funding: create new AuxFundingController interface

In this commit, we make a new `AuxFundingController` interface capable of processing messages off the wire. In addition, we can use it to abstract away details w.r.t how we obtain a `AuxFundingDesc` for a given channel.

We'll now use this whenever we get a channel funding request, to make sure we pass along the custom state that a channel may require.

funding/aux_funding.go

@@ -0,0 +1,51 @@
+package funding
+
+import (
+	"github.com/btcsuite/btcd/chaincfg/chainhash"
+	"github.com/lightningnetwork/lnd/fn"
+	"github.com/lightningnetwork/lnd/lntypes"
+	"github.com/lightningnetwork/lnd/lnwallet"
+	"github.com/lightningnetwork/lnd/msgmux"
+)
+
+// AuxFundingDescResult is a type alias for a function that returns an optional
+// aux funding desc.
+type AuxFundingDescResult = fn.Result[fn.Option[lnwallet.AuxFundingDesc]]
+
+// AuxTapscriptResult is a type alias for a function that returns an optional
+// tapscript root.
+type AuxTapscriptResult = fn.Result[fn.Option[chainhash.Hash]]
+
+// AuxFundingController permits the implementation of the funding of custom
+// channels types. The controller serves as a MsgEndpoint which allows it to
+// intercept custom messages, or even the regular funding messages. The
+// controller might also pass along an aux funding desc based on an existing
+// pending channel ID.
+type AuxFundingController interface {
+	// Endpoint is the embedded interface that signals that the funding
+	// controller is also a message endpoint. This'll allow it to handle
+	// custom messages specific to the funding type.
+	msgmux.Endpoint
+
+	// DescFromPendingChanID takes a pending channel ID, that may already be
+	// known due to prior custom channel messages, and maybe returns an aux
+	// funding desc which can be used to modify how a channel is funded.
+	DescFromPendingChanID(pid PendingChanID, openChan lnwallet.AuxChanState,
+		keyRing lntypes.Dual[lnwallet.CommitmentKeyRing],
+		initiator bool) AuxFundingDescResult
+
+	// DeriveTapscriptRoot takes a pending channel ID and maybe returns a
+	// tapscript root that should be used when creating any MuSig2 sessions
+	// for a channel.
+	DeriveTapscriptRoot(PendingChanID) AuxTapscriptResult
+
+	// ChannelReady is called when a channel has been fully opened (multiple
+	// confirmations) and is ready to be used. This can be used to perform
+	// any final setup or cleanup.
+	ChannelReady(openChan lnwallet.AuxChanState) error
+
+	// ChannelFinalized is called when a channel has been fully finalized.
+	// In this state, we've received the commitment sig from the remote
+	// party, so we are safe to broadcast the funding transaction.
+	ChannelFinalized(PendingChanID) error
+}

funding/manager.go

@@ -549,6 +549,12 @@ type Config struct {
 	// AuxLeafStore is an optional store that can be used to store auxiliary
 	// leaves for certain custom channel types.
 	AuxLeafStore fn.Option[lnwallet.AuxLeafStore]
+
+	// AuxFundingController is an optional controller that can be used to
+	// modify the way we handle certain custom channel types. It's also
+	// able to automatically handle new custom protocol messages related to
+	// the funding process.
+	AuxFundingController fn.Option[AuxFundingController]
 }
 
 // Manager acts as an orchestrator/bridge between the wallet's
@@ -1626,6 +1632,23 @@ func (f *Manager) fundeeProcessOpenChannel(peer lnpeer.Peer,
 		return
 	}
 
+	// At this point, if we have an AuxFundingController active, we'll
+	// check to see if we have a special tapscript root to use in our
+	// MuSig funding output.
+	tapscriptRoot, err := fn.MapOptionZ(
+		f.cfg.AuxFundingController,
+		func(c AuxFundingController) AuxTapscriptResult {
+			return c.DeriveTapscriptRoot(msg.PendingChannelID)
+		},
+	).Unpack()
+	if err != nil {
+		err = fmt.Errorf("error deriving tapscript root: %w", err)
+		log.Error(err)
+		f.failFundingFlow(peer, cid, err)
+
+		return
+	}
+
 	req := &lnwallet.InitFundingReserveMsg{
 		ChainHash:        &msg.ChainHash,
 		PendingChanID:    msg.PendingChannelID,
@@ -1642,6 +1665,7 @@ func (f *Manager) fundeeProcessOpenChannel(peer lnpeer.Peer,
 		ZeroConf:         zeroConf,
 		OptionScidAlias:  scid,
 		ScidAliasFeature: scidFeatureVal,
+		TapscriptRoot:    tapscriptRoot,
 	}
 
 	reservation, err := f.cfg.Wallet.InitChannelReservation(req)
@@ -4634,6 +4658,23 @@ func (f *Manager) handleInitFundingMsg(msg *InitFundingMsg) {
 		scidFeatureVal = true
 	}
 
+	// At this point, if we have an AuxFundingController active, we'll check
+	// to see if we have a special tapscript root to use in our MuSig2
+	// funding output.
+	tapscriptRoot, err := fn.MapOptionZ(
+		f.cfg.AuxFundingController,
+		func(c AuxFundingController) AuxTapscriptResult {
+			return c.DeriveTapscriptRoot(chanID)
+		},
+	).Unpack()
+	if err != nil {
+		err = fmt.Errorf("error deriving tapscript root: %w", err)
+		log.Error(err)
+		msg.Err <- err
+
+		return
+	}
+
 	req := &lnwallet.InitFundingReserveMsg{
 		ChainHash:         &msg.ChainHash,
 		PendingChanID:     chanID,
@@ -4673,6 +4714,7 @@ func (f *Manager) handleInitFundingMsg(msg *InitFundingMsg) {
 		OptionScidAlias:  scid,
 		ScidAliasFeature: scidFeatureVal,
 		Memo:             msg.Memo,
+		TapscriptRoot:    tapscriptRoot,
 	}
 
 	reservation, err := f.cfg.Wallet.InitChannelReservation(req)