contractcourt: use the sweeper for HTLC offered remote timeout resolution

In this commit, we bring the timeout resolver more in line with the
success resolver by using the sweeper to handle the HTLC offered remote
timeout outputs. These are outputs that we can sweep directly from the
remote party's commitment transaction when they broadcast their version
of the commitment transaction.

With this change, we slim down the scope slightly by only doing this for
anchor channels. Non-anchor channels will continue to use the
utxonursery for this output type for now.

Author: Roasbeef

contractcourt/htlc_timeout_resolver.go

@@ -538,7 +538,6 @@ func (h *htlcTimeoutResolver) sweepSecondLevelTx(immediate bool) error {
 		return err
 	}
 
-	// TODO(yy): checkpoint here?
 	return err
 }
 
@@ -562,6 +561,59 @@ func (h *htlcTimeoutResolver) sendSecondLevelTxLegacy() error {
 	return h.Checkpoint(h)
 }
 
+// sweepDirectHtlcOutput sends the direct spend of the HTLC output to the
+// sweeper. This is used when the remote party goes on chain, and we're able to
+// sweep an HTLC we offered after a timeout. Only the CLTV encumbered outputs
+// are resolved via this path.
+func (h *htlcTimeoutResolver) sweepDirectHtlcOutput(immediate bool) error {
+	var htlcWitnessType input.StandardWitnessType
+	if h.isTaproot() {
+		htlcWitnessType = input.TaprootHtlcOfferedRemoteTimeout
+	} else {
+		htlcWitnessType = input.HtlcOfferedRemoteTimeout
+	}
+
+	sweepInput := input.NewCsvInputWithCltv(
+		&h.htlcResolution.ClaimOutpoint, htlcWitnessType,
+		&h.htlcResolution.SweepSignDesc, h.broadcastHeight,
+		h.htlcResolution.CsvDelay, h.htlcResolution.Expiry,
+	)
+
+	// Calculate the budget.
+	//
+	// TODO(yy): the budget is twice the output's value, which is needed as
+	// we don't force sweep the output now. To prevent cascading force
+	// closes, we use all its output value plus a wallet input as the
+	// budget. This is a temporary solution until we can optionally cancel
+	// the incoming HTLC, more details in,
+	// - https://github.com/lightningnetwork/lnd/issues/7969
+	budget := calculateBudget(
+		btcutil.Amount(sweepInput.SignDesc().Output.Value), 2, 0,
+	)
+
+	log.Infof("%T(%x): offering offered remote timeout HTLC output to "+
+		"sweeper with deadline %v and budget=%v at height=%v",
+		h, h.htlc.RHash[:], h.incomingHTLCExpiryHeight, budget,
+		h.broadcastHeight)
+
+	_, err := h.Sweeper.SweepInput(
+		sweepInput,
+		sweep.Params{
+			Budget: budget,
+
+			// This is an outgoing HTLC, so we want to make sure
+			// that we sweep it before the incoming HTLC expires.
+			DeadlineHeight: h.incomingHTLCExpiryHeight,
+			Immediate:      immediate,
+		},
+	)
+	if err != nil {
+		return err
+	}
+
+	return nil
+}
+
 // spendHtlcOutput handles the initial spend of an HTLC output via the timeout
 // clause. If this is our local commitment, the second-level timeout TX will be
 // used to spend the output into the next stage. If this is the remote
@@ -582,8 +634,18 @@ func (h *htlcTimeoutResolver) spendHtlcOutput(
 			return nil, err
 		}
 
-	// If we have no SignDetails, and we haven't already sent the output to
-	// the utxo nursery, then we'll do so now.
+	// If this is a remote commitment there's no second level timeout txn,
+	// and we can just send this directly to the sweeper.
+	case h.htlcResolution.SignedTimeoutTx == nil && !h.outputIncubating:
+		if err := h.sweepDirectHtlcOutput(immediate); err != nil {
+			log.Errorf("Sending direct spend to sweeper: %v", err)
+
+			return nil, err
+		}
+
+	// If we have a SignedTimeoutTx but no SignDetails, this is a local
+	// commitment for a non-anchor channel, so we'll send it to the utxo
+	// nursery.
 	case h.htlcResolution.SignDetails == nil && !h.outputIncubating:
 		if err := h.sendSecondLevelTxLegacy(); err != nil {
 			log.Errorf("Sending timeout tx to nursery: %v", err)
@@ -690,6 +752,13 @@ func (h *htlcTimeoutResolver) handleCommitSpend(
 	)
 
