wtdb: let DeleteCommitedUpdates move session to terminal

In this commit, we adjust the DeleteCommitmentUpdate method so that it
marks a session as Terminal (if there are updates to delete) since once
we have deleted a commitment update from a session - the session is no
longer useable.

watchtower/wtdb/client_db.go

@@ -1860,9 +1860,10 @@ func (c *ClientDB) MarkChannelClosed(chanID lnwire.ChannelID,
 
 // isSessionClosable returns true if a session is considered closable. A session
 // is considered closable only if all the following points are true:
-// 1) It has no un-acked updates.
-// 2) It is exhausted (ie it can't accept any more updates)
-// 3) All the channels that it has acked updates for are closed.
+//  1. It has no un-acked updates.
+//  2. It is exhausted (ie it can't accept any more updates) OR it has been
+//     marked as terminal.
+//  3. All the channels that it has acked updates for are closed.
 func isSessionClosable(sessionsBkt, chanDetailsBkt, chanIDIndexBkt kvdb.RBucket,
 	id *SessionID) (bool, error) {
 
@@ -1893,10 +1894,14 @@ func isSessionClosable(sessionsBkt, chanDetailsBkt, chanIDIndexBkt kvdb.RBucket,
 		return false, err
 	}
 
+	isTerminal := session.Status == CSessionTerminal
+
 	// We have already checked that the session has no more committed
-	// updates. So now we can check if the session is exhausted.
-	if session.SeqNum < session.Policy.MaxUpdates {
-		// If the session is not yet exhausted, it is not yet closable.
+	// updates. So now we can check if the session is exhausted or has a
+	// terminal state.
+	if !isTerminal && session.SeqNum < session.Policy.MaxUpdates {
+		// If the session is not yet exhausted, and it is not yet in a
+		// terminal state then it is not yet closable.
 		return false, nil
 	}
 
@@ -1916,11 +1921,16 @@ func isSessionClosable(sessionsBkt, chanDetailsBkt, chanIDIndexBkt kvdb.RBucket,
 		}
 	}
 
-	// If the session has no acked-updates, then something is wrong since
-	// the above check ensures that this session has been exhausted meaning
-	// that it should have MaxUpdates acked updates.
 	ackedRangeBkt := sessBkt.NestedReadBucket(cSessionAckRangeIndex)
 	if ackedRangeBkt == nil {
+		if isTerminal {
+			return true, nil
+		}
+
+		// If the session has no acked-updates, and it is not in a
+		// terminal state then something is wrong since the above check
+		// ensures that this session has been exhausted meaning that it
+		// should have MaxUpdates acked updates.
 		return false, fmt.Errorf("no acked-updates found for "+
 			"exhausted session %s", id)
 	}
@@ -2068,7 +2078,6 @@ func (c *ClientDB) CommitUpdate(id *SessionID,
 		lastApplied = session.TowerLastApplied
 
 		return nil
-
 	}, func() {
 		lastApplied = 0
 	})
@@ -2334,6 +2343,20 @@ func (c *ClientDB) DeleteCommittedUpdates(id *SessionID) error {
 			return err
 		}
 
+		session, err := getClientSessionBody(sessions, id[:])
+		if err != nil {
+			return err
+		}
+
+		// Once we delete a committed update from the session, the
+		// SeqNum of the session will be incorrect and so the session
+		// should be marked as terminal.
+		session.Status = CSessionTerminal
+		err = putClientSessionBody(sessionBkt, session)
+		if err != nil {
+			return err
+		}
+
 		// Delete all the committed updates in one go by deleting the
 		// session commits bucket.
 		return sessionBkt.DeleteNestedBucket(cSessionCommits)

watchtower/wtdb/client_db_test.go

@@ -1002,6 +1002,44 @@ func testMarkChannelClosed(h *clientDBHarness) {
 	// Assert that we now can delete the session.
 	h.deleteSession(session1.ID, nil)
 	require.Empty(h.t, h.listClosableSessions(nil))
+
+	// We also want to test that a session can be deleted if it has been
+	// marked as terminal.
+
+	// Create session2 with MaxUpdates set to 5.
+	session2 := h.randSession(h.t, tower.ID, 5)
+	h.insertSession(session2, nil)
+
+	// Create and register channel 7.
+	chanID7 := randChannelID(h.t)
+	h.registerChan(chanID7, nil, nil)
+
+	// Add two updates for channel 7 in session 2. Ack one of them so that
+	// the mapping from this channel to this session is created but don't
+	// ack the other since we want to delete the committed update later.
+	update = randCommittedUpdateForChannel(h.t, chanID7, 1)
+	h.commitUpdate(&session2.ID, update, nil)
+	h.ackUpdate(&session2.ID, 1, 1, nil)
+
+	update = randCommittedUpdateForChannel(h.t, chanID7, 2)
+	h.commitUpdate(&session2.ID, update, nil)
+
+	// Check that attempting to delete the session will fail since it is not
+	// yet considered closable.
+	h.deleteSession(session2.ID, wtdb.ErrSessionNotClosable)
+
+	// Now delete the added committed updates. This should put the
+	// session in the terminal state after which we should be able to
+	// delete the session.
+	h.deleteCommittedUpdates(&session2.ID, nil)
+
+	// Marking channel 7 as closed will now return session 2 since it has
+	// been marked as terminal.
+	sl = h.markChannelClosed(chanID7, 1, nil)
+	require.ElementsMatch(h.t, sl, []wtdb.SessionID{session2.ID})
+
+	// We should now be able to delete the session.
+	h.deleteSession(session2.ID, nil)
 }
 
 // testAckUpdate asserts the behavior of AckUpdate.