watchtower: handle channel closures

Add a channel-close handler that waits for channel close events and
marks channels as closed in the tower client DB.

server.go

@@ -1512,7 +1512,16 @@ func newServer(cfg *Config, listenAddrs []net.Addr,
 			)
 		}
 
+		fetchClosedChannel := s.chanStateDB.FetchClosedChannelForID
+
 		s.towerClient, err = wtclient.New(&wtclient.Config{
+			FetchClosedChannel: fetchClosedChannel,
+			SubscribeChannelEvents: func() (subscribe.Subscription,
+				error) {
+
+				return s.channelNotifier.
+					SubscribeChannelEvents()
+			},
 			Signer:         cc.Wallet.Cfg.Signer,
 			NewAddress:     newSweepPkScriptGen(cc.Wallet),
 			SecretKeyRing:  s.cc.KeyRing,
@@ -1536,6 +1545,13 @@ func newServer(cfg *Config, listenAddrs []net.Addr,
 			blob.Type(blob.FlagAnchorChannel)
 
 		s.anchorTowerClient, err = wtclient.New(&wtclient.Config{
+			FetchClosedChannel: fetchClosedChannel,
+			SubscribeChannelEvents: func() (subscribe.Subscription,
+				error) {
+
+				return s.channelNotifier.
+					SubscribeChannelEvents()
+			},
 			Signer:         cc.Wallet.Cfg.Signer,
 			NewAddress:     newSweepPkScriptGen(cc.Wallet),
 			SecretKeyRing:  s.cc.KeyRing,

watchtower/wtclient/client.go

@@ -2,6 +2,7 @@ package wtclient
 
 import (
 	"bytes"
+	"errors"
 	"fmt"
 	"net"
 	"sync"
@@ -12,10 +13,12 @@ import (
 	"github.com/btcsuite/btclog"
 	"github.com/lightningnetwork/lnd/build"
 	"github.com/lightningnetwork/lnd/channeldb"
+	"github.com/lightningnetwork/lnd/channelnotifier"
 	"github.com/lightningnetwork/lnd/input"
 	"github.com/lightningnetwork/lnd/keychain"
 	"github.com/lightningnetwork/lnd/lnwallet"
 	"github.com/lightningnetwork/lnd/lnwire"
+	"github.com/lightningnetwork/lnd/subscribe"
 	"github.com/lightningnetwork/lnd/tor"
 	"github.com/lightningnetwork/lnd/watchtower/wtdb"
 	"github.com/lightningnetwork/lnd/watchtower/wtpolicy"
@@ -146,6 +149,16 @@ type Config struct {
 	// transaction.
 	Signer input.Signer
 
+	// SubscribeChannelEvents can be used to subscribe to channel event
+	// notifications.
+	SubscribeChannelEvents func() (subscribe.Subscription, error)
+
+	// FetchClosedChannel can be used to fetch the info about a closed
+	// channel. If the channel is not found or not yet closed then
+	// channeldb.ErrClosedChannelNotFound will be returned.
+	FetchClosedChannel func(cid lnwire.ChannelID) (
+		*channeldb.ChannelCloseSummary, error)
+
 	// NewAddress generates a new on-chain sweep pkscript.
 	NewAddress func() ([]byte, error)
 
@@ -269,6 +282,7 @@ type TowerClient struct {
 	staleTowers chan *staleTowerMsg
 
 	wg        sync.WaitGroup
+	quit      chan struct{}
 	forceQuit chan struct{}
 }
 
@@ -319,6 +333,7 @@ func New(config *Config) (*TowerClient, error) {
 		newTowers:         make(chan *newTowerMsg),
 		staleTowers:       make(chan *staleTowerMsg),
 		forceQuit:         make(chan struct{}),
+		quit:              make(chan struct{}),
 	}
 
 	// perUpdate is a callback function that will be used to inspect the
@@ -364,7 +379,7 @@ func New(config *Config) (*TowerClient, error) {
 			return
 		}
 
-		log.Infof("Using private watchtower %s, offering policy %s",
+		c.log.Infof("Using private watchtower %s, offering policy %s",
 			tower, cfg.Policy)
 
 		// Add the tower to the set of candidate towers.
@@ -540,10 +555,45 @@ func (c *TowerClient) Start() error {
 			}
 		}
 
