Reset trailers on recycled response

Whilst investigating #4711 for jetty-10, it was noticed that trailers are not nulled on recycled Response instances, nor on reset.

Signed-off-by: Greg Wilkins <gregw@webtide.com>

jetty-server/src/main/java/org/eclipse/jetty/server/Response.java

@@ -132,6 +132,7 @@ public class Response implements HttpServletResponse
         _out.recycle();
         _fields.clear();
         _encodingFrom = EncodingFrom.NOT_SET;
+        _trailers = null;
     }
 
     public HttpOutput getHttpOutput()
@@ -1080,6 +1081,7 @@ public class Response implements HttpServletResponse
         _mimeType = null;
         _characterEncoding = null;
         _encodingFrom = EncodingFrom.NOT_SET;
+        _trailers = null;
 
         // Clear all response headers
         _fields.clear();

tests/test-http-client-transport/src/test/java/org/eclipse/jetty/http/client/HttpTrailersTest.java

@@ -24,6 +24,7 @@ import java.io.InputStream;
 import java.nio.charset.StandardCharsets;
 import java.util.Random;
 import java.util.concurrent.TimeUnit;
+import java.util.concurrent.atomic.AtomicBoolean;
 import java.util.concurrent.atomic.AtomicReference;
 import javax.servlet.ServletInputStream;
 import javax.servlet.http.HttpServletRequest;
@@ -165,6 +166,7 @@ public class HttpTrailersTest extends AbstractTest<TransportScenario>
 
     private void testResponseTrailers(byte[] content) throws Exception
     {
+        final AtomicBoolean once = new AtomicBoolean(false);
         String trailerName = "Trailer";
         String trailerValue = "value";
         scenario.start(new AbstractHandler()
@@ -173,12 +175,15 @@ public class HttpTrailersTest extends AbstractTest<TransportScenario>
             public void handle(String target, Request jettyRequest, HttpServletRequest request, HttpServletResponse response) throws IOException
             {
                 jettyRequest.setHandled(true);
-
-                HttpFields trailers = new HttpFields();
-                trailers.put(trailerName, trailerValue);
-
                 Response jettyResponse = (Response)response;
-                jettyResponse.setTrailers(() -> trailers);
+
+                if (once.compareAndSet(false, true))
+                {
+                    HttpFields trailers = new HttpFields();
+                    trailers.put(trailerName, trailerValue);
+                    jettyResponse.setTrailers(() -> trailers);
+                }
+
                 if (content != null)
                     response.getOutputStream().write(content);
             }
@@ -205,6 +210,26 @@ public class HttpTrailersTest extends AbstractTest<TransportScenario>
             .send();
         assertEquals(HttpStatus.OK_200, response.getStatus());
         assertNull(failure.get());
+
+        // subsequent requests should not have trailers
+        response = scenario.client.newRequest(scenario.newURI())
+            .onResponseSuccess(r ->
+            {
+                try
+                {
+                    HttpResponse httpResponse = (HttpResponse)r;
+                    assertNull(httpResponse.getTrailers());
+                    failure.set(null);
+                }
+                catch (Throwable x)
+                {
+                    failure.set(x);
+                }
+            })
+            .timeout(5, TimeUnit.SECONDS)
+            .send();
+        assertEquals(HttpStatus.OK_200, response.getStatus());
+        assertNull(failure.get());
     }
 
     @ParameterizedTest