GzipHandler fails to set Vary header on 304 responses

[markslater]

Jetty version(s)
11.0.12

Java version/vendor (use: java -version)
openjdk version "11.0.17" 2022-10-18
OpenJDK Runtime Environment (build 11.0.17+8-post-Ubuntu-1ubuntu220.04)
OpenJDK 64-Bit Server VM (build 11.0.17+8-post-Ubuntu-1ubuntu220.04, mixed mode, sharing)

OS type/version
Linux 5.4.0-120-generic

Description
RFC 9110 states that a 304 response must generate a Vary header matching that generated by the equivalent 200 response:

The server generating a 304 response MUST generate any of the following header fields that would have been sent in a 200 (OK) response to the same request:

Content-Location, Date, ETag, and Vary
Cache-Control and Expires (see [CACHING])

Jetty's GzipHandler adds a Vary header for 200 responses it gzips, but doesn't add it for corresponding 304 responses.

GzipHttpOutputInterceptor, which GzipHandler uses to modify responses, infers from the ETag whether a 304 response it is handling refers to a resource it previously compressed, so perhaps the Vary header could be added at this point?

How to reproduce?
Using the following class:

package com.example;

import jakarta.servlet.http.HttpServlet;
import jakarta.servlet.http.HttpServletRequest;
import jakarta.servlet.http.HttpServletResponse;
import org.eclipse.jetty.server.Server;
import org.eclipse.jetty.server.handler.gzip.GzipHandler;
import org.eclipse.jetty.servlet.ServletContextHandler;
import org.eclipse.jetty.servlet.ServletHolder;

import java.io.IOException;
import java.io.Writer;
import java.util.UUID;

public class GzipETagServer {
    public static void main(String[] args) throws Exception {
        final Server server = new Server(8080);
        final ServletContextHandler servletContextHandler = new ServletContextHandler();
        servletContextHandler.addServlet(new ServletHolder(new FooServlet()), "/foo");
        final GzipHandler gzipHandler = new GzipHandler();
        gzipHandler.setHandler(servletContextHandler);
        server.setHandler(gzipHandler);
        server.start();
    }

    private static final class FooServlet extends HttpServlet {

        private final String eTag = '"' + UUID.randomUUID().toString() + '"';

        @Override
        protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws IOException {
            resp.setHeader("etag", eTag);
            if (eTag.equals(req.getHeader("if-none-match"))) {
                resp.setStatus(304);
            } else {
                resp.setStatus(200);
                try (Writer responseWriter = resp.getWriter()) {
                    responseWriter.write("Foo".repeat(100));
                }
            }
        }
    }
}

Visiting http://localhost:8080/foo in a browser will result in a gzipped 200 response with Vary header Vary: Accept-Encoding (and ETag e.g. ETag: "9af0190a-7fb3-4511-873c-75a6fad51345--gzip"). Visiting the same address again will result in a 304 response, but with no Vary header (with the same ETag

[joakime]

Opened PR #8906