sweeper: relax anchor sweeping when there's no deadline pressure (#7965)

* sweep: use longer variable name for clarity in `addToState`

* sweeper: add more docs and debug logs

* sweep: prioritize smaller inputs when adding wallet UTXOs

This commit sorts wallet UTXOs by their values when using them for
sweeping inputs. This way we'd avoid locking large UTXOs when sweeping
inputs and also provide an opportunity to aggregate wallet UTXOs.

* contractcourt+itest: relax anchor sweeping for CPFP purpose

This commit changes from always sweeping anchor for a local force close
to only do so when there is an actual time pressure. After this change,
a forced anchor sweeping will only be attempted when the deadline is
less than 144 blocks.

* docs: update release notes

* itest: update test `testMultiHopHtlcLocalChainClaim` to skip CPFP

Since we now only perform CPFP when both the fee rate is higher and the
deadline is less than 144, we need to update the test to reflect that
Bob will not CPFP the force close tx for the channle Alice->Bob.

* itest: fix `testMultiHopRemoteForceCloseOnChainHtlcTimeout`

* itest: update related tests to reflect anchor sweeping

This commit updates all related tests to reflect the latest anchor
sweeping behavior. Previously, anchor sweeping is always attempted as
CPFP when a force close is broadcast, while now it only happens when the
deadline is less than 144. For non-CPFP purpose sweeping, it will happen
after one block is mined after the force close transaction is confirmed
as the anchor will be resent to the sweeper with a floor fee rate, hence
making it economical to sweep.

contractcourt/channel_arbitrator.go

@@ -1310,9 +1310,23 @@ func (c *ChannelArbitrator) sweepAnchors(anchors *lnwallet.AnchorResolutions,
 			return err
 		}
 
+		// Create a force flag that's used to indicate whether we
+		// should force sweeping this anchor.
+		var force bool
+
+		// Check the deadline against the default value. If it's less
+		// than the default value of 144, it means there is a deadline
+		// and we will perform a CPFP for this commitment tx.
+		if deadline < anchorSweepConfTarget {
+			// Signal that this is a force sweep, so that the
+			// anchor will be swept even if it isn't economical
+			// purely based on the anchor value.
+			force = true
+		}
+
 		log.Debugf("ChannelArbitrator(%v): pre-confirmation sweep of "+
-			"anchor of %s commit tx %v", c.cfg.ChanPoint,
-			anchorPath, anchor.CommitAnchor)
+			"anchor of %s commit tx %v, force=%v", c.cfg.ChanPoint,
+			anchorPath, anchor.CommitAnchor, force)
 
 		witnessType := input.CommitmentAnchor
 
@@ -1338,20 +1352,17 @@ func (c *ChannelArbitrator) sweepAnchors(anchors *lnwallet.AnchorResolutions,
 		)
 
 		// Sweep anchor output with a confirmation target fee
-		// preference. Because this is a cpfp-operation, the anchor will
-		// only be attempted to sweep when the current fee estimate for
-		// the confirmation target exceeds the commit fee rate.
-		//
-		// Also signal that this is a force sweep, so that the anchor
-		// will be swept even if it isn't economical purely based on the
-		// anchor value.
+		// preference. Because this is a cpfp-operation, the anchor
+		// will only be attempted to sweep when the current fee
+		// estimate for the confirmation target exceeds the commit fee
+		// rate.
 		_, err = c.cfg.Sweeper.SweepInput(
 			&anchorInput,
 			sweep.Params{
 				Fee: sweep.FeePreference{
 					ConfTarget: deadline,
 				},
-				Force:          true,
+				Force:          force,
 				ExclusiveGroup: &exclusiveGroup,
 			},
 		)
@@ -1428,6 +1439,9 @@ func (c *ChannelArbitrator) findCommitmentDeadline(heightHint uint32,
 
 		if htlc.RefundTimeout < deadlineMinHeight {
 			deadlineMinHeight = htlc.RefundTimeout
+			log.Tracef("ChannelArbitrator(%v): outgoing HTLC has "+
+				"deadline: %v", c.cfg.ChanPoint,
+				deadlineMinHeight)
 		}
 	}
 