 	switch {
+
+	// If we swept an HTLC directly off the remote party's commitment
+	// transaction, then we can exit here as there's no second level sweep
+	// to do.
+	case h.htlcResolution.SignedTimeoutTx == nil:
+		break
+
 	// If the sweeper is handling the second level transaction, wait for
 	// the CSV and possible CLTV lock to expire, before sweeping the output
 	// on the second-level.
@@ -763,6 +832,7 @@ func (h *htlcTimeoutResolver) handleCommitSpend(
 			h.htlcResolution.CsvDelay,
 			uint32(commitSpend.SpendingHeight), h.htlc.RHash,
 		)
+
 		// Calculate the budget for this sweep.
 		budget := calculateBudget(
 			btcutil.Amount(inp.SignDesc().Output.Value),
@@ -800,6 +870,7 @@ func (h *htlcTimeoutResolver) handleCommitSpend(
 	case h.htlcResolution.SignedTimeoutTx != nil:
 		log.Infof("%T(%v): waiting for nursery/sweeper to spend CSV "+
 			"delayed output", h, claimOutpoint)
+
 		sweepTx, err := waitForSpend(
 			&claimOutpoint,
 			h.htlcResolution.SweepSignDesc.Output.PkScript,
@@ -866,9 +937,11 @@ func (h *htlcTimeoutResolver) IsResolved() bool {
 
 // report returns a report on the resolution state of the contract.
 func (h *htlcTimeoutResolver) report() *ContractReport {
-	// If the sign details are nil, the report will be created by handled
-	// by the nursery.
-	if h.htlcResolution.SignDetails == nil {
+	// If we have a SignedTimeoutTx but no SignDetails, this is a local
+	// commitment for a non-anchor channel, which was handled by the utxo
+	// nursery.
+	if h.htlcResolution.SignDetails == nil && h.
+		htlcResolution.SignedTimeoutTx != nil {
 		return nil
 	}
 
@@ -888,13 +961,20 @@ func (h *htlcTimeoutResolver) initReport() {
 		)
 	}
 
+	// If there's no timeout transaction, then we're already effectively in
+	// level two.
+	stage := uint32(1)
+	if h.htlcResolution.SignedTimeoutTx == nil {
+		stage = 2
+	}
+
 	h.currentReport = ContractReport{
 		Outpoint:       h.htlcResolution.ClaimOutpoint,
 		Type:           ReportOutputOutgoingHtlc,
 		Amount:         finalAmt,
 		MaturityHeight: h.htlcResolution.Expiry,
 		LimboBalance:   finalAmt,
-		Stage:          1,
+		Stage:          stage,
 	}
 }
 

contractcourt/htlc_timeout_resolver_test.go

@@ -294,12 +294,13 @@ func testHtlcTimeoutResolver(t *testing.T, testCase htlcTimeoutTestCase) {
 	chainCfg := ChannelArbitratorConfig{
 		ChainArbitratorConfig: ChainArbitratorConfig{
 			Notifier:   notifier,
+			Sweeper:    newMockSweeper(),
 			PreimageDB: witnessBeacon,
 			IncubateOutputs: func(wire.OutPoint,
 				fn.Option[lnwallet.OutgoingHtlcResolution],
 				fn.Option[lnwallet.IncomingHtlcResolution],
-				uint32, fn.Option[int32],
-			) error {
+				uint32, fn.Option[int32]) error {
+
 				incubateChan <- struct{}{}
 				return nil
 			},
@@ -311,26 +312,28 @@ func testHtlcTimeoutResolver(t *testing.T, testCase htlcTimeoutTestCase) {
 				}
 
 				resolutionChan <- msgs[0]
+
 				return nil
 			},
 			Budget: *DefaultBudgetConfig(),
 			QueryIncomingCircuit: func(circuit models.CircuitKey,
 			) *models.CircuitKey {
+
 				return nil
 			},
 		},
 		PutResolverReport: func(_ kvdb.RwTx,
-			_ *channeldb.ResolverReport,
-		) error {
+			_ *channeldb.ResolverReport) error {
+
 			return nil
 		},
 	}
 
 	cfg := ResolverConfig{
 		ChannelArbitratorConfig: chainCfg,
 		Checkpoint: func(_ ContractResolver,
-			reports ...*channeldb.ResolverReport,
-		) error {
+			reports ...*channeldb.ResolverReport) error {
+
 			checkPointChan <- struct{}{}
 
