Merge pull request #8946 from ziggie1984/fix-bumpforceclose-behaviour

Fix bumpforceclose behavior for force closes without htlcs.

Author: ProofOfKeags

contractcourt/channel_arbitrator.go

@@ -40,6 +40,12 @@ const (
 	// arbitratorBlockBufferSize is the size of the buffer we give to each
 	// channel arbitrator.
 	arbitratorBlockBufferSize = 20
+
+	// AnchorOutputValue is the output value for the anchor output of an
+	// anchor channel.
+	// See BOLT 03 for more details:
+	// https://github.com/lightning/bolts/blob/master/03-transactions.md
+	AnchorOutputValue = btcutil.Amount(330)
 )
 
 // WitnessSubscription represents an intent to be notified once new witnesses
@@ -1313,13 +1319,14 @@ func (c *ChannelArbitrator) sweepAnchors(anchors *lnwallet.AnchorResolutions,
 		}
 
 		// If we cannot find a deadline, it means there's no HTLCs at
-		// stake, which means we can relax our anchor sweeping as we
-		// don't have any time sensitive outputs to sweep.
+		// stake, which means we can relax our anchor sweeping
+		// conditions as we don't have any time sensitive outputs to
+		// sweep. However we need to register the anchor output with the
+		// sweeper so we are later able to bump the close fee.
 		if deadline.IsNone() {
 			log.Infof("ChannelArbitrator(%v): no HTLCs at stake, "+
-				"skipped anchor CPFP", c.cfg.ChanPoint)
-
-			return nil
+				"sweeping anchor with default deadline",
+				c.cfg.ChanPoint)
 		}
 
 		witnessType := input.CommitmentAnchor
@@ -1357,10 +1364,13 @@ func (c *ChannelArbitrator) sweepAnchors(anchors *lnwallet.AnchorResolutions,
 		// Calculate the budget based on the value under protection,
 		// which is the sum of all HTLCs on this commitment subtracted
 		// by their budgets.
+		// The anchor output in itself has a small output value of 330
+		// sats so we also include it in the budget to pay for the
+		// cpfp transaction.
 		budget := calculateBudget(
 			value, c.cfg.Budget.AnchorCPFPRatio,
 			c.cfg.Budget.AnchorCPFP,
-		)
+		) + AnchorOutputValue
 
 		log.Infof("ChannelArbitrator(%v): offering anchor from %s "+
 			"commitment %v to sweeper with deadline=%v, budget=%v",

docs/release-notes/release-notes-0.18.3.md

@@ -39,6 +39,10 @@
 * The fee limit for payments [was made
   compatible](https://github.com/lightningnetwork/lnd/pull/8941) with inbound
   fees.
+  
+* [Fixed](https://github.com/lightningnetwork/lnd/pull/8946) a case where 
+bumping an anchor channel closing was not possible when no HTLCs were on the
+commitment when the channel was force closed.
 
 # New Features
 ## Functional Enhancements

itest/lnd_sweep_test.go

@@ -73,9 +73,11 @@ func testSweepCPFPAnchorOutgoingTimeout(ht *lntest.HarnessTest) {
 	htlcBudget := htlcValue.MulF64(contractcourt.DefaultBudgetRatio)
 
 	// cpfpBudget is the budget used to sweep the CPFP anchor.
+	// In addition to the htlc amount to protect we also need to include
+	// the anchor amount itself for the budget.
 	cpfpBudget := (htlcValue - htlcBudget).MulF64(
 		contractcourt.DefaultBudgetRatio,
-	)
+	) + contractcourt.AnchorOutputValue
 
 	// Create a preimage, that will be held by Carol.
 	var preimage lntypes.Preimage
@@ -488,9 +490,11 @@ func testSweepCPFPAnchorIncomingTimeout(ht *lntest.HarnessTest) {
 	htlcBudget := htlcValue.MulF64(contractcourt.DefaultBudgetRatio)
 
 	// cpfpBudget is the budget used to sweep the CPFP anchor.
+	// In addition to the htlc amount to protect we also need to include
+	// the anchor amount itself for the budget.
 	cpfpBudget := (htlcValue - htlcBudget).MulF64(
 		contractcourt.DefaultBudgetRatio,
-	)
+	) + contractcourt.AnchorOutputValue
 
 	// Carol should have one incoming HTLC on channel Bob -> Carol.
 	ht.AssertIncomingHTLCActive(carol, bcChanPoint, payHash[:])
@@ -1342,9 +1346,13 @@ func testSweepCommitOutputAndAnchor(ht *lntest.HarnessTest) {
 	// PendingChannels RPC under the waiting close section.
 	ht.AssertChannelWaitingClose(alice, chanPoint)
 
-	// We should see neither Alice or Bob has any pending sweeps as there
-	// are no time-sensitive HTLCs.
-	ht.AssertNumPendingSweeps(alice, 0)
+	// Alice should see 2 anchor sweeps for the local and remote commitment.
+	// Even without HTLCs at stake the anchors are registered with the
+	// sweeper subsytem.
+	ht.AssertNumPendingSweeps(alice, 2)
+
+	// Bob did not force close the channel therefore he should have no
+	// pending sweeps.
 	ht.AssertNumPendingSweeps(bob, 0)
 
 	// Mine a block to confirm Alice's force closing tx. Once it's