Lazy Initialization of epoll splice queue

Motivation:
AbstractEpollStreamChannel has a queue which collects splice events. Splice is assumed not to be the most common use case of this class and thus the splice queue could be initialized in a lazy fashion to save memory. This becomes more significant when the number of connections grows.

Modifications:
- AbstractEpollStreamChannel.spliceQueue will be initialized in a lazy fashion

Result:
Less memory consumption for most use cases

transport-native-epoll/src/main/java/io/netty/channel/epoll/AbstractEpollStreamChannel.java

@@ -69,7 +69,7 @@ public abstract class AbstractEpollStreamChannel extends AbstractEpollChannel {
     private ChannelPromise connectPromise;
     private ScheduledFuture<?> connectTimeoutFuture;
     private SocketAddress requestedRemoteAddress;
-    private final Queue<SpliceInTask> spliceQueue = PlatformDependent.newMpscQueue();
+    private Queue<SpliceInTask> spliceQueue;
 
     // Lazy init these if we need to splice(...)
     private FileDescriptor pipeIn;
@@ -172,8 +172,7 @@ public abstract class AbstractEpollStreamChannel extends AbstractEpollChannel {
         if (!isOpen()) {
             promise.tryFailure(CLOSED_CHANNEL_EXCEPTION);
         } else {
-            SpliceInTask task = new SpliceInChannelTask(ch, len, checkNotNull(promise, "promise"));
-            spliceQueue.add(task);
+            addToSpliceQueue(new SpliceInChannelTask(ch, len, promise));
             failSpliceIfClosed(promise);
         }
         return promise;
@@ -190,6 +189,7 @@ public abstract class AbstractEpollStreamChannel extends AbstractEpollChannel {
      *   <li>{@link EpollChannelConfig#getEpollMode()} must be {@link EpollMode#LEVEL_TRIGGERED} for this
      *   {@link AbstractEpollStreamChannel}</li>
      *   <li>the {@link FileDescriptor} will not be closed after the {@link ChannelFuture} is notified</li>
+     *   <li>this channel must be registered to an event loop or {@link IllegalStateException} will be thrown.</li>
      * </ul>
      */
     public final ChannelFuture spliceTo(final FileDescriptor ch, final int offset, final int len) {
@@ -207,6 +207,7 @@ public abstract class AbstractEpollStreamChannel extends AbstractEpollChannel {
      *   <li>{@link EpollChannelConfig#getEpollMode()} must be {@link EpollMode#LEVEL_TRIGGERED} for this
      *   {@link AbstractEpollStreamChannel}</li>
      *   <li>the {@link FileDescriptor} will not be closed after the {@link ChannelPromise} is notified</li>
+     *   <li>this channel must be registered to an event loop or {@link IllegalStateException} will be thrown.</li>
      * </ul>
      */
     public final ChannelFuture spliceTo(final FileDescriptor ch, final int offset, final int len,
@@ -224,8 +225,7 @@ public abstract class AbstractEpollStreamChannel extends AbstractEpollChannel {
         if (!isOpen()) {
             promise.tryFailure(CLOSED_CHANNEL_EXCEPTION);
         } else {
-            SpliceInTask task = new SpliceFdTask(ch, offset, len, checkNotNull(promise, "promise"));
-            spliceQueue.add(task);
+            addToSpliceQueue(new SpliceFdTask(ch, offset, len, promise));
             failSpliceIfClosed(promise);
         }
         return promise;
@@ -560,6 +560,9 @@ public abstract class AbstractEpollStreamChannel extends AbstractEpollChannel {
     }
 
     private void clearSpliceQueue() {
+        if (spliceQueue == null) {
+            return;
+        }
         for (;;) {
             SpliceInTask task = spliceQueue.poll();
             if (task == null) {
@@ -788,17 +791,19 @@ public abstract class AbstractEpollStreamChannel extends AbstractEpollChannel {
             boolean close = false;
             try {
                 do {
-                    SpliceInTask spliceTask = spliceQueue.peek();
-                    if (spliceTask != null) {
-                        if (spliceTask.spliceIn(allocHandle)) {
-                            // We need to check if it is still active as if not we removed all SpliceTasks in
-                            // doClose(...)
-                            if (isActive()) {
-                                spliceQueue.remove();
+                    if (spliceQueue != null) {
+                        SpliceInTask spliceTask = spliceQueue.peek();
+                        if (spliceTask != null) {
+                            if (spliceTask.spliceIn(allocHandle)) {
+                                // We need to check if it is still active as if not we removed all SpliceTasks in
+                                // doClose(...)
+                                if (isActive()) {
+                                    spliceQueue.remove();
+                                }
+                                continue;
+                            } else {
+                                break;
                             }
-                            continue;
-                        } else {
-                            break;
                         }
                     }
 
@@ -843,6 +848,27 @@ public abstract class AbstractEpollStreamChannel extends AbstractEpollChannel {
         }
     }
 
+    private void addToSpliceQueue(final SpliceInTask task) {
+        EventLoop eventLoop = eventLoop();
+        if (eventLoop.inEventLoop()) {
+            addToSpliceQueue0(task);
+        } else {
+            eventLoop.execute(new OneTimeTask() {
+                @Override
+                public void run() {
+                    addToSpliceQueue0(task);
+                }
+            });
+        }
+    }
+
+    private void addToSpliceQueue0(SpliceInTask task) {
+        if (spliceQueue == null) {
+            spliceQueue = PlatformDependent.newMpscQueue();
+        }
+        spliceQueue.add(task);
+    }
+
     protected abstract class SpliceInTask extends MpscLinkedQueueNode<SpliceInTask> {
         final ChannelPromise promise;
         int len;