graph/db: move cache read checks to ChannelGraph.

This commit moves the graph cache checks for FetchNodeFeatures,
ForEachNodeDirectedChannel, GraphSession and ForEachNodeCached from the
KVStore to the ChannelGraph. Since the ChannelGraph is currently just a
pass-through for any of the KVStore methods, all that needs to be done
for calls to go via the ChannelGraph instead directly to the KVStore is
for the ChannelGraph to go and implement those methods.

graph/db/graph.go

@@ -91,3 +91,65 @@ func NewChannelGraph(cfg *Config, options ...ChanGraphOption) (*ChannelGraph,
 		graphCache: graphCache,
 	}, nil
 }
+
+// ForEachNodeDirectedChannel iterates through all channels of a given node,
+// executing the passed callback on the directed edge representing the channel
+// and its incoming policy. If the callback returns an error, then the iteration
+// is halted with the error propagated back up to the caller. If the graphCache
+// is available, then it will be used to retrieve the node's channels instead
+// of the database.
+//
+// Unknown policies are passed into the callback as nil values.
+//
+// NOTE: this is part of the graphdb.NodeTraverser interface.
+func (c *ChannelGraph) ForEachNodeDirectedChannel(node route.Vertex,
+	cb func(channel *DirectedChannel) error) error {
+
+	if c.graphCache != nil {
+		return c.graphCache.ForEachChannel(node, cb)
+	}
+
+	return c.KVStore.ForEachNodeDirectedChannel(node, cb)
+}
+
+// FetchNodeFeatures returns the features of the given node. If no features are
+// known for the node, an empty feature vector is returned.
+// If the graphCache is available, then it will be used to retrieve the node's
+// features instead of the database.
+//
+// NOTE: this is part of the graphdb.NodeTraverser interface.
+func (c *ChannelGraph) FetchNodeFeatures(node route.Vertex) (
+	*lnwire.FeatureVector, error) {
+
+	if c.graphCache != nil {
+		return c.graphCache.GetFeatures(node), nil
+	}
+
+	return c.KVStore.FetchNodeFeatures(node)
+}
+
+// GraphSession will provide the call-back with access to a NodeTraverser
+// instance which can be used to perform queries against the channel graph. If
+// the graph cache is not enabled, then the call-back will be provided with
+// access to the graph via a consistent read-only transaction.
+func (c *ChannelGraph) GraphSession(cb func(graph NodeTraverser) error) error {
+	if c.graphCache != nil {
+		return cb(c)
+	}
+
+	return c.KVStore.GraphSession(cb)
+}
+
+// ForEachNodeCached iterates through all the stored vertices/nodes in the
+// graph, executing the passed callback with each node encountered.
+//
+// NOTE: The callback contents MUST not be modified.
+func (c *ChannelGraph) ForEachNodeCached(cb func(node route.Vertex,
+	chans map[uint64]*DirectedChannel) error) error {
+
+	if c.graphCache != nil {
+		return c.graphCache.ForEachNode(cb)
+	}
+
+	return c.KVStore.ForEachNodeCached(cb)
+}