fix: correct reply content length

CHANGELOG.md

@@ -5,6 +5,26 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.1.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## [1.0.4] - 2026-08-20
+
+### Added
+
+- `nhttp_h1:encode_response/2` takes `t:nhttp_h1:enc_opts/0`.
+  `#{content_length => omit}` suppresses the automatic `Content-Length`
+  field. A server that answers a `CONNECT` request with a 2xx status uses
+  it, because RFC 9110 Section 8.6 forbids the field there and the response
+  map carries no request method
+
+### Fixed
+
+- `nhttp_h1:encode_response/1` emits `Content-Length: 0` on a response with
+  an empty body. The call omitted the field before, so a client on a
+  persistent connection read the next response as content
+- `nhttp_h1:encode_response/1` emits no `Content-Length` at a 1xx, 204, or
+  304 status (RFC 9110 Section 8.6). At 304 the field is valid only at the
+  length that a 200 response carries, which the encoder cannot compute, so a
+  caller that knows the value supplies it in the header list
+
 ## [1.0.3] - 2026-08-10
 
 ### Added

src/nhttp_h1.erl

@@ -110,6 +110,7 @@ pattern applies to chunked requests.
     encode_last_chunk/0,
     encode_request/1,
     encode_response/1,
+    encode_response/2,
     encode_response_head/3
 ]).
 
@@ -128,6 +129,7 @@ pattern applies to chunked requests.
     body_mode/0,
     body_stream/0,
     chunked_st/0,
+    enc_opts/0,
     opts/0,
     parse_error/0,
     parse_result/1,
@@ -158,6 +160,21 @@ pattern applies to chunked requests.
     | until_close
     | none.
 
+-doc """
+Encoder options for `encode_response/2`.
+
+`content_length` selects how the encoder frames a response:
+
+- `auto` (the default) adds `Content-Length` when the header list carries
+  neither `content-length` nor `transfer-encoding`, and the status permits
+  the field.
+- `omit` suppresses the automatic field at any status. A server that
+  answers a `CONNECT` request with a 2xx status uses it, because RFC 9110
+  Section 8.6 forbids the field there and the response map carries no
+  request method.
+""".
+-type enc_opts() :: #{content_length => auto | omit}.
+
 -type opts() :: #{
     max_header_size => pos_integer(),
     max_headers_count => pos_integer(),
@@ -597,7 +614,8 @@ encode_request(#{method := Method, path := Path} = Req) ->
     Version = maps:get(version, Req, http1_1),
     Headers = maps:get(headers, Req, []),
     Body = maps:get(body, Req, <<>>),
-    FinalHeaders = maybe_add_content_length(Headers, Body),
+    Len = iolist_size(Body),
+    FinalHeaders = maybe_add_content_length(Headers, Len, Len > 0),
     [
         nhttp_lib:encode_method(Method),
         <<" ">>,
@@ -610,14 +628,42 @@ encode_request(#{method := Method, path := Path} = Req) ->
         Body
     ].
 
--doc "Encode an HTTP/1.1 response to iolist.".
+-doc """
+Encode an HTTP/1.1 response to iolist.
+
+Equivalent to `encode_response(Resp, #{})`. The encoder adds
+`Content-Length` when the header list carries neither `content-length` nor
+`transfer-encoding`, including a `Content-Length: 0` on an empty body.
+
+The encoder adds no `Content-Length` at a 1xx, 204, or 304 status. RFC 9110
+Section 8.6 forbids the field at 1xx and 204. It permits the field at 304
+only at the length that a 200 response would have carried, which this
+encoder cannot compute, so a caller that knows the value supplies it in the
+header list.
+
+A 2xx response to a `CONNECT` request also carries no `Content-Length`. The
+response map holds no request method, so that case needs
+`encode_response/2` with `#{content_length => omit}`.
+""".
 -spec encode_response(resp()) -> iolist().
-encode_response(#{status := Status} = Resp) ->
+encode_response(Resp) ->
+    encode_response(Resp, #{}).
+
+-doc """
+Encode an HTTP/1.1 response to iolist under the given encoder options.
+
+See `encode_response/1` for the framing rules and `t:enc_opts/0` for the
+options.
+""".
+-spec encode_response(resp(), enc_opts()) -> iolist().
+encode_response(#{status := Status} = Resp, EncOpts) ->
     Version = maps:get(version, Resp, http1_1),
     Reason = maps:get(reason, Resp, <<>>),
     Headers = maps:get(headers, Resp, []),
     Body = maps:get(body, Resp, <<>>),
-    FinalHeaders = maybe_add_content_length(Headers, Body),
+    FinalHeaders = maybe_add_content_length(
+        Headers, iolist_size(Body), allows_content_length(Status, EncOpts)
+    ),
     [
         encode_version(Version),
         <<" ">>,
@@ -1024,23 +1070,34 @@ is_token_chars(<<C, Rest/binary>>) ->
 is_valid_chunk_ext_tail(<<>>) -> true;
 is_valid_chunk_ext_tail(Bin) -> skip_bws_to_semi(Bin).
 
