Clean up the pipeline implementation / Ensure Embedded*Channel does not run pending tasks immediately

- Replace ugly 'prev != null' check with explicit event scheduling
- Fix an incorrect flag operation in freeHandlerBuffersAfterRemoval()
- Fix a bug in AbstractEmbeddedChannel.doRegister where it makes pending tasks immediately, where the pending tasks actually triggers inbound events
- Remove unnecessary suppression of inboundBufferUpdated() event in DefaultChannelPipeline, which potentially hides an event ordering bug. Unfortunately, I don't remember why I added it in cca35454d2.

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

@@ -675,8 +675,13 @@ public abstract class AbstractChannel extends DefaultAttributeMap implements Cha
                     doDisconnect();
                     promise.setSuccess();
                     if (wasActive && !isActive()) {
+                        invokeLater(new Runnable() {
+                            @Override
+                            public void run() {
                                 pipeline.fireChannelInactive();
                             }
+                        });
+                    }
                 } catch (Throwable t) {
                     promise.setFailure(t);
                     closeIfClosed();
@@ -710,8 +715,13 @@ public abstract class AbstractChannel extends DefaultAttributeMap implements Cha
                     flushFutureNotifier.notifyFlushFutures(closedChannelException);
 
                     if (wasActive && !isActive()) {
+                        invokeLater(new Runnable() {
+                            @Override
+                            public void run() {
                                 pipeline.fireChannelInactive();
                             }
+                        });
+                    }
 
                     deregister(voidFuture());
                 } else {
@@ -754,7 +764,12 @@ public abstract class AbstractChannel extends DefaultAttributeMap implements Cha
                     if (registered) {
                         registered = false;
                         promise.setSuccess();
+                        invokeLater(new Runnable() {
+                            @Override
+                            public void run() {
                                 pipeline.fireChannelUnregistered();
+                            }
+                        });
                     } else {
                         // Some transports like local and AIO does not allow the deregistration of
                         // an open channel.  Their doDeregister() calls close().  Consequently,
@@ -785,8 +800,13 @@ public abstract class AbstractChannel extends DefaultAttributeMap implements Cha
             if (eventLoop().inEventLoop()) {
                 try {
                     doBeginRead();
-                } catch (Exception e) {
+                } catch (final Exception e) {
-                    pipeline().fireExceptionCaught(e);
+                    invokeLater(new Runnable() {
+                        @Override
+                        public void run() {
+                            pipeline.fireExceptionCaught(e);
+                        }
+                    });
                     close(unsafe().voidFuture());
                 }
             } else {
@@ -849,10 +869,11 @@ public abstract class AbstractChannel extends DefaultAttributeMap implements Cha
         private void flush0() {
             if (!inFlushNow) { // Avoid re-entrance
                 try {
+                    // Flush immediately only when there's no pending flush.
+                    // If there's a pending flush operation, event loop will call flushNow() later,
+                    // and thus there's no need to call it now.
                     if (!isFlushPending()) {
                         flushNow();
-                    } else {
-                        // Event loop will call flushNow() later by itself.
                     }
                 } catch (Throwable t) {
                     flushFutureNotifier.notifyFlushFutures(t);
@@ -930,6 +951,21 @@ public abstract class AbstractChannel extends DefaultAttributeMap implements Cha
             }
             close(voidFuture());
         }
+
+        private void invokeLater(Runnable task) {
+            // This method is used by outbound operation implementations to trigger an inbound event later.
+            // They do not trigger an inbound event immediately because an outbound operation might have been
+            // triggered by another inbound event handler method.  If fired immediately, the call stack
+            // will look like this for example:
+            //
+            //   handlerA.inboundBufferUpdated() - (1) an inbound handler method closes a connection.
+            //   -> handlerA.ctx.close()
+            //      -> channel.unsafe.close()
+            //         -> handlerA.channelInactive() - (2) another inbound handler method called while in (1) yet
+            //
+            // which means the execution of two inbound handler methods of the same handler overlap undesirably.
+            eventLoop().execute(task);
+        }
     }
 
     /**

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

@@ -24,11 +24,14 @@ import io.netty.buffer.Unpooled;
 import io.netty.util.DefaultAttributeMap;
 import io.netty.util.concurrent.EventExecutor;
 import io.netty.util.concurrent.EventExecutorGroup;
+import io.netty.util.concurrent.Future;
+import io.netty.util.internal.PlatformDependent;
 
 import java.net.SocketAddress;
 import java.nio.channels.ClosedChannelException;
 import java.util.Queue;
 import java.util.concurrent.ConcurrentLinkedQueue;
+import java.util.concurrent.ExecutionException;
 import java.util.concurrent.atomic.AtomicReferenceFieldUpdater;
 
 import static io.netty.channel.DefaultChannelPipeline.*;
@@ -309,17 +312,42 @@ final class DefaultChannelHandlerContext extends DefaultAttributeMap implements
 
     void initHeadHandler() {
         // Must be called for the head handler.
+        EventExecutor executor = executor();
+        if (executor.inEventLoop()) {
             HeadHandler h = (HeadHandler) handler;
             if (h.initialized) {
                 return;
             }
 
