MINOR: conn_stream: Add a flag to notify the mux it must respect the reserve

By setting the flag CO_RFL_KEEP_RSV when calling mux->rcv_buf, the
stream-interface notifies the mux it must keep some space to preserve the
buffer's reserve. This flag is only useful for multiplexers handling structured
data, because in such case, the stream-interface cannot know the real amount of
free space in the channel's buffer.

include/types/connection.h

@@ -259,6 +259,7 @@ 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_KEEP_RSV    = 0x0004,    /* Don't fill the reserved space */
 };
 
 /* flags that can be passed to xprt->snd_buf() and mux->snd_buf() */

src/stream_interface.c

@@ -1210,7 +1210,10 @@ int si_cs_recv(struct conn_stream *cs)
 		 * CS_FL_RCV_MORE on the CS if more space is needed.
 		 */
 		max = channel_recv_max(ic);
-		ret = cs->conn->mux->rcv_buf(cs, &ic->buf, max, flags | (co_data(ic) ? CO_RFL_BUF_WET : 0));
+		ret = cs->conn->mux->rcv_buf(cs, &ic->buf, max,
+		          flags |
+		          (co_data(ic) ? CO_RFL_BUF_WET : 0) |
+		          ((channel_recv_limit(ic) < b_size(&ic->buf)) ? CO_RFL_KEEP_RSV : 0));
 		if (cs->flags & CS_FL_RCV_MORE)
 			si_rx_room_blk(si);