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.

contractcourt/htlc_timeout_resolver.go

@@ -549,7 +549,6 @@ func (h *htlcTimeoutResolver) sweepSecondLevelTx(immediate bool) error {
 		return err
 	}
 
-	// TODO(yy): checkpoint here?
 	return err
 }
 
@@ -573,6 +572,59 @@ 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
@@ -593,8 +645,18 @@ 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)
@@ -701,6 +763,13 @@ 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.
@@ -774,6 +843,7 @@ 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),
@@ -811,6 +881,7 @@ 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,
@@ -877,9 +948,11 @@ 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
 	}
 
@@ -899,13 +972,20 @@ 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,
 	}
 }