-        assert executor().inEventLoop();
-
             h.init(this);
             h.initialized = true;
             outByteBuf = h.byteSink;
             outMsgBuf = h.msgSink;
+        } else {
+            Future<?> f = executor.submit(new Runnable() {
+                @Override
+                public void run() {
+                    initHeadHandler();
+                }
+            });
+
+            boolean interrupted = false;
+            try {
+                while (!f.isDone()) {
+                    try {
+                        f.get();
+                    } catch (InterruptedException e) {
+                        interrupted = true;
+                    } catch (ExecutionException e) {
+                        PlatformDependent.throwException(e);
+                    }
+                }
+            } finally {
+                if (interrupted) {
+                    Thread.currentThread().interrupt();
+                }
+            }
+        }
     }
 
     private boolean flushInboundBridge() {
@@ -401,8 +429,9 @@ final class DefaultChannelHandlerContext extends DefaultAttributeMap implements
     }
 
     private void freeHandlerBuffersAfterRemoval() {
-        if (flags == FLAG_REMOVED) { // Removed, but not freed yet
+        int flags = this.flags;
-            flags |= FLAG_FREED;
+        if ((flags & FLAG_REMOVED) != 0 && (flags & FLAG_FREED) == 0) { // Removed, but not freed yet
+            this.flags |= FLAG_FREED;
 
             final ChannelHandler handler = handler();
 
@@ -691,7 +720,7 @@ final class DefaultChannelHandlerContext extends DefaultAttributeMap implements
     public ChannelHandlerContext fireChannelUnregistered() {
         final DefaultChannelHandlerContext next = findContextInbound();
         EventExecutor executor = next.executor();
-        if (prev != null && executor.inEventLoop()) {
+        if (executor.inEventLoop()) {
             next.invokeChannelUnregistered();
         } else {
             executor.execute(new Runnable() {
@@ -743,7 +772,7 @@ final class DefaultChannelHandlerContext extends DefaultAttributeMap implements
     public ChannelHandlerContext fireChannelInactive() {
         final DefaultChannelHandlerContext next = findContextInbound();
         EventExecutor executor = next.executor();
-        if (prev != null && executor.inEventLoop()) {
+        if (executor.inEventLoop()) {
             next.invokeChannelInactive();
         } else {
             executor.execute(new Runnable() {
@@ -778,7 +807,7 @@ final class DefaultChannelHandlerContext extends DefaultAttributeMap implements
 
     private void invokeExceptionCaught(final Throwable cause) {
         EventExecutor executor = executor();
-        if (prev != null && executor.inEventLoop()) {
+        if (executor.inEventLoop()) {
             invokeExceptionCaught0(cause);
         } else {
             try {
@@ -897,7 +926,6 @@ final class DefaultChannelHandlerContext extends DefaultAttributeMap implements
 
     private void invokeInboundBufferUpdated() {
         ChannelStateHandler handler = (ChannelStateHandler) handler();
-
         if (handler instanceof ChannelInboundHandler) {
             for (;;) {
                 try {
@@ -1424,7 +1452,7 @@ final class DefaultChannelHandlerContext extends DefaultAttributeMap implements
 
     void invokeFreeInboundBuffer() {
         EventExecutor executor = executor();
-        if (prev != null && executor.inEventLoop()) {
+        if (executor.inEventLoop()) {
             invokeFreeInboundBuffer0();
         } else {
             executor.execute(new Runnable() {
@@ -1517,11 +1545,7 @@ final class DefaultChannelHandlerContext extends DefaultAttributeMap implements
             return;
         }
 
-        if (handler() instanceof ChannelStateHandler) {
         invokeExceptionCaught(cause);
-        } else {
-            findContextInbound().invokeExceptionCaught(cause);
-        }
     }
 
     private static boolean inExceptionCaught(Throwable cause) {

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

@@ -65,8 +65,6 @@ final class DefaultChannelPipeline implements ChannelPipeline {
 
     private final Map<String, DefaultChannelHandlerContext> name2ctx =
         new HashMap<String, DefaultChannelHandlerContext>(4);
-    private boolean firedChannelActive;
-    private boolean fireInboundBufferUpdatedOnActivation;
 
     final Map<EventExecutorGroup, EventExecutor> childExecutors =
             new IdentityHashMap<EventExecutorGroup, EventExecutor>();
@@ -814,7 +812,6 @@ final class DefaultChannelPipeline implements ChannelPipeline {
 
     @Override
     public ChannelPipeline fireChannelActive() {
-        firedChannelActive = true;
         head.initHeadHandler();
         head.fireChannelActive();
 
@@ -822,10 +819,6 @@ final class DefaultChannelPipeline implements ChannelPipeline {
             channel.read();
         }
 
-        if (fireInboundBufferUpdatedOnActivation) {
-            fireInboundBufferUpdatedOnActivation = false;
-            head.fireInboundBufferUpdated();
-        }
         return this;
     }
 
@@ -853,10 +846,6 @@ final class DefaultChannelPipeline implements ChannelPipeline {
 
     @Override
     public ChannelPipeline fireInboundBufferUpdated() {
-        if (!firedChannelActive) {
-            fireInboundBufferUpdatedOnActivation = true;
-            return this;
-        }
         head.fireInboundBufferUpdated();
         return this;
     }

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

@@ -226,12 +226,7 @@ public abstract class AbstractEmbeddedChannel<O> extends AbstractChannel {
 
     @Override
     protected Runnable doDeregister() throws Exception {
-        return new Runnable() {
+        return null;
-            @Override
-            public void run() {
-                runPendingTasks();
-            }
-        };
     }
 
     @Override