CDRIVER-4055 Update server selection for load balancer

Author: kevinAlbs

src/libmongoc/src/mongoc/mongoc-cmd.c

@@ -627,6 +627,7 @@ _mongoc_cmd_parts_assemble_mongod (mongoc_cmd_parts_t *parts,
       case MONGOC_TOPOLOGY_SHARDED:
       case MONGOC_TOPOLOGY_UNKNOWN:
       case MONGOC_TOPOLOGY_DESCRIPTION_TYPES:
+      case MONGOC_TOPOLOGY_LOADBALANCED:
       default:
          /* must not call this function w/ sharded or unknown topology type */
          BSON_ASSERT (false);

src/libmongoc/src/mongoc/mongoc-read-prefs.c

@@ -383,6 +383,12 @@ assemble_query (const mongoc_read_prefs_t *read_prefs,
 
    case MONGOC_TOPOLOGY_UNKNOWN:
    case MONGOC_TOPOLOGY_DESCRIPTION_TYPES:
+   case MONGOC_TOPOLOGY_LOADBALANCED:
+   /* TODO (CDRIVER-4056): this is pending a question on pre-5.0 support and testing.
+    * The spec says 5.0 is the minimum server version.
+    * It is unclear if we can assume the backing server always supports OP_MSG.
+    * I think the intent is we can, but there is no max wire version check.
+    */
    default:
       /* must not call _apply_read_preferences with unknown topology type */
       BSON_ASSERT (false);

src/libmongoc/src/mongoc/mongoc-topology-description.c

@@ -36,6 +36,7 @@ _is_data_node (mongoc_server_description_t *sd)
    case MONGOC_SERVER_STANDALONE:
    case MONGOC_SERVER_RS_SECONDARY:
    case MONGOC_SERVER_RS_PRIMARY:
+   case MONGOC_SERVER_LOADBALANCER:
       return true;
    case MONGOC_SERVER_RS_OTHER:
    case MONGOC_SERVER_RS_ARBITER:
@@ -351,6 +352,10 @@ _mongoc_topology_description_server_is_candidate (
       default:
          return false;
       }
+
+   case MONGOC_TOPOLOGY_LOADBALANCED:
+      return desc_type == MONGOC_SERVER_LOADBALANCER;
+
    default:
       return false;
    }
@@ -711,6 +716,16 @@ mongoc_topology_description_suitable_servers (
          topology->servers, _mongoc_find_suitable_mongos_cb, &data);
    }
 
+   /* Load balanced clusters --
+    * Always select the only server. */
+   if (topology->type == MONGOC_TOPOLOGY_LOADBALANCED) {
+      BSON_ASSERT (topology->servers->items_len == 1);
+      server = (mongoc_server_description_t *) mongoc_set_get_item (
+         topology->servers, 0);
+      _mongoc_array_append_val (set, server);
+      goto DONE;
+   }
+
    /* Ways to get here:
     *   - secondary read
     *   - secondary preferred read
@@ -1792,6 +1807,8 @@ _mongoc_topology_description_type (mongoc_topology_description_t *topology)
       return "RSWithPrimary";
    case MONGOC_TOPOLOGY_SINGLE:
       return "Single";
+   case MONGOC_TOPOLOGY_LOADBALANCED:
+      return "LoadBalanced";
    case MONGOC_TOPOLOGY_DESCRIPTION_TYPES:
    default:
       MONGOC_ERROR ("Invalid mongoc_topology_description_type_t type");

src/libmongoc/tests/test-mongoc-server-selection.c

@@ -60,7 +60,12 @@ test_all_spec_tests (TestSuite *suite)
    install_json_test_suite (suite, resolved, &test_rtt_calculation_cb);
 
    /* SS logic */
-   test_framework_resolve_path (JSON_DIR "/server_selection/server_selection", resolved);
+   test_framework_resolve_path (JSON_DIR "/server_selection/server_selection",
+                                resolved);
+   install_json_test_suite (suite, resolved, &test_server_selection_logic_cb);
+
+   test_framework_resolve_path (
+      JSON_DIR "/server_selection/server_selection/LoadBalanced", resolved);
    install_json_test_suite (suite, resolved, &test_server_selection_logic_cb);
 }