ADDITIONAL SYSTEM INFORMATION :
Oracle Linux 9
Oracle JDK 17.0.6

A DESCRIPTION OF THE PROBLEM :
When making calls using HttpClient to an endpoint which returns a 204 and no content, the jdk.internal.net.http.ResponseSubscribers.HttpResponseInputStream.available() method returns 1 instead of the expected result of 0 when called on the result.

REGRESSION : Last worked in version 11.0.18

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Execute a call against an endpoint returning a 204 and no content like this:
      HttpClient client = HttpClient.newHttpClient();
      HttpResponse<InputStream> resp = client.send(req, HttpResponse.BodyHandlers.ofInputStream());
      try (InputStream in = resp.body()) {
        int available = in.available();
        if (available > 0) {
          throw new RuntimeException("Received available()" + available);
        }
      }

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
available() should return 0.
ACTUAL -
available() returns 1. This occurs because the buffers queue within the client contains a single HeapByteBuffer with a capacity of 0.

---------- BEGIN SOURCE ----------
import java.io.IOException;
import java.io.InputStream;
import java.net.*;
import java.net.http.*;

import com.sun.net.httpserver.HttpServer;

public class TestHttpClient {
  public static void main(String... args)
      throws IOException, URISyntaxException, InterruptedException {
    // start a basic server that responds with 204 to all requests
    InetAddress address = InetAddress.getLoopbackAddress();
    InetSocketAddress socketAddress = new InetSocketAddress(address.getHostName(), 8080);
    HttpServer server = HttpServer.create(socketAddress, 20);
    server.createContext("/", e -> e.sendResponseHeaders(204, -1));
    server.start();

    // call it with a client
    try {
      HttpRequest req = HttpRequest.newBuilder()
          .uri(new URI("http", null, address.getHostName(), 8080, null, null, null)).GET().build();
      HttpClient client = HttpClient.newHttpClient();
      HttpResponse<InputStream> resp = client.send(req, HttpResponse.BodyHandlers.ofInputStream());
      try (InputStream in = resp.body()) {
        int available = in.available();
        if (available > 0) {
          throw new RuntimeException("Received available()" + available);
        }
      }
    }
    finally {
      server.stop(0);
    }
  }
}
---------- END SOURCE ----------

FREQUENCY : always