tapchannel: ensure the root commit asset has a valid witness

In this commit, we fix a very subtle issue related to split witnesses
and root assets.

Before this commit, when we went to sign a second level txn, which is a
split, we would create a virtual txn, which will commit to the _entire_
witness of the root asset in the vIn. Note that this bypasses the normal
segwit encoding logic, as that currently doesn't apply to the root asset
encoded in the split commit proof.

In the future, we may want to expand the segwit encoding semantics to
root assets after careful consideration.

In this commit, we fix a bug that would cause an invalid 2nd level sig
by ensuring that before we create the output commitments, we update the
root asset witness in each split, to the funding witness script, which
is just OP_TRUE.

Author: Roasbeef

tapchannel/commitment.go

@@ -528,6 +528,15 @@ func GenerateCommitmentAllocations(prevState *cmsg.Commitment,
 			err)
 	}
 
+	// The root asset of the split commitment will still commit to the full
+	// witness value. Therefore, we need to update the root asset witness to
+	// what it would be at broadcast time.
+	fundingWitness, err := fundingSpendWitness().Unpack()
+	if err != nil {
+		return nil, nil, fmt.Errorf("unable to make funding "+
+			"witness: %w", err)
+	}
+
 	// Prepare the output assets for each virtual packet, then create the
 	// output commitments.
 	ctx := context.Background()
@@ -537,6 +546,23 @@ func GenerateCommitmentAllocations(prevState *cmsg.Commitment,
 			return nil, nil, fmt.Errorf("unable to prepare output "+
 				"assets: %w", err)
 		}
+
+		// With the packets prepared, we'll swap in the correct witness
+		// for each of them.
+		for outIdx := range vPackets[idx].Outputs {
+			outAsset := vPackets[idx].Outputs[outIdx].Asset
+
+			// There is always only a single input, as we're
+			// sweeping a single contract w/ each vPkt.
+			const inputIndex = 0
+			err := outAsset.UpdateTxWitness(
+				inputIndex, fundingWitness,
+			)
+			if err != nil {
+				return nil, nil, fmt.Errorf("error updating "+
+					"witness: %w", err)
+			}
+		}
 	}
 
 	outCommitments, err := tapsend.CreateOutputCommitments(vPackets)