lnwallet: modify NewUnilateralCloseSummary to be aware of pending remote commits

In this commit, we modify the NewUnilateralCloseSummary to be able to
distinguish between a unilateral closure using the lowest+highest
commitment the remote party possesses. Before this commit, if the remote
party broadcast their highest commitment, when they have a lower
unrevoked commitment, then this function would fail to find the proper
output, leaving funds on the chain.

To fix this, it's now the duty of the caller to pass remotePendingCommit
with the proper value. The caller should use the lowest unrevoked
commitment, and the height hint of the broadcast commitment to discern
if this is a pending commitment or not.

lnwallet/channel.go

@@ -1234,10 +1234,10 @@ func compactLogs(ourLog, theirLog *updateLog,
 //
 // See the individual comments within the above methods for further details.
 type LightningChannel struct {
-	// signer is the main signer instances that will be responsible for
+	// Signer is the main signer instances that will be responsible for
 	// signing any HTLC and commitment transaction generated by the state
 	// machine.
-	signer Signer
+	Signer Signer
 
 	// signDesc is the primary sign descriptor that is capable of signing
 	// the commitment transaction that spends the multi-sig output.
@@ -1350,7 +1350,7 @@ func NewLightningChannel(signer Signer, pCache PreimageCache,
 	lc := &LightningChannel{
 		// TODO(roasbeef): tune num sig workers?
 		sigPool:           newSigPool(runtime.NumCPU(), signer),
-		signer:            signer,
+		Signer:            signer,
 		pCache:            pCache,
 		currentHeight:     localCommit.CommitHeight,
 		remoteCommitChain: newCommitmentChain(remoteCommit.CommitHeight),
@@ -2920,7 +2920,7 @@ func (lc *LightningChannel) SignNextCommitment() (lnwire.Sig, []lnwire.Sig, erro
 	// the new commitment transaction while we're waiting for the rest of
 	// the HTLC signatures to be processed.
 	lc.signDesc.SigHashes = txscript.NewTxSigHashes(newCommitView.txn)
-	rawSig, err := lc.signer.SignOutputRaw(newCommitView.txn, lc.signDesc)
+	rawSig, err := lc.Signer.SignOutputRaw(newCommitView.txn, lc.signDesc)
 	if err != nil {
 		close(cancelChan)
 		return sig, htlcSigs, err
@@ -4601,7 +4601,7 @@ func (lc *LightningChannel) getSignedCommitTx() (*wire.MsgTx, error) {
 	// With this, we then generate the full witness so the caller can
 	// broadcast a fully signed transaction.
 	lc.signDesc.SigHashes = txscript.NewTxSigHashes(commitTx)
-	ourSigRaw, err := lc.signer.SignOutputRaw(commitTx, lc.signDesc)
+	ourSigRaw, err := lc.Signer.SignOutputRaw(commitTx, lc.signDesc)
 	if err != nil {
 		return nil, err
 	}
@@ -4678,14 +4678,24 @@ type UnilateralCloseSummary struct {
 
 // NewUnilateralCloseSummary creates a new summary that provides the caller
 // with all the information required to claim all funds on chain in the event
-// that the remote party broadcasts their commitment.
+// that the remote party broadcasts their commitment. If the
+// remotePendingCommit value is set to true, then we'll use the next (second)
+// unrevoked commitment point to construct the summary. Otherwise, we assume
+// that the remote party broadcast the lower of their two possible commits.
 func NewUnilateralCloseSummary(chanState *channeldb.OpenChannel, signer Signer,
 	pCache PreimageCache, commitSpend *chainntnfs.SpendDetail,
-	remoteCommit channeldb.ChannelCommitment) (*UnilateralCloseSummary, error) {
+	remoteCommit channeldb.ChannelCommitment,
+	remotePendingCommit bool) (*UnilateralCloseSummary, error) {
 
 	// First, we'll generate the commitment point and the revocation point
-	// so we can re-construct the HTLC state and also our payment key.
+	// so we can re-construct the HTLC state and also our payment key. If
+	// this is the pending remote commitment, then we'll use the second
+	// unrevoked commit point in order to properly reconstruct the scripts
+	// we need to locate.
 	commitPoint := chanState.RemoteCurrentRevocation
+	if remotePendingCommit {
+		commitPoint = chanState.RemoteNextRevocation
+	}
 	keyRing := deriveCommitmentKeys(
 		commitPoint, false, &chanState.LocalChanCfg,
 		&chanState.RemoteChanCfg,
@@ -5258,7 +5268,7 @@ func (lc *LightningChannel) ForceClose() (*LocalForceCloseSummary, error) {
 
