watchtower: make use of the new tower-to-session index

In this commit, the towerID-to-sessionID index added in the previous
commit is put to use in order to make session lookup more efficient in
certain places. In the process, 2 TODO's are also removed from the code.

watchtower/wtdb/client_db.go

@@ -293,27 +293,32 @@ func (c *ClientDB) CreateTower(lnAddr *lnwire.NetAddress) (*Tower, error) {
 			// If there are any client sessions that correspond to
 			// this tower, we'll mark them as active to ensure we
 			// load them upon restarts.
-			//
-			// TODO(wilmer): with an index of tower -> sessions we
-			// can avoid the linear lookup.
+			towerSessIndex := towerToSessionIndex.NestedReadBucket(
+				tower.ID.Bytes(),
+			)
+			if towerSessIndex == nil {
+				return ErrTowerNotFound
+			}
+
 			sessions := tx.ReadWriteBucket(cSessionBkt)
 			if sessions == nil {
 				return ErrUninitializedDB
 			}
-			towerID := TowerIDFromBytes(towerIDBytes)
-			towerSessions, err := listClientSessions(
-				sessions, towers, &towerID,
-			)
-			if err != nil {
-				return err
-			}
-			for _, session := range towerSessions {
-				err := markSessionStatus(
-					sessions, session, CSessionActive,
+
+			err = towerSessIndex.ForEach(func(k, _ []byte) error {
+				session, err := getClientSessionBody(
+					sessions, k,
 				)
 				if err != nil {
 					return err
 				}
+
+				return markSessionStatus(
+					sessions, session, CSessionActive,
+				)
+			})
+			if err != nil {
+				return err
 			}
 		} else {
 			// No such tower exists, create a new tower id for our
@@ -410,16 +415,13 @@ func (c *ClientDB) RemoveTower(pubKey *btcec.PublicKey, addr net.Addr) error {
 
 		// Otherwise, we should attempt to mark the tower's sessions as
 		// inactive.
-		//
-		// TODO(wilmer): with an index of tower -> sessions we can avoid
-		// the linear lookup.
 		sessions := tx.ReadWriteBucket(cSessionBkt)
 		if sessions == nil {
 			return ErrUninitializedDB
 		}
 		towerID := TowerIDFromBytes(towerIDBytes)
-		towerSessions, err := listClientSessions(
-			sessions, towers, &towerID,
+		towerSessions, err := listTowerSessions(
+			towerID, sessions, towers, towersToSessionsIndex,
 		)
 		if err != nil {
 			return err
@@ -750,7 +752,25 @@ func (c *ClientDB) ListClientSessions(id *TowerID) (
 		}
 
 		var err error
-		clientSessions, err = listClientSessions(sessions, towers, id)
+
+		// If no tower ID is specified, then fetch all the sessions
+		// known to the db.
+		if id == nil {
+			clientSessions, err = listClientAllSessions(
+				sessions, towers,
+			)
+			return err
+		}
+
+		// Otherwise, fetch the sessions for the given tower.
+		towerToSessionIndex := tx.ReadBucket(cTowerToSessionIndexBkt)
+		if towerToSessionIndex == nil {
+			return ErrUninitializedDB
+		}
+
+		clientSessions, err = listTowerSessions(
+			*id, sessions, towers, towerToSessionIndex,
+		)
 		return err
 	}, func() {
 		clientSessions = nil
@@ -762,11 +782,9 @@ func (c *ClientDB) ListClientSessions(id *TowerID) (
 	return clientSessions, nil
 }
 
-// listClientSessions returns the set of all client sessions known to the db. An
-// optional tower ID can be used to filter out any client sessions in the
-// response that do not correspond to this tower.
-func listClientSessions(sessions, towers kvdb.RBucket,
-	id *TowerID) (map[SessionID]*ClientSession, error) {
+// listClientAllSessions returns the set of all client sessions known to the db.
+func listClientAllSessions(sessions,
+	towers kvdb.RBucket) (map[SessionID]*ClientSession, error) {
 
 	clientSessions := make(map[SessionID]*ClientSession)
 	err := sessions.ForEach(func(k, _ []byte) error {
@@ -779,14 +797,40 @@ func listClientSessions(sessions, towers kvdb.RBucket,
 			return err
 		}
 
-		// Filter out any sessions that don't correspond to the given
-		// tower if one was set.
-		if id != nil && session.TowerID != *id {
-			return nil
+		clientSessions[session.ID] = session
+
+		return nil
+	})
+	if err != nil {
+		return nil, err
+	}
+
+	return clientSessions, nil
+}
+
+// listTowerSessions returns the set of all client sessions known to the db
+// that are associated with the given tower id.
+func listTowerSessions(id TowerID, sessionsBkt, towersBkt,
+	towerToSessionIndex kvdb.RBucket) (map[SessionID]*ClientSession,
+	error) {
+
+	towerIndexBkt := towerToSessionIndex.NestedReadBucket(id.Bytes())
+	if towerIndexBkt == nil {
+		return nil, ErrTowerNotFound
+	}
+
+	clientSessions := make(map[SessionID]*ClientSession)
+	err := towerIndexBkt.ForEach(func(k, _ []byte) error {
+		// We'll load the full client session since the client will need
+		// the CommittedUpdates and AckedUpdates on startup to resume
+		// committed updates and compute the highest known commit height
+		// for each channel.
+		session, err := getClientSession(sessionsBkt, towersBkt, k)
+		if err != nil {
+			return err
 		}
 
 		clientSessions[session.ID] = session
-
 		return nil
 	})
 	if err != nil {