[#1557] Make the contract of Channel.Unsafe.flush() more clear

* Remove boolean parameter from Channel.Unsafe.flush() method
* Move NIO related things to AbstractNioChannel.NioUnsafe

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

@@ -54,7 +54,7 @@ public abstract class AbstractChannel extends DefaultAttributeMap implements Cha
     private volatile boolean registered;
 
     private ClosedChannelException closedChannelException;
-    private boolean inFlushNow;
+    private boolean inFlush0;
 
     /** Cache for the string representation of this channel */
     private boolean strValActive;
@@ -592,25 +592,18 @@ public abstract class AbstractChannel extends DefaultAttributeMap implements Cha
         }
 
         @Override
-        public void flush(boolean force) {
+        public void flush() {
             outboundBuffer.addFlush();
-            flush0(force);
+            flush0();
         }
 
-        private void flush0(boolean force) {
-            if (inFlushNow) {
+        protected void flush0() {
+            if (inFlush0) {
                 // Avoid re-entrance
                 return;
             }
 
-            // 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 (!force && isFlushPending()) {
-                return;
-            }
-
-            inFlushNow = true;
+            inFlush0 = true;
 
             final ChannelOutboundBuffer outboundBuffer = AbstractChannel.this.outboundBuffer;
 
@@ -621,7 +614,7 @@ public abstract class AbstractChannel extends DefaultAttributeMap implements Cha
                 } else {
                     outboundBuffer.fail(new ClosedChannelException());
                 }
-                inFlushNow = false;
+                inFlush0 = false;
                 return;
             }
 
@@ -667,7 +660,7 @@ public abstract class AbstractChannel extends DefaultAttributeMap implements Cha
                     close(voidPromise());
                 }
             } finally {
-                inFlushNow = false;
+                inFlush0 = false;
             }
         }
 
@@ -803,11 +796,6 @@ public abstract class AbstractChannel extends DefaultAttributeMap implements Cha
         return 0;
     }
 
-    /**
-     * Return {@code true} if a flush to the {@link Channel} is currently pending.
-     */
-    protected abstract boolean isFlushPending();
-
     final class CloseFuture extends DefaultChannelPromise {
 
         CloseFuture(AbstractChannel ch) {

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

@@ -61,11 +61,6 @@ public abstract class AbstractServerChannel extends AbstractChannel implements S
         throw new UnsupportedOperationException();
     }
 
-    @Override
-    protected boolean isFlushPending() {
-        return false;
-    }
-
     @Override
     protected AbstractUnsafe newUnsafe() {
         return new DefaultServerUnsafe();
@@ -84,7 +79,7 @@ public abstract class AbstractServerChannel extends AbstractChannel implements S
         }
 
         @Override
-        public void flush(boolean force) {
+        public void flush() {
             // 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(boolean force);
+        void flush();
 
         /**
          * 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(false);
+            unsafe.flush();
         }
 
         @Override

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

@@ -301,11 +301,6 @@ public class EmbeddedChannel extends AbstractChannel {
         return new DefaultUnsafe();
     }
 
-    @Override
-    protected boolean isFlushPending() {
-        return false;
-    }
-
     @Override
     protected int doWrite(Object[] msgs, int msgsLength, int startIndex) throws Exception {
         for (int i = startIndex; i < msgsLength; i ++) {

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

@@ -315,11 +315,6 @@ public class LocalChannel extends AbstractChannel {
         }
     }
 
-    @Override
-    protected boolean isFlushPending() {
-        return false;
-    }
-
     private class LocalUnsafe extends AbstractUnsafe {
 
         @Override

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

@@ -145,6 +145,8 @@ public abstract class AbstractNioChannel extends AbstractChannel {
          * Read from underlying {@link SelectableChannel}
          */
         void read();
+
+        void forceFlush();
     }
 
     protected abstract class AbstractNioUnsafe extends AbstractUnsafe implements NioUnsafe {
@@ -245,6 +247,23 @@ public abstract class AbstractNioChannel extends AbstractChannel {
                 connectPromise = null;
             }
         }
+
+        @Override
+        protected void flush0() {
+            // Flush immediately only when there's no pending flush.
+            // If there's a pending flush operation, event loop will call forceFlush() later,
+            // and thus there's no need to call it now.
+            if (isFlushPending()) {
+                return;
+            }
+            super.flush0();
+        }
+
+        @Override
+        public void forceFlush() {
+            // directly call super.flush0() to force a flush now
+            super.flush0();
+        }
     }
 
     @Override
@@ -252,8 +271,7 @@ public abstract class AbstractNioChannel extends AbstractChannel {
         return loop instanceof NioEventLoop;
     }
 
-    @Override
-    protected boolean isFlushPending() {
+    private boolean isFlushPending() {
         SelectionKey selectionKey = this.selectionKey;
         return selectionKey.isValid() && (selectionKey.interestOps() & SelectionKey.OP_WRITE) != 0;
     }

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

@@ -528,8 +528,8 @@ public final class NioEventLoop extends SingleThreadEventLoop {
             processSelectedKey(ch.selectionKey(), task);
         }
 
-        // Call flushNow which will also take care of clear the OP_WRITE once there is nothing left to write
-        ch.unsafe().flush(true);
+        // Call forceFlush which will also take care of clear the OP_WRITE once there is nothing left to write
+        ch.unsafe().forceFlush();
     }
 
     private static void unregisterWritableTasks(AbstractNioChannel ch) {

transport/src/main/java/io/netty/channel/oio/AbstractOioChannel.java

@@ -91,11 +91,6 @@ public abstract class AbstractOioChannel extends AbstractChannel {
         return loop instanceof ThreadPerChannelEventLoop;
     }
 
-    @Override
-    protected boolean isFlushPending() {
-        return false;
-    }
-
     /**
      * Connect to the remote peer using the given localAddress if one is specified or {@code null} otherwise.
      */