+		chanSub, err := c.cfg.SubscribeChannelEvents()
+		if err != nil {
+			returnErr = err
+			return
+		}
+
+		// Iterate over the list of registered channels and check if
+		// any of them can be marked as closed.
+		for id := range c.summaries {
+			isClosed, closedHeight, err := c.isChannelClosed(id)
+			if err != nil {
+				returnErr = err
+				return
+			}
+
+			if !isClosed {
+				continue
+			}
+
+			_, err = c.cfg.DB.MarkChannelClosed(id, closedHeight)
+			if err != nil {
+				c.log.Errorf("could not mark channel(%s) as "+
+					"closed: %v", id, err)
+
+				continue
+			}
+
+			// Since the channel has been marked as closed, we can
+			// also remove it from the channel summaries map.
+			delete(c.summaries, id)
+		}
+
+		c.wg.Add(1)
+		go c.handleChannelCloses(chanSub)
+
 		// Now start the session negotiator, which will allow us to
 		// request new session as soon as the backupDispatcher starts
 		// up.
-		err := c.negotiator.Start()
+		err = c.negotiator.Start()
 		if err != nil {
 			returnErr = err
 			return
@@ -591,6 +641,7 @@ func (c *TowerClient) Stop() error {
 		// dispatcher to exit. The backup queue will signal it's
 		// completion to the dispatcher, which releases the wait group
 		// after all tasks have been assigned to session queues.
+		close(c.quit)
 		c.wg.Wait()
 
 		// 4. Since all valid tasks have been assigned to session
@@ -772,6 +823,82 @@ func (c *TowerClient) nextSessionQueue() (*sessionQueue, error) {
 	return c.getOrInitActiveQueue(candidateSession, updates), nil
 }
 
+// handleChannelCloses listens for channel close events and marks channels as
+// closed in the DB.
+//
+// NOTE: This method MUST be run as a goroutine.
+func (c *TowerClient) handleChannelCloses(chanSub subscribe.Subscription) {
+	defer c.wg.Done()
+
+	c.log.Debugf("Starting channel close handler")
+	defer c.log.Debugf("Stopping channel close handler")
+
+	for {
+		select {
+		case update, ok := <-chanSub.Updates():
+			if !ok {
+				c.log.Debugf("Channel notifier has exited")
+				return
+			}
+
+			// We only care about channel-close events.
+			event, ok := update.(channelnotifier.ClosedChannelEvent)
+			if !ok {
+				continue
+			}
+
+			chanID := lnwire.NewChanIDFromOutPoint(
+				&event.CloseSummary.ChanPoint,
+			)
+
+			c.log.Debugf("Received ClosedChannelEvent for "+
+				"channel: %s", chanID)
+
+			err := c.handleClosedChannel(
+				chanID, event.CloseSummary.CloseHeight,
+			)
+			if err != nil {
+				c.log.Errorf("Could not handle channel close "+
+					"event for channel(%s): %v", chanID,
+					err)
+			}
+
+		case <-c.forceQuit:
+			return
+
+		case <-c.quit:
+			return
+		}
+	}
+}
+
+// handleClosedChannel handles the closure of a single channel. It will mark the
+// channel as closed in the DB.
+func (c *TowerClient) handleClosedChannel(chanID lnwire.ChannelID,
+	closeHeight uint32) error {
+
+	c.backupMu.Lock()
+	defer c.backupMu.Unlock()
+
+	// We only care about channels registered with the tower client.
+	if _, ok := c.summaries[chanID]; !ok {
+		return nil
+	}
+
+	c.log.Debugf("Marking channel(%s) as closed", chanID)
+
+	_, err := c.cfg.DB.MarkChannelClosed(chanID, closeHeight)
+	if err != nil {
+		return fmt.Errorf("could not mark channel(%s) as closed: %w",
+			chanID, err)
+	}
+
+	delete(c.summaries, chanID)
+	delete(c.chanCommitHeights, chanID)
+
+	return nil
+}
+
 // backupDispatcher processes events coming from the taskPipeline and is
 // responsible for detecting when the client needs to renegotiate a session to
 // fulfill continuing demand. The event loop exits after all tasks have been
@@ -1145,6 +1272,22 @@ func (c *TowerClient) initActiveQueue(s *ClientSession,
 	return sq
 }
 
