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.

watchtower/wtdb/client_db_test.go

@@ -207,6 +207,17 @@ func (h *clientDBHarness) markChannelClosed(id lnwire.ChannelID,
 	return closableSessions
 }
 
+func (h *clientDBHarness) listClosableSessions(
+	expErr error) map[wtdb.SessionID]uint32 {
+
+	h.t.Helper()
+
+	closableSessions, err := h.db.ListClosableSessions()
+	require.ErrorIs(h.t, err, expErr)
+
+	return closableSessions
+}
+
 // newTower is a helper function that creates a new tower with a randomly
 // generated public key and inserts it into the client DB.
 func (h *clientDBHarness) newTower() *wtdb.Tower {
@@ -711,11 +722,16 @@ func testMarkChannelClosed(h *clientDBHarness) {
 	// since it has an update for channel 6 which is still open.
 	sl = h.markChannelClosed(chanID5, 1, nil)
 	require.Empty(h.t, sl)
+	require.Empty(h.t, h.listClosableSessions(nil))
 
 	// Finally, if we close channel 6, session 1 _should_ be in the closable
 	// list.
-	sl = h.markChannelClosed(chanID6, 1, nil)
+	sl = h.markChannelClosed(chanID6, 100, nil)
 	require.ElementsMatch(h.t, sl, []wtdb.SessionID{session1.ID})
+	slMap := h.listClosableSessions(nil)
+	require.InDeltaMapValues(h.t, slMap, map[wtdb.SessionID]uint32{
+		session1.ID: 100,
+	}, 0)
 }
 
 // testAckUpdate asserts the behavior of AckUpdate.