onionmessage: add BFS pathfinding for onion messages

In this commit we add FindPath, a BFS-based shsortest-path
algorithm that finds routes through the channel graph for
onion messages. The search filters nodes by the
OnionMessage feature bits (38/39).

We also add a  unit tests covering: direct neighbor routing,
multi-hop paths, feature-bit filtering, missing destination
nodes, destination without onion support, max hop limits,
cycle handling, and shortest-path selection.
choice of BFS is because there isn't any weight involve.

Author: Abdulkbk

onionmessage/errors.go

@@ -14,4 +14,13 @@ var (
 	// ErrSCIDEmpty is returned when the short channel ID is missing from
 	// the route data.
 	ErrSCIDEmpty = errors.New("short channel ID empty")
+
+	// ErrNoPathFound is returned when no path exists between the source
+	// and destination nodes that supports onion messaging.
+	ErrNoPathFound = errors.New("no path found to destination")
+
+	// ErrDestinationNoOnionSupport is returned when the destination node
+	// does not advertise support for onion messages.
+	ErrDestinationNoOnionSupport = errors.New("destination does not " +
+		"support onion messages")
 )

onionmessage/pathfind.go

@@ -0,0 +1,162 @@
+package onionmessage
+
+import (
+	"context"
+
+	graphdb "github.com/lightningnetwork/lnd/graph/db"
+	"github.com/lightningnetwork/lnd/lnwire"
+	"github.com/lightningnetwork/lnd/routing/route"
+)
+
+// OnionMessagePath represents a route found for an onion message.
+type OnionMessagePath struct {
+	// Hops is ordered from the first-hop peer to the destination.
+	Hops []route.Vertex
+}
+
+// FindPath finds the shortest path (by hop count) from source to destination
+// through nodes that support onion messaging (feature bit 38/39). It uses a
+// standard BFS on the channel graph filtered by the OnionMessagesOptional
+// feature bit.
+func FindPath(ctx context.Context, graph graphdb.NodeTraverser, source,
+	destination route.Vertex, maxHops int) (*OnionMessagePath, error) {
+
+	// Check that the destination supports onion messaging.
+	destFeatures, err := graph.FetchNodeFeatures(ctx, destination)
+	if err != nil {
+		return nil, err
+	}
+
+	if !destFeatures.HasFeature(lnwire.OnionMessagesOptional) {
+		return nil, ErrDestinationNoOnionSupport
+	}
+
+	// If source == destination, return empty path.
+	if source == destination {
+		return &OnionMessagePath{}, nil
+	}
+
+	parent := make(map[route.Vertex]route.Vertex)
+	visited := make(map[route.Vertex]bool)
+	featureCache := make(map[route.Vertex]bool)
+
+	// Cache the destination as supporting onion messages (checked above).
+	featureCache[destination] = true
+
+	// supportsOnionMessages is a closure that checks (with caching) whether
+	// a node advertises the onion messages feature bit.
+	supportsOnionMessages := func(node route.Vertex) bool {
+		if cached, ok := featureCache[node]; ok {
+			return cached
+		}
+
+		features, err := graph.FetchNodeFeatures(ctx, node)
+		if err != nil {
+			log.Tracef("Unable to fetch features for node %v: %v",
+				node.String(), err)
+			featureCache[node] = false
+			return false
+		}
+
+		supports := features.HasFeature(
+			lnwire.OnionMessagesOptional,
+		)
+
+		featureCache[node] = supports
+
+		return supports
+	}
+
+	visited[source] = true
+
+	queue := []route.Vertex{source}
+	depth := 0
+
+	for len(queue) > 0 {
+		depth++
+		if depth > maxHops {
+			break
+		}
+
+		nextQueue := make([]route.Vertex, 0)
+
+		for _, current := range queue {
+			err := graph.ForEachNodeDirectedChannel(ctx, current,
+				func(channel *graphdb.DirectedChannel) error {
+					neighbor := channel.OtherNode
+
+					if visited[neighbor] {
+						return nil
+					}
+
+					// Skip nodes that don't support
+					// onion messaging.
+					if !supportsOnionMessages(neighbor) {
+						return nil
+					}
+
+					visited[neighbor] = true
+					parent[neighbor] = current
+
+					if neighbor == destination {
+						return errBFSDone
+					}
+
+					nextQueue = append(
+						nextQueue, neighbor,
+					)
+
+					return nil
+				},
+				func() {
+					// Reset callback - nothing to
+					// reset for BFS.
+				},
+			)
+
+			// Check if we found the destination.
+			if err == errBFSDone {
+				return reconstructPath(
+					parent, source, destination,
+				), nil
+			}
+
+			if err != nil {
+				return nil, err
+			}
+		}
+
+		queue = nextQueue
+	}
+
+	return nil, ErrNoPathFound
+}
+
+// errBFSDone is a sentinel error used internally to break out of the
+// ForEachNodeDirectedChannel callback when the destination is found.
+var errBFSDone = &bfsDoneError{}
+
+type bfsDoneError struct{}
+
+func (e *bfsDoneError) Error() string { return "bfs done" }
+
+// reconstructPath rebuilds the path from destination back to source using the
+// parent map, returning the hops in forward order (excluding source).
+func reconstructPath(parent map[route.Vertex]route.Vertex,
+	source, destination route.Vertex) *OnionMessagePath {
+
+	var path []route.Vertex
+
+	current := destination
+	for current != source {
+		path = append(path, current)
+		current = parent[current]
+	}
+
+	// Reverse the path to get source-to-destination order.
+	for i, j := 0, len(path)-1; i < j; i, j = i+1, j-1 {
+		path[i], path[j] = path[j], path[i]
+	}
+
+	return &OnionMessagePath{Hops: path}
+}