funding: use p2tr by default for batch_open_channel

In LND v0.15.1, batch_open_channel used p2tr by default
for change output. However, in a later version, a breaking
change has been fixed in FundPSBT to make the change type
configurable (default to p2wkh). As batch_open_channel
uses FundPsbt, we need to put back p2tr as default for
change output

funding/batch.go

@@ -320,6 +320,7 @@ func (b *Batcher) BatchFund(ctx context.Context,
 	// anyway.
 	firstReq := b.channels[0].fundingReq
 	feeRateSatPerKVByte := firstReq.FundingFeePerKw.FeePerKVByte()
+	changeType := walletrpc.ChangeAddressType_CHANGE_ADDRESS_TYPE_P2TR
 	fundPsbtReq := &walletrpc.FundPsbtRequest{
 		Template: &walletrpc.FundPsbtRequest_Raw{
 			Raw: txTemplate,
@@ -329,6 +330,7 @@ func (b *Batcher) BatchFund(ctx context.Context,
 		},
 		MinConfs:         firstReq.MinConfs,
 		SpendUnconfirmed: firstReq.MinConfs == 0,
+		ChangeType:       changeType,
 	}
 	fundPsbtResp, err := b.cfg.WalletKitServer.FundPsbt(ctx, fundPsbtReq)
 	if err != nil {

itest/lnd_funding_test.go

@@ -7,6 +7,7 @@ import (
 
 	"github.com/btcsuite/btcd/btcutil"
 	"github.com/btcsuite/btcd/chaincfg/chainhash"
+	"github.com/btcsuite/btcd/txscript"
 	"github.com/btcsuite/btcd/wire"
 	"github.com/lightningnetwork/lnd/funding"
 	"github.com/lightningnetwork/lnd/input"
@@ -777,6 +778,21 @@ func testBatchChanFunding(ht *lntest.HarnessTest) {
 	ht.AssertTopologyChannelOpen(alice, chanPoint2)
 	ht.AssertTopologyChannelOpen(alice, chanPoint3)
 
+	// Check if the change type from the batch_open_channel funding is P2TR.
+	rawTx := ht.Miner.GetRawTransaction(txHash)
+	require.Len(ht, rawTx.MsgTx().TxOut, 4)
+
+	// For calculating the change output index we use the formula for the
+	// sum of consecutive of integers (n(n+1)/2). All the channel point
+	// indexes are known, so we just calculate the difference to get the
+	// change output index.
+	changeIndex := uint32(6) - (chanPoint1.OutputIndex +
+		chanPoint2.OutputIndex + chanPoint3.OutputIndex)
+
+	ht.AssertOutputScriptClass(
+		rawTx, changeIndex, txscript.WitnessV1TaprootTy,
+	)
+
 	// With the channel open, ensure that it is counted towards Alice's
 	// total channel balance.
 	balRes := alice.RPC.ChannelBalance()

lntest/harness_assertion.go

@@ -377,6 +377,20 @@ func (h *HarnessTest) AssertChannelExists(hn *node.HarnessNode,
 	return channel
 }
 
+// AssertOutputScriptClass checks that the specified transaction output has the
+// expected script class.
+func (h *HarnessTest) AssertOutputScriptClass(tx *btcutil.Tx,
+	outputIndex uint32, scriptClass txscript.ScriptClass) {
+
+	require.Greater(h, len(tx.MsgTx().TxOut), int(outputIndex))
+
+	txOut := tx.MsgTx().TxOut[outputIndex]
+
+	pkScript, err := txscript.ParsePkScript(txOut.PkScript)
+	require.NoError(h, err)
+	require.Equal(h, pkScript.Class(), scriptClass)
+}
+
 // findChannel tries to find a target channel in the node using the given
 // channel point.
 func (h *HarnessTest) findChannel(hn *node.HarnessNode,