graph/db: let FetchChannelEdgesByID behave as promised

The comment of FetchChannelEdgesByID says that if the ErrZombieEdge
error is returned, then the ChannelEdgeInfo return parameter will also
still be populated and returned (ie, wont be nil). This commit updates
the SQLStore implementation of FetchChannelEdgesByID to do this. This is
needed to prevent nil dereference panics at any call-sites that depend
on the method working as it describes.

graph/db/graph_test.go

@@ -451,8 +451,14 @@ func TestEdgeInsertionDeletion(t *testing.T) {
 	// properly deleted.
 	_, _, _, err = graph.FetchChannelEdgesByOutpoint(&outpoint)
 	require.ErrorIs(t, err, ErrEdgeNotFound)
-	_, _, _, err = graph.FetchChannelEdgesByID(chanID)
+
+	// Assert that if the edge is a zombie, then FetchChannelEdgesByID
+	// still returns a populated models.ChannelEdgeInfo as its comment
+	// description promises.
+	edge, _, _, err := graph.FetchChannelEdgesByID(chanID)
 	require.ErrorIs(t, err, ErrZombieEdge)
+	require.NotNil(t, edge)
+
 	isZombie, _, _, err := graph.IsZombieEdge(chanID)
 	require.NoError(t, err)
 	require.True(t, isZombie)

graph/db/sql_store.go

@@ -1852,20 +1852,29 @@ func (s *SQLStore) FetchChannelEdgesByID(chanID uint64) (
 		if errors.Is(err, sql.ErrNoRows) {
 			// First check if this edge is perhaps in the zombie
 			// index.
-			isZombie, err := db.IsZombieChannel(
-				ctx, sqlc.IsZombieChannelParams{
+			zombie, err := db.GetZombieChannel(
+				ctx, sqlc.GetZombieChannelParams{
 					Scid:    chanIDB[:],
 					Version: int16(ProtocolV1),
 				},
 			)
-			if err != nil {
+			if errors.Is(err, sql.ErrNoRows) {
+				return ErrEdgeNotFound
+			} else if err != nil {
 				return fmt.Errorf("unable to check if "+
 					"channel is zombie: %w", err)
-			} else if isZombie {
-				return ErrZombieEdge
 			}
 
-			return ErrEdgeNotFound
+			// At this point, we know the channel is a zombie, so
+			// we'll return an error indicating this, and we will
+			// populate the edge info with the public keys of each
+			// party as this is the only information we have about
+			// it.
+			edge = &models.ChannelEdgeInfo{}
+			copy(edge.NodeKey1Bytes[:], zombie.NodeKey1)
+			copy(edge.NodeKey2Bytes[:], zombie.NodeKey2)
+
+			return ErrZombieEdge
 		} else if err != nil {
 			return fmt.Errorf("unable to fetch channel: %w", err)
 		}
@@ -1903,7 +1912,10 @@ func (s *SQLStore) FetchChannelEdgesByID(chanID uint64) (
 		return nil
 	}, sqldb.NoOpReset)
 	if err != nil {
-		return nil, nil, nil, fmt.Errorf("could not fetch channel: %w",
+		// If we are returning the ErrZombieEdge, then we also need to
+		// return the edge info as the method comment indicates that
+		// this will be populated when the edge is a zombie.
+		return edge, nil, nil, fmt.Errorf("could not fetch channel: %w",
 			err)
 	}