multi: update FetchChanInfos to take in an optional tx

In this commit, the FetchChanInfos ChannelGraph method is updated to
take in an optional read transaction for the case where it is called
from within another transaction.

channeldb/graph.go

@@ -2357,7 +2357,11 @@ func (c *ChannelGraph) FilterChannelRange(startHeight,
 // skipped and the result will contain only those edges that exist at the time
 // of the query. This can be used to respond to peer queries that are seeking to
 // fill in gaps in their view of the channel graph.
-func (c *ChannelGraph) FetchChanInfos(chanIDs []uint64) ([]ChannelEdge, error) {
+//
+// NOTE: An optional transaction may be provided. If none is provided, then a
+// new one will be created.
+func (c *ChannelGraph) FetchChanInfos(tx kvdb.RTx, chanIDs []uint64) (
+	[]ChannelEdge, error) {
 	// TODO(roasbeef): sort cids?
 
 	var (
@@ -2365,7 +2369,7 @@ func (c *ChannelGraph) FetchChanInfos(chanIDs []uint64) ([]ChannelEdge, error) {
 		cidBytes  [8]byte
 	)
 
-	err := kvdb.View(c.db, func(tx kvdb.RTx) error {
+	fetchChanInfos := func(tx kvdb.RTx) error {
 		edges := tx.ReadBucket(edgeBucket)
 		if edges == nil {
 			return ErrGraphNoEdgesFound
@@ -2427,9 +2431,20 @@ func (c *ChannelGraph) FetchChanInfos(chanIDs []uint64) ([]ChannelEdge, error) {
 			})
 		}
 		return nil
-	}, func() {
-		chanEdges = nil
-	})
+	}
+
+	if tx == nil {
+		err := kvdb.View(c.db, fetchChanInfos, func() {
+			chanEdges = nil
+		})
+		if err != nil {
+			return nil, err
+		}
+
+		return chanEdges, nil
+	}
+
+	err := fetchChanInfos(tx)
 	if err != nil {
 		return nil, err
 	}
@@ -3673,7 +3688,7 @@ func (c *ChannelGraph) markEdgeLiveUnsafe(tx kvdb.RwTx, chanID uint64) error {
 	// We need to add the channel back into our graph cache, otherwise we
 	// won't use it for path finding.
 	if c.graphCache != nil {
-		edgeInfos, err := c.FetchChanInfos([]uint64{chanID})
+		edgeInfos, err := c.FetchChanInfos(tx, []uint64{chanID})
 		if err != nil {
 			return err
 		}

channeldb/graph_test.go

@@ -2685,7 +2685,7 @@ func TestFetchChanInfos(t *testing.T) {
 	// We'll now attempt to query for the range of channel ID's we just
 	// inserted into the database. We should get the exact same set of
 	// edges back.
-	resp, err := graph.FetchChanInfos(edgeQuery)
+	resp, err := graph.FetchChanInfos(nil, edgeQuery)
 	require.NoError(t, err, "unable to fetch chan edges")
 	if len(resp) != len(edges) {
 		t.Fatalf("expected %v edges, instead got %v", len(edges),