lnwallet: update internal co-op close flow to support musig2 keyspend

In this commit, we update the co-op close flow to support the new musig2
keyspend flow. We'll use some new functional options to allow a caller
to pass in an active musig2 session. If this is present, then we'll use
that to complete the musig2 flow by signing with a partial signature,
and then ultimately combining the signatures at the end.

lnwallet/chancloser/interface.go

@@ -0,0 +1,109 @@
+package chancloser
+
+import (
+	"github.com/btcsuite/btcd/btcec/v2/schnorr/musig2"
+	"github.com/btcsuite/btcd/btcutil"
+	"github.com/btcsuite/btcd/chaincfg/chainhash"
+	"github.com/btcsuite/btcd/wire"
+	"github.com/lightningnetwork/lnd/channeldb"
+	"github.com/lightningnetwork/lnd/input"
+	"github.com/lightningnetwork/lnd/lnwallet"
+	"github.com/lightningnetwork/lnd/lnwallet/chainfee"
+	"github.com/lightningnetwork/lnd/lnwire"
+)
+
+// CoopFeeEstimator is used to estimate the fee of a co-op close transaction.
+type CoopFeeEstimator interface {
+	// EstimateFee estimates an _absolute_ fee for a co-op close transaction
+	// given the local+remote tx outs (for the co-op close transaction),
+	// channel type, and ideal fee rate. If a passed TxOut is nil, then
+	// that indicates that an output is dust on the co-op close transaction
+	// _before_ fees are accounted for.
+	EstimateFee(chanType channeldb.ChannelType,
+		localTxOut, remoteTxOut *wire.TxOut,
+		idealFeeRate chainfee.SatPerKWeight) btcutil.Amount
+}
+
+// Channel abstracts away from the core channel state machine by exposing an
+// interface that requires only the methods we need to carry out the channel
+// closing process.
+type Channel interface {
+	// ChannelPoint returns the channel point of the target channel.
+	ChannelPoint() *wire.OutPoint
+
+	// MarkCoopBroadcasted persistently marks that the channel close
+	// transaction has been broadcast.
+	MarkCoopBroadcasted(*wire.MsgTx, bool) error
+
+	// IsInitiator returns true we are the initiator of the channel.
+	IsInitiator() bool
+
+	// ShortChanID returns the scid of the channel.
+	ShortChanID() lnwire.ShortChannelID
+
+	// ChanType returns the channel type of the channel.
+	ChanType() channeldb.ChannelType
+
+	// FundingTxOut returns the funding output of the channel.
+	FundingTxOut() *wire.TxOut
+
+	// AbsoluteThawHeight returns the absolute thaw height of the channel.
+	// If the channel is pending, or an unconfirmed zero conf channel, then
+	// an error should be returned.
+	AbsoluteThawHeight() (uint32, error)
+
+	// LocalBalanceDust returns true if when creating a co-op close
+	// transaction, the balance of the local party will be dust after
+	// accounting for any anchor outputs.
+	LocalBalanceDust() bool
+
+	// RemoteBalanceDust returns true if when creating a co-op close
+	// transaction, the balance of the remote party will be dust after
+	// accounting for any anchor outputs.
+	RemoteBalanceDust() bool
+
+	// RemoteUpfrontShutdownScript returns the upfront shutdown script of
+	// the remote party. If the remote party didn't specify such a script,
+	// an empty delivery address should be returned.
+	RemoteUpfrontShutdownScript() lnwire.DeliveryAddress
+
+	// CreateCloseProposal creates a new co-op close proposal in the form
+	// of a valid signature, the chainhash of the final txid, and our final
+	// balance in the created state.
+	CreateCloseProposal(proposedFee btcutil.Amount, localDeliveryScript []byte,
+		remoteDeliveryScript []byte,
+		closeOpt ...lnwallet.ChanCloseOpt) (input.Signature, *chainhash.Hash,
+		btcutil.Amount, error)
+
+	// CompleteCooperativeClose persistently "completes" the cooperative
+	// close by producing a fully signed co-op close transaction.
+	CompleteCooperativeClose(localSig, remoteSig input.Signature,
+		localDeliveryScript, remoteDeliveryScript []byte,
+		proposedFee btcutil.Amount, closeOpt ...lnwallet.ChanCloseOpt,
+	) (*wire.MsgTx, btcutil.Amount, error)
+}
+
+// MusigSession is an interface that abstracts away the details of the musig2
+// session details. A session is used to generate the necessary closing options
+// needed to close a channel cooperatively.
+type MusigSession interface {
+	// ProposalClosingOpts generates the set of closing options needed to
+	// generate a new musig2 proposal signature.
+	ProposalClosingOpts() ([]lnwallet.ChanCloseOpt, error)
+
+	// CombineClosingOpts returns the options that should be used when
+	// combining the final musig partial signature. The method also maps
+	// the lnwire partial signatures into an input.Signature that can be
+	// used more generally.
+	CombineClosingOpts(localSig, remoteSig lnwire.PartialSig,
+	) (input.Signature, input.Signature, []lnwallet.ChanCloseOpt, error)
+
+	// ClosingNonce generates the nonce we'll use to generate the musig2
+	// partial signatures for the co-op close transaction.
+	ClosingNonce() (*musig2.Nonces, error)
+
+	// InitRemoteNonce saves the remote nonce the party sent during their
+	// shutdown message so it can be used later to generate and verify
+	// signatures.
+	InitRemoteNonce(nonce *musig2.Nonces)
+}