Response headers not sent when returning empty body tuple

[lightuponcloud]

Cowboy version: (e.g. 2.9.0)

Erlang/OTP version: (e.g. OTP 25.2)

Problem
When returning {<<>>, Req, State} after setting response headers, Cowboy does not send the headers.
Example:

Req1 = cowboy_req:set_resp_header(<<"content-type">>, <<"application/x-ndjson">>, Req0),
Req2 = cowboy_req:set_resp_header(<<"x-change-count">>, <<"0">>, Req1),
{<<>>, Req2, State}.

No response headers appear in the client response.

However, if the body is non-empty, e.g.:

Result = <<"line1\n">>,
Req1 = cowboy_req:set_resp_header(<<"content-type">>, <<"application/x-ndjson">>, Req0),
Req2 = cowboy_req:set_resp_header(<<"x-change-count">>, <<"1">>, Req1),
{Result, Req2, State}.

…the headers are included.

Expected Behavior
Setting headers followed by returning a body tuple—even if the body is empty—should result in headers being sent, or Cowboy documentation should clarify the behavior.

Actual Behavior
Returning {<<>>, Req, State} results in no headers being sent, suggesting Cowboy considers this as “no reply yet.”

Question
Is this intended behavior?
If so, what is the correct way to send an empty body with custom headers?

[essen]

I assume this is cowboy_rest, right?

I don't know about Cowboy 2.9, it's old, but I would expect that if you return an empty body, that x-change-count is set in the response in all cases, and that content-type is set if the response is a 200 response.

Best would be a small module replicating the behavior.

[lightuponcloud]

I cannot reproduce the issue anymore. Maybe Nginx was replacing some headers.
I'm attaching small module anyway:
toy.tar.gz

[essen]

Thanks I will create a test case when I work on the next release.