graph/db: move cache writes for Prune methods

This commit moves the cache writes for PruneGraphNodes and PruneGraph
from the KVStore to the ChannelGraph.

graph/db/graph.go

@@ -4,6 +4,8 @@ import (
 	"sync"
 	"time"
 
+	"github.com/btcsuite/btcd/chaincfg/chainhash"
+	"github.com/btcsuite/btcd/wire"
 	"github.com/lightningnetwork/lnd/batch"
 	"github.com/lightningnetwork/lnd/graph/db/models"
 	"github.com/lightningnetwork/lnd/kvdb"
@@ -322,3 +324,65 @@ func (c *ChannelGraph) DisconnectBlockAtHeight(height uint32) (
 
 	return edges, nil
 }
+
+// PruneGraph prunes newly closed channels from the channel graph in response
+// to a new block being solved on the network. Any transactions which spend the
+// funding output of any known channels within he graph will be deleted.
+// Additionally, the "prune tip", or the last block which has been used to
+// prune the graph is stored so callers can ensure the graph is fully in sync
+// with the current UTXO state. A slice of channels that have been closed by
+// the target block are returned if the function succeeds without error.
+func (c *ChannelGraph) PruneGraph(spentOutputs []*wire.OutPoint,
+	blockHash *chainhash.Hash, blockHeight uint32) (
+	[]*models.ChannelEdgeInfo, error) {
+
+	c.cacheMu.Lock()
+	defer c.cacheMu.Unlock()
+
+	edges, nodes, err := c.KVStore.PruneGraph(
+		spentOutputs, blockHash, blockHeight,
+	)
+	if err != nil {
+		return nil, err
+	}
+
+	if c.graphCache != nil {
+		for _, edge := range edges {
+			c.graphCache.RemoveChannel(
+				edge.NodeKey1Bytes, edge.NodeKey2Bytes,
+				edge.ChannelID,
+			)
+		}
+
+		for _, node := range nodes {
+			c.graphCache.RemoveNode(node)
+		}
+
+		log.Debugf("Pruned graph, cache now has %s",
+			c.graphCache.Stats())
+	}
+
+	return edges, nil
+}
+
+// PruneGraphNodes is a garbage collection method which attempts to prune out
+// any nodes from the channel graph that are currently unconnected. This ensure
+// that we only maintain a graph of reachable nodes. In the event that a pruned
+// node gains more channels, it will be re-added back to the graph.
+func (c *ChannelGraph) PruneGraphNodes() error {
+	c.cacheMu.Lock()
+	defer c.cacheMu.Unlock()
+
+	nodes, err := c.KVStore.PruneGraphNodes()
+	if err != nil {
+		return err
+	}
+
+	if c.graphCache != nil {
+		for _, node := range nodes {
+			c.graphCache.RemoveNode(node)
+		}
+	}
+
+	return nil
+}