Improve the filter test for ResourceServletTest to expose Issue jetty…

…#12766

jetty-ee10/jetty-ee10-servlet/src/test/java/org/eclipse/jetty/ee10/servlet/ResourceServletTest.java

@@ -106,6 +106,7 @@
 import static org.hamcrest.Matchers.notNullValue;
 import static org.hamcrest.Matchers.nullValue;
 import static org.hamcrest.Matchers.startsWith;
+import static org.junit.jupiter.api.Assertions.assertFalse;
 import static org.junit.jupiter.api.Assertions.assertNotNull;
 import static org.junit.jupiter.api.Assumptions.assumeTrue;
 
@@ -2407,6 +2408,7 @@ public void testOutputStreamAndCharsetFiltered() throws Exception
         assertThat(response, containsHeaderValue(HttpHeader.CONTENT_LENGTH, String.valueOf(body.length())));
         assertThat(response, containsHeaderValue(HttpHeader.CONTENT_TYPE, "text/plain;charset=UTF-8"));
         assertThat(body, containsString("Extra Info"));
+        assertThat("https://github.com/jetty/jetty.project/issues/12766", response, containsHeaderValue(OutputFilter.POST_DO_FILTER_HEADER_NAME, "true"));
 
         rawResponse = connector.getResponse("""
             GET /context/image.jpg HTTP/1.1\r
@@ -2420,6 +2422,7 @@ public void testOutputStreamAndCharsetFiltered() throws Exception
         assertThat(response, containsHeaderValue(HttpHeader.CONTENT_LENGTH, String.valueOf(body.length())));
         assertThat(response, containsHeaderValue(HttpHeader.CONTENT_TYPE, "image/jpeg;charset=utf-8"));
         assertThat(body, containsString("Extra Info"));
+        assertThat("https://github.com/jetty/jetty.project/issues/12766", response, containsHeaderValue(OutputFilter.POST_DO_FILTER_HEADER_NAME, "true"));
     }
 
     /**
@@ -3555,6 +3558,8 @@ private static String decompressGzip(byte[] compressedContent) throws IOExceptio
 
     public static class OutputFilter implements Filter
     {
+        public static final String POST_DO_FILTER_HEADER_NAME = "OutputFilterPostDoFilter";
+
         @Override
         public void init(FilterConfig filterConfig)
         {
@@ -3566,6 +3571,9 @@ public void doFilter(ServletRequest request, ServletResponse response, FilterCha
             response.getOutputStream().println("Extra Info");
             response.setCharacterEncoding("utf-8");
             chain.doFilter(request, response);
+            assertFalse(response.isCommitted());
+            // https://github.com/jetty/jetty.project/issues/12766
+            ((HttpServletResponse)response).addHeader(POST_DO_FILTER_HEADER_NAME, "true");
         }
     }