 	localCommitment := lc.channelState.LocalCommitment
 	summary, err := NewLocalForceCloseSummary(lc.channelState,
-		lc.signer, lc.pCache, commitTx, localCommitment)
+		lc.Signer, lc.pCache, commitTx, localCommitment)
 	if err != nil {
 		return nil, err
 	}
@@ -5427,7 +5437,7 @@ func (lc *LightningChannel) CreateCloseProposal(proposedFee btcutil.Amount,
 	// using the generated txid to be notified once the closure transaction
 	// has been confirmed.
 	lc.signDesc.SigHashes = txscript.NewTxSigHashes(closeTx)
-	sig, err := lc.signer.SignOutputRaw(closeTx, lc.signDesc)
+	sig, err := lc.Signer.SignOutputRaw(closeTx, lc.signDesc)
 	if err != nil {
 		return nil, nil, 0, err
 	}

lnwallet/channel_test.go

@@ -1443,13 +1443,13 @@ func TestStateUpdatePersistence(t *testing.T) {
 		t.Fatalf("unable to fetch channel: %v", err)
 	}
 	aliceChannelNew, err := NewLightningChannel(
-		aliceChannel.signer, nil, aliceChannels[0],
+		aliceChannel.Signer, nil, aliceChannels[0],
 	)
 	if err != nil {
 		t.Fatalf("unable to create new channel: %v", err)
 	}
 	bobChannelNew, err := NewLightningChannel(
-		bobChannel.signer, nil, bobChannels[0],
+		bobChannel.Signer, nil, bobChannels[0],
 	)
 	if err != nil {
 		t.Fatalf("unable to create new channel: %v", err)
@@ -2543,14 +2543,14 @@ func TestChanSyncFullySynced(t *testing.T) {
 		t.Fatalf("unable to fetch channel: %v", err)
 	}
 	aliceChannelNew, err := NewLightningChannel(
-		aliceChannel.signer, nil, aliceChannels[0],
+		aliceChannel.Signer, nil, aliceChannels[0],
 	)
 	if err != nil {
 		t.Fatalf("unable to create new channel: %v", err)
 	}
 	defer aliceChannelNew.Stop()
 	bobChannelNew, err := NewLightningChannel(
-		bobChannel.signer, nil, bobChannels[0],
+		bobChannel.Signer, nil, bobChannels[0],
 	)
 	if err != nil {
 		t.Fatalf("unable to create new channel: %v", err)
@@ -2573,7 +2573,7 @@ func restartChannel(channelOld *LightningChannel) (*LightningChannel, error) {
 	}
 
 	channelNew, err := NewLightningChannel(
-		channelOld.signer, channelOld.pCache, nodeChannels[0],
+		channelOld.Signer, channelOld.pCache, nodeChannels[0],
 	)
 	if err != nil {
 		return nil, err
@@ -4173,8 +4173,8 @@ func TestChannelUnilateralCloseHtlcResolution(t *testing.T) {
 		SpenderTxHash: &commitTxHash,
 	}
 	aliceCloseSummary, err := NewUnilateralCloseSummary(
-		aliceChannel.channelState, aliceChannel.signer, aliceChannel.pCache,
-		spendDetail, aliceChannel.channelState.RemoteCommitment,
+		aliceChannel.channelState, aliceChannel.Signer, aliceChannel.pCache,
+		spendDetail, aliceChannel.channelState.RemoteCommitment, false,
 	)
 	if err != nil {
 		t.Fatalf("unable to create alice close summary: %v", err)
@@ -4216,7 +4216,7 @@ func TestChannelUnilateralCloseHtlcResolution(t *testing.T) {
 	// With the transaction constructed, we'll generate a witness that
 	// should be valid for it, and verify using an instance of Script.
 	sweepTx.TxIn[0].Witness, err = receiverHtlcSpendTimeout(
-		aliceChannel.signer, &outHtlcResolution.SweepSignDesc,
+		aliceChannel.Signer, &outHtlcResolution.SweepSignDesc,
 		sweepTx, int32(outHtlcResolution.Expiry),
 	)
 	if err != nil {
@@ -4250,7 +4250,7 @@ func TestChannelUnilateralCloseHtlcResolution(t *testing.T) {
 		sweepTx,
 	)
 	sweepTx.TxIn[0].Witness, err = SenderHtlcSpendRedeem(
-		aliceChannel.signer, &inHtlcResolution.SweepSignDesc,
+		aliceChannel.Signer, &inHtlcResolution.SweepSignDesc,
 		sweepTx, preimageBob[:],
 	)
 	if err != nil {

lnwallet/transactions_test.go

@@ -419,7 +419,7 @@ func TestCommitmentAndHTLCTransactions(t *testing.T) {
 	// of the dependencies.
 	channel := LightningChannel{
 		channelState:  &channelState,
-		signer:        signer,
+		Signer:        signer,
 		localChanCfg:  &channelState.LocalChanCfg,
 		remoteChanCfg: &channelState.RemoteChanCfg,
 	}