mongoc_client_pool_pop (mongoc_client_pool_t *pool)
{
   mongoc_client_t *client;

   ENTRY;

   BSON_ASSERT (pool);

   bson_mutex_lock (&pool->mutex);

again:
   if (!(client = (mongoc_client_t *) _mongoc_queue_pop_head (&pool->queue))) {
      if (pool->size < pool->max_pool_size) {
         client = _mongoc_client_new_from_uri (pool->topology);

         /* for tests */
         mongoc_client_set_stream_initiator (
            client,
            pool->topology->scanner->initiator,
            pool->topology->scanner->initiator_context);

         client->error_api_version = pool->error_api_version;
         _mongoc_client_set_apm_callbacks_private (
            client, &pool->apm_callbacks, pool->apm_context);