--spec maybe_add_content_length(nhttp_lib:headers(), iodata()) -> nhttp_lib:headers().
-maybe_add_content_length(Headers, Body) ->
-    case iolist_size(Body) of
-        0 ->
+-spec maybe_add_content_length(nhttp_lib:headers(), non_neg_integer(), boolean()) ->
+    nhttp_lib:headers().
+maybe_add_content_length(Headers, _Len, false) ->
+    Headers;
+maybe_add_content_length(Headers, Len, true) ->
+    case
+        nhttp_headers:has(<<"content-length">>, Headers) orelse
+            nhttp_headers:has(<<"transfer-encoding">>, Headers)
+    of
+        true ->
             Headers;
-        Len ->
-            case
-                nhttp_headers:has(<<"content-length">>, Headers) orelse
-                    nhttp_headers:has(<<"transfer-encoding">>, Headers)
-            of
-                true ->
-                    Headers;
-                false ->
-                    [{<<"content-length">>, integer_to_binary(Len)} | Headers]
-            end
+        false ->
+            [{<<"content-length">>, integer_to_binary(Len)} | Headers]
     end.
 
+-spec allows_content_length(nhttp_lib:status(), enc_opts()) -> boolean().
+allows_content_length(Status, EncOpts) ->
+    case maps:get(content_length, EncOpts, auto) of
+        auto -> not forbids_content_length(Status);
+        omit -> false
+    end.
+
+-spec forbids_content_length(nhttp_lib:status()) -> boolean().
+forbids_content_length(Status) when Status >= 100, Status =< 199 -> true;
+forbids_content_length(204) -> true;
+forbids_content_length(304) -> true;
+forbids_content_length(_Status) -> false.
+
 -spec parse_chunk_body_after_size(binary(), non_neg_integer(), non_neg_integer(), binary()) ->
     {ok, binary(), pos_integer()}
     | {final, pos_integer()}

src/nhttp_lib.app.src

@@ -1,6 +1,6 @@
 {application, nhttp_lib, [
     {description, "HTTP protocol primitives for Erlang/OTP 27+ (HTTP/1.1, HTTP/2, HTTP/3, QPACK)"},
-    {vsn, "1.0.3"},
+    {vsn, "1.0.4"},
     {registered, []},
     {applications, [
         kernel,

test/compliance/nhttp_h1_rfc9110_SUITE.erl

@@ -34,6 +34,11 @@ groups() ->
         ]},
         {section_8_content_length, [parallel], [
             no_content_length_1xx_204,
+            no_content_length_304,
+            content_length_zero_on_empty_body,
+            caller_content_length_survives_on_204,
+            no_content_length_with_transfer_encoding,
+            content_length_omit_opt_out,
             reject_malformed_content_length
         ]},
         {section_9_methods, [parallel], [
@@ -106,24 +111,88 @@ reject_or_replace_invalid_chars_in_field_value(_Config) ->
 %%% Section 8 - Content-Length
 %%%-----------------------------------------------------------------------------
 
