Architectural notes

Acurl creates an event loop to manage its tasks. It communicates among those tasks by using pipes (pairs of file descriptors). Onto the write half of these file descriptors, it writes a stream of (usually) pointers to objects in the processʼs memory. The read half receives these pointers and acts on them. There are 4 pipes hat acurl uses, which are set up in Eventloop_new:

• req_in connects Session_request (write) to start_request (read).
• req_out connects response_complete (write) to Eventloop_get_completed (read). There is a secondary codepath through start_request that also writes to req_out if the request is a so-called dummy request. Dummy requests are an internal API used to convince curl to do (something related to cookie management).
• stop connects Eventloop_stop(write) to stop_eventloop (read). (The implementation of Eventloop_stop was broken until the refactoring at the end of July 2019, indicating that it probably never actually worked.)
• curl_easy_cleanup Response_dealloc (write) to curl_easy_cleanup_in_eventloop (read).