Send Content-Length: 0 for empty POST/PUT/PATCH bodies

Like curl, add an explicit Content-Length: 0 when a body-bearing method
(POST/PUT/PATCH) is sent with an empty body, unless the caller already set
one. Bodyless methods (GET/HEAD/DELETE) stay without it. Fixes servers such
as AWS that require Content-Length even for a 0-length body.

Author: benoitc

src/hackney_conn.erl

@@ -2115,7 +2115,7 @@ compute_netloc(Host, Port, Transport) ->
     end.
 
 %% @private Build request headers
-build_headers(_Method, Headers0, Body, Netloc) ->
+build_headers(Method, Headers0, Body, Netloc) ->
     %% Start with user headers
     Headers1 = hackney_headers:new(Headers0),
 
@@ -2127,8 +2127,16 @@ build_headers(_Method, Headers0, Body, Netloc) ->
 
     %% Add Content-Length for bodies
     case Body of
-        <<>> -> Headers3;
-        [] -> Headers3;
+        B when B =:= <<>>; B =:= [] ->
+            %% Empty body: like curl, send Content-Length: 0 for methods that
+            %% carry a body (POST/PUT/PATCH) so a server that requires the
+            %% header (e.g. AWS) still gets it; leave bodyless methods
+            %% (GET/HEAD/DELETE/...) without one.
+            case body_method(Method) andalso
+                 not hackney_headers:is_key(<<"content-length">>, Headers3) of
+                true -> hackney_headers:store(<<"Content-Length">>, <<"0">>, Headers3);
+                false -> Headers3
+            end;
         _ when is_binary(Body) ->
             Len = byte_size(Body),
             case hackney_headers:is_key(<<"content-length">>, Headers3) of
@@ -2148,6 +2156,16 @@ build_headers(_Method, Headers0, Body, Netloc) ->
             Headers3
     end.
 
+%% @private Methods that carry a request body, so an empty body still gets an
+%% explicit Content-Length: 0 (curl does the same for POST/PUT/PATCH).
+body_method(Method) ->
+    case hackney_bstr:to_upper(hackney_bstr:to_binary(Method)) of
+        <<"POST">> -> true;
+        <<"PUT">> -> true;
+        <<"PATCH">> -> true;
+        _ -> false
+    end.
+
 %% @private Convert headers to binary
 headers_to_binary(Headers) ->
     hackney_headers:to_binary(Headers).

test/hackney_conn_content_length_tests.erl

@@ -0,0 +1,72 @@
+%%% Content-Length on empty request bodies, curl-style: body-bearing methods
+%%% (POST/PUT/PATCH) get an explicit Content-Length: 0; bodyless methods do not,
+%%% and a caller-supplied Content-Length is never duplicated.
+-module(hackney_conn_content_length_tests).
+
+-include_lib("eunit/include/eunit.hrl").
+
+post_empty_body_gets_content_length_zero_test() ->
+    Req = capture(<<"POST">>, [], <<>>),
+    ?assertNotEqual(nomatch, cl(Req, <<"0">>)).
+
+put_empty_body_gets_content_length_zero_test() ->
+    Req = capture(<<"PUT">>, [], <<>>),
+    ?assertNotEqual(nomatch, cl(Req, <<"0">>)).
+
+get_empty_body_has_no_content_length_test() ->
+    Req = capture(<<"GET">>, [], <<>>),
+    ?assertEqual(nomatch, binary:match(lower(Req), <<"content-length:">>)).
+
+delete_empty_body_has_no_content_length_test() ->
+    Req = capture(<<"DELETE">>, [], <<>>),
+    ?assertEqual(nomatch, binary:match(lower(Req), <<"content-length:">>)).
+
+user_content_length_not_duplicated_test() ->
+    Req = capture(<<"POST">>, [{<<"Content-Length">>, <<"5">>}], <<>>),
+    %% Exactly one content-length header, and it is the caller's value.
+    Matches = binary:matches(lower(Req), <<"content-length:">>),
+    ?assertEqual(1, length(Matches)),
+    ?assertNotEqual(nomatch, cl(Req, <<"5">>)).
+
+%% Find "content-length: <V>" case-insensitively.
+cl(Req, V) ->
+    binary:match(lower(Req), <<"content-length: ", V/binary, "\r\n">>).
+
+lower(Bin) -> hackney_bstr:to_lower(Bin).
+
+%% Drive a real buffered request into a raw listener and return the request
+%% bytes the server received.
+capture(Method, Headers, Body) ->
+    {ok, _} = application:ensure_all_started(hackney),
+    {ok, LSock} = gen_tcp:listen(0, [binary, {active, false},
+                                     {reuseaddr, true}, {ip, {127, 0, 0, 1}}]),
+    {ok, Port} = inet:port(LSock),
+    Parent = self(),
+    spawn(fun() ->
+        case gen_tcp:accept(LSock, 3000) of
+            {ok, S} ->
+                ReqBytes = recv_headers(S, <<>>),
+                gen_tcp:send(S, <<"HTTP/1.1 200 OK\r\nContent-Length: 0\r\n\r\n">>),
+                Parent ! {captured, ReqBytes},
+                gen_tcp:close(S);
+            _ -> Parent ! {captured, <<>>}
+        end,
+        gen_tcp:close(LSock)
+    end),
+    {ok, Pid} = hackney_conn:start_link(#{host => "127.0.0.1", port => Port,
+                                          transport => hackney_tcp,
+                                          connect_timeout => 1000}),
+    ok = hackney_conn:connect(Pid, 1000),
+    {ok, 200, _} = hackney_conn:request(Pid, Method, <<"/">>, Headers, Body),
+    catch hackney_conn:stop(Pid),
+    receive {captured, Req} -> Req after 3000 -> error(no_capture) end.
+
+recv_headers(S, Acc) ->
+    case binary:match(Acc, <<"\r\n\r\n">>) of
+        nomatch ->
+            case gen_tcp:recv(S, 0, 2000) of
+                {ok, D} -> recv_headers(S, <<Acc/binary, D/binary>>);
+                {error, _} -> Acc
+            end;
+        _ -> Acc
+    end.