+%% RFC 9110 Section 8.6: "A server MUST NOT send a Content-Length header
+%% field in any response with a status code of 1xx (Informational) or 204
+%% (No Content)."
 no_content_length_1xx_204(_Config) ->
-    Resp100 = #{
-        status => 100,
-        reason => <<"Continue">>,
-        headers => []
-    },
-    Io100 = nhttp_h1:encode_response(Resp100),
-    Encoded100 = iolist_to_binary(Io100),
-    ?assertEqual(nomatch, binary:match(Encoded100, <<"Content-Length">>)),
+    lists:foreach(
+        fun({Status, Reason}) ->
+            Resp = #{status => Status, reason => Reason, headers => []},
+            Encoded = iolist_to_binary(nhttp_h1:encode_response(Resp)),
+            ?assertEqual(false, has_content_length(Encoded))
+        end,
+        [
+            {100, <<"Continue">>},
+            {101, <<"Switching Protocols">>},
+            {199, <<"Informational">>},
+            {204, <<"No Content">>}
+        ]
+    ).
+
+%% RFC 9110 Section 8.6: a 304 permits Content-Length only at the length a
+%% 200 response would have carried, which the encoder cannot compute.
+no_content_length_304(_Config) ->
+    Resp = #{status => 304, reason => <<"Not Modified">>, headers => []},
+    Encoded = iolist_to_binary(nhttp_h1:encode_response(Resp)),
+    ?assertEqual(false, has_content_length(Encoded)).
+
+%% RFC 9110 Section 8.6: "in the absence of Transfer-Encoding, an origin
+%% server SHOULD send a Content-Length header field when the content size is
+%% known prior to sending the complete header section."
+content_length_zero_on_empty_body(_Config) ->
+    NoBodyKey = #{status => 200, reason => <<"OK">>, headers => []},
+    ?assertEqual(
+        {true, <<"0">>},
+        content_length_value(iolist_to_binary(nhttp_h1:encode_response(NoBodyKey)))
+    ),
+
+    EmptyBody = NoBodyKey#{body => <<>>},
+    ?assertEqual(
+        {true, <<"0">>},
+        content_length_value(iolist_to_binary(nhttp_h1:encode_response(EmptyBody)))
+    ),
+
+    NotFound = #{status => 404, reason => <<"Not Found">>, headers => [], body => <<>>},
+    ?assertEqual(
+        {true, <<"0">>},
+        content_length_value(iolist_to_binary(nhttp_h1:encode_response(NotFound)))
+    ).
 
-    Resp204 = #{
+caller_content_length_survives_on_204(_Config) ->
+    Resp = #{
         status => 204,
         reason => <<"No Content">>,
-        headers => []
+        headers => [{<<"content-length">>, <<"42">>}]
+    },
+    Encoded = iolist_to_binary(nhttp_h1:encode_response(Resp)),
+    ?assertEqual({true, <<"42">>}, content_length_value(Encoded)),
+    ?assertEqual(1, count_content_length(Encoded)).
+
+no_content_length_with_transfer_encoding(_Config) ->
+    Resp = #{
+        status => 200,
+        reason => <<"OK">>,
+        headers => [{<<"transfer-encoding">>, <<"chunked">>}],
+        body => <<>>
     },
-    Io204 = nhttp_h1:encode_response(Resp204),
-    Encoded204 = iolist_to_binary(Io204),
-    ?assertEqual(nomatch, binary:match(Encoded204, <<"Content-Length">>)).
+    Encoded = iolist_to_binary(nhttp_h1:encode_response(Resp)),
+    ?assertEqual(false, has_content_length(Encoded)).
+
+%% RFC 9110 Section 8.6: "A server MUST NOT send a Content-Length header
+%% field in any 2xx (Successful) response to a CONNECT request."
+content_length_omit_opt_out(_Config) ->
+    Resp = #{status => 200, reason => <<"Connection Established">>, headers => []},
+    Encoded = iolist_to_binary(nhttp_h1:encode_response(Resp, #{content_length => omit})),
+    ?assertEqual(false, has_content_length(Encoded)),
+
+    WithBody = Resp#{body => <<"hello">>},
+    EncodedWithBody = iolist_to_binary(
+        nhttp_h1:encode_response(WithBody, #{content_length => omit})
+    ),
+    ?assertEqual(false, has_content_length(EncodedWithBody)),
+
+    EncodedAuto = iolist_to_binary(nhttp_h1:encode_response(Resp, #{content_length => auto})),
+    ?assertEqual({true, <<"0">>}, content_length_value(EncodedAuto)).
 
