HTTP/2 HelloWorld Client Example Bug

Motivation:
The HTTP/2 helloworld client example has 2 bugs:
1. HttpResponseHandler has a map which is accessed from multiple threads, but the map is not thread safe.
2. Requests are flushed and maybe completely written and the responses may be received/processed by Netty before an element is inserted into the HttpResponseHandler map. This may result in an 'unexpected message' error even though the message has actually been sent.

Modifications:
- HttpResponseHandler should use a thread safe map
- Http2Client shouldn't flush until entries are added to the HttpResponseHandler map

Result:
Fixes netty#6165.

example/src/main/java/io/netty/example/http2/helloworld/client/Http2Client.java

@@ -15,7 +15,6 @@
 package io.netty.example.http2.helloworld.client;
 
 import io.netty.bootstrap.Bootstrap;
-import io.netty.buffer.Unpooled;
 import io.netty.channel.Channel;
 import io.netty.channel.ChannelOption;
 import io.netty.channel.EventLoopGroup;
@@ -44,6 +43,7 @@
 
 import java.util.concurrent.TimeUnit;
 
+import static io.netty.buffer.Unpooled.wrappedBuffer;
 import static io.netty.handler.codec.http.HttpMethod.GET;
 import static io.netty.handler.codec.http.HttpMethod.POST;
 import static io.netty.handler.codec.http.HttpVersion.HTTP_1_1;
@@ -118,20 +118,20 @@ public static void main(String[] args) throws Exception {
                 request.headers().add(HttpConversionUtil.ExtensionHeaderNames.SCHEME.text(), scheme.name());
                 request.headers().add(HttpHeaderNames.ACCEPT_ENCODING, HttpHeaderValues.GZIP);
                 request.headers().add(HttpHeaderNames.ACCEPT_ENCODING, HttpHeaderValues.DEFLATE);
-                responseHandler.put(streamId, channel.writeAndFlush(request), channel.newPromise());
+                responseHandler.put(streamId, channel.write(request), channel.newPromise());
                 streamId += 2;
             }
             if (URL2 != null) {
                 // Create a simple POST request with a body.
                 FullHttpRequest request = new DefaultFullHttpRequest(HTTP_1_1, POST, URL2,
-                                Unpooled.copiedBuffer(URL2DATA.getBytes(CharsetUtil.UTF_8)));
+                        wrappedBuffer(URL2DATA.getBytes(CharsetUtil.UTF_8)));
                 request.headers().add(HttpHeaderNames.HOST, hostName);
                 request.headers().add(HttpConversionUtil.ExtensionHeaderNames.SCHEME.text(), scheme.name());
                 request.headers().add(HttpHeaderNames.ACCEPT_ENCODING, HttpHeaderValues.GZIP);
                 request.headers().add(HttpHeaderNames.ACCEPT_ENCODING, HttpHeaderValues.DEFLATE);
-                responseHandler.put(streamId, channel.writeAndFlush(request), channel.newPromise());
-                streamId += 2;
+                responseHandler.put(streamId, channel.write(request), channel.newPromise());
             }
+            channel.flush();
             responseHandler.awaitResponses(5, TimeUnit.SECONDS);
             System.out.println("Finished HTTP/2 request(s)");
 

example/src/main/java/io/netty/example/http2/helloworld/client/HttpResponseHandler.java

@@ -22,23 +22,25 @@
 import io.netty.handler.codec.http.FullHttpResponse;
 import io.netty.handler.codec.http2.HttpConversionUtil;
 import io.netty.util.CharsetUtil;
+import io.netty.util.internal.PlatformDependent;
 
 import java.util.AbstractMap.SimpleEntry;
 import java.util.Iterator;
+import java.util.Map;
 import java.util.Map.Entry;
-import java.util.SortedMap;
-import java.util.TreeMap;
 import java.util.concurrent.TimeUnit;
 
 /**
  * Process {@link io.netty.handler.codec.http.FullHttpResponse} translated from HTTP/2 frames
  */
 public class HttpResponseHandler extends SimpleChannelInboundHandler<FullHttpResponse> {
 
-    private SortedMap<Integer, Entry<ChannelFuture, ChannelPromise>> streamidPromiseMap;
+    private Map<Integer, Entry<ChannelFuture, ChannelPromise>> streamidPromiseMap;
 
     public HttpResponseHandler() {
-        streamidPromiseMap = new TreeMap<Integer, Entry<ChannelFuture, ChannelPromise>>();
+        // Use a concurrent map because we add and iterate from the main thread (just for the purposes of the example),
+        // but Netty also does a get on the map when messages are received in a EventLoop thread.
+        streamidPromiseMap = PlatformDependent.newConcurrentHashMap();
     }
 
     /**