+ if (idl->txn) {
+ /* We're being called from ovsdb_idl_txn_write(). We must not update
+ * any arcs, because the transaction will be backed out at commit or
+ * abort time and we don't want our graph screwed up.
+ *
+ * Just return the destination row, if there is one and it has not been
+ * deleted. */
+ if (dst && (hmap_node_is_null(&dst->txn_node) || dst->new)) {
+ return dst;
+ }
+ return NULL;
+ } else {
+ /* We're being called from some other context. Update the graph. */
+ if (!dst) {
+ dst = ovsdb_idl_row_create(dst_table, dst_uuid);
+ }
+
+ /* Add a new arc, if it wouldn't be a self-arc or a duplicate arc. */
+ if (may_add_arc(src, dst)) {
+ /* The arc *must* be added at the front of the dst_arcs list. See
+ * ovsdb_idl_row_reparse_backrefs() for details. */
+ arc = xmalloc(sizeof *arc);
+ list_push_front(&src->src_arcs, &arc->src_node);
+ list_push_front(&dst->dst_arcs, &arc->dst_node);
+ arc->src = src;
+ arc->dst = dst;
+ }