watchtower: add GetClientSession func to DB

This commit adds a new `GetClientSession` method to the tower client DB
which can be used to fetch a session by its ID from the DB.

watchtower/wtclient/interface.go

@@ -64,6 +64,11 @@ type DB interface {
 		...wtdb.ClientSessionListOption) (
 		map[wtdb.SessionID]*wtdb.ClientSession, error)
 
+	// GetClientSession loads the ClientSession with the given ID from the
+	// DB.
+	GetClientSession(wtdb.SessionID,
+		...wtdb.ClientSessionListOption) (*wtdb.ClientSession, error)
+
 	// FetchSessionCommittedUpdates retrieves the current set of un-acked
 	// updates of the given session.
 	FetchSessionCommittedUpdates(id *wtdb.SessionID) (

watchtower/wtdb/client_db.go

@@ -1009,6 +1009,36 @@ func getSessionKeyIndex(keyIndexes kvdb.RwBucket, towerID TowerID,
 	return byteOrder.Uint32(keyIndexBytes), nil
 }
 
+// GetClientSession loads the ClientSession with the given ID from the DB.
+func (c *ClientDB) GetClientSession(id SessionID,
+	opts ...ClientSessionListOption) (*ClientSession, error) {
+
+	var sess *ClientSession
+	err := kvdb.View(c.db, func(tx kvdb.RTx) error {
+		sessionsBkt := tx.ReadBucket(cSessionBkt)
+		if sessionsBkt == nil {
+			return ErrUninitializedDB
+		}
+
+		chanIDIndexBkt := tx.ReadBucket(cChanIDIndexBkt)
+		if chanIDIndexBkt == nil {
+			return ErrUninitializedDB
+		}
+
+		session, err := c.getClientSession(
+			sessionsBkt, chanIDIndexBkt, id[:], nil, opts...,
+		)
+		if err != nil {
+			return err
+		}
+
+		sess = session
+		return nil
+	}, func() {})
+
+	return sess, err
+}
+
 // ListClientSessions returns the set of all client sessions known to the db. An
 // optional tower ID can be used to filter out any client sessions in the
 // response that do not correspond to this tower.

watchtower/wtmock/client_db.go

@@ -554,6 +554,36 @@ func (m *ClientDB) FetchChanSummaries() (wtdb.ChannelSummaries, error) {
 	return summaries, nil
 }
 
+// GetClientSession loads the ClientSession with the given ID from the DB.
+func (m *ClientDB) GetClientSession(id wtdb.SessionID,
+	opts ...wtdb.ClientSessionListOption) (*wtdb.ClientSession, error) {
+
+	cfg := wtdb.NewClientSessionCfg()
+	for _, o := range opts {
+		o(cfg)
+	}
+
+	session, ok := m.activeSessions[id]
+	if !ok {
+		return nil, wtdb.ErrClientSessionNotFound
+	}
+
+	if cfg.PerMaxHeight != nil {
+		for chanID, index := range m.ackedUpdates[session.ID] {
+			cfg.PerMaxHeight(&session, chanID, index.MaxHeight())
+		}
+	}
+
+	if cfg.PerCommittedUpdate != nil {
+		for _, update := range m.committedUpdates[session.ID] {
+			update := update
+			cfg.PerCommittedUpdate(&session, &update)
+		}
+	}
+
+	return &session, nil
+}
+
 // RegisterChannel registers a channel for use within the client database. For
 // now, all that is stored in the channel summary is the sweep pkscript that
 // we'd like any tower sweeps to pay into. In the future, this will be extended