wtclient: refactor existing candidate session filtering into method

watchtower/wtclient/client.go

@@ -38,6 +38,14 @@ const (
 	DefaultForceQuitDelay = 10 * time.Second
 )
 
+var (
+	// activeSessionFilter is a filter that ignored any sessions which are
+	// not active.
+	activeSessionFilter = func(s *wtdb.ClientSession) bool {
+		return s.Status == wtdb.CSessionActive
+	}
+)
+
 // RegisteredTower encompasses information about a registered watchtower with
 // the client.
 type RegisteredTower struct {
@@ -268,49 +276,18 @@ func New(config *Config) (*TowerClient, error) {
 	// the client. We will use any of these session if their policies match
 	// the current policy of the client, otherwise they will be ignored and
 	// new sessions will be requested.
-	sessions, err := cfg.DB.ListClientSessions(nil)
+	candidateSessions, err := getClientSessions(
+		cfg.DB, cfg.SecretKeyRing, nil, activeSessionFilter,
+	)
 	if err != nil {
 		return nil, err
 	}
 
-	candidateSessions := make(map[wtdb.SessionID]*wtdb.ClientSession)
-	sessionTowers := make(map[wtdb.TowerID]*wtdb.Tower)
-	for _, s := range sessions {
-		// Candidate sessions must be in an active state.
-		if s.Status != wtdb.CSessionActive {
-			continue
-		}
-
-		// Reload the tower from disk using the tower ID contained in
-		// each candidate session. We will also rederive any session
-		// keys needed to be able to communicate with the towers and
-		// authenticate session requests. This prevents us from having
-		// to store the private keys on disk.
-		tower, ok := sessionTowers[s.TowerID]
-		if !ok {
-			var err error
-			tower, err = cfg.DB.LoadTowerByID(s.TowerID)
-			if err != nil {
-				return nil, err
-			}
-		}
-		s.Tower = tower
-
-		sessionKey, err := DeriveSessionKey(cfg.SecretKeyRing, s.KeyIndex)
-		if err != nil {
-			return nil, err
-		}
-		s.SessionPrivKey = sessionKey
-
-		candidateSessions[s.ID] = s
-		sessionTowers[tower.ID] = tower
-	}
-
 	var candidateTowers []*wtdb.Tower
-	for _, tower := range sessionTowers {
+	for _, s := range candidateSessions {
 		log.Infof("Using private watchtower %s, offering policy %s",
-			tower, cfg.Policy)
-		candidateTowers = append(candidateTowers, tower)
+			s.Tower, cfg.Policy)
+		candidateTowers = append(candidateTowers, s.Tower)
 	}
 
 	// Load the sweep pkscripts that have been generated for all previously
@@ -353,6 +330,50 @@ func New(config *Config) (*TowerClient, error) {
 	return c, nil
 }
 
+// getClientSessions retrieves the client sessions for a particular tower if
+// specified, otherwise all client sessions for all towers are retrieved. An
+// optional filter can be provided to filter out any undesired client sessions.
+//
+// NOTE: This method should only be used when deserialization of a
+// ClientSession's Tower and SessionPrivKey fields is desired, otherwise, the
+// existing ListClientSessions method should be used.
+func getClientSessions(db DB, keyRing SecretKeyRing, forTower *wtdb.TowerID,
+	passesFilter func(*wtdb.ClientSession) bool) (
+	map[wtdb.SessionID]*wtdb.ClientSession, error) {
+
+	sessions, err := db.ListClientSessions(forTower)
+	if err != nil {
+		return nil, err
+	}
+
+	// Reload the tower from disk using the tower ID contained in each
+	// candidate session. We will also rederive any session keys needed to
+	// be able to communicate with the towers and authenticate session
+	// requests. This prevents us from having to store the private keys on
+	// disk.
+	for _, s := range sessions {
+		tower, err := db.LoadTowerByID(s.TowerID)
+		if err != nil {
+			return nil, err
+		}
+		s.Tower = tower
+
+		sessionKey, err := DeriveSessionKey(keyRing, s.KeyIndex)
+		if err != nil {
+			return nil, err
+		}
+		s.SessionPrivKey = sessionKey
+
+		// If an optional filter was provided, use it to filter out any
+		// undesired sessions.
+		if passesFilter != nil && !passesFilter(s) {
+			delete(sessions, s.ID)
+		}
+	}
+
+	return sessions, nil
+}
+
 // buildHighestCommitHeights inspects the full set of candidate client sessions
 // loaded from disk, and determines the highest known commit height for each
 // channel. This allows the client to reject backups that it has already