+// isChanClosed can be used to check if the channel with the given ID has been
+// closed. If it has been, the block height in which its closing transaction was
+// mined will also be returned.
+func (c *TowerClient) isChannelClosed(id lnwire.ChannelID) (bool, uint32,
+	error) {
+
+	chanSum, err := c.cfg.FetchClosedChannel(id)
+	if errors.Is(err, channeldb.ErrClosedChannelNotFound) {
+		return false, 0, nil
+	} else if err != nil {
+		return false, 0, err
+	}
+
+	return true, chanSum.CloseHeight, nil
+}
+
 // AddTower adds a new watchtower reachable at the given address and considers
 // it for new sessions. If the watchtower already exists, then any new addresses
 // included will be considered when dialing it for session negotiations and

watchtower/wtclient/client_test.go

@@ -1,6 +1,7 @@
 package wtclient_test
 
 import (
+	"bytes"
 	"encoding/binary"
 	"errors"
 	"fmt"
@@ -16,11 +17,13 @@ import (
 	"github.com/btcsuite/btcd/txscript"
 	"github.com/btcsuite/btcd/wire"
 	"github.com/lightningnetwork/lnd/channeldb"
+	"github.com/lightningnetwork/lnd/channelnotifier"
 	"github.com/lightningnetwork/lnd/input"
 	"github.com/lightningnetwork/lnd/keychain"
 	"github.com/lightningnetwork/lnd/lntest/wait"
 	"github.com/lightningnetwork/lnd/lnwallet"
 	"github.com/lightningnetwork/lnd/lnwire"
+	"github.com/lightningnetwork/lnd/subscribe"
 	"github.com/lightningnetwork/lnd/tor"
 	"github.com/lightningnetwork/lnd/watchtower/blob"
 	"github.com/lightningnetwork/lnd/watchtower/wtclient"
@@ -393,8 +396,12 @@ type testHarness struct {
 	server     *wtserver.Server
 	net        *mockNet
 
+	channelEvents *mockSubscription
+	sendUpdatesOn bool
+
 	mu             sync.Mutex
 	channels       map[lnwire.ChannelID]*mockChannel
+	closedChannels map[lnwire.ChannelID]uint32
 
 	quit chan struct{}
 }
@@ -441,8 +448,45 @@ func newHarness(t *testing.T, cfg harnessCfg) *testHarness {
 	mockNet := newMockNet()
 	clientDB := wtmock.NewClientDB()
 
-	clientCfg := &wtclient.Config{
+	h := &testHarness{
+		t:              t,
+		cfg:            cfg,
+		signer:         signer,
+		capacity:       cfg.localBalance + cfg.remoteBalance,
+		clientDB:       clientDB,
+		serverAddr:     towerAddr,
+		serverDB:       serverDB,
+		serverCfg:      serverCfg,
+		net:            mockNet,
+		channelEvents:  newMockSubscription(t),
+		channels:       make(map[lnwire.ChannelID]*mockChannel),
+		closedChannels: make(map[lnwire.ChannelID]uint32),
+		quit:           make(chan struct{}),
+	}
+	t.Cleanup(func() {
+		close(h.quit)
+	})
+
+	fetchChannel := func(id lnwire.ChannelID) (
+		*channeldb.ChannelCloseSummary, error) {
+
+		h.mu.Lock()
+		defer h.mu.Unlock()
+
+		height, ok := h.closedChannels[id]
+		if !ok {
+			return nil, channeldb.ErrClosedChannelNotFound
+		}
+
+		return &channeldb.ChannelCloseSummary{CloseHeight: height}, nil
+	}
+
+	h.clientCfg = &wtclient.Config{
 		Signer: signer,
+		SubscribeChannelEvents: func() (subscribe.Subscription, error) {
+			return h.channelEvents, nil
+		},
+		FetchClosedChannel: fetchChannel,
 		Dial:               mockNet.Dial,
 		DB:                 clientDB,
 		AuthDial:           mockNet.AuthDial,
@@ -458,24 +502,6 @@ func newHarness(t *testing.T, cfg harnessCfg) *testHarness {
 		ForceQuitDelay: 10 * time.Second,
 	}
 
