Correctly set writerIndex when EpollChannelOption.MAX_DATAGRAM_PAYLOAD_SIZE is used in all cases (#9819)

Motivation:

Due a bug we did not correctly set the writerIndex of the ByteBuf when a
user specified EpollChannelOption.MAX_DATAGRAM_PAYLOAD_SIZE but we ended
up with a non scattering read.

Modifications:

- Set writerIndex to the correct value
- Add unit tests

Result:

Fixes https://github.com/netty/netty/issues/9788

transport-native-epoll/src/main/java/io/netty/channel/epoll/EpollDatagramChannel.java

@@ -671,9 +671,10 @@ public final class EpollDatagramChannel extends AbstractEpollChannel implements
             if (localAddress == null) {
                 localAddress = localAddress();
             }
+            int received = remoteAddress.receivedAmount();
             allocHandle.lastBytesRead(maxDatagramPacketSize <= 0 ?
-                    remoteAddress.receivedAmount() : writable);
-            byteBuf.writerIndex(byteBuf.writerIndex() + allocHandle.lastBytesRead());
+                    received : writable);
+            byteBuf.writerIndex(writerIndex + received);
             allocHandle.incMessagesRead(1);
 
             pipeline().fireChannelRead(new DatagramPacket(byteBuf, localAddress, remoteAddress));

transport-native-epoll/src/test/java/io/netty/channel/epoll/EpollDatagramScatteringReadTest.java

@@ -184,4 +184,93 @@ public class EpollDatagramScatteringReadTest extends AbstractDatagramTest  {
             }
         }
     }
+
+    @Test
+    public void testScatteringReadWithSmallBuffer() throws Throwable {
+        run();
+    }
+
+    public void testScatteringReadWithSmallBuffer(Bootstrap sb, Bootstrap cb) throws Throwable {
+        testScatteringReadWithSmallBuffer0(sb, cb, false);
+    }
+
+    @Test
+    public void testScatteringConnectedReadWithSmallBuffer() throws Throwable {
+        run();
+    }
+
+    public void testScatteringConnectedReadWithSmallBuffer(Bootstrap sb, Bootstrap cb) throws Throwable {
+        testScatteringReadWithSmallBuffer0(sb, cb, true);
+    }
+
+    private void testScatteringReadWithSmallBuffer0(Bootstrap sb, Bootstrap cb, boolean connected) throws Throwable {
+        int packetSize = 16;
+
+        sb.option(ChannelOption.RCVBUF_ALLOCATOR, new AdaptiveRecvByteBufAllocator(1400, 1400, 64 * 1024));
+        sb.option(EpollChannelOption.MAX_DATAGRAM_PAYLOAD_SIZE, 1400);
+
+        Channel sc = null;
+        Channel cc = null;
+
+        try {
+            cb.handler(new SimpleChannelInboundHandler<Object>() {
+                @Override
+                public void channelRead0(ChannelHandlerContext ctx, Object msgs) {
+                    // Nothing will be sent.
+                }
+            });
+            cc = cb.bind(newSocketAddress()).sync().channel();
+            final SocketAddress ccAddress = cc.localAddress();
+
+            final AtomicReference<Throwable> errorRef = new AtomicReference<Throwable>();
+            final byte[] bytes = new byte[packetSize];
+            PlatformDependent.threadLocalRandom().nextBytes(bytes);
+
+            final CountDownLatch latch = new CountDownLatch(1);
+            sb.handler(new SimpleChannelInboundHandler<DatagramPacket>() {
+
+                @Override
+                protected void channelRead0(ChannelHandlerContext ctx, DatagramPacket msg) {
+                    assertEquals(ccAddress, msg.sender());
+
+                    assertEquals(bytes.length, msg.content().readableBytes());
+                    byte[] receivedBytes = new byte[bytes.length];
+                    msg.content().readBytes(receivedBytes);
+                    assertArrayEquals(bytes, receivedBytes);
+
+                    latch.countDown();
+                }
+
+                @Override
+                public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause)  {
+                    errorRef.compareAndSet(null, cause);
+                }
+            });
+
+            sc = sb.bind(newSocketAddress()).sync().channel();
+
+            if (connected) {
+                sc.connect(cc.localAddress()).syncUninterruptibly();
+            }
+
+            InetSocketAddress addr = (InetSocketAddress) sc.localAddress();
+
+            cc.writeAndFlush(new DatagramPacket(cc.alloc().directBuffer().writeBytes(bytes), addr)).sync();
+
+            if (!latch.await(10, TimeUnit.SECONDS)) {
+                Throwable error = errorRef.get();
+                if (error != null) {
+                    throw error;
+                }
+                fail("Timeout while waiting for packets");
+            }
+        } finally {
+            if (cc != null) {
+                cc.close().syncUninterruptibly();
+            }
+            if (sc != null) {
+                sc.close().syncUninterruptibly();
+            }
+        }
+    }
 }