rfnoc: Fix back-edge consistency check

On back-edges, no properties are forwarded, but properties must be
consistent after property resolution. This breaks when the source edge
on a back-edge has an edge property which the destination block does
not. Consider the following graph:

    DDC -> Replay -> DDC

where both instances of 'DDC' refer to the same block. Now, assume the
first edge is declared a back edge (in principle, it shouldn't matter).
The DDC block has an edge property `samp_rate` which the Replay block
does not. Therefore, it can't forward this edge property to the Replay
block's input edge property list.

In the consistency check code, we don't check for the existence of edge
nodes, because it is assumed edge properties where either forwarded, or
aligned through some other manner. This leads to a property lookup
failure.

With this fix, we skip the consistency check for edge properties which
don't exist on the destination node. This is safe because the
destination block can not have a property resolver defined for undefined
properties. This means the destination block can either:
- Drop the property. In this case, there is no value in checking
  consistency. Even if we could forward edge properties on back-edges,
  they would always have the same value.
- Forward the property. In that case, the consistency check would happen
  elsewhere in the graph where there's no back-edge.

1 files changed, 7 insertions, 0 deletions

diff --git a/host/lib/rfnoc/graph.cpp b/host/lib/rfnoc/graph.cpp
index 931777629..9b8b9ec59 100644
--- a/host/lib/rfnoc/graph.cpp
+++ b/host/lib/rfnoc/graph.cpp
@@ -669,6 +669,13 @@ bool graph_t::_assert_edge_props_consistent(rfnoc_graph_t::edge_descriptor edge)
     for (auto src_prop_it = src_prop_map.begin(); src_prop_it != src_prop_map.end();
          ++src_prop_it) {
         auto src_prop = src_prop_it->second;
+        if (dst_prop_map.count(src_prop->get_id()) == 0) {
+            UHD_LOG_DEBUG(LOG_ID,
+                "On back-edge "
+                    << edge_info.to_string() << ", source block has edge property `"
+                    << src_prop->get_id() << "', but destination block does not.");
+            continue;
+        }
         auto dst_prop = dst_prop_map.at(src_prop->get_id());
         if (!src_prop->equal(dst_prop)) {
             UHD_LOG_ERROR(LOG_ID,