-	h := &testHarness{
-		t:          t,
-		cfg:        cfg,
-		signer:     signer,
-		capacity:   cfg.localBalance + cfg.remoteBalance,
-		clientDB:   clientDB,
-		clientCfg:  clientCfg,
-		serverAddr: towerAddr,
-		serverDB:   serverDB,
-		serverCfg:  serverCfg,
-		net:        mockNet,
-		channels:   make(map[lnwire.ChannelID]*mockChannel),
-		quit:       make(chan struct{}),
-	}
-	t.Cleanup(func() {
-		close(h.quit)
-	})
-
 	if !cfg.noServerStart {
 		h.startServer()
 		t.Cleanup(h.stopServer)
@@ -576,6 +602,41 @@ func (h *testHarness) channel(id uint64) *mockChannel {
 	return c
 }
 
+// closeChannel marks a channel as closed.
+//
+// NOTE: The method fails if a channel for id does not exist.
+func (h *testHarness) closeChannel(id uint64, height uint32) {
+	h.t.Helper()
+
+	h.mu.Lock()
+	defer h.mu.Unlock()
+
+	chanID := chanIDFromInt(id)
+
+	_, ok := h.channels[chanID]
+	require.Truef(h.t, ok, "unable to fetch channel %d", id)
+
+	h.closedChannels[chanID] = height
+	delete(h.channels, chanID)
+
+	chanPointHash, err := chainhash.NewHash(chanID[:])
+	require.NoError(h.t, err)
+
+	if !h.sendUpdatesOn {
+		return
+	}
+
+	h.channelEvents.sendUpdate(channelnotifier.ClosedChannelEvent{
+		CloseSummary: &channeldb.ChannelCloseSummary{
+			ChanPoint: wire.OutPoint{
+				Hash:  *chanPointHash,
+				Index: 0,
+			},
+			CloseHeight: height,
+		},
+	})
+}
+
 // registerChannel registers the channel identified by id with the client.
 func (h *testHarness) registerChannel(id uint64) {
 	h.t.Helper()
@@ -624,7 +685,7 @@ func (h *testHarness) backupState(id, i uint64, expErr error) {
 	err := h.client.BackupState(
 		&chanID, retribution, channeldb.SingleFunderBit,
 	)
-	require.ErrorIs(h.t, expErr, err)
+	require.ErrorIs(h.t, err, expErr)
 }
 
 // sendPayments instructs the channel identified by id to send amt to the remote
@@ -770,11 +831,94 @@ func (h *testHarness) removeTower(pubKey *btcec.PublicKey, addr net.Addr) {
 	require.NoError(h.t, err)
 }
 
+// relevantSessions returns a list of session IDs that have acked updates for
+// the given channel ID.
+func (h *testHarness) relevantSessions(chanID uint64) []wtdb.SessionID {
+	h.t.Helper()
+
+	var (
+		sessionIDs []wtdb.SessionID
+		cID        = chanIDFromInt(chanID)
+	)
+
+	collectSessions := wtdb.WithPerNumAckedUpdates(
+		func(session *wtdb.ClientSession, id lnwire.ChannelID,
+			_ uint16) {
+
+			if !bytes.Equal(id[:], cID[:]) {
+				return
+			}
+
+			sessionIDs = append(sessionIDs, session.ID)
+		},
+	)
+
+	_, err := h.clientDB.ListClientSessions(nil, nil, collectSessions)
+	require.NoError(h.t, err)
+
+	return sessionIDs
+}
+
+// isSessionClosable returns true if the given session has been marked as
+// closable in the DB.
+func (h *testHarness) isSessionClosable(id wtdb.SessionID) bool {
+	h.t.Helper()
+
+	cs, err := h.clientDB.ListClosableSessions()
+	require.NoError(h.t, err)
+
+	_, ok := cs[id]
+
+	return ok
+}
+
+// mockSubscription is a mock subscription client that blocks on sends into the
+// updates channel.
+type mockSubscription struct {
+	t       *testing.T
+	updates chan interface{}
+
+	// Embed the subscription interface in this mock so that we satisfy it.
+	subscribe.Subscription
+}
+
+// newMockSubscription creates a mock subscription.
+func newMockSubscription(t *testing.T) *mockSubscription {
+	t.Helper()
+
+	return &mockSubscription{
+		t:       t,
+		updates: make(chan interface{}),
+	}
+}
+
+// sendUpdate sends an update into our updates channel, mocking the dispatch of
+// an update from a subscription server. This call will fail the test if the
+// update is not consumed within our timeout.
+func (m *mockSubscription) sendUpdate(update interface{}) {
+	select {
+	case m.updates <- update:
+
+	case <-time.After(waitTime):
+		m.t.Fatalf("update: %v timeout", update)
+	}
+}
+
+// Updates returns the updates channel for the mock.
+func (m *mockSubscription) Updates() <-chan interface{} {
+	return m.updates
+}
+
 const (
 	localBalance  = lnwire.MilliSatoshi(100000000)
 	remoteBalance = lnwire.MilliSatoshi(200000000)
 )
 