@@ -1456,6 +1470,9 @@ func (c *ChannelArbitrator) findCommitmentDeadline(heightHint uint32,
 
 		if htlc.RefundTimeout < deadlineMinHeight {
 			deadlineMinHeight = htlc.RefundTimeout
+			log.Tracef("ChannelArbitrator(%v): incoming HTLC has "+
+				"deadline: %v", c.cfg.ChanPoint,
+				deadlineMinHeight)
 		}
 	}
 

docs/release-notes/release-notes-0.17.1.md

@@ -25,6 +25,16 @@
 
 # New Features
 ## Functional Enhancements
+
+- Previously, when a channel was force closed locally, its anchor was always
+  force-swept when the CPFP requirements are met. This is now
+  [changed](https://github.com/lightningnetwork/lnd/pull/7965) to only attempt
+  CPFP based on the deadline of the commitment transaction. The anchor output
+  will still be offered to the sweeper during channel force close, while the
+  actual sweeping won't be forced(CPFP) unless a relevant HTLC will timeout in
+  144 blocks. If CPFP before this deadline is needed, user can use `BumpFee`
+  instead.
+
 ## RPC Additions
 ## lncli Additions
 
@@ -44,5 +54,5 @@
 ## Tooling and Documentation
 
 # Contributors (Alphabetical Order)
-
-* Eugene Siegel
+* Eugene Siegel
+* Yong Yu

itest/lnd_channel_backup_test.go

@@ -783,16 +783,15 @@ func runChanRestoreScenarioForceClose(ht *lntest.HarnessTest, zeroConf bool) {
 		ht, crs.password, crs.mnemonic, multi, dave,
 	)
 
-	// We now wait until both Dave's closing tx and sweep tx have shown in
-	// mempool.
-	ht.Miner.AssertNumTxsInMempool(2)
+	// We now wait until both Dave's closing tx.
+	ht.Miner.AssertNumTxsInMempool(1)
 
 	// Now that we're able to make our restored now, we'll shutdown the old
 	// Dave node as we'll be storing it shortly below.
 	ht.Shutdown(dave)
 
 	// Mine a block to confirm the closing tx from Dave.
-	ht.MineBlocksAndAssertNumTxes(1, 2)
+	ht.MineBlocksAndAssertNumTxes(1, 1)
 
 	// To make sure the channel state is advanced correctly if the channel
 	// peer is not online at first, we also shutdown Carol.
@@ -1412,8 +1411,9 @@ func chanRestoreViaRPC(ht *lntest.HarnessTest, password []byte,
 func assertTimeLockSwept(ht *lntest.HarnessTest, carol, dave *node.HarnessNode,
 	carolStartingBalance, daveStartingBalance int64) {
 
-	// We expect Carol to sweep her funds and also the anchor tx.
-	expectedTxes := 2
+	// We expect Carol to sweep her funds and also the anchor tx. In
+	// addition, Dave will also sweep his anchor output.
+	expectedTxes := 3
 
 	// Carol should sweep her funds immediately, as they are not
 	// timelocked.
@@ -1495,11 +1495,7 @@ func assertDLPExecuted(ht *lntest.HarnessTest,
 
 	// Upon reconnection, the nodes should detect that Dave is out of sync.
 	// Carol should force close the channel using her latest commitment.
-	expectedTxes := 1
-	if lntest.CommitTypeHasAnchors(commitType) {
-		expectedTxes = 2
-	}
-	ht.Miner.AssertNumTxsInMempool(expectedTxes)
+	ht.Miner.AssertNumTxsInMempool(1)
 
 	// Channel should be in the state "waiting close" for Carol since she
 	// broadcasted the force close tx.
@@ -1515,7 +1511,8 @@ func assertDLPExecuted(ht *lntest.HarnessTest,
 	ht.RestartNode(dave)
 
 	// Generate a single block, which should confirm the closing tx.
-	ht.MineBlocksAndAssertNumTxes(1, expectedTxes)
+	ht.MineBlocksAndAssertNumTxes(1, 1)
+	blocksMined := uint32(1)
 
