routerrpc: allow dest_features graph fallback

lnrpc/routerrpc/router.pb.go

@@ -258,7 +258,9 @@ type SendPaymentRequest struct {
 	//*
 	//Features assumed to be supported by the final node. All transitive feature
 	//depdencies must also be set properly. For a given feature bit pair, either
-	//optional or remote may be set, but not both.
+	//optional or remote may be set, but not both. If this field is nil or empty,
+	//the router will try to load destination features from the graph as a
+	//fallback.
 	DestFeatures         []lnrpc.FeatureBit `protobuf:"varint,16,rep,packed,name=dest_features,json=destFeatures,proto3,enum=lnrpc.FeatureBit" json:"dest_features,omitempty"`
 	XXX_NoUnkeyedLiteral struct{}           `json:"-"`
 	XXX_unrecognized     []byte             `json:"-"`

lnrpc/routerrpc/router.proto

@@ -109,7 +109,9 @@ message SendPaymentRequest {
     /**
     Features assumed to be supported by the final node. All transitive feature
     depdencies must also be set properly. For a given feature bit pair, either
-    optional or remote may be set, but not both.
+    optional or remote may be set, but not both. If this field is nil or empty,
+    the router will try to load destination features from the graph as a
+    fallback.
     */
     repeated lnrpc.FeatureBit dest_features = 16;
 }

lnrpc/routerrpc/router_backend.go

@@ -710,7 +710,15 @@ func unmarshallHopHint(rpcHint *lnrpc.HopHint) (zpay32.HopHint, error) {
 // UnmarshalFeatures converts a list of uint32's into a valid feature vector.
 // This method checks that feature bit pairs aren't assigned toegether, and
 // validates transitive depdencies.
-func UnmarshalFeatures(rpcFeatures []lnrpc.FeatureBit) (*lnwire.FeatureVector, error) {
+func UnmarshalFeatures(
+	rpcFeatures []lnrpc.FeatureBit) (*lnwire.FeatureVector, error) {
+
+	// If no destination features are specified we'll return nil to signal
+	// that the router should try to use the graph as a fallback.
+	if rpcFeatures == nil {
+		return nil, nil
+	}
+
 	raw := lnwire.NewRawFeatureVector()
 	for _, bit := range rpcFeatures {
 		err := raw.SafeSet(lnwire.FeatureBit(bit))