It calls S__fillbuf() but only if there’s not enough data in the stream buffer, which in my case, there is 
My use case is possibly a bit weird - and the underlying issue may have actually been fixed in a later version of Prolog. The gist is (according to my notes from 2 years ago on the ticket) that I’m dealing with HTTP post requests in keep-alive connections. In the case where the handler reads only one part of the multipart document (for example, if the same content is represented in both text and HTML), processes it, and succeeds, the socket is left with the rest of the multipart document pending on it, meaning the thread reading the next request receives a lot of unexpected stuff (the remainder of the previous request). To fix this, I wrapped the multipart handler in a predicate which gets the current stream position at the start of the handler, gets the stream position at the end, looks at the content-length (it’s more complex with chunked, but you get the idea) and then discards the unread part of the body. Obviously this doesn’t work if the stream pointer hasn’t been updated as the stream has been processed.
It might be as simple as moving stream->bufp after successfully consuming the data in the stream…