tapchannel: add test to derive limits

This test attempts to find out what the maximum number of HTLC
signatures we can pack into a CommitSig message is. This number depends
on the number of different fungible asset pieces (asset IDs) we have, as
we need to provide a signature for each piece.

The resulting values are:

    aux_leaf_signer_test.go:111: Last valid commit sig msg size with: numAssetIDs=0, numHTLCs=966
    aux_leaf_signer_test.go:103: Last valid commit sig msg size with: numAssetIDs=1, numHTLCs=373
    aux_leaf_signer_test.go:103: Last valid commit sig msg size with: numAssetIDs=2, numHTLCs=231
    aux_leaf_signer_test.go:103: Last valid commit sig msg size with: numAssetIDs=3, numHTLCs=166
    aux_leaf_signer_test.go:103: Last valid commit sig msg size with: numAssetIDs=4, numHTLCs=130
    aux_leaf_signer_test.go:103: Last valid commit sig msg size with: numAssetIDs=5, numHTLCs=107
    aux_leaf_signer_test.go:103: Last valid commit sig msg size with: numAssetIDs=6, numHTLCs=91
    aux_leaf_signer_test.go:103: Last valid commit sig msg size with: numAssetIDs=7, numHTLCs=79
    aux_leaf_signer_test.go:103: Last valid commit sig msg size with: numAssetIDs=8, numHTLCs=70
    aux_leaf_signer_test.go:103: Last valid commit sig msg size with: numAssetIDs=9, numHTLCs=63
    aux_leaf_signer_test.go:103: Last valid commit sig msg size with: numAssetIDs=10, numHTLCs=57

