Reduce object creation on Http2FrameCodec (#9333)

Motivation:

We don't need the extra ChannelPromise when writing headers anymore in Http2FrameCodec. This also means we cal re-use a ChannelFutureListener and so not need to create new instances all the time.

Modifications:

- Just pass the original ChannelPromise when writing headers
- Reuse the ChannelFutureListener

Result:

Two less objects created when writing headers for an not-yet created stream.

codec-http2/src/main/java/io/netty/handler/codec/http2/Http2FrameCodec.java

@@ -17,7 +17,6 @@ package io.netty.handler.codec.http2;
 
 import io.netty.buffer.ByteBuf;
 import io.netty.channel.Channel;
-import io.netty.channel.ChannelFuture;
 import io.netty.channel.ChannelFutureListener;
 import io.netty.channel.ChannelHandlerContext;
 import io.netty.channel.ChannelInboundHandler;
@@ -159,6 +158,7 @@ public class Http2FrameCodec extends Http2ConnectionHandler {
     private int numBufferedStreams;
     private final IntObjectMap<DefaultHttp2FrameStream> frameStreamToInitializeMap =
             new IntObjectHashMap<DefaultHttp2FrameStream>(8);
+    private final ChannelFutureListener bufferedStreamsListener = future -> numBufferedStreams--;
 
     Http2FrameCodec(Http2ConnectionEncoder encoder, Http2ConnectionDecoder decoder, Http2Settings initialSettings,
                     boolean decoupleCloseAndGoAway) {
@@ -393,34 +393,15 @@ public class Http2FrameCodec extends Http2ConnectionHandler {
             // We should not re-use ids.
             assert old == null;
 
-            // TODO(buchgr): Once Http2FrameStream and Http2Stream are merged this is no longer necessary.
-            final ChannelPromise writePromise = ctx.newPromise();
-
             encoder().writeHeaders(ctx, streamId, headersFrame.headers(), headersFrame.padding(),
-                    headersFrame.isEndStream(), writePromise);
+                    headersFrame.isEndStream(), promise);
-            if (writePromise.isDone()) {
+            if (!promise.isDone()) {
-                notifyHeaderWritePromise(writePromise, promise);
-            } else {
                 numBufferedStreams++;
-
+                promise.addListener(bufferedStreamsListener);
-                writePromise.addListener((ChannelFutureListener) future -> {
-                    numBufferedStreams--;
-
-                    notifyHeaderWritePromise(future, promise);
-                });
             }
         }
     }
 
-    private static void notifyHeaderWritePromise(ChannelFuture future, ChannelPromise promise) {
-        Throwable cause = future.cause();
-        if (cause == null) {
-            promise.setSuccess();
-        } else {
-            promise.setFailure(cause);
-        }
-    }
-
     private void onStreamActive0(Http2Stream stream) {
         if (stream.id() != Http2CodecUtil.HTTP_UPGRADE_STREAM_ID &&
                 connection().local().isValidStreamId(stream.id())) {
@@ -507,7 +488,7 @@ public class Http2FrameCodec extends Http2ConnectionHandler {
         }
     }
 
-    void onHttp2UnknownStreamError(@SuppressWarnings("unused") ChannelHandlerContext ctx, Throwable cause,
+    private void onHttp2UnknownStreamError(@SuppressWarnings("unused") ChannelHandlerContext ctx, Throwable cause,
                                    Http2Exception.StreamException streamException) {
         // Just log....
         LOG.warn("Stream exception thrown for unknown stream {}.", streamException.streamId(), cause);
@@ -610,11 +591,11 @@ public class Http2FrameCodec extends Http2ConnectionHandler {
         }
     }
 
-    void onUpgradeEvent(ChannelHandlerContext ctx, UpgradeEvent evt) {
+    private void onUpgradeEvent(ChannelHandlerContext ctx, UpgradeEvent evt) {
         ctx.fireUserEventTriggered(evt);
     }
 
-    void onHttp2StreamWritabilityChanged(ChannelHandlerContext ctx, Http2FrameStream stream,
+    private void onHttp2StreamWritabilityChanged(ChannelHandlerContext ctx, Http2FrameStream stream,
                                          @SuppressWarnings("unused") boolean writable) {
         ctx.fireUserEventTriggered(Http2FrameStreamEvent.writabilityChanged(stream));
     }