+var defaultTxPolicy = wtpolicy.TxPolicy{
+	BlobType:     blob.TypeAltruistCommit,
+	SweepFeeRate: wtpolicy.DefaultSweepFeeRate,
+}
+
 type clientTest struct {
 	name string
 	cfg  harnessCfg
@@ -791,10 +935,7 @@ var clientTests = []clientTest{
 			localBalance:  localBalance,
 			remoteBalance: remoteBalance,
 			policy: wtpolicy.Policy{
-				TxPolicy: wtpolicy.TxPolicy{
-					BlobType:     blob.TypeAltruistCommit,
-					SweepFeeRate: wtpolicy.DefaultSweepFeeRate,
-				},
+				TxPolicy:   defaultTxPolicy,
 				MaxUpdates: 20000,
 			},
 			noRegisterChan0: true,
@@ -825,10 +966,7 @@ var clientTests = []clientTest{
 			localBalance:  localBalance,
 			remoteBalance: remoteBalance,
 			policy: wtpolicy.Policy{
-				TxPolicy: wtpolicy.TxPolicy{
-					BlobType:     blob.TypeAltruistCommit,
-					SweepFeeRate: wtpolicy.DefaultSweepFeeRate,
-				},
+				TxPolicy:   defaultTxPolicy,
 				MaxUpdates: 20000,
 			},
 		},
@@ -860,10 +998,7 @@ var clientTests = []clientTest{
 			localBalance:  localBalance,
 			remoteBalance: remoteBalance,
 			policy: wtpolicy.Policy{
-				TxPolicy: wtpolicy.TxPolicy{
-					BlobType:     blob.TypeAltruistCommit,
-					SweepFeeRate: wtpolicy.DefaultSweepFeeRate,
-				},
+				TxPolicy:   defaultTxPolicy,
 				MaxUpdates: 5,
 			},
 		},
@@ -927,10 +1062,7 @@ var clientTests = []clientTest{
 			localBalance:  localBalance,
 			remoteBalance: remoteBalance,
 			policy: wtpolicy.Policy{
-				TxPolicy: wtpolicy.TxPolicy{
-					BlobType:     blob.TypeAltruistCommit,
-					SweepFeeRate: wtpolicy.DefaultSweepFeeRate,
-				},
+				TxPolicy:   defaultTxPolicy,
 				MaxUpdates: 20000,
 			},
 		},
@@ -1006,10 +1138,7 @@ var clientTests = []clientTest{
 			localBalance:  localBalance,
 			remoteBalance: remoteBalance,
 			policy: wtpolicy.Policy{
-				TxPolicy: wtpolicy.TxPolicy{
-					BlobType:     blob.TypeAltruistCommit,
-					SweepFeeRate: wtpolicy.DefaultSweepFeeRate,
-				},
+				TxPolicy:   defaultTxPolicy,
 				MaxUpdates: 5,
 			},
 		},
@@ -1062,10 +1191,7 @@ var clientTests = []clientTest{
 			localBalance:  100000001, // ensure (% amt != 0)
 			remoteBalance: 200000001, // ensure (% amt != 0)
 			policy: wtpolicy.Policy{
-				TxPolicy: wtpolicy.TxPolicy{
-					BlobType:     blob.TypeAltruistCommit,
-					SweepFeeRate: wtpolicy.DefaultSweepFeeRate,
-				},
+				TxPolicy:   defaultTxPolicy,
 				MaxUpdates: 1000,
 			},
 		},
@@ -1106,10 +1232,7 @@ var clientTests = []clientTest{
 			localBalance:  localBalance,
 			remoteBalance: remoteBalance,
 			policy: wtpolicy.Policy{
-				TxPolicy: wtpolicy.TxPolicy{
-					BlobType:     blob.TypeAltruistCommit,
-					SweepFeeRate: wtpolicy.DefaultSweepFeeRate,
-				},
+				TxPolicy:   defaultTxPolicy,
 				MaxUpdates: 5,
 			},
 		},
@@ -1156,10 +1279,7 @@ var clientTests = []clientTest{
 			localBalance:  localBalance,
 			remoteBalance: remoteBalance,
 			policy: wtpolicy.Policy{
-				TxPolicy: wtpolicy.TxPolicy{
-					BlobType:     blob.TypeAltruistCommit,
-					SweepFeeRate: wtpolicy.DefaultSweepFeeRate,
-				},
+				TxPolicy:   defaultTxPolicy,
 				MaxUpdates: 5,
 			},
 			noAckCreateSession: true,
@@ -1212,10 +1332,7 @@ var clientTests = []clientTest{
