watchtower: add ListClosableSessions method

This commit adds a new ListClosableSessions method to the tower client
DB. This method will return a map of sessionIDs to block heights. The
IDs belong to sessions that are considered closable and the block
heights are the block height at which the last associated channel for
the session was closed in.

watchtower/wtdb/client_db.go

@@ -1385,6 +1385,46 @@ func (c *ClientDB) MarkBackupIneligible(chanID lnwire.ChannelID,
 	return nil
 }
 
+// ListClosableSessions fetches and returns the IDs for all sessions marked as
+// closable.
+func (c *ClientDB) ListClosableSessions() (map[SessionID]uint32, error) {
+	sessions := make(map[SessionID]uint32)
+	err := kvdb.View(c.db, func(tx kvdb.RTx) error {
+		csBkt := tx.ReadBucket(cClosableSessionsBkt)
+		if csBkt == nil {
+			return ErrUninitializedDB
+		}
+
+		sessIDIndexBkt := tx.ReadBucket(cSessionIDIndexBkt)
+		if sessIDIndexBkt == nil {
+			return ErrUninitializedDB
+		}
+
+		return csBkt.ForEach(func(dbIDBytes, heightBytes []byte) error {
+			dbID, err := readBigSize(dbIDBytes)
+			if err != nil {
+				return err
+			}
+
+			sessID, err := getRealSessionID(sessIDIndexBkt, dbID)
+			if err != nil {
+				return err
+			}
+
+			sessions[*sessID] = byteOrder.Uint32(heightBytes)
+
+			return nil
+		})
+	}, func() {
+		sessions = make(map[SessionID]uint32)
+	})
+	if err != nil {
+		return nil, err
+	}
+
+	return sessions, nil
+}
+
 // MarkChannelClosed will mark a registered channel as closed by setting its
 // closed-height as the given block height. It returns a list of session IDs for
 // sessions that are now considered closable due to the close of this channel.