 			// Send all of our reports into the channel.
@@ -367,13 +370,15 @@ func testHtlcTimeoutResolver(t *testing.T, testCase htlcTimeoutTestCase) {
 
 		if testCase.timeout {
 			timeoutTxID := timeoutTx.TxHash()
-			reports = append(reports, &channeldb.ResolverReport{
-				OutPoint:        timeoutTx.TxIn[0].PreviousOutPoint,
+			report := &channeldb.ResolverReport{
+				OutPoint:        timeoutTx.TxIn[0].PreviousOutPoint, //nolint:lll
 				Amount:          testHtlcAmt.ToSatoshis(),
-				ResolverType:    channeldb.ResolverTypeOutgoingHtlc,
-				ResolverOutcome: channeldb.ResolverOutcomeFirstStage,
+				ResolverType:    channeldb.ResolverTypeOutgoingHtlc,  //nolint:lll
+				ResolverOutcome: channeldb.ResolverOutcomeFirstStage, //nolint:lll
 				SpendTxID:       &timeoutTxID,
-			})
+			}
+
+			reports = append(reports, report)
 		}
 	}
 
@@ -391,10 +396,21 @@ func testHtlcTimeoutResolver(t *testing.T, testCase htlcTimeoutTestCase) {
 		}
 	}()
 
-	// As the output isn't yet in the nursery, we expect that we
-	// should receive an incubation request.
+	// If this is a remote commit, then we expct the outputs should receive
+	// an incubation request to go through the sweeper, otherwise the
+	// nursery.
+	var sweepChan chan input.Input
+	if testCase.remoteCommit {
+		mockSweeper, ok := resolver.Sweeper.(*mockSweeper)
+		require.True(t, ok)
+		sweepChan = mockSweeper.sweptInputs
+	}
+
+	// The output should be offered to either the sweeper or
+	// the nursery.
 	select {
 	case <-incubateChan:
+	case <-sweepChan:
 	case err := <-resolveErr:
 		t.Fatalf("unable to resolve HTLC: %v", err)
 	case <-time.After(time.Second * 5):
@@ -450,7 +466,6 @@ func testHtlcTimeoutResolver(t *testing.T, testCase htlcTimeoutTestCase) {
 			t.Fatalf("resolution not sent")
 		}
 	} else {
-
 		// Otherwise, the HTLC should now timeout.  First, we
 		// should get a resolution message with a populated
 		// failure message.
@@ -559,10 +574,6 @@ func TestHtlcTimeoutSingleStage(t *testing.T) {
 	}
 
 	checkpoints := []checkpoint{
-		{
-			// Output should be handed off to the nursery.
-			incubating: true,
-		},
 		{
 			// We send a confirmation the sweep tx from published
 			// by the nursery.
@@ -594,7 +605,7 @@ func TestHtlcTimeoutSingleStage(t *testing.T) {
 			// After the sweep has confirmed, we expect the
 			// checkpoint to be resolved, and with the above
 			// report.
-			incubating: true,
+			incubating: false,
 			resolved:   true,
 			reports: []*channeldb.ResolverReport{
 				claim,
@@ -849,9 +860,9 @@ func TestHtlcTimeoutSingleStageRemoteSpend(t *testing.T) {
 	)
 }
 
-// TestHtlcTimeoutSecondStageRemoteSpend tests that when a remite commitment
-// confirms, and the remote spends the output using the success tx, we
-// properly detect this and extract the preimage.
+// TestHtlcTimeoutSecondStageRemoteSpend tests that when a remote commitment
+// confirms, and the remote spends the output using the success tx, we properly
+// detect this and extract the preimage.
 func TestHtlcTimeoutSecondStageRemoteSpend(t *testing.T) {
 	commitOutpoint := wire.OutPoint{Index: 2}
 
@@ -895,10 +906,6 @@ func TestHtlcTimeoutSecondStageRemoteSpend(t *testing.T) {
 	}
 
 	checkpoints := []checkpoint{
-		{
-			// Output should be handed off to the nursery.
-			incubating: true,
-		},
 		{
 			// We send a confirmation for the remote's second layer
 			// success transcation.
@@ -944,7 +951,7 @@ func TestHtlcTimeoutSecondStageRemoteSpend(t *testing.T) {
 			// After the sweep has confirmed, we expect the
 			// checkpoint to be resolved, and with the above
 			// report.
-			incubating: true,
+			incubating: false,
 			resolved:   true,
 			reports: []*channeldb.ResolverReport{
 				claim,
@@ -1321,8 +1328,8 @@ func TestHtlcTimeoutSecondStageSweeperRemoteSpend(t *testing.T) {
 }
 
 func testHtlcTimeout(t *testing.T, resolution lnwallet.OutgoingHtlcResolution,
-	checkpoints []checkpoint,
-) {
+	checkpoints []checkpoint) {
+
 	t.Helper()
 
 	defer timeout()()