 			localBalance:  localBalance,
 			remoteBalance: remoteBalance,
 			policy: wtpolicy.Policy{
-				TxPolicy: wtpolicy.TxPolicy{
-					BlobType:     blob.TypeAltruistCommit,
-					SweepFeeRate: wtpolicy.DefaultSweepFeeRate,
-				},
+				TxPolicy:   defaultTxPolicy,
 				MaxUpdates: 5,
 			},
 			noAckCreateSession: true,
@@ -1274,10 +1391,7 @@ var clientTests = []clientTest{
 			localBalance:  localBalance,
 			remoteBalance: remoteBalance,
 			policy: wtpolicy.Policy{
-				TxPolicy: wtpolicy.TxPolicy{
-					BlobType:     blob.TypeAltruistCommit,
-					SweepFeeRate: wtpolicy.DefaultSweepFeeRate,
-				},
+				TxPolicy:   defaultTxPolicy,
 				MaxUpdates: 10,
 			},
 		},
@@ -1333,10 +1447,7 @@ var clientTests = []clientTest{
 			localBalance:  localBalance,
 			remoteBalance: remoteBalance,
 			policy: wtpolicy.Policy{
-				TxPolicy: wtpolicy.TxPolicy{
-					BlobType:     blob.TypeAltruistCommit,
-					SweepFeeRate: wtpolicy.DefaultSweepFeeRate,
-				},
+				TxPolicy:   defaultTxPolicy,
 				MaxUpdates: 5,
 			},
 		},
@@ -1381,10 +1492,7 @@ var clientTests = []clientTest{
 			localBalance:  localBalance,
 			remoteBalance: remoteBalance,
 			policy: wtpolicy.Policy{
-				TxPolicy: wtpolicy.TxPolicy{
-					BlobType:     blob.TypeAltruistCommit,
-					SweepFeeRate: wtpolicy.DefaultSweepFeeRate,
-				},
+				TxPolicy:   defaultTxPolicy,
 				MaxUpdates: 5,
 			},
 		},
@@ -1489,10 +1597,7 @@ var clientTests = []clientTest{
 			localBalance:  localBalance,
 			remoteBalance: remoteBalance,
 			policy: wtpolicy.Policy{
-				TxPolicy: wtpolicy.TxPolicy{
-					BlobType:     blob.TypeAltruistCommit,
-					SweepFeeRate: wtpolicy.DefaultSweepFeeRate,
-				},
+				TxPolicy:   defaultTxPolicy,
 				MaxUpdates: 5,
 			},
 		},
@@ -1557,10 +1662,7 @@ var clientTests = []clientTest{
 			localBalance:  localBalance,
 			remoteBalance: remoteBalance,
 			policy: wtpolicy.Policy{
-				TxPolicy: wtpolicy.TxPolicy{
-					BlobType:     blob.TypeAltruistCommit,
-					SweepFeeRate: wtpolicy.DefaultSweepFeeRate,
-				},
+				TxPolicy:   defaultTxPolicy,
 				MaxUpdates: 5,
 			},
 			noServerStart: true,
@@ -1654,6 +1756,142 @@ var clientTests = []clientTest{
 			}, waitTime)
 			require.NoError(h.t, err)
 		},
+	}, {
+		name: "assert that sessions are correctly marked as closable",
+		cfg: harnessCfg{
+			localBalance:  localBalance,
+			remoteBalance: remoteBalance,
+			policy: wtpolicy.Policy{
+				TxPolicy:   defaultTxPolicy,
+				MaxUpdates: 5,
+			},
+		},
+		fn: func(h *testHarness) {
+			const numUpdates = 5
+
+			// In this test we assert that a channel is correctly
+			// marked as closed and that sessions are also correctly
+			// marked as closable.
+
+			// We start with the sendUpdatesOn parameter set to
+			// false so that we can test that channels are correctly
+			// evaluated at startup.
+			h.sendUpdatesOn = false
+
+			// Advance channel 0 to create all states and back them
+			// all up. This will saturate the session with updates
+			// for channel 0 which means that the session should be
+			// considered closable when channel 0 is closed.
+			hints := h.advanceChannelN(0, numUpdates)
+			h.backupStates(0, 0, numUpdates, nil)