 	// Dave should consider the channel pending force close (since he is
 	// waiting for his sweep to confirm).
@@ -1532,14 +1529,22 @@ func assertDLPExecuted(ht *lntest.HarnessTest,
 
 		// Mine Dave's anchor sweep tx.
 		ht.MineBlocksAndAssertNumTxes(1, 1)
+		blocksMined++
+
+		// The above block will trigger Carol's sweeper to reconsider
+		// the anchor sweeping. Because we are now sweeping at the fee
+		// rate floor, the sweeper will consider this input has
+		// positive yield thus attempts the sweeping.
+		ht.MineBlocksAndAssertNumTxes(1, 1)
+		blocksMined++
 
 		// After Carol's output matures, she should also reclaim her
 		// funds.
 		//
 		// The commit sweep resolver publishes the sweep tx at
 		// defaultCSV-1 and we already mined one block after the
 		// commitmment was published, so take that into account.
-		ht.MineBlocks(defaultCSV - 1 - 1)
+		ht.MineBlocks(defaultCSV - blocksMined)
 		ht.MineBlocksAndAssertNumTxes(1, 1)
 
 		// Now the channel should be fully closed also from Carol's POV.
@@ -1561,21 +1566,33 @@ func assertDLPExecuted(ht *lntest.HarnessTest,
 		// Dave should sweep his funds immediately, as they are not
 		// timelocked. We also expect Dave to sweep his anchor, if
 		// present.
-		ht.Miner.AssertNumTxsInMempool(expectedTxes)
+		if lntest.CommitTypeHasAnchors(commitType) {
+			ht.MineBlocksAndAssertNumTxes(1, 2)
+		} else {
+			ht.MineBlocksAndAssertNumTxes(1, 1)
+		}
 
-		// Mine the sweep tx.
-		ht.MineBlocksAndAssertNumTxes(1, expectedTxes)
+		blocksMined++
 
 		// Now Dave should consider the channel fully closed.
 		ht.AssertNumPendingForceClose(dave, 0)
 
+		// The above block will trigger Carol's sweeper to reconsider
+		// the anchor sweeping. Because we are now sweeping at the fee
+		// rate floor, the sweeper will consider this input has
+		// positive yield thus attempts the sweeping.
+		if lntest.CommitTypeHasAnchors(commitType) {
+			ht.MineBlocksAndAssertNumTxes(1, 1)
+			blocksMined++
+		}
+
 		// After Carol's output matures, she should also reclaim her
 		// funds.
 		//
 		// The commit sweep resolver publishes the sweep tx at
-		// defaultCSV-1 and we already mined one block after the
+		// defaultCSV-1 and we already have blocks mined after the
 		// commitmment was published, so take that into account.
-		ht.MineBlocks(defaultCSV - 1 - 1)
+		ht.MineBlocks(defaultCSV - blocksMined)
 		ht.MineBlocksAndAssertNumTxes(1, 1)
 
 		// Now the channel should be fully closed also from Carol's

itest/lnd_channel_force_close_test.go

@@ -23,10 +23,13 @@ import (
 )
 
 // testCommitmentTransactionDeadline tests that the anchor sweep transaction is
-// taking account of the deadline of the commitment transaction. It tests two
+// taking account of the deadline of the commitment transaction. It tests three
 // scenarios:
 //  1. when the CPFP is skipped, checks that the deadline is not used.
-//  2. when the CPFP is used, checks that the deadline is applied.
+//  2. when the CPFP is used, checks that the deadline is NOT applied when it's
+//     larger than 144.
+//  3. when the CPFP is used, checks that the deadline is applied when it's
+//     less than 144.
 //
 // Note that whether the deadline is used or not is implicitly checked by its
 // corresponding fee rates.
@@ -43,13 +46,13 @@ func testCommitmentTransactionDeadline(ht *lntest.HarnessTest) {
 		// DefaultAnchorsCommitMaxFeeRateSatPerVByte.
 		feeRateDefault = 20000
 
