watchtower/wtdb: add new towerID-to-sessionID index

This commit adds a new towerID-to-sessionID index to the wtclient DB.
The commit also contains the necessary migration required in order to
build the index for an existing client.
This index will greatly improve the lookup of sessions for a given tower
ID.

watchtower/wtdb/client_db.go

@@ -48,6 +48,12 @@ var (
 	//    tower-pubkey -> tower-id.
 	cTowerIndexBkt = []byte("client-tower-index-bucket")
 
+	// cTowerToSessionIndexBkt is a top-level bucket storing:
+	// 	tower-id -> session-id -> 1
+	cTowerToSessionIndexBkt = []byte(
+		"client-tower-to-session-index-bucket",
+	)
+
 	// ErrTowerNotFound signals that the target tower was not found in the
 	// database.
 	ErrTowerNotFound = errors.New("tower not found")
@@ -196,6 +202,7 @@ func initClientDBBuckets(tx kvdb.RwTx) error {
 		cSessionBkt,
 		cTowerBkt,
 		cTowerIndexBkt,
+		cTowerToSessionIndexBkt,
 	}
 
 	for _, bucket := range buckets {
@@ -260,6 +267,13 @@ func (c *ClientDB) CreateTower(lnAddr *lnwire.NetAddress) (*Tower, error) {
 			return ErrUninitializedDB
 		}
 
+		towerToSessionIndex := tx.ReadWriteBucket(
+			cTowerToSessionIndexBkt,
+		)
+		if towerToSessionIndex == nil {
+			return ErrUninitializedDB
+		}
+
 		// Check if the tower index already knows of this pubkey.
 		towerIDBytes := towerIndex.Get(towerPubKey[:])
 		if len(towerIDBytes) == 8 {
@@ -321,6 +335,13 @@ func (c *ClientDB) CreateTower(lnAddr *lnwire.NetAddress) (*Tower, error) {
 			if err != nil {
 				return err
 			}
+
+			// Create a new bucket for this tower in the
+			// tower-to-sessions index.
+			_, err = towerToSessionIndex.CreateBucket(towerIDBytes)
+			if err != nil {
+				return err
+			}
 		}
 
 		// Store the new or updated tower under its tower id.
@@ -349,11 +370,19 @@ func (c *ClientDB) RemoveTower(pubKey *btcec.PublicKey, addr net.Addr) error {
 		if towers == nil {
 			return ErrUninitializedDB
 		}
+
 		towerIndex := tx.ReadWriteBucket(cTowerIndexBkt)
 		if towerIndex == nil {
 			return ErrUninitializedDB
 		}
 
+		towersToSessionsIndex := tx.ReadWriteBucket(
+			cTowerToSessionIndexBkt,
+		)
+		if towersToSessionsIndex == nil {
+			return ErrUninitializedDB
+		}
+
 		// Don't return an error if the watchtower doesn't exist to act
 		// as a NOP.
 		pubKeyBytes := pubKey.SerializeCompressed()
@@ -402,7 +431,14 @@ func (c *ClientDB) RemoveTower(pubKey *btcec.PublicKey, addr net.Addr) error {
 			if err := towerIndex.Delete(pubKeyBytes); err != nil {
 				return err
 			}
-			return towers.Delete(towerIDBytes)
+
+			if err := towers.Delete(towerIDBytes); err != nil {
+				return err
+			}
+
+			return towersToSessionsIndex.DeleteNestedBucket(
+				towerIDBytes,
+			)
 		}
 
 		// We'll mark its sessions as inactive as long as they don't
@@ -581,6 +617,13 @@ func (c *ClientDB) CreateClientSession(session *ClientSession) error {
 			return ErrUninitializedDB
 		}
 
+		towerToSessionIndex := tx.ReadWriteBucket(
+			cTowerToSessionIndexBkt,
+		)
+		if towerToSessionIndex == nil {
+			return ErrUninitializedDB
+		}
+
 		// Check that  client session with this session id doesn't
 		// already exist.
 		existingSessionBytes := sessions.NestedReadWriteBucket(
@@ -625,6 +668,19 @@ func (c *ClientDB) CreateClientSession(session *ClientSession) error {
 			}
 		}
 
+		// Add the new entry to the towerID-to-SessionID index.
+		indexBkt := towerToSessionIndex.NestedReadWriteBucket(
+			towerID.Bytes(),
+		)
+		if indexBkt == nil {
+			return ErrTowerNotFound
+		}
+
+		err = indexBkt.Put(session.ID[:], []byte{1})
+		if err != nil {
+			return err
+		}
+
 		// Finally, write the client session's body in the sessions
 		// bucket.
 		return putClientSessionBody(sessions, session)