Always add vary: accept-encoding in cowboy_compress_h

We must add it even if we don't end up compressing because
it indicates that we might. This indication doesn't mean
that the user agent's accept-encoding values will ever
result in content encoding being applied.

doc/src/manual/cowboy_compress_h.asciidoc

@@ -57,6 +57,8 @@ The compress stream handler does not produce any event.
 
 * *2.11*: Compression is now disabled when the etag
   header is in the response headers.
+* *2.11*: The vary: accept-encoding header is now
+  always set when this handler is enabled.
 * *2.6*: The options `compress_buffering` and
   `compress_threshold` were added.
 * *2.0*: Module introduced.

src/cowboy_compress_h.erl

@@ -103,40 +103,40 @@ check_resp_headers(_, State) ->
 	State.
 
 fold(Commands, State=#state{compress=undefined}) ->
-	{Commands, State};
+	fold_vary_only(Commands, State, []);
 fold(Commands, State) ->
 	fold(Commands, State, []).
 
 fold([], State, Acc) ->
 	{lists:reverse(Acc), State};
 %% We do not compress full sendfile bodies.
 fold([Response={response, _, _, {sendfile, _, _, _}}|Tail], State, Acc) ->
-	fold(Tail, State, [Response|Acc]);
+	fold(Tail, State, [vary_response(Response)|Acc]);
 %% We compress full responses directly, unless they are lower than
 %% the configured threshold or we find we are not able to by looking at the headers.
 fold([Response0={response, _, Headers, Body}|Tail],
 		State0=#state{threshold=CompressThreshold}, Acc) ->
 	case check_resp_headers(Headers, State0) of
 		State=#state{compress=undefined} ->
-			fold(Tail, State, [Response0|Acc]);
+			fold(Tail, State, [vary_response(Response0)|Acc]);
 		State1 ->
 			BodyLength = iolist_size(Body),
 			if
 				BodyLength =< CompressThreshold ->
-					fold(Tail, State1, [Response0|Acc]);
+					fold(Tail, State1, [vary_response(Response0)|Acc]);
 				true ->
 					{Response, State} = gzip_response(Response0, State1),
-					fold(Tail, State, [Response|Acc])
+					fold(Tail, State, [vary_response(Response)|Acc])
 			end
 	end;
 %% Check headers and initiate compression...
 fold([Response0={headers, _, Headers}|Tail], State0, Acc) ->
 	case check_resp_headers(Headers, State0) of
 		State=#state{compress=undefined} ->
-			fold(Tail, State, [Response0|Acc]);
+			fold(Tail, State, [vary_headers(Response0)|Acc]);
 		State1 ->
 			{Response, State} = gzip_headers(Response0, State1),
-			fold(Tail, State, [Response|Acc])
+			fold(Tail, State, [vary_headers(Response)|Acc])
 	end;
 %% then compress each data commands individually.
 fold([Data0={data, _, _}|Tail], State0=#state{compress=gzip}, Acc) ->
@@ -164,6 +164,15 @@ fold([SetOptions={set_options, Opts}|Tail], State=#state{
 fold([Command|Tail], State, Acc) ->
 	fold(Tail, State, [Command|Acc]).
 
+fold_vary_only([], State, Acc) ->
+	{lists:reverse(Acc), State};
+fold_vary_only([Response={response, _, _, _}|Tail], State, Acc) ->
+	fold_vary_only(Tail, State, [vary_response(Response)|Acc]);
+fold_vary_only([Response={headers, _, _}|Tail], State, Acc) ->
+	fold_vary_only(Tail, State, [vary_headers(Response)|Acc]);
+fold_vary_only([Command|Tail], State, Acc) ->
+	fold_vary_only(Tail, State, [Command|Acc]).
+
 buffering_to_zflush(true) -> none;
 buffering_to_zflush(false) -> sync.
 
@@ -183,20 +192,26 @@ gzip_response({response, Status, Headers, Body}, State) ->
 	after
 		zlib:close(Z)
 	end,
-	{{response, Status, vary(Headers#{
+	{{response, Status, Headers#{
 		<<"content-length">> => integer_to_binary(iolist_size(GzBody)),
 		<<"content-encoding">> => <<"gzip">>
-	}), GzBody}, State}.
+	}, GzBody}, State}.
 
 gzip_headers({headers, Status, Headers0}, State) ->
 	Z = zlib:open(),
 	%% We use the same arguments as when compressing the body fully.
 	%% @todo It might be good to allow them to be configured?
 	zlib:deflateInit(Z, default, deflated, 31, 8, default),
 	Headers = maps:remove(<<"content-length">>, Headers0),
-	{{headers, Status, vary(Headers#{
+	{{headers, Status, Headers#{
 		<<"content-encoding">> => <<"gzip">>
-	})}, State#state{deflate=Z}}.
+	}}, State#state{deflate=Z}}.
+
+vary_response({response, Status, Headers, Body}) ->
+	{response, Status, vary(Headers), Body}.
+
+vary_headers({headers, Status, Headers}) ->
+	{headers, Status, vary(Headers)}.
 
 %% We must add content-encoding to vary if it's not already there.
 vary(Headers=#{<<"vary">> := Vary}) ->

test/compress_SUITE.erl

@@ -67,7 +67,7 @@ gzip_accept_encoding_malformed(Config) ->
 	{200, Headers, _} = do_get("/reply/large",
 		[{<<"accept-encoding">>, <<";">>}], Config),
 	false = lists:keyfind(<<"content-encoding">>, 1, Headers),
-	false = lists:keyfind(<<"vary">>, 1, Headers),
+	{_, <<"accept-encoding">>} = lists:keyfind(<<"vary">>, 1, Headers),
 	{_, <<"100000">>} = lists:keyfind(<<"content-length">>, 1, Headers),
 	ok.
 
