graph/db: remove unnecessary AddNode method on GraphCache

The AddNode method on the GraphCache calls `AddNodeFeatures` underneath
and then iterates through all the node's persisted channels and adds
them to the cache too via `AddChannel`.

This is, however, not required since at the time the cache is populated
in `NewChannelGraph`, the cache is populated will all persisted nodes
and all persisted channels. Then, once any new channels come in, via
`AddChannelEdge`, they are added to the cache via AddChannel. If any new
nodes come in via `AddLightningNode`, then currently the cache's AddNode
method is called which the both adds the node and again iterates through
all persisted channels and re-adds them to the cache. This is definitely
redundent since the initial cache population and updates via
AddChannelEdge should keep the cache fresh in terms of channels.

So we remove this for 2 reasons: 1) to remove the redundent DB calls and
2) this requires a kvdb.RTx to be passed in to the GraphCache calls
   which will make it hard to extract the cache out of the CRUD layer
and be used more generally.

The AddNode method made sense when the cache was first added in the
code-base
[here](369c09be61 (diff-ae36bdb6670644d20c4e43f3a0ed47f71886c2bcdf3cc2937de24315da5dc072R213))
since then during graph cache population, nodes and channels would be
added to the cache in a single DB transaction. This was, however,
changed [later
on](352008a0c2)
to be done in 2 separate DB calls for efficiency reasons.

graph/db/graph.go

@@ -904,10 +904,7 @@ func (c *ChannelGraph) AddLightningNode(node *models.LightningNode,
 				cNode := newGraphCacheNode(
 					node.PubKeyBytes, node.Features,
 				)
-				err := c.graphCache.AddNode(tx, cNode)
-				if err != nil {
-					return err
-				}
+				c.graphCache.AddNodeFeatures(cNode)
 			}
 
 			return addLightningNode(tx, node)

graph/db/graph_cache.go

@@ -136,23 +136,6 @@ func (c *GraphCache) AddNodeFeatures(node GraphCacheNode) {
 	c.mtx.Unlock()
 }
 
-// AddNode adds a graph node, including all the (directed) channels of that
-// node.
-func (c *GraphCache) AddNode(tx kvdb.RTx, node GraphCacheNode) error {
-	c.AddNodeFeatures(node)
-
-	return node.ForEachChannel(
-		tx, func(tx kvdb.RTx, info *models.ChannelEdgeInfo,
-			outPolicy *models.ChannelEdgePolicy,
-			inPolicy *models.ChannelEdgePolicy) error {
-
-			c.AddChannel(info, outPolicy, inPolicy)
-
-			return nil
-		},
-	)
-}
-
 // AddChannel adds a non-directed channel, meaning that the order of policy 1
 // and policy 2 does not matter, the directionality is extracted from the info
 // and policy flags automatically. The policy will be set as the outgoing policy

graph/db/graph_cache_test.go

@@ -88,18 +88,16 @@ func TestGraphCacheAddNode(t *testing.T) {
 		node := &node{
 			pubKey:   nodeA,
 			features: lnwire.EmptyFeatureVector(),
-			edgeInfos: []*models.ChannelEdgeInfo{{
-				ChannelID: 1000,
-				// Those are direction independent!
-				NodeKey1Bytes: pubKey1,
-				NodeKey2Bytes: pubKey2,
-				Capacity:      500,
-			}},
-			outPolicies: []*models.ChannelEdgePolicy{outPolicy1},
-			inPolicies:  []*models.ChannelEdgePolicy{inPolicy1},
 		}
 		cache := NewGraphCache(10)
-		require.NoError(t, cache.AddNode(nil, node))
+		cache.AddNodeFeatures(node)
+		cache.AddChannel(&models.ChannelEdgeInfo{
+			ChannelID: 1000,
+			// Those are direction independent!
+			NodeKey1Bytes: pubKey1,
+			NodeKey2Bytes: pubKey2,
+			Capacity:      500,
+		}, outPolicy1, inPolicy1)
 
 		var fromChannels, toChannels []*DirectedChannel
 		_ = cache.ForEachChannel(nodeA, func(c *DirectedChannel) error {