shortcut server selection

Author: kevinAlbs

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

@@ -629,7 +629,7 @@ _mongoc_cmd_parts_assemble_mongod (mongoc_cmd_parts_t *parts,
       case MONGOC_TOPOLOGY_DESCRIPTION_TYPES:
       case MONGOC_TOPOLOGY_LOADBALANCED:
       default:
-         /* must not call this function w/ sharded or unknown topology type */
+         /* must not call this function w/ sharded, load balanced, or unknown topology type */
          BSON_ASSERT (false);
       }
    } /* if (!parts->is_write_command) */

src/libmongoc/src/mongoc/mongoc-server-description-private.h

@@ -109,8 +109,7 @@ struct _mongoc_server_description_t {
    2. an app thread receives any network error before completing a handshake
    3. an app thread receives a non-timeout network error after the handshake
    4. an app thread receives a "not primary" or "node is recovering" error from
-   a
-   pre-4.2 server.
+   a pre-4.2 server.
    */
    uint32_t generation;
 };

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

@@ -131,6 +131,7 @@ _mongoc_topology_description_monitor_opening (mongoc_topology_description_t *td)
                                                                 (int) 0);
       prev_sd = mongoc_server_description_new_copy (sd);
       if (td->apm_callbacks.topology_changed) {
+         mongoc_topology_description_destroy (prev_td);
          _mongoc_topology_description_copy_to (td, prev_td);
       }
       sd->type = MONGOC_SERVER_LOADBALANCER;

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

@@ -816,10 +816,6 @@ _mongoc_topology_do_blocking_scan (mongoc_topology_t *topology,
 {
    _mongoc_handshake_freeze ();
 
-   if (topology->description.type == MONGOC_TOPOLOGY_LOADBALANCED) {
-      MONGOC_DEBUG ("bypassing server selection logic for single threaded");
-      return;
-   }
    bson_mutex_lock (&topology->mutex);
    mongoc_topology_scan_once (topology, true /* obey cooldown */);
    bson_mutex_unlock (&topology->mutex);
@@ -1013,6 +1009,20 @@ mongoc_topology_select_server_id (mongoc_topology_t *topology,
    if (topology->single_threaded) {
       _mongoc_topology_description_monitor_opening (&topology->description);
 
+      if (topology->description.type == MONGOC_TOPOLOGY_LOADBALANCED) {
+         /* Bypass server selection loop. Always select the only server. */
+         selected_server = mongoc_topology_description_select (
+            &topology->description, optype, read_prefs, local_threshold_ms);
+
+         if (!selected_server) {
+            _mongoc_server_selection_error (
+                  "No suitable server found in load balanced deployment",
+                  NULL,
+                  error);
+            return 0;
+         }
+      }
+
       tried_once = false;
       next_update = topology->last_scan + heartbeat_msec * 1000;
       if (next_update < loop_start) {
@@ -1110,6 +1120,17 @@ mongoc_topology_select_server_id (mongoc_topology_t *topology,
       selected_server = mongoc_topology_description_select (
          &topology->description, optype, read_prefs, local_threshold_ms);
 
+      if (topology->description.type == MONGOC_TOPOLOGY_LOADBALANCED) {
+         /* Bypass server selection loop. Always select the only server. */
+         if (!selected_server) {
+            _mongoc_server_selection_error (
+                  "No suitable server found in load balanced deployment",
+                  NULL,
+                  error);
+            return 0;
+         }
+      }
+
       if (!selected_server) {
          TRACE (
             "server selection requesting an immediate scan, want %s",