multi: add ability to fund+use musig2 channels that commit to a tapscript root

lnwallet/chanfunding/canned_assembler.go

@@ -5,7 +5,9 @@ import (
 
 	"github.com/btcsuite/btcd/btcec/v2"
 	"github.com/btcsuite/btcd/btcutil"
+	"github.com/btcsuite/btcd/chaincfg/chainhash"
 	"github.com/btcsuite/btcd/wire"
+	"github.com/lightningnetwork/lnd/fn"
 	"github.com/lightningnetwork/lnd/input"
 	"github.com/lightningnetwork/lnd/keychain"
 )
@@ -56,6 +58,14 @@ type ShimIntent struct {
 	// generate an aggregate key to use as the taproot-native multi-sig
 	// output.
 	musig2 bool
+
+	// tapscriptRoot is the root of the tapscript tree that will be used to
+	// create the funding output. This field will only be utilized if the
+	// musig2 flag above is set to true.
+	//
+	// TODO(roasbeef): fold above into new chan type? sum type like thing,
+	// includes the tapscript root, etc
+	tapscriptRoot fn.Option[chainhash.Hash]
 }
 
 // FundingOutput returns the witness script, and the output that creates the
@@ -73,12 +83,18 @@ func (s *ShimIntent) FundingOutput() ([]byte, *wire.TxOut, error) {
 	// If musig2 is active, then we'll return a single aggregated key
 	// rather than using the "existing" funding script.
 	if s.musig2 {
+		var scriptOpts []input.FundingScriptOpt
+		s.tapscriptRoot.WhenSome(func(root chainhash.Hash) {
+			scriptOpts = append(
+				scriptOpts, input.WithTapscriptRoot(root),
+			)
+		})
+
 		// Similar to the existing p2wsh script, we'll always ensure
 		// the keys are sorted before use.
 		return input.GenTaprootFundingScript(
-			s.localKey.PubKey,
-			s.remoteKey,
-			int64(totalAmt),
+			s.localKey.PubKey, s.remoteKey, int64(totalAmt),
+			scriptOpts...,
 		)
 	}
 

lnwallet/chanfunding/psbt_assembler.go

@@ -534,6 +534,7 @@ func (p *PsbtAssembler) ProvisionChannel(req *Request) (Intent, error) {
 		ShimIntent: ShimIntent{
 			localFundingAmt: p.fundingAmt,
 			musig2:          req.Musig2,
+			tapscriptRoot:   req.TapscriptRoot,
 		},
 		State:         PsbtShimRegistered,
 		BasePsbt:      p.basePsbt,

lnwallet/chanfunding/wallet_assembler.go

@@ -362,10 +362,10 @@ func (w *WalletAssembler) ProvisionChannel(r *Request) (Intent, error) {
 		// we will call the specialized coin selection function for
 		// that.
 		case r.FundUpToMaxAmt != 0 && r.MinFundAmt != 0:
-
-			// We need to ensure that manually selected coins, which
-			// are spent entirely on the channel funding, leave
-			// enough funds in the wallet to cover for a reserve.
+			// We need to ensure that manually selected coins,
+			// which are spent entirely on the channel funding,
+			// leave enough funds in the wallet to cover for a
+			// reserve.
 			reserve := r.WalletReserve
 			if len(manuallySelectedCoins) > 0 {
 				sumCoins := func(
@@ -386,8 +386,8 @@ func (w *WalletAssembler) ProvisionChannel(r *Request) (Intent, error) {
 
 				// If sufficient reserve funds are available we
 				// don't have to provide for it during coin
-				// selection. The manually selected coins can be
-				// spent entirely on the channel funding. If
+				// selection. The manually selected coins can
+				// be spent entirely on the channel funding. If
 				// the excess of coins cover the reserve
 				// partially then we have to provide for the
 				// rest during coin selection.
@@ -501,6 +501,7 @@ func (w *WalletAssembler) ProvisionChannel(r *Request) (Intent, error) {
 				localFundingAmt:  localContributionAmt,
 				remoteFundingAmt: r.RemoteAmt,
 				musig2:           r.Musig2,
+				tapscriptRoot:    r.TapscriptRoot,
 			},
 			InputCoins: selectedCoins,
 			coinLocker: w.cfg.CoinLocker,