Porting away from SoupSessionAsync made soup_session_send_message block the whole server, so it is now impossible to do self-introspection beacause of this note in the Soup porting guide:

Differences in SoupMessage-sending APIs

SoupSessionAsync always uses asynchronous I/O, and SoupSessionSync always uses blocking I/O, regardless of the operation. In the new SoupSession, soup_session_queue_message() uses asynchronous I/O (like SoupSessionAsync), and soup_session_send_message() uses blocking I/O (like SoupSessionSync). There is no API on the plain SoupSession that simulates the effect of calling soup_session_send_message() on a SoupSessionAsync (ie, running the main loop internally), or of calling soup_session_queue_message() on a SoupSessionSync (ie, automatically sending the request in another thread).