graph+discovery: calculate funding tx script in gossiper

In preparation for an upcoming commit which will move all channel
funding tx validation to the gossiper, we first move the helper method
which helps build the expected funding transaction script based on the
fields in the channel announcement. We will still want this script later
on in the builder for updating the ChainView though, and so we pass this
field along with the ChannelEdgeInfo. With this change, we can remove
the TapscriptRoot field from the ChannelEdgeInfo since the only reason
it was there was so that the builder could reconstruct the full funding
script.

discovery/gossiper.go

@@ -23,6 +23,7 @@ import (
 	"github.com/lightningnetwork/lnd/graph"
 	graphdb "github.com/lightningnetwork/lnd/graph/db"
 	"github.com/lightningnetwork/lnd/graph/db/models"
+	"github.com/lightningnetwork/lnd/input"
 	"github.com/lightningnetwork/lnd/keychain"
 	"github.com/lightningnetwork/lnd/lnpeer"
 	"github.com/lightningnetwork/lnd/lnutils"
@@ -2619,6 +2620,7 @@ func (d *AuthenticatedGossiper) handleChanAnnouncement(nMsg *networkMsg,
 
 	// If there were any optional message fields provided, we'll include
 	// them in its serialized disk representation now.
+	var tapscriptRoot fn.Option[chainhash.Hash]
 	if nMsg.optionalMsgFields != nil {
 		if nMsg.optionalMsgFields.capacity != nil {
 			edge.Capacity = *nMsg.optionalMsgFields.capacity
@@ -2629,7 +2631,24 @@ func (d *AuthenticatedGossiper) handleChanAnnouncement(nMsg *networkMsg,
 		}
 
 		// Optional tapscript root for custom channels.
-		edge.TapscriptRoot = nMsg.optionalMsgFields.tapscriptRoot
+		tapscriptRoot = nMsg.optionalMsgFields.tapscriptRoot
+	}
+
+	// We only make use of the funding script later on during funding
+	// transaction validation if AssumeChannelValid is not true.
+	if !(d.cfg.AssumeChannelValid || d.cfg.IsAlias(scid)) {
+		fundingPkScript, err := makeFundingScript(
+			ann.BitcoinKey1[:], ann.BitcoinKey2[:], ann.Features,
+			tapscriptRoot,
+		)
+		if err != nil {
+			log.Errorf("Unable to make funding script %v", err)
+			nMsg.err <- err
+
+			return nil, false
+		}
+
+		edge.FundingScript = fn.Some(fundingPkScript)
 	}
 
 	log.Debugf("Adding edge for short_chan_id: %v", scid.ToUint64())
@@ -3598,3 +3617,57 @@ func (d *AuthenticatedGossiper) ShouldDisconnect(pubkey *btcec.PublicKey) (
 
 	return false, nil
 }
+
+// makeFundingScript is used to make the funding script for both segwit v0 and
+// segwit v1 (taproot) channels.
+func makeFundingScript(bitcoinKey1, bitcoinKey2 []byte,
+	features *lnwire.RawFeatureVector,
+	tapscriptRoot fn.Option[chainhash.Hash]) ([]byte, error) {
+
+	legacyFundingScript := func() ([]byte, error) {
+		witnessScript, err := input.GenMultiSigScript(
+			bitcoinKey1, bitcoinKey2,
+		)
+		if err != nil {
+			return nil, err
+		}
+		pkScript, err := input.WitnessScriptHash(witnessScript)
+		if err != nil {
+			return nil, err
+		}
+
+		return pkScript, nil
+	}
+
+	if features.IsEmpty() {
+		return legacyFundingScript()
+	}
+
+	chanFeatureBits := lnwire.NewFeatureVector(features, lnwire.Features)
+	if chanFeatureBits.HasFeature(
+		lnwire.SimpleTaprootChannelsOptionalStaging,
+	) {
+
+		pubKey1, err := btcec.ParsePubKey(bitcoinKey1)
+		if err != nil {
+			return nil, err
+		}
+		pubKey2, err := btcec.ParsePubKey(bitcoinKey2)
+		if err != nil {
+			return nil, err
+		}
+
+		fundingScript, _, err := input.GenTaprootFundingScript(
+			pubKey1, pubKey2, 0, tapscriptRoot,
+		)
+		if err != nil {
+			return nil, err
+		}
+
+		// TODO(roasbeef): add tapscript root to gossip v1.5
+
+		return fundingScript, nil
+	}
+
+	return legacyFundingScript()
+}