rpc: ensure we don't leak unadvertised nodes within invoice routing hints

In this commit, we ensure that we don't include routing hints for
unadvertised nodes at the time of invoice creation. Otherwise, this
would lead us to leak these unadvertised nodes to anyone who can get
their hands on the invoice being created. To prevent this, we'll now
look at the network graph and ensure that the node in unadvertised if
all of their edges are unadvertised and only extend to us.

rpcserver.go

@@ -2759,12 +2759,34 @@ func (r *rpcServer) AddInvoice(ctx context.Context,
 			}
 
 			if !link.EligibleToForward() {
-				rpcsLog.Debugf("Skipping link %v due to not "+
+				rpcsLog.Debugf("Skipping channel %v due to not "+
 					"being eligible to forward payments",
 					chanPoint)
 				continue
 			}
 
+			// To ensure we don't leak unadvertised nodes, we'll
+			// make sure our counterparty is publicly advertised
+			// within the network. Otherwise, we'll end up leaking
+			// information about nodes that intend to stay
+			// unadvertised, like in the case of a node only having
+			// private channels.
+			var remotePub [33]byte
+			copy(remotePub[:], channel.IdentityPub.SerializeCompressed())
+			isRemoteNodePublic, err := graph.IsPublicNode(remotePub)
+			if err != nil {
+				rpcsLog.Errorf("Unable to determine if node %x "+
+					"is advertised: %v", remotePub, err)
+				continue
+			}
+
+			if !isRemoteNodePublic {
+				rpcsLog.Debugf("Skipping channel %v due to "+
+					"counterparty %x being unadvertised",
+					chanPoint, remotePub)
+				continue
+			}
+
 			// Fetch the policies for each end of the channel.
 			chanID := channel.ShortChanID().ToUint64()
 			info, p1, p2, err := graph.FetchChannelEdgesByID(chanID)
@@ -2778,8 +2800,7 @@ func (r *rpcServer) AddInvoice(ctx context.Context,
 			// Now, we'll need to determine which is the correct
 			// policy for HTLCs being sent from the remote node.
 			var remotePolicy *channeldb.ChannelEdgePolicy
-			remotePub := channel.IdentityPub.SerializeCompressed()
+			if bytes.Equal(remotePub[:], info.NodeKey1Bytes[:]) {
-			if bytes.Equal(remotePub, info.NodeKey1Bytes[:]) {
 				remotePolicy = p1
 			} else {
 				remotePolicy = p2