[#5566] Ensure using a ChannelInitializer via ServerBootstrap.handler(...) produce correct ordering.

Motivation:

When a ChannelInitializer is used via ServerBootstrap.handler(...) the users handlers may be added after the internal ServerBootstrapAcceptor. This should not happen.

Modifications:

Delay the adding of the ServerBootstrapAcceptor until the initChannel(....) method returns.

Result:

Correct order of handlers in the ServerChannels ChannelPipeline.

transport/src/main/java/io/netty/bootstrap/ServerBootstrap.java

@@ -169,16 +169,26 @@ public class ServerBootstrap extends AbstractBootstrap<ServerBootstrap, ServerCh
         p.addLast(new ChannelInitializer<Channel>() {
             @Override
             public void initChannel(Channel ch) throws Exception {
-                ChannelPipeline pipeline = ch.pipeline();
+                final ChannelPipeline pipeline = ch.pipeline();
                 ChannelHandler handler = config.handler();
                 if (handler != null) {
                     pipeline.addLast(handler);
                 }
+
+                // We add this handler via the EventLoop as the user may have used a ChannelInitializer as handler.
+                // In this case the initChannel(...) method will only be called after this method returns. Because
+                // of this we need to ensure we add our handler in a delayed fashion so all the users handler are
+                // placed in front of the ServerBootstrapAcceptor.
+                ch.eventLoop().execute(new Runnable() {
+                    @Override
+                    public void run() {
                         pipeline.addLast(new ServerBootstrapAcceptor(
                                 currentChildGroup, currentChildHandler, currentChildOptions, currentChildAttrs));
                     }
                 });
             }
+        });
+    }
 
     @Override
     public ServerBootstrap validate() {

transport/src/test/java/io/netty/bootstrap/ServerBootstrapTest.java

@@ -15,13 +15,21 @@
  */
 package io.netty.bootstrap;
 
+import io.netty.channel.Channel;
+import io.netty.channel.ChannelHandler;
 import io.netty.channel.ChannelHandlerAdapter;
 import io.netty.channel.ChannelHandlerContext;
 import io.netty.channel.ChannelInboundHandlerAdapter;
+import io.netty.channel.ChannelInitializer;
+import io.netty.channel.DefaultEventLoopGroup;
+import io.netty.channel.EventLoopGroup;
+import io.netty.channel.local.LocalAddress;
+import io.netty.channel.local.LocalChannel;
 import io.netty.channel.local.LocalEventLoopGroup;
 import io.netty.channel.local.LocalServerChannel;
 import org.junit.Test;
 
+import java.util.UUID;
 import java.util.concurrent.CountDownLatch;
 import java.util.concurrent.atomic.AtomicReference;
 
@@ -59,4 +67,74 @@ public class ServerBootstrapTest {
             group.shutdownGracefully();
         }
     }
+
+    @Test(timeout = 3000)
+    public void testParentHandler() throws Exception {
+        testParentHandler(false);
+    }
+
+    @Test(timeout = 3000)
+    public void testParentHandlerViaChannelInitializer() throws Exception {
+        testParentHandler(true);
+    }
+
+    private static void testParentHandler(boolean channelInitializer) throws Exception {
+        final LocalAddress addr = new LocalAddress(UUID.randomUUID().toString());
+        final CountDownLatch readLatch = new CountDownLatch(1);
+        final CountDownLatch initLatch = new CountDownLatch(1);
+
+        final ChannelHandler handler = new ChannelInboundHandlerAdapter() {
+            @Override
+            public void handlerAdded(ChannelHandlerContext ctx) throws Exception {
+                initLatch.countDown();
+                super.handlerAdded(ctx);
+            }
+
+            @Override
+            public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception {
+                readLatch.countDown();
+                super.channelRead(ctx, msg);
+            }
+        };
+
+        EventLoopGroup group = new DefaultEventLoopGroup(1);
+        Channel sch = null;
+        Channel cch = null;
+        try {
+            ServerBootstrap sb = new ServerBootstrap();
+            sb.channel(LocalServerChannel.class)
+                    .group(group)
+                    .childHandler(new ChannelInboundHandlerAdapter());
+            if (channelInitializer) {
+                sb.handler(new ChannelInitializer<Channel>() {
+                    @Override
+                    protected void initChannel(Channel ch) throws Exception {
+                        ch.pipeline().addLast(handler);
+                    }
+                });
+            } else {
+                sb.handler(handler);
+            }
+
+            Bootstrap cb = new Bootstrap();
+            cb.group(group)
+                    .channel(LocalChannel.class)
+                    .handler(new ChannelInboundHandlerAdapter());
+
+            sch = sb.bind(addr).syncUninterruptibly().channel();
+
+            cch = cb.connect(addr).syncUninterruptibly().channel();
+
+            initLatch.await();
+            readLatch.await();
+        } finally {
+            if (sch != null) {
+                sch.close().syncUninterruptibly();
+            }
+            if (cch != null) {
+                cch.close().syncUninterruptibly();
+            }
+            group.shutdownGracefully();
+        }
+    }
 }