MINOR: connection: introduce a new receive flag: CO_RFL_READ_ONCE

This flag is currently supported by raw_sock to perform a single recv()
attempt and avoid subscribing. Typically on the request and response
paths with keep-alive, with short messages we know that it's very likely
that the first message is enough.

include/types/connection.h

@@ -284,6 +284,7 @@ enum {
 enum {
 	CO_RFL_BUF_WET     = 0x0001,    /* Buffer still has some output data present */
 	CO_RFL_BUF_FLUSH   = 0x0002,    /* Flush mux's buffers but don't read more data */
+	CO_RFL_READ_ONCE   = 0x0004,    /* don't loop even if the request/response is small */
 };
 
 /* flags that can be passed to xprt->snd_buf() and mux->snd_buf() */

src/raw_sock.c

@@ -292,6 +292,9 @@ static size_t raw_sock_to_buf(struct connection *conn, void *xprt_ctx, struct bu
 				}
 			}
 			count -= ret;
+
+			if (flags & CO_RFL_READ_ONCE)
+				break;
 		}
 		else if (ret == 0) {
 			goto read0;