tapchannel: add CommitmentToPackets function

This commit adds a new helper function that creates a set of virtual
packets from an aux commitment state and a set of input proofs (as well
as some negotiated values during the coop channel close flow).

The purpose of this is to use the asset allocation that has already
happened to distribute the assets of different asset ID pieces within
the given channel commitment again for the virtual packets of a coop
close transaction.

The reason we want to use the already allocated outputs vs.
re-allocating them again is to make sure that the distribution matches
exactly that of the commitment.
See explanation in next commit for more details.

Author: guggero

tapchannel/commitment.go

@@ -4,6 +4,7 @@ import (
 	"bytes"
 	"context"
 	"fmt"
+	"net/url"
 	"sort"
 
 	"github.com/btcsuite/btcd/btcec/v2/schnorr"
@@ -28,6 +29,7 @@ import (
 	"github.com/lightningnetwork/lnd/lnwallet"
 	"github.com/lightningnetwork/lnd/lnwallet/chainfee"
 	"github.com/lightningnetwork/lnd/lnwire"
+	"golang.org/x/exp/maps"
 )
 
 // DecodedDescriptor is a wrapper around a PaymentDescriptor that also includes
@@ -1505,6 +1507,131 @@ func deriveFundingScriptKey(ctx context.Context, addrBook address.Storage,
 	return fundingScriptKey, nil
 }
 
+// CommitmentToPackets converts a commitment to a list of vPackets. The
+// commitment must not contain any HTLCs, as this only works for coop-closed
+// channels.
+func CommitmentToPackets(c *cmsg.Commitment, inputs []*proof.Proof,
+	chainParams *address.ChainParams, localShutdownMsg,
+	remoteShutdownMsg cmsg.AuxShutdownMsg, localOutputIndex,
+	remoteOutputIndex uint32,
+	vPktVersion tappsbt.VPacketVersion) ([]*tappsbt.VPacket, error) {
+
+	if len(c.IncomingHtlcAssets.Val.HtlcOutputs) > 0 ||
+		len(c.OutgoingHtlcAssets.Val.HtlcOutputs) > 0 {
+
+		return nil, fmt.Errorf("commitment contains HTLCs, cannot " +
+			"create vPackets")
+	}
+
+	// We group the inputs by asset ID, so we can create a vPacket for each
+	// asset ID. The number of vPackets is dictated by the number of
+	// different asset IDs in the commitment transaction.
+	groupedInputs := tapsend.GroupProofsByAssetID(inputs)
+	vPackets := make(map[asset.ID]*tappsbt.VPacket, len(groupedInputs))
+	for assetID, proofsByID := range groupedInputs {
+		pkt, err := tappsbt.FromProofs(
+			proofsByID, chainParams, vPktVersion,
+		)
+		if err != nil {
+			return nil, fmt.Errorf("error creating vPacket: %w",
+				err)
+		}
+
+		vPackets[assetID] = pkt
+	}
+
+	localOutputs := c.LocalOutputs()
+	remoteOutputs := c.RemoteOutputs()
+
+	// We now distribute the outputs to the vPackets.
+	if len(localOutputs) > 0 {
+		for _, output := range localOutputs {
+			pkt, ok := vPackets[output.AssetID.Val]
+			if !ok {
+				return nil, fmt.Errorf("no vPacket found "+
+					"for asset ID %s", output.AssetID.Val)
+			}
+
+			outAsset := output.Proof.Val.Asset
+			vOut, err := assetToInteractiveVOutput(
+				outAsset, asset.V0, localShutdownMsg,
+				localOutputIndex,
+			)
+			if err != nil {
+				return nil, fmt.Errorf("error creating "+
+					"vOutput: %w", err)
+			}
+
+			pkt.Outputs = append(pkt.Outputs, vOut)
+		}
+	}
+
+	if len(remoteOutputs) > 0 {
+		for _, output := range remoteOutputs {
+			pkt, ok := vPackets[output.AssetID.Val]
+			if !ok {
+				return nil, fmt.Errorf("no vPacket found "+
+					"for asset ID %s", output.AssetID.Val)
+			}
+
+			outAsset := output.Proof.Val.Asset
+			vOut, err := assetToInteractiveVOutput(
+				outAsset, asset.V0, remoteShutdownMsg,
+				remoteOutputIndex,
+			)
+			if err != nil {
+				return nil, fmt.Errorf("error creating "+
+					"vOutput: %w", err)
+			}
+
+			pkt.Outputs = append(pkt.Outputs, vOut)
+		}
+	}
+
+	return maps.Values(vPackets), nil
+}
+
+// assetToInteractiveVOutput creates a VOutput for an asset that is part of an
+// interactive transaction.
+func assetToInteractiveVOutput(a asset.Asset, version asset.Version,
+	shutdownMsg cmsg.AuxShutdownMsg,
+	anchorOutputIndex uint32) (*tappsbt.VOutput, error) {
+
+	scriptKey, ok := shutdownMsg.ScriptKeys.Val[a.ID()]
+	if !ok {
+		return nil, fmt.Errorf("no script key for asset %s", a.ID())
+	}
+
+	var proofDeliveryUrl *url.URL
+	err := lfn.MapOptionZ(
+		shutdownMsg.ProofDeliveryAddr.ValOpt(), func(u []byte) error {
+			var err error
+			proofDeliveryUrl, err = url.Parse(string(u))
+			return err
+		},
+	)
+	if err != nil {
+		return nil, fmt.Errorf("unable to decode proof delivery "+
+			"address: %w", err)
+	}
+
+	return &tappsbt.VOutput{
+		Amount:       a.Amount,
+		AssetVersion: version,
+		Type: func() tappsbt.VOutputType {
+			if a.SplitCommitmentRoot == nil {
+				return tappsbt.TypeSimple
+			}
+			return tappsbt.TypeSplitRoot
+		}(),
+		Interactive:             true,
+		AnchorOutputIndex:       anchorOutputIndex,
+		AnchorOutputInternalKey: shutdownMsg.AssetInternalKey.Val,
+		ScriptKey:               asset.NewScriptKey(&scriptKey),
+		ProofDeliveryAddress:    proofDeliveryUrl,
+	}, nil
+}
+
 // InPlaceCustomCommitSort performs an in-place sort of a transaction, given a
 // list of allocations. The sort is applied to the transaction outputs, using
 // the allocation's OutputIndex. The transaction inputs are sorted by the