Both 'Content-Length: 0' and 'Transfer-Encoding: chunked' found when 'Accept-encoding: gzip' header not sent

[ShaharTrasmitSecurity]

According to the http 1.1 spec:

A sender MUST NOT send a Content-Length header field in any message
that contains a Transfer-Encoding header field. (https://tools.ietf.org/html/rfc7230#section-3.3.2).

When testing prometheus metrics exposition, we are creating HTTPServer (in io.prometheus.client.exporter.HTTPServer) to expose metrics to, and sending HTTP requests to query the exposed metrics

When no metrics matches the query and an empty response should return, the following exception occur:

java.lang.IllegalArgumentException: Both 'Content-Length: 0' and 'Transfer-Encoding: chunked' found

After debugging, i found that when gzip encoding is not allowed (through Accept-encoding header), the response will always be encoded as chunked, and will contain the Content-Length: 0 header which conflicts with the HTTP 1.1 spec

** The issue occur only when the content length is 0

[dhoard]

@ShaharTrasmitSecurity I suspect it's in HTTPMetricHandler in this code...

            } else {
                long contentLength = response.size();
                t.getResponseHeaders().set("Content-Length", String.valueOf(contentLength));
                if (t.getRequestMethod().equals("HEAD")) {
                    contentLength = -1;
                }
                t.sendResponseHeaders(HttpURLConnection.HTTP_OK, contentLength);
                response.writeTo(t.getResponseBody());
            }

A JUnit test will need to be written in TestHTTPServer to confirm.

[dhoard]

@ShaharTrasmitSecurity @fstab I have an initial branch of code https://github.com/dhoard/client_java/tree/issue_732 that appears to resolve the issue, but requires refactoring of TestHTTPServer to be able to get HTTP response headers.

Implementing #700 would make this type of test easier.

@ShaharTrasmitSecurity @fstab I have created a fix and PR #738

[dhoard]

@ShaharTrasmitSecurity PR #738 has been merged to resolve the issue in the next release.