-		// finalCTLV is used when Alice sends payment to Bob.
-		finalCTLV = 144
+		// defaultDeadline is the anchorSweepConfTarget, which is used
+		// when the commitment has no deadline pressure.
+		defaultDeadline = 144
 
-		// deadline is used when Alice sweep the anchor. Notice there
-		// is a block padding of 3 added, such that the value of
-		// deadline is 147.
-		deadline = uint32(finalCTLV + routing.BlockPadding)
+		// deadline is one block below the default deadline. A forced
+		// anchor sweep will be performed when seeing this value.
+		deadline = defaultDeadline - 1
 	)
 
 	// feeRateSmall(sat/kw) is used when we want to skip the CPFP
@@ -91,7 +94,7 @@ func testCommitmentTransactionDeadline(ht *lntest.HarnessTest) {
 
 	// calculateSweepFeeRate runs multiple steps to calculate the fee rate
 	// used in sweeping the transactions.
-	calculateSweepFeeRate := func(expectedSweepTxNum int) int64 {
+	calculateSweepFeeRate := func(expectedSweepTxNum, deadline int) int64 {
 		// Create two nodes, Alice and Bob.
 		alice := setupNode("Alice")
 		defer ht.Shutdown(alice)
@@ -110,14 +113,18 @@ func testCommitmentTransactionDeadline(ht *lntest.HarnessTest) {
 			},
 		)
 
+		// Calculate the final ctlv delta based on the expected
+		// deadline.
+		finalCltvDelta := int32(deadline - int(routing.BlockPadding))
+
 		// Send a payment with a specified finalCTLVDelta, which will
 		// be used as our deadline later on when Alice force closes the
 		// channel.
 		req := &routerrpc.SendPaymentRequest{
 			Dest:           bob.PubKey[:],
 			Amt:            10e4,
 			PaymentHash:    ht.Random32Bytes(),
-			FinalCltvDelta: finalCTLV,
+			FinalCltvDelta: finalCltvDelta,
 			TimeoutSeconds: 60,
 			FeeLimitMsat:   noFeeLimitMsat,
 		}
@@ -154,8 +161,27 @@ func testCommitmentTransactionDeadline(ht *lntest.HarnessTest) {
 	// we should see only one sweep tx in the mempool.
 	ht.SetFeeEstimateWithConf(feeRateSmall, deadline)
 
-	// Calculate fee rate used.
-	feeRate := calculateSweepFeeRate(1)
+	// Calculate fee rate used and assert only the force close tx is
+	// broadcast.
+	feeRate := calculateSweepFeeRate(1, deadline)
+
+	// We expect the default max fee rate is used. Allow some deviation
+	// because weight estimates during tx generation are estimates.
+	require.InEpsilonf(
+		ht, int64(maxPerKw), feeRate, 0.01,
+		"expected fee rate:%d, got fee rate:%d", maxPerKw, feeRate,
+	)
+
+	// Setup our fee estimation for the deadline. Because the fee rate is
+	// greater than the parent tx's fee rate, this value will be used to
+	// sweep the anchor transaction. However, due to the default value
+	// being used, we should not attempt CPFP here because we are not force
+	// sweeping the anchor output.
+	ht.SetFeeEstimateWithConf(feeRateLarge, defaultDeadline)
+
+	// Calculate fee rate used and assert only the force close tx is
+	// broadcast.
+	feeRate = calculateSweepFeeRate(1, defaultDeadline)
 
 	// We expect the default max fee rate is used. Allow some deviation
 	// because weight estimates during tx generation are estimates.
@@ -170,8 +196,9 @@ func testCommitmentTransactionDeadline(ht *lntest.HarnessTest) {
 	// transactions in the mempool.
 	ht.SetFeeEstimateWithConf(feeRateLarge, deadline)
 
-	// Calculate fee rate used.
-	feeRate = calculateSweepFeeRate(2)
+	// Calculate fee rate used and assert both the force close tx and the
+	// anchor sweeping tx are broadcast.
+	feeRate = calculateSweepFeeRate(2, deadline)
 
 	// We expect the anchor to be swept with the deadline, which has the
 	// fee rate of feeRateLarge.