+			h.waitServerUpdates(hints, waitTime)
+
+			// We expect only 1 session to have updates for this
+			// channel.
+			sessionIDs := h.relevantSessions(0)
+			require.Len(h.t, sessionIDs, 1)
+
+			// Since channel 0 is still open, the session should not
+			// yet be closable.
+			require.False(h.t, h.isSessionClosable(sessionIDs[0]))
+
+			// Close the channel.
+			h.closeChannel(0, 1)
+
+			// Since updates are currently not being sent, we expect
+			// the session to still not be marked as closable.
+			require.False(h.t, h.isSessionClosable(sessionIDs[0]))
+
+			// Restart the client.
+			h.client.ForceQuit()
+			h.startClient()
+
+			// The session should now have been marked as closable.
+			err := wait.Predicate(func() bool {
+				return h.isSessionClosable(sessionIDs[0])
+			}, waitTime)
+			require.NoError(h.t, err)
+
+			// Now we set sendUpdatesOn to true and do the same with
+			// a new channel. A restart should now not be necessary
+			// anymore.
+			h.sendUpdatesOn = true
+
+			h.makeChannel(
+				1, h.cfg.localBalance, h.cfg.remoteBalance,
+			)
+			h.registerChannel(1)
+
+			hints = h.advanceChannelN(1, numUpdates)
+			h.backupStates(1, 0, numUpdates, nil)
+			h.waitServerUpdates(hints, waitTime)
+
+			// Determine the ID of the session of interest.
+			sessionIDs = h.relevantSessions(1)
+
+			// We expect only 1 session to have updates for this
+			// channel.
+			require.Len(h.t, sessionIDs, 1)
+
+			// Assert that the session is not yet closable since
+			// the channel is still open.
+			require.False(h.t, h.isSessionClosable(sessionIDs[0]))
+
+			// Now close the channel.
+			h.closeChannel(1, 1)
+
+			// Since the updates have been turned on, the session
+			// should now show up as closable.
+			err = wait.Predicate(func() bool {
+				return h.isSessionClosable(sessionIDs[0])
+			}, waitTime)
+			require.NoError(h.t, err)
+
+			// Now we test that a session must be exhausted with all
+			// channels closed before it is seen as closable.
+			h.makeChannel(
+				2, h.cfg.localBalance, h.cfg.remoteBalance,
+			)
+			h.registerChannel(2)
+
+			// Fill up only half of the session updates.
+			hints = h.advanceChannelN(2, numUpdates)
+			h.backupStates(2, 0, numUpdates/2, nil)
+			h.waitServerUpdates(hints[:numUpdates/2], waitTime)
+
+			// Determine the ID of the session of interest.
+			sessionIDs = h.relevantSessions(2)
+
+			// We expect only 1 session to have updates for this
+			// channel.
+			require.Len(h.t, sessionIDs, 1)
+
+			// Now close the channel.
+			h.closeChannel(2, 1)
+
+			// The session should _not_ be closable due to it not
+			// being exhausted yet.
+			require.False(h.t, h.isSessionClosable(sessionIDs[0]))
+
+			// Create a new channel.
+			h.makeChannel(
+				3, h.cfg.localBalance, h.cfg.remoteBalance,
+			)
+			h.registerChannel(3)
+
+			hints = h.advanceChannelN(3, numUpdates)
+			h.backupStates(3, 0, numUpdates, nil)
+			h.waitServerUpdates(hints, waitTime)
+
+			// Close it.
+			h.closeChannel(3, 1)
+
+			// Now the session should be closable.
+			err = wait.Predicate(func() bool {
+				return h.isSessionClosable(sessionIDs[0])
+			}, waitTime)
+			require.NoError(h.t, err)
+		},
 	},
 }