channeldb/graph: filter zombie channels in FilterKnownChanIDs

This commit modifies FilterKnownChanIDs to skip edges that
we ourselves have deemed zombies. This prevents us from requesting
the updates from them, as this wastes bandwidth and cpu cycles.

channeldb/graph.go

@@ -1513,10 +1513,10 @@ func (c *ChannelGraph) NodeUpdatesInHorizon(startTime, endTime time.Time) ([]Lig
 }
 
 // FilterKnownChanIDs takes a set of channel IDs and return the subset of chan
-// ID's that we don't know of in the passed set. In other words, we perform a
-// set difference of our set of chan ID's and the ones passed in. This method
-// can be used by callers to determine the set of channels ta peer knows of
-// that we don't.
+// ID's that we don't know and are not known zombies of the passed set. In other
+// words, we perform a set difference of our set of chan ID's and the ones
+// passed in. This method can be used by callers to determine the set of
+// channels another peer knows of that we don't.
 func (c *ChannelGraph) FilterKnownChanIDs(chanIDs []uint64) ([]uint64, error) {
 	var newChanIDs []uint64
 
@@ -1530,15 +1530,31 @@ func (c *ChannelGraph) FilterKnownChanIDs(chanIDs []uint64) ([]uint64, error) {
 			return ErrGraphNoEdgesFound
 		}
 
+		// Fetch the zombie index, it may not exist if no edges have
+		// ever been marked as zombies. If the index has been
+		// initialized, we will use it later to skip known zombie edges.
+		zombieIndex := edges.Bucket(zombieBucket)
+
 		// We'll run through the set of chanIDs and collate only the
 		// set of channel that are unable to be found within our db.
 		var cidBytes [8]byte
 		for _, cid := range chanIDs {
 			byteOrder.PutUint64(cidBytes[:], cid)
 
-			if v := edgeIndex.Get(cidBytes[:]); v == nil {
-				newChanIDs = append(newChanIDs, cid)
+			// If the edge is already known, skip it.
+			if v := edgeIndex.Get(cidBytes[:]); v != nil {
+				continue
 			}
+
+			// If the edge is a known zombie, skip it.
+			if zombieIndex != nil {
+				isZombie, _, _ := isZombieEdge(zombieIndex, cid)
+				if isZombie {
+					continue
+				}
+			}
+
+			newChanIDs = append(newChanIDs, cid)
 		}
 
 		return nil

channeldb/graph_test.go

@@ -1741,6 +1741,24 @@ func TestFilterKnownChanIDs(t *testing.T) {
 		chanIDs = append(chanIDs, chanID.ToUint64())
 	}
 
+	const numZombies = 5
+	zombieIDs := make([]uint64, 0, numZombies)
+	for i := 0; i < numZombies; i++ {
+		channel, chanID := createEdge(
+			uint32(i*10+1), 0, 0, 0, node1, node2,
+		)
+		if err := graph.AddChannelEdge(&channel); err != nil {
+			t.Fatalf("unable to create channel edge: %v", err)
+		}
+		if err := graph.MarkEdgeZombie(
+			chanID.ToUint64(), node1.PubKeyBytes, node2.PubKeyBytes,
+		); err != nil {
+			t.Fatalf("unable to mark edge zombie: %v", err)
+		}
+
+		zombieIDs = append(zombieIDs, chanID.ToUint64())
+	}
+
 	queryCases := []struct {
 		queryIDs []uint64
 
@@ -1751,6 +1769,11 @@ func TestFilterKnownChanIDs(t *testing.T) {
 		{
 			queryIDs: chanIDs,
 		},
+		// If we attempt to filter out all zombies that we know of, the
+		// response should be the empty set.
+		{
+			queryIDs: zombieIDs,
+		},
 
 		// If we query for a set of ID's that we didn't insert, we
 		// should get the same set back.