lnwallet/chancloser: remove raw state machine pointer, add channel interface

In this commit, we remove the raw channel state machine pointer from the
chan closer and instead replace that with an interface that captures
*just* the methods we need in order to do the co-op close dance.

This is a preparatory refactoring for some upcoming unit tests.

lnwallet/chancloser/chancloser.go

@@ -6,10 +6,12 @@ import (
 
 	"github.com/btcsuite/btcd/btcutil"
 	"github.com/btcsuite/btcd/chaincfg"
+	"github.com/btcsuite/btcd/chaincfg/chainhash"
 	"github.com/btcsuite/btcd/txscript"
 	"github.com/btcsuite/btcd/wire"
 	"github.com/davecgh/go-spew/spew"
 	"github.com/lightningnetwork/lnd/htlcswitch"
+	"github.com/lightningnetwork/lnd/input"
 	"github.com/lightningnetwork/lnd/labels"
 	"github.com/lightningnetwork/lnd/lnwallet"
 	"github.com/lightningnetwork/lnd/lnwallet/chainfee"
@@ -87,11 +89,55 @@ const (
 	defaultMaxFeeMultiplier = 3
 )
 
+// 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 {
+	// CalcFee returns the absolute fee for the given fee rate.
+	CalcFee(chainfee.SatPerKWeight) btcutil.Amount
+
+	// 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
+
+	// 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)
+
+	// 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) (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) (*wire.MsgTx, btcutil.Amount, error)
+}
+
 // ChanCloseCfg holds all the items that a ChanCloser requires to carry out its
 // duties.
 type ChanCloseCfg struct {
 	// Channel is the channel that should be closed.
-	Channel *lnwallet.LightningChannel
+	Channel Channel
 
 	// BroadcastTx broadcasts the passed transaction to the network.
 	BroadcastTx func(*wire.MsgTx, string) error
@@ -106,6 +152,7 @@ type ChanCloseCfg struct {
 	// MaxFee, is non-zero represents the highest fee that the initiator is
 	// willing to pay to close the channel.
 	MaxFee chainfee.SatPerKWeight
+
 	// Quit is a channel that should be sent upon in the occasion the state
 	// machine should cease all progress and shutdown.
 	Quit chan struct{}
@@ -184,10 +231,8 @@ func NewChanCloser(cfg ChanCloseCfg, deliveryScript []byte,
 	idealFeePerKw chainfee.SatPerKWeight, negotiationHeight uint32,
 	closeReq *htlcswitch.ChanClose, locallyInitiated bool) *ChanCloser {
 
-	// Given the target fee-per-kw, we'll compute what our ideal _total_ fee
+	// Given the target fee-per-kw, we'll compute what our ideal _total_
-	// will be starting at for this fee negotiation.
+	// fee will be starting at for this fee negotiation.
-	//
-	// TODO(roasbeef): should factor in minimal commit
 	idealFeeSat := cfg.Channel.CalcFee(idealFeePerKw)
 
 	// When we're the initiator, we'll want to also factor in the highest
@@ -300,9 +345,13 @@ func (c *ChanCloser) CloseRequest() *htlcswitch.ChanClose {
 	return c.closeReq
 }
 
-// Channel returns the channel stored in the config.
+// Channel returns the channel stored in the config as a
+// *lnwallet.LightningChannel.
+//
+// NOTE: This method will PANIC if the underlying channel implementation isn't
+// the desired type.
 func (c *ChanCloser) Channel() *lnwallet.LightningChannel {
-	return c.cfg.Channel
+	return c.cfg.Channel.(*lnwallet.LightningChannel)
 }
 
 // NegotiationHeight returns the negotiation height.
@@ -370,9 +419,8 @@ func (c *ChanCloser) ProcessCloseMsg(msg lnwire.Message) ([]lnwire.Message,
 		// initiator of the channel opening, then we'll deny their close
 		// attempt.
 		chanInitiator := c.cfg.Channel.IsInitiator()
-		chanState := c.cfg.Channel.State()
 		if !chanInitiator {
-			absoluteThawHeight, err := chanState.AbsoluteThawHeight()
+			absoluteThawHeight, err := c.cfg.Channel.AbsoluteThawHeight()
 			if err != nil {
 				return nil, false, err
 			}