Add the set_options stream handler command

The first two options to benefit from this are the
cowboy_compress_h options.

src/cowboy_compress_h.erl

@@ -34,10 +34,7 @@
 init(StreamID, Req, Opts) ->
 	State0 = check_req(Req),
 	CompressThreshold = maps:get(compress_threshold, Opts, 300),
-	DeflateFlush = case maps:get(compress_buffering, Opts, false) of
-		false -> sync;
-		true -> none
-	end,
+	DeflateFlush = buffering_to_zflush(maps:get(compress_buffering, Opts, false)),
 	{Commands0, Next} = cowboy_stream:init(StreamID, Req, Opts),
 	fold(Commands0, State0#state{next=Next,
 		threshold=CompressThreshold,
@@ -143,10 +140,24 @@ fold([Data0={data, _, _}|Tail], State0=#state{compress=gzip}, Acc) ->
 fold([Trailers={trailers, _}|Tail], State0=#state{compress=gzip}, Acc) ->
 	{{data, fin, Data}, State} = gzip_data({data, fin, <<>>}, State0),
 	fold(Tail, State, [Trailers, {data, nofin, Data}|Acc]);
+%% All the options from this handler can be updated for the current stream.
+fold([{set_options, Opts}|Tail], State=#state{
+		threshold=CompressThreshold0, deflate_flush=DeflateFlush0}, Acc) ->
+	CompressThreshold = maps:get(compress_threshold, Opts, CompressThreshold0),
+	DeflateFlush = case Opts of
+		#{compress_buffering := CompressBuffering} ->
+			buffering_to_zflush(CompressBuffering);
+		_ ->
+			DeflateFlush0
+	end,
+	fold(Tail, State#state{threshold=CompressThreshold, deflate_flush=DeflateFlush}, Acc);
 %% Otherwise, we have an unrelated command or compression is disabled.
 fold([Command|Tail], State, Acc) ->
 	fold(Tail, State, [Command|Acc]).
 
+buffering_to_zflush(true) -> none;
+buffering_to_zflush(false) -> sync.
+
 gzip_response({response, Status, Headers, Body}, State) ->
 	%% We can't call zlib:gzip/1 because it does an
 	%% iolist_to_binary(GzBody) at the end to return

src/cowboy_stream.erl

@@ -41,6 +41,7 @@
 	| {error_response, cowboy:http_status(), cowboy:http_headers(), iodata()}
 	| {switch_protocol, cowboy:http_headers(), module(), state()}
 	| {internal_error, any(), human_reason()}
+	| {set_options, map()}
 	| {log, logger:level(), io:format(), list()}
 	| stop].
 -export_type([commands/0]).

src/cowboy_stream_h.erl

@@ -227,6 +227,9 @@ info(StreamID, Push={push, _, _, _, _, _, _, _}, State) ->
 	do_info(StreamID, Push, [Push], State);
 info(StreamID, SwitchProtocol={switch_protocol, _, _, _}, State) ->
 	do_info(StreamID, SwitchProtocol, [SwitchProtocol], State#state{expect=undefined});
+%% Convert the set_options message to a command.
+info(StreamID, SetOptions={set_options, _}, State) ->
+	do_info(StreamID, SetOptions, [SetOptions], State);
 %% Unknown message, either stray or meant for a handler down the line.
 info(StreamID, Info, State) ->
 	do_info(StreamID, Info, [], State).