Undo AbstractChannelHandlerContext extend PausableChannelEventExecutor. Fixes #2814

Motivation:
Our last-minute change that made AbstractChannelHandlerContext implement PausableChannelEventExecutor broke the socksproxy example.

Modifications:
AbstractChannelHandlerContext does not inherit from PausableChannelEventExecutor anymore. Instead we'll allocate an extra object on demand.

Result:

AbstractChannelHandlerContext.executor().newPromise() returns the correct type.

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

@@ -28,16 +28,12 @@ import io.netty.util.internal.PlatformDependent;
 import io.netty.util.internal.StringUtil;
 import java.net.SocketAddress;
 import java.util.WeakHashMap;
+import java.util.concurrent.atomic.AtomicReferenceFieldUpdater;
 
 /**
  * Abstract base class for {@link ChannelHandlerContext} implementations.
- * <p>
- * The unusual is-a relationship with {@link PausableChannelEventExecutor} was put in place to avoid
- * additional object allocations that would have been required to wrap the return values of {@link #executor()}
- * and {@link #invoker()}.
  */
-abstract class AbstractChannelHandlerContext
-        extends PausableChannelEventExecutor implements ChannelHandlerContext, ResourceLeakHint {
+abstract class AbstractChannelHandlerContext implements ChannelHandlerContext, ResourceLeakHint {
 
     // This class keeps an integer member field 'skipFlags' whose each bit tells if the corresponding handler method
     // is annotated with @Skip. 'skipFlags' is retrieved in runtime via the reflection API and is cached.
@@ -92,10 +88,21 @@ abstract class AbstractChannelHandlerContext
     private static final WeakHashMap<Class<?>, Integer>[] skipFlagsCache =
             new WeakHashMap[Runtime.getRuntime().availableProcessors()];
 
+    private static final AtomicReferenceFieldUpdater<AbstractChannelHandlerContext, PausableChannelEventExecutor>
+            WRAPPED_EVENTEXECUTOR_UPDATER;
+
     static {
         for (int i = 0; i < skipFlagsCache.length; i ++) {
             skipFlagsCache[i] = new WeakHashMap<Class<?>, Integer>();
         }
+        AtomicReferenceFieldUpdater<AbstractChannelHandlerContext, PausableChannelEventExecutor> updater =
+               PlatformDependent.newAtomicReferenceFieldUpdater(
+                       AbstractChannelHandlerContext.class, "wrappedEventLoop");
+        if (updater == null) {
+            updater = AtomicReferenceFieldUpdater.newUpdater(
+                    AbstractChannelHandlerContext.class, PausableChannelEventExecutor.class, "wrappedEventLoop");
+        }
+        WRAPPED_EVENTEXECUTOR_UPDATER = updater;
     }
 
     /**
@@ -226,6 +233,11 @@ abstract class AbstractChannelHandlerContext
     volatile Runnable invokeFlushTask;
     volatile Runnable invokeChannelWritableStateChangedTask;
 
+    /**
+     * Wrapped {@link EventLoop} and {@link ChannelHandlerInvoker} to support {@link Channel#deregister()}.
+     */
+    private volatile PausableChannelEventExecutor wrappedEventLoop;
+
     AbstractChannelHandlerContext(
             DefaultChannelPipeline pipeline, ChannelHandlerInvoker invoker, String name, int skipFlags) {
 
@@ -286,12 +298,32 @@ abstract class AbstractChannelHandlerContext
 
     @Override
     public final EventExecutor executor() {
-        return this;
+        if (invoker == null) {
+            return channel().eventLoop();
+        } else {
+            return wrappedEventLoop();
+        }
     }
 
     @Override
     public final ChannelHandlerInvoker invoker() {
-        return this;
+        if (invoker == null) {
+            return channel().eventLoop().asInvoker();
+        } else {
+            return wrappedEventLoop();
+        }
+    }
+
+    private PausableChannelEventExecutor wrappedEventLoop() {
+        PausableChannelEventExecutor wrapped = wrappedEventLoop;
+        if (wrapped == null) {
+            wrapped = new PausableChannelEventExecutor0();
+            if (!WRAPPED_EVENTEXECUTOR_UPDATER.compareAndSet(this, null, wrapped)) {
+                // Set in the meantime so we need to issue another volatile read
+                return wrappedEventLoop;
+            }
+        }
+        return wrapped;
     }
 
     @Override
@@ -554,35 +586,44 @@ abstract class AbstractChannelHandlerContext
         return StringUtil.simpleClassName(ChannelHandlerContext.class) + '(' + name + ", " + channel + ')';
     }
 
