discovery+lnwallet: add fetchPKScript helper to gossiper

This commit makes an lnwallet.BlockChainIO available to the gossiper and
uses it to construct a helper that can be used to fetch the pk script
for a given SCID. This will be used for channel announcement
verification in an upcoming commit.

discovery/gossiper.go

@@ -171,6 +171,10 @@ type Config struct {
 	// order to be included in the LN graph.
 	Graph graph.ChannelGraphSource
 
+	// ChainIO represents an abstraction over a source that can query the
+	// blockchain.
+	ChainIO lnwallet.BlockChainIO
+
 	// ChanSeries is an interfaces that provides access to a time series
 	// view of the current known channel graph. Each GossipSyncer enabled
 	// peer will utilize this in order to create and respond to channel
@@ -1930,6 +1934,13 @@ func (d *AuthenticatedGossiper) processRejectedEdge(
 	return announcements, nil
 }
 
+// fetchPKScript fetches the output script for the given SCID.
+func (d *AuthenticatedGossiper) fetchPKScript(chanID *lnwire.ShortChannelID) (
+	[]byte, error) {
+
+	return lnwallet.FetchPKScriptWithQuit(d.cfg.ChainIO, chanID, d.quit)
+}
+
 // addNode processes the given node announcement, and adds it to our channel
 // graph.
 func (d *AuthenticatedGossiper) addNode(msg *lnwire.NodeAnnouncement,

lnwallet/interface.go

@@ -21,6 +21,7 @@ import (
 	"github.com/lightningnetwork/lnd/fn"
 	"github.com/lightningnetwork/lnd/keychain"
 	"github.com/lightningnetwork/lnd/lnwallet/chainfee"
+	"github.com/lightningnetwork/lnd/lnwallet/chanvalidate"
 	"github.com/lightningnetwork/lnd/lnwire"
 )
 
@@ -737,3 +738,26 @@ func FetchFundingTx(chain BlockChainIO,
 
 	return fundingBlock.Transactions[chanID.TxIndex].Copy(), nil
 }
+
+// FetchPKScriptWithQuit fetches the output script for the given SCID and exits
+// early with an error if the provided quit channel is closed before
+// completion.
+func FetchPKScriptWithQuit(chain BlockChainIO, chanID *lnwire.ShortChannelID,
+	quit chan struct{}) ([]byte, error) {
+
+	tx, err := FetchFundingTxWrapper(chain, chanID, quit)
+	if err != nil {
+		return nil, err
+	}
+
+	outputLocator := chanvalidate.ShortChanIDChanLocator{
+		ID: *chanID,
+	}
+
+	output, _, err := outputLocator.Locate(tx)
+	if err != nil {
+		return nil, err
+	}
+
+	return output.PkScript, nil
+}

server.go

@@ -1042,6 +1042,7 @@ func newServer(cfg *Config, listenAddrs []net.Addr,
 
 	s.authGossiper = discovery.New(discovery.Config{
 		Graph:                 s.graphBuilder,
+		ChainIO:               s.cc.ChainIO,
 		Notifier:              s.cc.ChainNotifier,
 		ChainHash:             *s.cfg.ActiveNetParams.GenesisHash,
 		Broadcast:             s.BroadcastMessage,