lnwallet: add new CommitmentTypeSimpleTaproot chan type

In this commit, we add a new wallet level channel type, along with the
new fields we'll need to accept from both parties within the
contribution messages. In this case, we now have a local nonce, along
with the internal musig session.

lnwallet/reservation.go

@@ -6,6 +6,7 @@ import (
 	"sync"
 
 	"github.com/btcsuite/btcd/btcec/v2"
+	"github.com/btcsuite/btcd/btcec/v2/schnorr/musig2"
 	"github.com/btcsuite/btcd/btcutil"
 	"github.com/btcsuite/btcd/chaincfg/chainhash"
 	"github.com/btcsuite/btcd/wire"
@@ -42,6 +43,11 @@ const (
 	// guarantee that the channel initiator has no incentives to close a
 	// leased channel before its maturity date.
 	CommitmentTypeScriptEnforcedLease
+
+	// CommitmentTypeSimpleTaproot is the base commitment type for the
+	// channels that use a musig2 funding output and the tapscript tree
+	// where relevant for the commitment transaciton pk scripts.
+	CommitmentTypeSimpleTaproot
 )
 
 // HasStaticRemoteKey returns whether the commitment type supports remote
@@ -50,7 +56,8 @@ func (c CommitmentType) HasStaticRemoteKey() bool {
 	switch c {
 	case CommitmentTypeTweakless,
 		CommitmentTypeAnchorsZeroFeeHtlcTx,
-		CommitmentTypeScriptEnforcedLease:
+		CommitmentTypeScriptEnforcedLease,
+		CommitmentTypeSimpleTaproot:
 		return true
 	default:
 		return false
@@ -61,13 +68,19 @@ func (c CommitmentType) HasStaticRemoteKey() bool {
 func (c CommitmentType) HasAnchors() bool {
 	switch c {
 	case CommitmentTypeAnchorsZeroFeeHtlcTx,
-		CommitmentTypeScriptEnforcedLease:
+		CommitmentTypeScriptEnforcedLease,
+		CommitmentTypeSimpleTaproot:
 		return true
 	default:
 		return false
 	}
 }
 
+// IsTaproot returns true if the channel type is a taproot channel.
+func (c CommitmentType) IsTaproot() bool {
+	return c == CommitmentTypeSimpleTaproot
+}
+
 // String returns the name of the CommitmentType.
 func (c CommitmentType) String() string {
 	switch c {
@@ -79,6 +92,8 @@ func (c CommitmentType) String() string {
 		return "anchors-zero-fee-second-level"
 	case CommitmentTypeScriptEnforcedLease:
 		return "script-enforced-lease"
+	case CommitmentTypeSimpleTaproot:
+		return "simple-taproot"
 	default:
 		return "invalid"
 	}
@@ -117,6 +132,11 @@ type ChannelContribution struct {
 	// UpfrontShutdown is an optional address to which the channel should be
 	// paid out to on cooperative close.
 	UpfrontShutdown lnwire.DeliveryAddress
+
+	// LocalNonce is populated if the channel type is a simple taproot
+	// channel. This stores the public (and secret) nonce that will be used
+	// to generate commitments for the local party.
+	LocalNonce *musig2.Nonces
 }
 
 // toChanConfig returns the raw channel configuration generated by a node's
@@ -200,6 +220,8 @@ type ChannelReservation struct {
 	// nextRevocationKeyLoc stores the key locator information for this
 	// channel.
 	nextRevocationKeyLoc keychain.KeyLocator
+
+	musigSessions *MusigPairSession
 }
 
 // NewChannelReservation creates a new channel reservation. This function is
@@ -372,6 +394,10 @@ func NewChannelReservation(capacity, localFundingAmt btcutil.Amount,
 		chanType |= channeldb.FrozenBit
 	}
 
+	if req.CommitType == CommitmentTypeSimpleTaproot {
+		chanType |= channeldb.SimpleTaprootFeatureBit
+	}
+
 	if req.ZeroConf {
 		chanType |= channeldb.ZeroConfBit
 	}
@@ -455,6 +481,15 @@ func (r *ChannelReservation) IsZeroConf() bool {
 	return r.partialState.IsZeroConf()
 }
 
+// IsTaproot returns if the reservation's underlying partial channel state is a
+// taproot channel.
+func (r *ChannelReservation) IsTaproot() bool {
+	r.RLock()
+	defer r.RUnlock()
+
+	return r.partialState.ChanType.IsTaproot()
+}
+
 // CommitConstraints takes the constraints that the remote party specifies for
 // the type of commitments that we can generate for them. These constraints
 // include several parameters that serve as flow control restricting the amount