lnwallet: add HTLC index to commitment sorting function

To avoid sorting issues with identical HTLCs (equal size, equal payment
hash, equal CLTV), we need to also use the HTLC index to be able to
distinguish between them.

lnwallet/commitment.go

@@ -613,7 +613,7 @@ func CommitScriptAnchors(chanType channeldb.ChannelType,
 // commitment transaction outputs. The second parameter is a list of CLTV
 // timeouts that must correspond to the number of transaction outputs, with the
 // value of 0 for non-HTLC outputs.
-type CommitSortFunc func(*wire.MsgTx, []uint32) error
+type CommitSortFunc func(*wire.MsgTx, []uint32, []input.HtlcIndex) error
 
 // CommitAuxLeaves stores two potential auxiliary leaves for the remote and
 // local output that may be used to argument the final tapscript trees of the
@@ -1003,6 +1003,7 @@ func (cb *CommitmentBuilder) createUnsignedCommitmentTx(ourBalance,
 	// commitment outputs and should correspond to zero values for the
 	// purposes of sorting.
 	cltvs := make([]uint32, len(commitTx.TxOut))
+	htlcIndexes := make([]input.HtlcIndex, len(commitTx.TxOut))
 	for _, htlc := range filteredHTLCView.OurUpdates {
 		if HtlcIsDust(
 			cb.chanState.ChanType, false, isOurs, feePerKw,
@@ -1025,7 +1026,11 @@ func (cb *CommitmentBuilder) createUnsignedCommitmentTx(ourBalance,
 		if err != nil {
 			return nil, err
 		}
-		cltvs = append(cltvs, htlc.Timeout) // nolint:makezero
+
+		// We want to add the CLTV and HTLC index to their respective
+		// slices, even if we already pre-allocated them.
+		cltvs = append(cltvs, htlc.Timeout)               //nolint
+		htlcIndexes = append(htlcIndexes, htlc.HtlcIndex) //nolint
 	}
 	for _, htlc := range filteredHTLCView.TheirUpdates {
 		if HtlcIsDust(
@@ -1049,7 +1054,11 @@ func (cb *CommitmentBuilder) createUnsignedCommitmentTx(ourBalance,
 		if err != nil {
 			return nil, err
 		}
-		cltvs = append(cltvs, htlc.Timeout) // nolint:makezero
+
+		// We want to add the CLTV and HTLC index to their respective
+		// slices, even if we already pre-allocated them.
+		cltvs = append(cltvs, htlc.Timeout)               //nolint
+		htlcIndexes = append(htlcIndexes, htlc.HtlcIndex) //nolint
 	}
 
 	// Set the state hint of the commitment transaction to facilitate
@@ -1071,7 +1080,7 @@ func (cb *CommitmentBuilder) createUnsignedCommitmentTx(ourBalance,
 				"sorting function")
 		}
 
-		err = customCommitSort(commitTx, cltvs)
+		err = customCommitSort(commitTx, cltvs, htlcIndexes)
 		if err != nil {
 			return nil, fmt.Errorf("unable to sort commitment "+
 				"transaction by custom order: %w", err)