Queue HTTP/2 trailers when there's still data in the buffer

src/cowboy_http2.erl

@@ -46,6 +46,7 @@
 		{cowboy_stream:fin(), non_neg_integer(), iolist()
 			| {sendfile, non_neg_integer(), pos_integer(), file:name_all()}}),
 	local_buffer_size = 0 :: non_neg_integer(),
+	local_trailers = undefined :: undefined | cowboy:http_headers(),
 	%% Whether we finished receiving data.
 	remote = nofin :: cowboy_stream:fin(),
 	%% Remote flow control window (how much we accept to receive).
@@ -663,6 +664,9 @@ resume_streams(State0, [Stream0|Tail], Acc) ->
 			resume_streams(State1, Tail, [Stream|Acc])
 	end.
 
+send_data(State, Stream=#stream{local=Local, local_buffer_size=0, local_trailers=Trailers})
+		when (Trailers =/= undefined) andalso ((Local =:= idle) orelse (Local =:= nofin)) ->
+	send_trailers(State, Stream#stream{local_trailers=undefined}, Trailers);
 %% @todo We might want to print an error if local=fin.
 %%
 %% @todo It's possible that the stream terminates. We must remove it.
@@ -681,12 +685,12 @@ send_data(State0, Stream0=#stream{local_buffer=Q0, local_buffer_size=BufferSize}
 send_data(State, Stream, IsFin, Data) ->
 	send_data(State, Stream, IsFin, Data, in).
 
-%% Always send trailer frames even if the window is empty.
-send_data(State=#state{socket=Socket, transport=Transport, encode_state=EncodeState0},
-		Stream=#stream{id=StreamID}, fin, {trailers, Trailers}, _) ->
-	{HeaderBlock, EncodeState} = headers_encode(Trailers, EncodeState0),
-	Transport:send(Socket, cow_http2:headers(StreamID, fin, HeaderBlock)),
-	{State#state{encode_state=EncodeState}, Stream#stream{local=fin}};
+%% We can send trailers immediately if the queue is empty, otherwise we queue.
+%% We always send trailer frames even if the window is empty.
+send_data(State, Stream=#stream{local_buffer_size=0}, fin, {trailers, Trailers}, _) ->
+	send_trailers(State, Stream, Trailers);
+send_data(State, Stream, fin, {trailers, Trailers}, _) ->
+	{State, Stream#stream{local_trailers=Trailers}};
 %% Send data immediately if we can, buffer otherwise.
 %% @todo We might want to print an error if local=fin.
 send_data(State=#state{local_window=ConnWindow},
@@ -725,6 +729,12 @@ send_data(State=#state{socket=Socket, transport=Transport, local_window=ConnWind
 			end
 	end.
 
+send_trailers(State=#state{socket=Socket, transport=Transport, encode_state=EncodeState0},
+		Stream=#stream{id=StreamID}, Trailers) ->
+	{HeaderBlock, EncodeState} = headers_encode(Trailers, EncodeState0),
+	Transport:send(Socket, cow_http2:headers(StreamID, fin, HeaderBlock)),
+	{State#state{encode_state=EncodeState}, Stream#stream{local=fin}}.
+
 queue_data(Stream=#stream{local_buffer=Q0, local_buffer_size=Size0}, IsFin, Data, In) ->
 	DataSize = case Data of
 		{sendfile, _, Bytes, _} -> Bytes;

test/handlers/resp_h.erl

@@ -212,6 +212,15 @@ do(<<"stream_body">>, Req0, Opts) ->
 	end;
 do(<<"stream_trailers">>, Req0, Opts) ->
 	case cowboy_req:binding(arg, Req0) of
+		<<"large">> ->
+			Req = cowboy_req:stream_reply(200, #{
+				<<"trailer">> => <<"grpc-status">>
+			}, Req0),
+			cowboy_req:stream_body(<<0:800000>>, nofin, Req),
+			cowboy_req:stream_trailers(#{
+				<<"grpc-status">> => <<"0">>
+			}, Req),
+			{ok, Req, Opts};
 		_ ->
 			Req = cowboy_req:stream_reply(200, #{
 				<<"trailer">> => <<"grpc-status">>

test/req_SUITE.erl

@@ -854,6 +854,14 @@ stream_trailers(Config) ->
 	{_, <<"grpc-status">>} = lists:keyfind(<<"trailer">>, 1, RespHeaders),
 	ok.
 
+stream_trailers_large(Config) ->
+	doc("Stream large body followed by trailer headers."),
+	{200, RespHeaders, <<0:800000>>, [
+		{<<"grpc-status">>, <<"0">>}
+	]} = do_trailers("/resp/stream_trailers/large", Config),
+	{_, <<"grpc-status">>} = lists:keyfind(<<"trailer">>, 1, RespHeaders),
+	ok.
+
 stream_trailers_no_te(Config) ->
 	doc("Stream body followed by trailer headers without a te header in the request."),
 	ConnPid = gun_open(Config),