resteasy-reactive-client: Streaming JAX-RS client response while also accessing response headers

[otaconix]

Related to #50813: how can I stream the response body using a JAX-RS Client, while also accessing the response headers?

Context: I am trying to build a resource that fetches a (potentially large) file from another service, and send that document to clients in the resource's response. One of the things I want to achieve is to minimize resource utilization during this operation (so no slurping the entire response into memory).

My current experiment looks like this:

@GET
@Path("/streaming-response")
@Produces(MediaType.APPLICATION_OCTET_STREAM)
public Response getStreamingResponse(@QueryParam("url") URI url, @QueryParam("filename") String fileName) {
    if (fileName == null) {
        fileName = UUID.randomUUID() + ".txt";
    }

    Client client = ClientBuilder.newClient();
    InputStream upstreamResponse =
            client.target(url).request(MediaType.APPLICATION_OCTET_STREAM).get(InputStream.class);

    Response.ResponseBuilder response = Response.ok();
    response.header(HttpHeaders.CONTENT_DISPOSITION, "attachment; filename=" + fileName);
    response.header(HttpHeaders.CONTENT_LENGTH, null);

    return response.entity((StreamingOutput) output -> {
                byte[] buffer = new byte[streamingResponseBufferSize]; // 1 MiB buffer

                try {
                    int readBytes;
                    do {
                        readBytes = upstreamResponse.read(buffer);
                        output.write(buffer, 0, readBytes);
                        output.flush();
                    } while (readBytes >= 0);
                } finally {
                    client.close();
                }
            })
            .build();
}

This "works", as in I'm only using an 8KiB buffer per request to this endpoint, but I have no access to the response headers from the upstream service. Is there a way to combine this streaming approach with the ability to see the response headers?

EDIT: fixed issue where I'd write the full buffer even if fewer bytes were read from the InputStream.

[otaconix]

I'd really appreciate it if someone could help explain this to me. I've tried running Quarkus in a debugger to find out where InputStream's special-casing happens, because I'd already tried writing a MessageBodyReader for some:

record HeadersAndInputStream(MultivaluedMap<String, String> headers, InputStream bodyStream) {}

to no avail. I mean, it works, but it seems to consume the InputStream before I can get to it.