Add keepalive function that resets the inactivity timer

Can be used in a streaming loop when we expect more data to arrive
and want to prevent an inactivity timeout.

src/cowboy_http.erl

@@ -255,6 +255,8 @@ loop(State=#state{parent=Parent, socket=Socket, transport=Transport, opts=Opts,
 		%% Messages pertaining to a stream.
 		{{Pid, StreamID}, Msg} when Pid =:= self() ->
 			loop(info(State, StreamID, Msg));
+		{{Pid, _StreamID}, keepalive} when Pid =:= self() ->
+			loop(State);
 		%% Exit signal from children.
 		Msg = {'EXIT', Pid, _} ->
 			loop(down(State, Pid, Msg));

src/cowboy_req.erl

@@ -82,6 +82,7 @@
 -export([stream_reply/3]).
 %% @todo stream_body/2 (nofin)
 -export([stream_body/3]).
+-export([stream_keepalive/1]).
 %% @todo stream_events/2 (nofin)
 -export([stream_events/3]).
 -export([stream_trailers/2]).
@@ -889,6 +890,10 @@ stream_body(Msg, Req=#{pid := Pid}) ->
 	cast(Msg, Req),
 	receive {data_ack, Pid} -> ok end.
 
+-spec stream_keepalive(req()) -> ok.
+stream_keepalive(Req) ->
+	cast(keepalive, Req).
+
 -spec stream_events(cow_sse:event() | [cow_sse:event()], fin | nofin, req()) -> ok.
 stream_events(Event, IsFin, Req) when is_map(Event) ->
 	stream_events([Event], IsFin, Req);