Config struct {
 	// ChainNotifier can be used to subscribe to block notifications.
 	ChainNotifier chainntnfs.ChainNotifier
 
+	// BuildBreachRetribution is a function closure that allows the client
+	// fetch the breach retribution info for a certain channel at a certain
+	// revoked commitment height.
+	BuildBreachRetribution BreachRetributionBuilder
+
 	// NewAddress generates a new on-chain sweep pkscript.
 	NewAddress func() ([]byte, error)
 
@@ -240,6 +245,12 @@ type Config struct {
 	SessionCloseRange uint32
 }
 
+// BreachRetributionBuilder is a function that can be used to construct a
+// BreachRetribution from a channel ID and a commitment height.
+type BreachRetributionBuilder func(id lnwire.ChannelID,
+	commitHeight uint64) (*lnwallet.BreachRetribution,
+	channeldb.ChannelType, error)
+
 // newTowerMsg is an internal message we'll use within the TowerClient to signal
 // that a new tower can be considered.
 type newTowerMsg struct {

watchtower/wtclient/client_test.go

@@ -517,6 +517,15 @@ func newHarness(t *testing.T, cfg harnessCfg) *testHarness {
 		SessionCloseRange: 1,
 	}
 
+	h.clientCfg.BuildBreachRetribution = func(id lnwire.ChannelID,
+		commitHeight uint64) (*lnwallet.BreachRetribution,
+		channeldb.ChannelType, error) {
+
+		_, retribution := h.channelFromID(id).getState(commitHeight)
+
+		return retribution, channeldb.SingleFunderBit, nil
+	}
+
 	if !cfg.noServerStart {
 		h.startServer()
 		t.Cleanup(h.stopServer)
@@ -627,6 +636,21 @@ func (h *testHarness) channel(id uint64) *mockChannel {
 	return c
 }
 
+// channelFromID retrieves the channel corresponding to id.
+//
+// NOTE: The method fails if a channel for id does not exist.
+func (h *testHarness) channelFromID(chanID lnwire.ChannelID) *mockChannel {
+	h.t.Helper()
+
+	h.mu.Lock()
+	defer h.mu.Unlock()
+
+	c, ok := h.channels[chanID]
+	require.Truef(h.t, ok, "unable to fetch channel %s", chanID)
+
+	return c
+}
+
 // closeChannel marks a channel as closed.
 //
 // NOTE: The method fails if a channel for id does not exist.