watchtower/wtmock/client_db: adjust mock clientdb behavior

In advance of the upcoming wtdb.ClientDB, we'll modify the behavior
of the mockdb to be more like the final bbolt backed one, and assert
that all or our tests are still passing.

watchtower/wtmock/client_db.go

@@ -65,7 +65,7 @@ func (m *ClientDB) CreateTower(lnAddr *lnwire.NetAddress) (*wtdb.Tower, error) {
 	m.towerIndex[towerPubKey] = towerID
 	m.towers[towerID] = tower
 
-	return tower, nil
+	return copyTower(tower), nil
 }
 
 // LoadTower retrieves a tower by its tower ID.
@@ -74,7 +74,7 @@ func (m *ClientDB) LoadTower(towerID wtdb.TowerID) (*wtdb.Tower, error) {
 	defer m.mu.Unlock()
 
 	if tower, ok := m.towers[towerID]; ok {
-		return tower, nil
+		return copyTower(tower), nil
 	}
 
 	return nil, wtdb.ErrTowerNotFound
@@ -106,6 +106,11 @@ func (m *ClientDB) CreateClientSession(session *wtdb.ClientSession) error {
 	m.mu.Lock()
 	defer m.mu.Unlock()
 
+	// Ensure that we aren't overwriting an existing session.
+	if _, ok := m.activeSessions[session.ID]; ok {
+		return wtdb.ErrClientSessionAlreadyExists
+	}
+
 	// Ensure that a session key index has been reserved for this tower.
 	keyIndex, ok := m.indexes[session.TowerID]
 	if !ok {
@@ -151,11 +156,10 @@ func (m *ClientDB) NextSessionKeyIndex(towerID wtdb.TowerID) (uint32, error) {
 		return index, nil
 	}
 
+	m.nextIndex++
 	index := m.nextIndex
 	m.indexes[towerID] = index
 
-	m.nextIndex++
-
 	return index, nil
 }
 
@@ -286,3 +290,14 @@ func cloneBytes(b []byte) []byte {
 
 	return bb
 }
+
+func copyTower(tower *wtdb.Tower) *wtdb.Tower {
+	t := &wtdb.Tower{
+		ID:          tower.ID,
+		IdentityKey: tower.IdentityKey,
+		Addresses:   make([]net.Addr, len(tower.Addresses)),
+	}
+	copy(t.Addresses, tower.Addresses)
+
+	return t
+}