Change AbstractChannel#doRegister return type from Runnable to void.

transport/src/main/java/io/netty/channel/AbstractChannel.java

@@ -428,13 +428,10 @@ public abstract class AbstractChannel extends DefaultAttributeMap implements Cha
                 if (!ensureOpen(promise)) {
                     return;
                 }
-                Runnable postRegisterTask = doRegister();
+                doRegister();
                 registered = true;
                 promise.setSuccess();
                 pipeline.fireChannelRegistered();
-                if (postRegisterTask != null) {
-                    postRegisterTask.run();
-                }
                 if (isActive()) {
                     pipeline.fireChannelActive();
                 }
@@ -731,12 +728,11 @@ public abstract class AbstractChannel extends DefaultAttributeMap implements Cha
 
     /**
      * Is called after the {@link Channel} is registered with its {@link EventLoop} as part of the register process.
-     * You can return a {@link Runnable} which will be run as post-task of the registration process.
      *
-     * Sub-classes may override this method as it will just return {@code null}
+     * Sub-classes may override this method
      */
-    protected Runnable doRegister() throws Exception {
+    protected void doRegister() throws Exception {
-        return null;
+        // NOOP
     }
 
     /**

transport/src/main/java/io/netty/channel/embedded/EmbeddedChannel.java

@@ -271,9 +271,8 @@ public class EmbeddedChannel extends AbstractChannel {
     }
 
     @Override
-    protected Runnable doRegister() throws Exception {
+    protected void doRegister() throws Exception {
         state = 1;
-        return null;
     }
 
     @Override

transport/src/main/java/io/netty/channel/local/LocalChannel.java

@@ -150,39 +150,26 @@ public class LocalChannel extends AbstractChannel {
     }
 
     @Override
-    protected Runnable doRegister() throws Exception {
+    protected void doRegister() throws Exception {
-        final LocalChannel peer = this.peer;
-        Runnable postRegisterTask;
-
         if (peer != null) {
             state = 2;
 
             peer.remoteAddress = parent().localAddress();
             peer.state = 2;
 
-            // Ensure the peer's channelActive event is triggered *after* this channel's
+            // Always call peer.eventLoop().execute() even if peer.eventLoop().inEventLoop() is true.
-            // channelRegistered event is triggered, so that this channel's pipeline is fully
+            // This ensures that if both channels are on the same event loop, the peer's channelActive
-            // initialized by ChannelInitializer.
+            // event is triggered *after* this channel's channelRegistered event, so that this channel's
-            final EventLoop peerEventLoop = peer.eventLoop();
+            // pipeline is fully initialized by ChannelInitializer before any channelRead events.
-            postRegisterTask = new Runnable() {
+            peer.eventLoop().execute(new Runnable() {
                 @Override
                 public void run() {
-                    peerEventLoop.execute(new Runnable() {
-                        @Override
-                        public void run() {
-                            peer.connectPromise.setSuccess();
                     peer.pipeline().fireChannelActive();
+                    peer.connectPromise.setSuccess();
                 }
             });
         }
-            };
-        } else {
-            postRegisterTask = null;
-        }
-
         ((SingleThreadEventExecutor) eventLoop()).addShutdownHook(shutdownHook);
-
-        return postRegisterTask;
     }
 
     @Override

transport/src/main/java/io/netty/channel/local/LocalServerChannel.java

@@ -82,9 +82,8 @@ public class LocalServerChannel extends AbstractServerChannel {
     }
 
     @Override
-    protected Runnable doRegister() throws Exception {
+    protected void doRegister() throws Exception {
         ((SingleThreadEventExecutor) eventLoop()).addShutdownHook(shutdownHook);
-        return null;
     }
 
     @Override
@@ -114,13 +113,13 @@ public class LocalServerChannel extends AbstractServerChannel {
             return;
         }
 
-        ChannelPipeline pipeline = pipeline();
         Queue<Object> inboundBuffer = this.inboundBuffer;
         if (inboundBuffer.isEmpty()) {
             acceptInProgress = true;
             return;
         }
 
+        ChannelPipeline pipeline = pipeline();
         for (;;) {
             Object m = inboundBuffer.poll();
             if (m == null) {
@@ -132,17 +131,25 @@ public class LocalServerChannel extends AbstractServerChannel {
     }
 
     LocalChannel serve(final LocalChannel peer) {
-        LocalChannel child = new LocalChannel(this, peer);
+        final LocalChannel child = new LocalChannel(this, peer);
+        if (eventLoop().inEventLoop()) {
             serve0(child);
+        } else {
+            eventLoop().execute(new Runnable() {
+              @Override
+              public void run() {
+                serve0(child);
+              }
+            });
+        }
         return child;
     }
 
     private void serve0(final LocalChannel child) {
-        if (eventLoop().inEventLoop()) {
-            final ChannelPipeline pipeline = pipeline();
         inboundBuffer.add(child);
         if (acceptInProgress) {
             acceptInProgress = false;
+            ChannelPipeline pipeline = pipeline();
             for (;;) {
                 Object m = inboundBuffer.poll();
                 if (m == null) {
@@ -152,13 +159,5 @@ public class LocalServerChannel extends AbstractServerChannel {
             }
             pipeline.fireChannelReadComplete();
         }
-        } else {
-            eventLoop().execute(new Runnable() {
-                @Override
-                public void run() {
-                    serve0(child);
-                }
-            });
-        }
     }
 }

transport/src/main/java/io/netty/channel/nio/AbstractNioChannel.java

@@ -277,12 +277,12 @@ public abstract class AbstractNioChannel extends AbstractChannel {
     }
 
     @Override
-    protected Runnable doRegister() throws Exception {
+    protected void doRegister() throws Exception {
         boolean selected = false;
         for (;;) {
             try {
                 selectionKey = javaChannel().register(eventLoop().selector, 0, this);
-                return null;
+                return;
             } catch (CancelledKeyException e) {
                 if (!selected) {
                     // Force the Selector to select now  as the "canceled" SelectionKey may still be