Add testcase to ensure NioEventLoop.rebuildSelector() works correctly.

Motivation:

We had recently a report that the issue [netty#6607] is still not fixed.

Modifications:

Add a testcase to prove the issue is fixed.

Result:

More tests.

transport/src/test/java/io/netty/channel/nio/NioEventLoopTest.java

@@ -16,9 +16,15 @@
 package io.netty.channel.nio;
 
 import io.netty.channel.AbstractEventLoopTest;
+import io.netty.channel.Channel;
 import io.netty.channel.EventLoopGroup;
 import io.netty.channel.socket.ServerSocketChannel;
 import io.netty.channel.socket.nio.NioServerSocketChannel;
+import org.junit.Test;
+
+import java.nio.channels.Selector;
+
+import static org.junit.Assert.*;
 
 public class NioEventLoopTest extends AbstractEventLoopTest {
 
@@ -31,4 +37,35 @@ protected EventLoopGroup newEventLoopGroup() {
     protected Class<? extends ServerSocketChannel> newChannel() {
         return NioServerSocketChannel.class;
     }
+
+    @Test
+    public void testRebuildSelector() throws Exception {
+        EventLoopGroup group = new NioEventLoopGroup(1);
+        final NioEventLoop loop = (NioEventLoop) group.next();
+        try {
+            Channel channel = new NioServerSocketChannel();
+            loop.register(channel).syncUninterruptibly();
+
+            Selector selector = loop.unwrappedSelector();
+            assertSame(selector, ((NioEventLoop) channel.eventLoop()).unwrappedSelector());
+            assertTrue(selector.isOpen());
+
+            // Submit to the EventLoop so we are sure its really executed in a non-async manner.
+            loop.submit(new Runnable() {
+                @Override
+                public void run() {
+                    loop.rebuildSelector();
+                }
+            }).syncUninterruptibly();
+
+            Selector newSelector = ((NioEventLoop) channel.eventLoop()).unwrappedSelector();
+            assertTrue(newSelector.isOpen());
+            assertNotSame(selector, newSelector);
+            assertFalse(selector.isOpen());
+
+            channel.close().syncUninterruptibly();
+        } finally {
+            group.shutdownGracefully();
+        }
+    }
 }