channeldb/graph: dedup channel edges returned from ChanUpdatesInHorizon

In this commit, we ensure that we de-duplicate the set of channel edges
returned from ChanUpdatesInHorizon. Other subsystems within lnd use this
method to retrieve and send all the channels with updates within a time
series to network peers. However, since the method looks at the edge
update index, which can include up to two entries per edge for each
policy, it's possible that we'd send channel announcements and updates
twice, causing extra bandwidth.

channeldb/graph.go

@@ -1176,6 +1176,10 @@ type ChannelEdge struct {
 // ChanUpdatesInHorizon returns all the known channel edges which have at least
 // one edge that has an update timestamp within the specified horizon.
 func (c *ChannelGraph) ChanUpdatesInHorizon(startTime, endTime time.Time) ([]ChannelEdge, error) {
+	// To ensure we don't return duplicate ChannelEdges, we'll use an
+	// additional map to keep track of the edges already seen to prevent
+	// re-adding it.
+	edgesSeen := make(map[uint64]struct{})
 	var edgesInHorizon []ChannelEdge
 
 	err := c.db.View(func(tx *bolt.Tx) error {
@@ -1219,6 +1223,14 @@ func (c *ChannelGraph) ChanUpdatesInHorizon(startTime, endTime time.Time) ([]Cha
 			// chan ID so we can query it in the DB.
 			chanID := indexKey[8:]
 
+			// If we've already retrieved the info and policies for
+			// this edge, then we can skip it as we don't need to do
+			// so again.
+			chanIDInt := byteOrder.Uint64(chanID)
+			if _, ok := edgesSeen[chanIDInt]; ok {
+				continue
+			}
+
 			// First, we'll fetch the static edge information.
 			edgeInfo, err := fetchChanEdgeInfo(edgeIndex, chanID)
 			if err != nil {
@@ -1237,6 +1249,7 @@ func (c *ChannelGraph) ChanUpdatesInHorizon(startTime, endTime time.Time) ([]Cha
 
 			// Finally, we'll collate this edge with the rest of
 			// edges to be returned.
+			edgesSeen[chanIDInt] = struct{}{}
 			edgesInHorizon = append(edgesInHorizon, ChannelEdge{
 				Info:    &edgeInfo,
 				Policy1: edge1,