tapchannel/aux_leaf_signer_test.go

@@ -0,0 +1,164 @@
+package tapchannel
+
+import (
+	"bytes"
+	"crypto/rand"
+	"encoding/hex"
+	"fmt"
+	"testing"
+
+	"github.com/btcsuite/btcd/btcec/v2"
+	"github.com/btcsuite/btcd/txscript"
+	"github.com/lightninglabs/taproot-assets/asset"
+	cmsg "github.com/lightninglabs/taproot-assets/tapchannelmsg"
+	"github.com/lightningnetwork/lnd/input"
+	"github.com/lightningnetwork/lnd/lnwire"
+	"github.com/lightningnetwork/lnd/tlv"
+	"github.com/stretchr/testify/require"
+)
+
+// Some of these test values and functions are from lnd's lnwire/lnwire_test.go.
+// We just need them to have some realistically sized values for the BTC-level
+// Schnorr signatures. The actual values are not important for the test.
+var (
+	testSchnorrSigStr, _ = hex.DecodeString(
+		"04e7f9037658a92afeb4f25bae5339e3ddca81a353493827d26f16d92308" +
+			"e49e2a25e92208678a2df86970da91b03a8af8815a8a60498b35" +
+			"8daf560b347aa557",
+	)
+	testSchnorrSig, _ = lnwire.NewSigFromSchnorrRawSignature(
+		testSchnorrSigStr,
+	)
+)
+
+func randPartialSigWithNonce() (*lnwire.PartialSigWithNonce, error) {
+	var sigBytes [32]byte
+	if _, err := rand.Read(sigBytes[:]); err != nil {
+		return nil, fmt.Errorf("unable to generate sig: %w", err)
+	}
+
+	var s btcec.ModNScalar
+	s.SetByteSlice(sigBytes[:])
+
+	var nonce lnwire.Musig2Nonce
+	if _, err := rand.Read(nonce[:]); err != nil {
+		return nil, fmt.Errorf("unable to generate nonce: %w", err)
+	}
+
+	return &lnwire.PartialSigWithNonce{
+		PartialSig: lnwire.NewPartialSig(s),
+		Nonce:      nonce,
+	}, nil
+}
+
+func somePartialSigWithNonce(t *testing.T) lnwire.OptPartialSigWithNonceTLV {
+	sig, err := randPartialSigWithNonce()
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	return tlv.SomeRecordT(tlv.NewRecordT[
+		lnwire.PartialSigWithNonceType,
+		lnwire.PartialSigWithNonce,
+	](*sig))
+}
+
+// TestMaxCommitSigMsgSize attempts to find values for the max number of asset
+// IDs we want to allow per channel and the resulting maximum number of HTLCs
+// that channel can allow. The maximum number of different asset IDs that can be
+// committed to a channel directly impacts the number of HTLCs that can be
+// created on that channel, because we have a limited message size to exchange
+// the second-stage HTLC signatures. The goal of this test is to find the right
+// number of asset IDs we should allow per channel to still give us a reasonable
+// amount of HTLCs.
+func TestMaxCommitSigMsgSize(t *testing.T) {
+	// This test is only relevant once, to find the values we want to use
+	// for the maximum number of asset IDs and the resulting maximum number
+	// of HTLCs. We only need to re-run this if any of the parameters
+	// change.
+	t.Skip("Test for manual execution only")
+
+	const (
+		maxNumAssetIDs = 10
+		startNumHTLCs  = 5
+		endNumHTLCs    = input.MaxHTLCNumber
+	)
+
+	var buf bytes.Buffer
+	for numID := 0; numID <= maxNumAssetIDs; numID++ {
+		for htlcs := startNumHTLCs; htlcs <= endNumHTLCs; htlcs++ {
+			buf.Reset()
+
+			msg := makeCommitSig(t, numID, htlcs)
+			err := msg.Encode(&buf, 0)
+			require.NoError(t, err)
+
+			if buf.Len() > lnwire.MaxMsgBody {
+				t.Logf("Last valid commit sig msg size with: "+
+					"numAssetIDs=%d, numHTLCs=%d",
+					numID, htlcs-1)
+
+				break
+			}
+
+			if htlcs == endNumHTLCs {
+				t.Logf("Last valid commit sig msg size with: "+
+					"numAssetIDs=%d, numHTLCs=%d",
+					numID, htlcs)
+			}
+		}
+	}
+}
+
+func makeCommitSig(t *testing.T, numAssetIDs, numHTLCs int) *lnwire.CommitSig {
+	var (
+		msg = &lnwire.CommitSig{
+			HtlcSigs: make([]lnwire.Sig, numHTLCs),
+		}
+		err error
+	)
+
+	// Static values that are always set for custom channels (which are
+	// Taproot channels, so have an all-zero legacy commit signature and a
+	// partial MuSig2 signature).
+	msg.PartialSig = somePartialSigWithNonce(t)
+	msg.CommitSig, err = lnwire.NewSigFromWireECDSA(
+		bytes.Repeat([]byte{0}, 64),
+	)
+
+	assetSigs := make([][]*cmsg.AssetSig, numHTLCs)
+	for i := range numHTLCs {
+		msg.HtlcSigs[i] = testSchnorrSig
+
+		assetSigs[i] = make([]*cmsg.AssetSig, numAssetIDs)
+		for j := range numAssetIDs {
+			var assetID asset.ID
+
+			_, err := rand.Read(assetID[:])
+			require.NoError(t, err)
+
+			assetSigs[i][j] = cmsg.NewAssetSig(
+				assetID, testSchnorrSig, txscript.SigHashAll,
+			)
+		}
+	}
+
+	if numAssetIDs == 0 {
+		return msg
+	}
+
+	commitSig := cmsg.NewCommitSig(assetSigs)
+
+	var buf bytes.Buffer
+	err = commitSig.Encode(&buf)
+	require.NoError(t, err)
+
+	msg.CustomRecords = lnwire.CustomRecords{
+		// The actual record type is not important for this test, it
+		// just needs to be in the correct range to be encoded with the
+		// correct number of bytes in the compact size encoding.
+		lnwire.MinCustomRecordsTlvType + 123: buf.Bytes(),
+	}
+
+	return msg
+}