Fix #6562 last written bytebuffer

Restored the last flush of an empty buffer as is needed for 0 length empty write.

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

@@ -863,10 +863,12 @@ public void write(byte[] b, int off, int len) throws IOException
         // Blocking write
         try
         {
+            boolean complete = false;
             // flush any content from the aggregate
             if (BufferUtil.hasContent(_aggregate))
             {
-                channelWrite(_aggregate, last && len == 0);
+                complete = last && len == 0;
+                channelWrite(_aggregate, complete);
 
                 // should we fill aggregate again from the buffer?
                 if (len > 0 && !last && len <= _commitSize && len <= maximizeAggregateSpace())
@@ -896,6 +898,10 @@ public void write(byte[] b, int off, int len) throws IOException
                 }
                 channelWrite(view, last);
             }
+            else if (last && !complete)
+            {
+                channelWrite(BufferUtil.EMPTY_BUFFER, true);
+            }
 
             onWriteComplete(last, null);
         }
@@ -963,12 +969,18 @@ public void write(ByteBuffer buffer) throws IOException
             {
                 // Blocking write
                 // flush any content from the aggregate
+                boolean complete = false;
                 if (BufferUtil.hasContent(_aggregate))
-                    channelWrite(_aggregate, last && len == 0);
+                {
+                    complete = last && len == 0;
+                    channelWrite(_aggregate, complete);
+                }
 
                 // write any remaining content in the buffer directly
                 if (len > 0)
                     channelWrite(buffer, last);
+                else if (last && !complete)
+                    channelWrite(BufferUtil.EMPTY_BUFFER, true);
 
                 onWriteComplete(last, null);
             }

jetty-server/src/test/java/org/eclipse/jetty/server/HttpOutputTest.java

@@ -28,6 +28,7 @@
 import java.nio.charset.StandardCharsets;
 import java.util.Arrays;
 import java.util.concurrent.TimeUnit;
+import java.util.concurrent.atomic.AtomicBoolean;
 import java.util.concurrent.atomic.AtomicInteger;
 import javax.servlet.AsyncContext;
 import javax.servlet.ServletException;
@@ -763,6 +764,102 @@ public void setNext(Interceptor interceptor)
         assertThat(response, containsString("400\tTHIS IS A BIGGER FILE"));
     }
 
+    @Test
+    public void testEmptyArray() throws Exception
+    {
+        AtomicBoolean committed = new AtomicBoolean();
+        AbstractHandler handler = new AbstractHandler()
+        {
+            @Override
+            public void handle(String target, Request baseRequest, HttpServletRequest request, HttpServletResponse response) throws IOException, ServletException
+            {
+                baseRequest.setHandled(true);
+                response.setStatus(200);
+                response.getOutputStream().write(new byte[0]);
+                committed.set(response.isCommitted());
+            }
+        };
+
+        _swap.setHandler(handler);
+        handler.start();
+        String response = _connector.getResponse("GET / HTTP/1.0\nHost: localhost:80\n\n");
+        assertThat(response, containsString("HTTP/1.1 200 OK"));
+        assertThat(committed.get(), is(false));
+    }
+
+    @Test
+    public void testEmptyArrayKnown() throws Exception
+    {
+        AtomicBoolean committed = new AtomicBoolean();
+        AbstractHandler handler = new AbstractHandler()
+        {
+            @Override
+            public void handle(String target, Request baseRequest, HttpServletRequest request, HttpServletResponse response) throws IOException, ServletException
+            {
+                baseRequest.setHandled(true);
+                response.setStatus(200);
+                response.setContentLength(0);
+                response.getOutputStream().write(new byte[0]);
+                committed.set(response.isCommitted());
+            }
+        };
+
+        _swap.setHandler(handler);
+        handler.start();
+        String response = _connector.getResponse("GET / HTTP/1.0\nHost: localhost:80\n\n");
+        assertThat(response, containsString("HTTP/1.1 200 OK"));
+        assertThat(response, containsString("Content-Length: 0"));
+        assertThat(committed.get(), is(true));
+    }
+
+    @Test
+    public void testEmptyBuffer() throws Exception
+    {
+        AtomicBoolean committed = new AtomicBoolean();
+        AbstractHandler handler = new AbstractHandler()
+        {
+            @Override
+            public void handle(String target, Request baseRequest, HttpServletRequest request, HttpServletResponse response) throws IOException, ServletException
+            {
+                baseRequest.setHandled(true);
+                response.setStatus(200);
+                ((HttpOutput)response.getOutputStream()).write(ByteBuffer.wrap(new byte[0]));
+                committed.set(response.isCommitted());
+            }
+        };
+
+        _swap.setHandler(handler);
+        handler.start();
+        String response = _connector.getResponse("GET / HTTP/1.0\nHost: localhost:80\n\n");
+        assertThat(response, containsString("HTTP/1.1 200 OK"));
+        assertThat(committed.get(), is(false));
+    }
+
+    @Test
+    public void testEmptyBufferKnown() throws Exception
+    {
+        AtomicBoolean committed = new AtomicBoolean();
+        AbstractHandler handler = new AbstractHandler()
+        {
+            @Override
+            public void handle(String target, Request baseRequest, HttpServletRequest request, HttpServletResponse response) throws IOException, ServletException
+            {
+                baseRequest.setHandled(true);
+                response.setStatus(200);
+                response.setContentLength(0);
+                ((HttpOutput)response.getOutputStream()).write(ByteBuffer.wrap(new byte[0]));
+                committed.set(response.isCommitted());
+            }
+        };
+
+        _swap.setHandler(handler);
+        handler.start();
+        String response = _connector.getResponse("GET / HTTP/1.0\nHost: localhost:80\n\n");
+        assertThat(response, containsString("HTTP/1.1 200 OK"));
+        assertThat(response, containsString("Content-Length: 0"));
+        assertThat(committed.get(), is(true));
+    }
+
     @Test
     public void testAggregation() throws Exception
     {