Add preliminary support for trailers

The code is definitely not the best, but as long as it doesn't
break anything it should be OK for now.

src/gun.erl

@@ -359,6 +359,8 @@ await(ServerPid, StreamRef, Timeout, MRef) ->
 			{response, IsFin, Status, Headers};
 		{gun_data, ServerPid, StreamRef, IsFin, Data} ->
 			{data, IsFin, Data};
+		{gun_trailers, ServerPid, StreamRef, Trailers} ->
+			{trailers, Trailers};
 		{gun_push, ServerPid, StreamRef, NewStreamRef, Method, URI, Headers} ->
 			{push, NewStreamRef, Method, URI, Headers};
 		{gun_error, ServerPid, StreamRef, Reason} ->
@@ -395,6 +397,10 @@ await_body(ServerPid, StreamRef, Timeout, MRef, Acc) ->
 				<< Acc/binary, Data/binary >>);
 		{gun_data, ServerPid, StreamRef, fin, Data} ->
 			{ok, << Acc/binary, Data/binary >>};
+		%% It's OK to return trailers here because the client
+		%% specifically requested them.
+		{gun_trailers, ServerPid, StreamRef, Trailers} ->
+			{ok, Acc, Trailers};
 		{gun_error, ServerPid, StreamRef, Reason} ->
 			{error, Reason};
 		{gun_error, ServerPid, Reason} ->

src/gun_http.erl

@@ -27,7 +27,7 @@
 -export([down/1]).
 -export([ws_upgrade/7]).
 
--type io() :: head | {body, non_neg_integer()} | body_close | body_chunked.
+-type io() :: head | {body, non_neg_integer()} | body_close | body_chunked | body_trailer.
 
 %% @todo Make that a record.
 -type websocket_info() :: {websocket, reference(), binary(), [binary()], gun:ws_opts()}. %% key, extensions, options
@@ -101,6 +101,7 @@ handle(Data, State=#http_state{in=head, buffer=Buffer}) ->
 %% Everything sent to the socket until it closes is part of the response body.
 handle(Data, State=#http_state{in=body_close}) ->
 	send_data_if_alive(Data, State, nofin);
+%% Chunked transfer-encoding may contain both data and trailers.
 handle(Data, State=#http_state{in=body_chunked, in_state=InState,
 		buffer=Buffer, connection=Conn}) ->
 	Buffer2 = << Buffer/binary, Data/binary >>,
@@ -121,20 +122,48 @@ handle(Data, State=#http_state{in=body_chunked, in_state=InState,
 			send_data_if_alive(Data2,
 				State#http_state{buffer=Rest, in_state=InState2},
 				nofin);
-		{done, _TotalLength, Rest} ->
+		{done, HasTrailers, Rest} ->
+			IsFin = case HasTrailers of
+				trailers -> nofin;
+				no_trailers -> fin
+			end,
 			%% I suppose it doesn't hurt to append an empty binary.
-			State1 = send_data_if_alive(<<>>, State, fin),
-			case Conn of
-				keepalive ->
+			State1 = send_data_if_alive(<<>>, State, IsFin),
+			case {HasTrailers, Conn} of
+				{trailers, _} ->
+					handle(Rest, State1#http_state{buffer = <<>>, in=body_trailer});
+				{no_trailers, keepalive} ->
 					handle(Rest, end_stream(State1#http_state{buffer= <<>>}));
-				close ->
+				{no_trailers, close} ->
 					close
 			end;
-		{done, Data2, _TotalLength, Rest} ->
-			State1 = send_data_if_alive(Data2, State, fin),
+		{done, Data2, HasTrailers, Rest} ->
+			IsFin = case HasTrailers of
+				trailers -> nofin;
+				no_trailers -> fin
+			end,
+			State1 = send_data_if_alive(Data2, State, IsFin),
+			case {HasTrailers, Conn} of
+				{trailers, _} ->
+					handle(Rest, State1#http_state{buffer = <<>>, in=body_trailer});
+				{no_trailers, keepalive} ->
+					handle(Rest, end_stream(State1#http_state{buffer= <<>>}));
+				{no_trailers, close} ->
+					close
+			end
+	end;
+handle(Data, State=#http_state{in=body_trailer, buffer=Buffer, connection=Conn,
+		streams=[#stream{ref=StreamRef, reply_to=ReplyTo}|_]}) ->
+	Data2 = << Buffer/binary, Data/binary >>,
+	case binary:match(Data2, <<"\r\n\r\n">>) of
+		nomatch -> State#http_state{buffer=Data2};
+		{_, _} ->
+			{Trailers, Rest} = cow_http:parse_headers(Data2),
+			%% @todo We probably want to pass this to gun_content_handler?
+			ReplyTo ! {gun_trailers, self(), stream_ref(StreamRef), Trailers},
 			case Conn of
 				keepalive ->
-					handle(Rest, end_stream(State1#http_state{buffer= <<>>}));
+					handle(Rest, end_stream(State#http_state{buffer= <<>>}));
 				close ->
 					close
 			end

src/gun_http2.erl

@@ -160,9 +160,15 @@ frame({headers, StreamID, IsFin, head_fin, HeaderBlock},
 									remote_fin(Stream#stream{handler_state=Handlers},
 										State#http2_state{decode_state=DecodeState}, IsFin)
 							end;
+						%% @todo For now we assume that it's a trailer if there's no :status.
+						%% A better state machine is needed to distinguish between that and errors.
 						false ->
-							stream_reset(State, StreamID, {stream_error, protocol_error,
-								'Malformed response; missing :status in HEADERS frame. (RFC7540 8.1.2.4)'})
+							%% @todo We probably want to pass this to gun_content_handler?
+							ReplyTo ! {gun_trailers, self(), StreamRef, Headers0},
+							remote_fin(Stream, State#http2_state{decode_state=DecodeState}, fin)
+%%						false ->
+%%							stream_reset(State, StreamID, {stream_error, protocol_error,
+%%								'Malformed response; missing :status in HEADERS frame. (RFC7540 8.1.2.4)'})
 					end
 			catch _:_ ->
 				terminate(State, StreamID, {connection_error, compression_error,