keep and follow node_refs when traversing HNSW graph

* cannot assert in HnswGraph read operations,
  because nodes may be removed or replaced by another
  thread
* read operations on the HNSW graph need to load the
  node_ref (and check that it's valid) when following
  links; keep and use that node_ref to ensure succeeding
  operations actually access information that belongs
  to the same version of the node.
* to ensure the entry point is internally consistent,
  check that the atomic value is the same after reading
  the corresponding node_ref, and retry if it was
  updated concurrently.

1 files changed, 3 insertions, 3 deletions

diff --git a/searchlib/src/tests/tensor/hnsw_saver/hnsw_save_load_test.cpp b/searchlib/src/tests/tensor/hnsw_saver/hnsw_save_load_test.cpp
index bcc886fccad..2db6437664e 100644
--- a/searchlib/src/tests/tensor/hnsw_saver/hnsw_save_load_test.cpp
+++ b/searchlib/src/tests/tensor/hnsw_saver/hnsw_save_load_test.cpp
@@ -37,7 +37,7 @@ using V = std::vector<uint32_t>;
 void populate(HnswGraph &graph) {
     // no 0
     graph.make_node_for_document(1, 1);
-    graph.make_node_for_document(2, 2);
+    auto er = graph.make_node_for_document(2, 2);
     // no 3
     graph.make_node_for_document(4, 2);
     graph.make_node_for_document(5, 0);
@@ -49,7 +49,7 @@ void populate(HnswGraph &graph) {
     graph.set_link_array(6, 0, V{1, 2, 4});
     graph.set_link_array(2, 1, V{4});
     graph.set_link_array(4, 1, V{2});
-    graph.set_entry_node({2, 1});
+    graph.set_entry_node({2, er, 1});
 }
 
 void modify(HnswGraph &graph) {
@@ -63,7 +63,7 @@ void modify(HnswGraph &graph) {
     graph.set_link_array(4, 1, V{7});
     graph.set_link_array(7, 1, V{4});
 
-    graph.set_entry_node({4, 1});
+    graph.set_entry_node({4, graph.get_node_ref(4), 1});
 }