Add a boolean parameter to Channel.Unsafe.flush() so that SelectionKey state check can be bypassed.

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

@@ -592,12 +592,12 @@ public abstract class AbstractChannel extends DefaultAttributeMap implements Cha
         }
 
         @Override
-        public void flush() {
+        public void flush(boolean force) {
             outboundBuffer.addFlush();
-            flush0();
+            flush0(force);
         }
 
-        private void flush0() {
+        private void flush0(boolean force) {
             if (inFlushNow) {
                 // Avoid re-entrance
                 return;
@@ -606,7 +606,7 @@ public abstract class AbstractChannel extends DefaultAttributeMap implements Cha
             // 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()) {
+            if (!force || isFlushPending()) {
                 return;
             }
 

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

@@ -84,7 +84,7 @@ public abstract class AbstractServerChannel extends AbstractChannel implements S
         }
 
         @Override
-        public void flush() {
+        public void flush(boolean force) {
             // ignore
         }
 

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

@@ -242,7 +242,7 @@ public interface Channel extends AttributeMap, ChannelOutboundInvoker, ChannelPr
         /**
          * Flush out all write operations scheduled via {@link #write(Object, ChannelPromise)}.
          */
-        void flush();
+        void flush(boolean force);
 
         /**
          * Return a special ChannelPromise which can be reused and passed to the operations in {@link Unsafe}.

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

@@ -1030,7 +1030,7 @@ final class DefaultChannelPipeline implements ChannelPipeline {
 
         @Override
         public void flush(ChannelHandlerContext ctx) throws Exception {
-            unsafe.flush();
+            unsafe.flush(false);
         }
 
         @Override

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

@@ -529,7 +529,7 @@ public final class NioEventLoop extends SingleThreadEventLoop {
         }
 
         // Call flushNow which will also take care of clear the OP_WRITE once there is nothing left to write
-        ch.unsafe().flush();
+        ch.unsafe().flush(true);
     }
 
     private static void unregisterWritableTasks(AbstractNioChannel ch) {