 reject_malformed_content_length(_Config) ->
     Req1 = <<
@@ -158,16 +227,18 @@ reject_malformed_content_length(_Config) ->
 %%% Section 9 - Methods
 %%%-----------------------------------------------------------------------------
 
+%% RFC 9110 Section 8.6: a 2xx response to CONNECT carries no Content-Length.
+%% The status alone does not identify the case, so the caller opts out.
 no_body_headers_2xx_connect(_Config) ->
     Resp = #{
         status => 200,
         reason => <<"Connection Established">>,
         headers => []
     },
-    Io = nhttp_h1:encode_response(Resp),
+    Io = nhttp_h1:encode_response(Resp, #{content_length => omit}),
     Encoded = iolist_to_binary(Io),
-    ?assertEqual(nomatch, binary:match(Encoded, <<"Content-Length">>)),
-    ?assertEqual(nomatch, binary:match(Encoded, <<"Transfer-Encoding">>)).
+    ?assertEqual(false, has_content_length(Encoded)),
+    ?assertEqual(false, lists:keymember(<<"transfer-encoding">>, 1, encoded_headers(Encoded))).
 
 client_ignore_body_headers_connect(_Config) ->
     Resp = <<
@@ -215,6 +286,31 @@ no_body_in_304(_Config) ->
 %%% Helpers
 %%%-----------------------------------------------------------------------------
 
+-spec encoded_headers(binary()) -> nhttp_lib:headers().
+encoded_headers(Encoded) ->
+    [_StatusLine | Lines] = binary:split(Encoded, <<"\r\n">>, [global]),
+    [
+        {string:lowercase(Name), string:trim(Value, leading, " ")}
+     || Line <- Lines,
+        Line =/= <<>>,
+        [Name, Value] <- [binary:split(Line, <<":">>)]
+    ].
+
+-spec has_content_length(binary()) -> boolean().
+has_content_length(Encoded) ->
+    lists:keymember(<<"content-length">>, 1, encoded_headers(Encoded)).
+
+-spec content_length_value(binary()) -> {true, binary()} | false.
+content_length_value(Encoded) ->
+    case lists:keyfind(<<"content-length">>, 1, encoded_headers(Encoded)) of
+        {_, Value} -> {true, Value};
+        false -> false
+    end.
+
+-spec count_content_length(binary()) -> non_neg_integer().
+count_content_length(Encoded) ->
+    length([V || {<<"content-length">>, V} <- encoded_headers(Encoded)]).
+
 -spec find_header(binary(), nhttp_lib:headers()) -> {ok, binary()} | error.
 find_header(Name, Headers) ->
     LowerName = string:lowercase(Name),

test/nhttp_h1_SUITE.erl

@@ -533,7 +533,8 @@ encode_response_no_body(_Config) ->
     },
     IOList = nhttp_h1:encode_response(Resp),
     Bin = iolist_to_binary(IOList),
-    ?assertMatch(<<"HTTP/1.1 204 No Content\r\n", _/binary>>, Bin).
+    ?assertMatch(<<"HTTP/1.1 204 No Content\r\n", _/binary>>, Bin),
+    ?assertEqual(nomatch, re:run(Bin, <<"content-length">>, [caseless])).
 
 encode_response_head(_Config) ->
     IOList = nhttp_h1:encode_response_head(

test/property_test/nhttp_h1_props.erl

@@ -130,10 +130,18 @@ h1_resp_gen() ->
             status => Status,
             reason => Reason,
             headers => Headers,
-            body => Body
+            body => body_for_status(Status, Body)
         }
     ).
 
+-spec body_for_status(nhttp_lib:status(), binary()) -> binary().
+body_for_status(Status, _Body) when
+    Status >= 100, Status =< 199; Status =:= 204; Status =:= 304
+->
+    <<>>;
+body_for_status(_Status, Body) ->
+    Body.
+
 -spec method_gen() -> triq_dom:domain().
 method_gen() ->
     oneof([get, post, put, delete, head, options, patch]).