Fix generation of inefficient MLD partitions

CHANGELOG.md

@@ -4,6 +4,8 @@
       - FIXED: Allow for special characters in the profile/method as part of the HTTP URL. [#6090](https://github.com/Project-OSRM/osrm-backend/pull/6090)
     - Build:
       - CHANGED: Replace Travis with Github Actions for CI builds [#6071](https://github.com/Project-OSRM/osrm-backend/pull/6071)
+    - Routing:
+      - FIXED: Fix generation of inefficient MLD partitions [#6084](https://github.com/Project-OSRM/osrm-backend/pull/6084)
 
 # 5.25.0
   - Changes from 5.24.0

src/extractor/edge_based_graph_factory.cpp

@@ -412,6 +412,13 @@ EdgeBasedGraphFactory::GenerateEdgeExpandedNodes(const WayRestrictionMap &way_re
             m_edge_based_node_distances.push_back(
                 m_edge_based_node_distances[nbe_to_ebn_mapping[eid]]);
 
+            // Include duplicate nodes in cnbg to ebg mapping. This means a
+            // compressed node pair (u,v) can appear multiple times in this list.
+            // This is needed by the MLD partition step to ensure duplicate nodes
+            // are also assigned to partitions (the MLD partitioner is currently
+            // the only consumer of this mapping).
+            mapping.push_back(NBGToEBG{node_u, node_v, edge_based_node_id, SPECIAL_NODEID});
+
             edge_based_node_id++;
             progress.PrintStatus(progress_counter++);
         }

src/partitioner/partitioner.cpp

@@ -118,6 +118,10 @@ int Partitioner::Run(const PartitionerConfig &config)
             edge_based_partition_ids[backward_node] = node_based_partition_ids[v];
     }
 
+    BOOST_ASSERT(std::none_of(edge_based_partition_ids.begin(),
+                              edge_based_partition_ids.end(),
+                              [](auto x) { return x == SPECIAL_NODEID; }));
+
     std::vector<Partition> partitions;
     std::vector<std::uint32_t> level_to_num_cells;
     std::tie(partitions, level_to_num_cells) =