watchtower/wtclient/interface: add LoadTower and mock impl

watchtower/wtclient/client.go

@@ -221,6 +221,17 @@ func New(config *Config) (*TowerClient, error) {
 		return nil, err
 	}
 
+	// Reload any towers from disk using the tower IDs contained in each
+	// candidate session.
+	for _, s := range c.candidateSessions {
+		tower, err := c.cfg.DB.LoadTower(s.TowerID)
+		if err != nil {
+			return nil, err
+		}
+
+		s.Tower = tower
+	}
+
 	// Finally, load the sweep pkscripts that have been generated for all
 	// previously registered channels.
 	c.sweepPkScripts, err = c.cfg.DB.FetchChanPkScripts()

watchtower/wtclient/interface.go

@@ -19,6 +19,9 @@ type DB interface {
 	// sessions.
 	CreateTower(*lnwire.NetAddress) (*wtdb.Tower, error)
 
+	// LoadTower retrieves a tower by its tower ID.
+	LoadTower(uint64) (*wtdb.Tower, error)
+
 	// CreateClientSession saves a newly negotiated client session to the
 	// client's database. This enables the session to be used across
 	// restarts.

watchtower/wtdb/tower.go

@@ -1,6 +1,7 @@
 package wtdb
 
 import (
+	"errors"
 	"net"
 	"sync"
 
@@ -8,6 +9,12 @@ import (
 	"github.com/lightningnetwork/lnd/lnwire"
 )
 
+var (
+	// ErrTowerNotFound signals that the target tower was not found in the
+	// database.
+	ErrTowerNotFound = errors.New("tower not found")
+)
+
 // Tower holds the necessary components required to connect to a remote tower.
 // Communication is handled by brontide, and requires both a public key and an
 // address.

watchtower/wtmock/client_db.go

@@ -64,6 +64,18 @@ func (m *ClientDB) CreateTower(lnAddr *lnwire.NetAddress) (*wtdb.Tower, error) {
 	return tower, nil
 }
 
+// LoadTower retrieves a tower by its tower ID.
+func (m *ClientDB) LoadTower(towerID uint64) (*wtdb.Tower, error) {
+	m.mu.Lock()
+	defer m.mu.Unlock()
+
+	if tower, ok := m.towers[towerID]; ok {
+		return tower, nil
+	}
+
+	return nil, wtdb.ErrTowerNotFound
+}
+
 // MarkBackupIneligible records that particular commit height is ineligible for
 // backup. This allows the client to track which updates it should not attempt
 // to retry after startup.
@@ -92,7 +104,6 @@ func (m *ClientDB) CreateClientSession(session *wtdb.ClientSession) error {
 
 	m.activeSessions[session.ID] = &wtdb.ClientSession{
 		TowerID:          session.TowerID,
-		Tower:            session.Tower,
 		SessionKeyDesc:   session.SessionKeyDesc,
 		SessionPrivKey:   session.SessionPrivKey,
 		ID:               session.ID,