channeldb: fix graph cache feature handling [skip ci]

Fixes #5830.
When a channel for a node is announced before the node itself is
announced on the network, it's possible that we have channels for a node
but no features defined yet. This was previously logged as a warning
which spammed the log unnecessarily.

channeldb/graph.go

@@ -376,7 +376,8 @@ func (c *ChannelGraph) ForEachNodeChannel(node route.Vertex,
 	return c.graphCache.ForEachChannel(node, cb)
 }
 
-// FetchNodeFeatures returns the features of a given node.
+// FetchNodeFeatures returns the features of a given node. If no features are
+// known for the node, an empty feature vector is returned.
 func (c *ChannelGraph) FetchNodeFeatures(
 	node route.Vertex) (*lnwire.FeatureVector, error) {
 

channeldb/graph_cache.go

@@ -415,9 +415,12 @@ func (c *GraphCache) ForEachChannel(node route.Vertex,
 
 	features, ok := c.nodeFeatures[node]
 	if !ok {
-		log.Warnf("Node %v has no features defined, falling back to "+
+		// If the features were set to nil explicitly, that's fine here.
-			"default feature vector for path finding", node)
+		// The router will overwrite the features of the destination
-
+		// node with those found in the invoice if necessary. But if we
+		// didn't yet get a node announcement we want to mimic the
+		// behavior of the old DB based code that would always set an
+		// empty feature vector instead of leaving it nil.
 		features = lnwire.EmptyFeatureVector()
 	}
 
@@ -444,7 +447,8 @@ func (c *GraphCache) ForEachChannel(node route.Vertex,
 	return nil
 }
 
-// GetFeatures returns the features of the node with the given ID.
+// GetFeatures returns the features of the node with the given ID. If no
+// features are known for the node, an empty feature vector is returned.
 func (c *GraphCache) GetFeatures(node route.Vertex) *lnwire.FeatureVector {
 	c.mtx.RLock()
 	defer c.mtx.RUnlock()