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/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)