Call discardReadBytes() on outbound byte buffers when possible

- Also fixed a test failure in codec-http

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

@@ -626,6 +626,9 @@ public abstract class AbstractChannel extends DefaultAttributeMap implements Cha
                     cause = t;
                 } finally {
                     writeCounter += oldSize - out.size();
+                    if (out.isEmpty() && out.hasByteBuffer()) {
+                        out.byteBuffer().discardReadBytes();
+                    }
                 }
 
                 if (cause == null) {

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

@@ -323,7 +323,7 @@ public class DefaultChannelFuture extends FlushCheckpoint implements ChannelFutu
     }
 
     private void checkDeadLock() {
-        if (channel().eventLoop().inEventLoop()) {
+        if (channel().isRegistered() && channel().eventLoop().inEventLoop()) {
             throw new BlockingOperationException();
         }
     }

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

@@ -927,6 +927,11 @@ public class DefaultChannelPipeline implements ChannelPipeline {
                 ((ChannelOutboundHandler<Object>) ctx.handler()).flush(ctx, future);
             } catch (Throwable t) {
                 notifyHandlerException(t);
+            } finally {
+                ChannelBufferHolder<Object> outbound = ctx.outbound();
+                if (!outbound.isBypass() && outbound.isEmpty() && outbound.hasByteBuffer()) {
+                    outbound.byteBuffer().discardReadBytes();
+                }
             }
         } else {
             channel().unsafe().flush(future);