Reduce calls to System.nanoTime() and object creation in IdleStateHandler. Related to [#3808]

Motivation:

Calling System.nanoTime() for each channelRead(...) is very expensive. See [#3808] for more detailed description.
Also we always do extra work for each write and read even if read or write idle states should not be handled.

Modifications:

- Move System.nanoTime() call to channelReadComplete(...).
- Reuse ChannelFutureListener for writes
- Only add ChannelFutureListener to writes if write and all idle states should be handled.
- Only call System.nanoTime() for reads if idle state events for read and all states should be handled.

Result:

Less overhead when using the IdleStateHandler.

handler/src/main/java/io/netty/handler/timeout/IdleStateHandler.java

@@ -97,6 +97,15 @@ import java.util.concurrent.TimeUnit;
 public class IdleStateHandler extends ChannelDuplexHandler {
     private static final long MIN_TIMEOUT_NANOS = TimeUnit.MILLISECONDS.toNanos(1);
 
+    // Not create a new ChannelFutureListener per write operation to reduce GC pressure.
+    private final ChannelFutureListener writeListener = new ChannelFutureListener() {
+        @Override
+        public void operationComplete(ChannelFuture future) throws Exception {
+            lastWriteTime = System.nanoTime();
+            firstWriterIdleEvent = firstAllIdleEvent = true;
+        }
+    };
+
     private final long readerIdleTimeNanos;
     private final long writerIdleTimeNanos;
     private final long allIdleTimeNanos;
@@ -113,6 +122,7 @@ public class IdleStateHandler extends ChannelDuplexHandler {
     private boolean firstAllIdleEvent = true;
 
     private volatile int state; // 0 - none, 1 - initialized, 2 - destroyed
+    private volatile boolean reading;
 
     /**
      * Creates a new instance firing {@link IdleStateEvent}s.
@@ -249,23 +259,28 @@ public class IdleStateHandler extends ChannelDuplexHandler {
 
     @Override
     public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception {
-        lastReadTime = System.nanoTime();
+        if (readerIdleTimeNanos > 0 || allIdleTimeNanos > 0) {
+            reading = true;
             firstReaderIdleEvent = firstAllIdleEvent = true;
+        }
         ctx.fireChannelRead(msg);
     }
 
+    @Override
+    public void channelReadComplete(ChannelHandlerContext ctx) throws Exception {
+        if (readerIdleTimeNanos > 0 || allIdleTimeNanos > 0) {
+            lastReadTime = System.nanoTime();
+            reading = false;
+        }
+        ctx.fireChannelReadComplete();
+    }
+
     @Override
     public void write(ChannelHandlerContext ctx, Object msg, ChannelPromise promise) throws Exception {
         // Allow writing with void promise if handler is only configured for read timeout events.
         if (writerIdleTimeNanos > 0 || allIdleTimeNanos > 0) {
             ChannelPromise unvoid = promise.unvoid();
-            unvoid.addListener(new ChannelFutureListener() {
-                @Override
-                public void operationComplete(ChannelFuture future) throws Exception {
-                    lastWriteTime = System.nanoTime();
-                    firstWriterIdleEvent = firstAllIdleEvent = true;
-                }
-            });
+            unvoid.addListener(writeListener);
             ctx.write(msg, unvoid);
         } else {
             ctx.write(msg, promise);
@@ -342,9 +357,11 @@ public class IdleStateHandler extends ChannelDuplexHandler {
                 return;
             }
 
-            long currentTime = System.nanoTime();
-            long lastReadTime = IdleStateHandler.this.lastReadTime;
-            long nextDelay = readerIdleTimeNanos - (currentTime - lastReadTime);
+            long nextDelay = readerIdleTimeNanos;
+            if (!reading) {
+                nextDelay -= System.nanoTime() - lastReadTime;
+            }
+
             if (nextDelay <= 0) {
                 // Reader is idle - set a new timeout and notify the callback.
                 readerIdleTimeout =
@@ -382,9 +399,8 @@ public class IdleStateHandler extends ChannelDuplexHandler {
                 return;
             }
 
-            long currentTime = System.nanoTime();
             long lastWriteTime = IdleStateHandler.this.lastWriteTime;
-            long nextDelay = writerIdleTimeNanos - (currentTime - lastWriteTime);
+            long nextDelay = writerIdleTimeNanos - (System.nanoTime() - lastWriteTime);
             if (nextDelay <= 0) {
                 // Writer is idle - set a new timeout and notify the callback.
                 writerIdleTimeout = ctx.executor().schedule(
@@ -422,9 +438,10 @@ public class IdleStateHandler extends ChannelDuplexHandler {
                 return;
             }
 
-            long currentTime = System.nanoTime();
-            long lastIoTime = Math.max(lastReadTime, lastWriteTime);
-            long nextDelay = allIdleTimeNanos - (currentTime - lastIoTime);
+            long nextDelay = allIdleTimeNanos;
+            if (!reading) {
+                nextDelay -= System.nanoTime() - Math.max(lastReadTime, lastWriteTime);
+            }
             if (nextDelay <= 0) {
                 // Both reader and writer are idle - set a new timeout and
                 // notify the callback.