lnwallet/chanfunding: update assemblers to support musig2

In this commit, we update the set of intents and assemblers to recognize
musig2. For this change, we use a new bool, `musig2`, then use that to
determine if we need to use the new taproot funding scripts or not.

lnwallet/chanfunding/assembler.go

@@ -110,6 +110,11 @@ type Request struct {
 	// ChangeAddr is a closure that will provide the Assembler with a
 	// change address for the funding transaction if needed.
 	ChangeAddr func() (btcutil.Address, error)
+
+	// Musig2 if true, then musig2 will be used to generate the funding
+	// output. By definition, this'll also use segwit v1 (taproot) for the
+	// funding output.
+	Musig2 bool
 }
 
 // Intent is returned by an Assembler and represents the base functionality the

lnwallet/chanfunding/canned_assembler.go

@@ -35,6 +35,11 @@ type ShimIntent struct {
 	// a normal channel. Until this height, it's considered frozen, so it
 	// can only be cooperatively closed by the responding party.
 	thawHeight uint32
+
+	// musig2 determines if the funding output should use musig2 to
+	// generate an aggregate key to use as the taproot-native multi-sig
+	// output.
+	musig2 bool
 }
 
 // FundingOutput returns the witness script, and the output that creates the
@@ -48,6 +53,19 @@ func (s *ShimIntent) FundingOutput() ([]byte, *wire.TxOut, error) {
 	}
 
 	totalAmt := s.localFundingAmt + s.remoteFundingAmt
+
+	// If musig2 is active, then we'll return a single aggregated key
+	// rather than using the "existing" funding script.
+	if s.musig2 {
+		// 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),
+		)
+	}
+
 	return input.GenFundingPkScript(
 		s.localKey.PubKey.SerializeCompressed(),
 		s.remoteKey.SerializeCompressed(),
@@ -171,13 +189,20 @@ type CannedAssembler struct {
 	// a normal channel. Until this height, it's considered frozen, so it
 	// can only be cooperatively closed by the responding party.
 	thawHeight uint32
+
+	// musig2 determines if the funding output should use musig2 to
+	// generate an aggregate key to use as the taproot-native multi-sig
+	// output.
+	musig2 bool
 }
 
 // NewCannedAssembler creates a new CannedAssembler from the material required
 // to construct a funding output and channel point.
+//
+// TODO(roasbeef): pass in chan type instead?
 func NewCannedAssembler(thawHeight uint32, chanPoint wire.OutPoint,
 	fundingAmt btcutil.Amount, localKey *keychain.KeyDescriptor,
-	remoteKey *btcec.PublicKey, initiator bool) *CannedAssembler {
+	remoteKey *btcec.PublicKey, initiator, musig2 bool) *CannedAssembler {
 
 	return &CannedAssembler{
 		initiator:  initiator,
@@ -186,6 +211,7 @@ func NewCannedAssembler(thawHeight uint32, chanPoint wire.OutPoint,
 		fundingAmt: fundingAmt,
 		chanPoint:  chanPoint,
 		thawHeight: thawHeight,
+		musig2:     musig2,
 	}
 }
 
@@ -215,6 +241,7 @@ func (c *CannedAssembler) ProvisionChannel(req *Request) (Intent, error) {
 		remoteKey:  c.remoteKey,
 		chanPoint:  &c.chanPoint,
 		thawHeight: c.thawHeight,
+		musig2:     c.musig2,
 	}
 
 	if c.initiator {

lnwallet/chanfunding/psbt_assembler.go

@@ -531,6 +531,7 @@ func (p *PsbtAssembler) ProvisionChannel(req *Request) (Intent, error) {
 	intent := &PsbtIntent{
 		ShimIntent: ShimIntent{
 			localFundingAmt: p.fundingAmt,
+			musig2:          req.Musig2,
 		},
 		State:         PsbtShimRegistered,
 		BasePsbt:      p.basePsbt,

lnwallet/chanfunding/wallet_assembler.go

@@ -476,6 +476,7 @@ func (w *WalletAssembler) ProvisionChannel(r *Request) (Intent, error) {
 			ShimIntent: ShimIntent{
 				localFundingAmt:  localContributionAmt,
 				remoteFundingAmt: r.RemoteAmt,
+				musig2:           r.Musig2,
 			},
 			InputCoins: selectedCoins,
 			coinLocker: w.cfg.CoinLocker,

lnwallet/test/test_interface.go

@@ -2974,11 +2974,11 @@ func testSingleFunderExternalFundingTx(miner *rpctest.Harness,
 	thawHeight := uint32(200)
 	aliceExternalFunder := chanfunding.NewCannedAssembler(
 		thawHeight, *chanPoint, btcutil.Amount(chanAmt), &aliceFundingKey,
-		bobFundingKey.PubKey, true,
+		bobFundingKey.PubKey, true, false,
 	)
 	bobShimIntent, err := chanfunding.NewCannedAssembler(
 		thawHeight, *chanPoint, btcutil.Amount(chanAmt), &bobFundingKey,
-		aliceFundingKey.PubKey, false,
+		aliceFundingKey.PubKey, false, false,
 	).ProvisionChannel(&chanfunding.Request{
 		LocalAmt: btcutil.Amount(chanAmt),
 		MinConfs: 1,

rpcserver.go

@@ -1819,10 +1819,12 @@ func newFundingShimAssembler(chanPointShim *lnrpc.ChanPointShim, initiator bool,
 
 	// With all the parts assembled, we can now make the canned assembler
 	// to pass into the wallet.
+	//
+	// TODO(roasbeef): update to support musig2
 	return chanfunding.NewCannedAssembler(
 		chanPointShim.ThawHeight, *chanPoint,
 		btcutil.Amount(chanPointShim.Amt), &localKeyDesc,
-		remoteKey, initiator,
+		remoteKey, initiator, false,
 	), nil
 }