-    @Override
-    public EventExecutor unwrap() {
-        return unwrapInvoker().executor();
-    }
+    private final class PausableChannelEventExecutor0 extends PausableChannelEventExecutor {
 
-    @Override
-    public void rejectNewTasks() {
-        /**
-         * This cast is correct because {@link #channel()} always returns an {@link AbstractChannel} and
-         * {@link AbstractChannel#eventLoop()} always returns a {@link PausableEventExecutor}.
-         */
-        ((PausableEventExecutor) channel().eventLoop()).rejectNewTasks();
-    }
-
-    @Override
-    public void acceptNewTasks() {
-        ((PausableEventExecutor) channel().eventLoop()).acceptNewTasks();
-    }
-
-    @Override
-    public boolean isAcceptingNewTasks() {
-        return ((PausableEventExecutor) channel().eventLoop()).isAcceptingNewTasks();
-    }
-
-    @Override
-    ChannelHandlerInvoker unwrapInvoker() {
-        if (invoker == null) {
-            return channel().unsafe().invoker();
+        @Override
+        public void rejectNewTasks() {
+            /**
+             * This cast is correct because {@link #channel()} always returns an {@link AbstractChannel} and
+             * {@link AbstractChannel#eventLoop()} always returns a {@link PausableChannelEventExecutor}.
+             */
+            ((PausableEventExecutor) channel().eventLoop()).rejectNewTasks();
+        }
+
+        @Override
+        public void acceptNewTasks() {
+            ((PausableEventExecutor) channel().eventLoop()).acceptNewTasks();
+        }
+
+        @Override
+        public boolean isAcceptingNewTasks() {
+            return ((PausableEventExecutor) channel().eventLoop()).isAcceptingNewTasks();
+        }
+
+        @Override
+        public Channel channel() {
+            return AbstractChannelHandlerContext.this.channel();
+        }
+
+        @Override
+        public EventExecutor unwrap() {
+            return unwrapInvoker().executor();
+        }
+
+        @Override
+        public ChannelHandlerInvoker unwrapInvoker() {
+            /**
+             * {@link #invoker} can not be {@code null}, because {@link PausableChannelEventExecutor0} will only be
+             * instantiated if {@link #invoker} is not {@code null}.
+             */
+            return invoker;
         }
-        return invoker;
     }
 }

transport/src/test/java/io/netty/channel/ChannelDeregistrationTest.java

@@ -17,6 +17,7 @@ package io.netty.channel;
 
 import io.netty.bootstrap.ServerBootstrap;
 import io.netty.buffer.Unpooled;
+import io.netty.channel.embedded.EmbeddedChannel;
 import io.netty.channel.nio.NioEventLoopGroup;
 import io.netty.channel.socket.ServerSocketChannel;
 import io.netty.channel.socket.SocketChannel;
@@ -25,7 +26,9 @@ import io.netty.util.NetUtil;
 import io.netty.util.concurrent.DefaultEventExecutorGroup;
 import io.netty.util.concurrent.EventExecutor;
 import io.netty.util.concurrent.EventExecutorGroup;
+import io.netty.util.concurrent.GlobalEventExecutor;
 import io.netty.util.concurrent.PausableEventExecutor;
+import io.netty.util.concurrent.Promise;
 import org.junit.Test;
 
 import java.net.InetSocketAddress;
@@ -199,6 +202,29 @@ public class ChannelDeregistrationTest {
                 .unwrap());
     }
 
+    /**
+     * See https://github.com/netty/netty/issues/2814
+     */
+    @Test(timeout = 1000)
+    public void testPromise() {
+        ChannelHandler handler = new TestChannelHandler1();
+        AbstractChannel ch = new EmbeddedChannel(handler);
+        DefaultChannelPipeline p = new DefaultChannelPipeline(ch);
+        DefaultChannelHandlerInvoker invoker = new DefaultChannelHandlerInvoker(GlobalEventExecutor.INSTANCE);
+
+        ChannelHandlerContext ctx = new DefaultChannelHandlerContext(p, invoker, "Test", handler);
+        // Make sure no ClassCastException is thrown
+        Promise<Integer> promise = ctx.executor().newPromise();
+        promise.setSuccess(0);
+        assertTrue(promise.isSuccess());
+
+        ctx = new DefaultChannelHandlerContext(p, null, "Test", handler);
+        // Make sure no ClassCastException is thrown
+        promise = ctx.executor().newPromise();
+        promise.setSuccess(0);
+        assertTrue(promise.isSuccess());
+    }
+
     private static final class TestChannelHandler1 extends ChannelHandlerAdapter { }
 
     private static final class TestChannelHandler2 extends ChannelHandlerAdapter {