watchtower: add filter function to ListTowers

And then only load active towers on client start up.

watchtower/wtclient/client.go

@@ -261,7 +261,10 @@ func getTowerAndSessionCandidates(db DB, keyRing ECDHKeyRing,
 	opts ...wtdb.ClientSessionListOption) (
 	map[wtdb.SessionID]*ClientSession, error) {
 
-	towers, err := db.ListTowers()
+	// Fetch all active towers from the DB.
+	towers, err := db.ListTowers(func(tower *wtdb.Tower) bool {
+		return tower.Status == wtdb.TowerStatusActive
+	})
 	if err != nil {
 		return nil, err
 	}

watchtower/wtclient/interface.go

@@ -41,8 +41,9 @@ type DB interface {
 	LoadTowerByID(wtdb.TowerID) (*wtdb.Tower, error)
 
 	// ListTowers retrieves the list of towers available within the
-	// database.
+	// database. The filter function may be set in order to filter out the
-	ListTowers() ([]*wtdb.Tower, error)
+	// towers to be returned.
+	ListTowers(filter wtdb.TowerFilterFn) ([]*wtdb.Tower, error)
 
 	// NextSessionKeyIndex reserves a new session key derivation index for a
 	// particular tower id and blob type. The index is reserved for that

watchtower/wtclient/manager.go

@@ -455,7 +455,7 @@ func (m *Manager) Stats() ClientStats {
 func (m *Manager) RegisteredTowers(opts ...wtdb.ClientSessionListOption) (
 	map[blob.Type][]*RegisteredTower, error) {
 
-	towers, err := m.cfg.DB.ListTowers()
+	towers, err := m.cfg.DB.ListTowers(nil)
 	if err != nil {
 		return nil, err
 	}

watchtower/wtdb/client_db.go

@@ -632,8 +632,14 @@ func (c *ClientDB) LoadTower(pubKey *btcec.PublicKey) (*Tower, error) {
 	return tower, nil
 }
 
-// ListTowers retrieves the list of towers available within the database.
+// TowerFilterFn is the signature of a call-back function that can be used to
-func (c *ClientDB) ListTowers() ([]*Tower, error) {
+// skip certain towers in the ListTowers method.
+type TowerFilterFn func(*Tower) bool
+
+// ListTowers retrieves the list of towers available within the database that
+// have a status matching the given status. The filter function may be set in
+// order to filter out the towers to be returned.
+func (c *ClientDB) ListTowers(filter TowerFilterFn) ([]*Tower, error) {
 	var towers []*Tower
 	err := kvdb.View(c.db, func(tx kvdb.RTx) error {
 		towerBucket := tx.ReadBucket(cTowerBkt)
@@ -646,7 +652,13 @@ func (c *ClientDB) ListTowers() ([]*Tower, error) {
 			if err != nil {
 				return err
 			}
+
+			if filter != nil && !filter(tower) {
+				return nil
+			}
+
 			towers = append(towers, tower)
+
 			return nil
 		})
 	}, func() {