watchtower: add wtclient.ClientSession constructor

watchtower/wtclient/client.go

@@ -435,27 +435,19 @@ func getTowerAndSessionCandidates(db DB, keyRing ECDHKeyRing,
 		}
 
 		for _, s := range sessions {
-			towerKeyDesc, err := keyRing.DeriveKey(
-				keychain.KeyLocator{
-					Family: keychain.KeyFamilyTowerSession,
-					Index:  s.KeyIndex,
-				},
+			if !sessionFilter(s) {
+				continue
+			}
+
+			cs, err := NewClientSessionFromDBSession(
+				s, tower, keyRing,
 			)
 			if err != nil {
 				return nil, err
 			}
 
-			sessionKeyECDH := keychain.NewPubKeyECDH(
-				towerKeyDesc, keyRing,
-			)
-
 			// Add the session to the set of candidate sessions.
-			candidateSessions[s.ID] = &ClientSession{
-				ID:                s.ID,
-				ClientSessionBody: s.ClientSessionBody,
-				Tower:             tower,
-				SessionKeyECDH:    sessionKeyECDH,
-			}
+			candidateSessions[s.ID] = cs
 
 			perActiveTower(tower)
 		}

watchtower/wtclient/interface.go

@@ -198,3 +198,30 @@ type ClientSession struct {
 	// key used to connect to the watchtower.
 	SessionKeyECDH keychain.SingleKeyECDH
 }
+
+// NewClientSessionFromDBSession converts a wtdb.ClientSession to a
+// ClientSession.
+func NewClientSessionFromDBSession(s *wtdb.ClientSession, tower *Tower,
+	keyRing ECDHKeyRing) (*ClientSession, error) {
+
+	towerKeyDesc, err := keyRing.DeriveKey(
+		keychain.KeyLocator{
+			Family: keychain.KeyFamilyTowerSession,
+			Index:  s.KeyIndex,
+		},
+	)
+	if err != nil {
+		return nil, err
+	}
+
+	sessionKeyECDH := keychain.NewPubKeyECDH(
+		towerKeyDesc, keyRing,
+	)
+
+	return &ClientSession{
+		ID:                s.ID,
+		ClientSessionBody: s.ClientSessionBody,
+		Tower:             tower,
+		SessionKeyECDH:    sessionKeyECDH,
+	}, nil
+}