mongoc_server_description_ismaster(3) on Linux
The text 25916337fc3f, as Debian 9 (Stretch) shipped it.
The client or client pool periodically runs an "isMaster" command on each server, to update its view of the MongoDB deployment. Use mongoc_client_get_server_descriptions() and mongoc_server_description_ismaster() to get the most recent "isMaster" response.