@@ -76,7 +76,7 @@ gzip_accept_encoding_missing(Config) ->
 	{200, Headers, _} = do_get("/reply/large",
 		[], Config),
 	false = lists:keyfind(<<"content-encoding">>, 1, Headers),
-	false = lists:keyfind(<<"vary">>, 1, Headers),
+	{_, <<"accept-encoding">>} = lists:keyfind(<<"vary">>, 1, Headers),
 	{_, <<"100000">>} = lists:keyfind(<<"content-length">>, 1, Headers),
 	ok.
 
@@ -85,7 +85,7 @@ gzip_accept_encoding_no_gzip(Config) ->
 	{200, Headers, _} = do_get("/reply/large",
 		[{<<"accept-encoding">>, <<"compress">>}], Config),
 	false = lists:keyfind(<<"content-encoding">>, 1, Headers),
-	false = lists:keyfind(<<"vary">>, 1, Headers),
+	{_, <<"accept-encoding">>} = lists:keyfind(<<"vary">>, 1, Headers),
 	{_, <<"100000">>} = lists:keyfind(<<"content-length">>, 1, Headers),
 	ok.
 
@@ -94,7 +94,7 @@ gzip_accept_encoding_not_supported(Config) ->
 	{200, Headers, _} = do_get("/reply/large",
 		[{<<"accept-encoding">>, <<"application/gzip">>}], Config),
 	false = lists:keyfind(<<"content-encoding">>, 1, Headers),
-	false = lists:keyfind(<<"vary">>, 1, Headers),
+	{_, <<"accept-encoding">>} = lists:keyfind(<<"vary">>, 1, Headers),
 	{_, <<"100000">>} = lists:keyfind(<<"content-length">>, 1, Headers),
 	ok.
 
@@ -105,7 +105,7 @@ gzip_reply_content_encoding(Config) ->
 	%% We set the content-encoding to compress; without actually compressing.
 	{_, <<"compress">>} = lists:keyfind(<<"content-encoding">>, 1, Headers),
 	%% The reply didn't include a vary header.
-	false = lists:keyfind(<<"vary">>, 1, Headers),
+	{_, <<"accept-encoding">>} = lists:keyfind(<<"vary">>, 1, Headers),
 	{_, <<"100000">>} = lists:keyfind(<<"content-length">>, 1, Headers),
 	ok.
 
@@ -116,7 +116,7 @@ gzip_reply_etag(Config) ->
 	%% We set a strong etag.
 	{_, <<"\"STRONK\"">>} = lists:keyfind(<<"etag">>, 1, Headers),
 	%% The reply didn't include a vary header.
-	false = lists:keyfind(<<"vary">>, 1, Headers),
+	{_, <<"accept-encoding">>} = lists:keyfind(<<"vary">>, 1, Headers),
 	{_, <<"100000">>} = lists:keyfind(<<"content-length">>, 1, Headers),
 	ok.
 
@@ -136,7 +136,7 @@ gzip_reply_sendfile(Config) ->
 	{200, Headers, Body} = do_get("/reply/sendfile",
 		[{<<"accept-encoding">>, <<"gzip">>}], Config),
 	false = lists:keyfind(<<"content-encoding">>, 1, Headers),
-	false = lists:keyfind(<<"vary">>, 1, Headers),
+	{_, <<"accept-encoding">>} = lists:keyfind(<<"vary">>, 1, Headers),
 	ct:log("Body received:~n~p~n", [Body]),
 	ok.
 
@@ -145,7 +145,7 @@ gzip_reply_small_body(Config) ->
 	{200, Headers, _} = do_get("/reply/small",
 		[{<<"accept-encoding">>, <<"gzip">>}], Config),
 	false = lists:keyfind(<<"content-encoding">>, 1, Headers),
-	false = lists:keyfind(<<"vary">>, 1, Headers),
+	{_, <<"accept-encoding">>} = lists:keyfind(<<"vary">>, 1, Headers),
 	{_, <<"100">>} = lists:keyfind(<<"content-length">>, 1, Headers),
 	ok.
 
@@ -181,7 +181,7 @@ gzip_stream_reply_content_encoding(Config) ->
 	{200, Headers, Body} = do_get("/stream_reply/content-encoding",
 		[{<<"accept-encoding">>, <<"gzip">>}], Config),
 	{_, <<"compress">>} = lists:keyfind(<<"content-encoding">>, 1, Headers),
-	false = lists:keyfind(<<"vary">>, 1, Headers),
+	{_, <<"accept-encoding">>} = lists:keyfind(<<"vary">>, 1, Headers),
 	100000 = iolist_size(Body),
 	ok.
 
@@ -190,7 +190,7 @@ gzip_stream_reply_etag(Config) ->
 	{200, Headers, Body} = do_get("/stream_reply/etag",
 		[{<<"accept-encoding">>, <<"gzip">>}], Config),
 	{_, <<"\"STRONK\"">>} = lists:keyfind(<<"etag">>, 1, Headers),
-	false = lists:keyfind(<<"vary">>, 1, Headers),
+	{_, <<"accept-encoding">>} = lists:keyfind(<<"vary">>, 1, Headers),
 	100000 = iolist_size(Body),
 	ok.