H2 client should send ENABLE_PUSH=false setting for new connections (…

…#940)

Motivation:

To notify server-side that ST client currently does not support
server push, client should send `ENABLE_PUSH=false` setting when it
connects to the server.
If server sends a `PUSH_PROMISE` frame, client should send `GO_AWAY`
before closing the connection.

Modifications:

- Add `ENABLE_PUSH=false` and `MAX_CONCURRENT_STREAMS=0` to
the set of initial h2 settings;
- Send `GO_AWAY` frame with `PROTOCOL_ERROR` if client receives a
`PUSH_PROMISE` frame from server;

Result:

H2 client notifies server that it does not support server push and
sends `GO_AWAY` frame before closing the connection, if server sends
a push.

servicetalk-http-netty/src/main/java/io/servicetalk/http/netty/H2ClientParentChannelInitializer.java

@@ -1,5 +1,5 @@
 /*
- * Copyright © 2019 Apple Inc. and the ServiceTalk project authors
+ * Copyright © 2019-2020 Apple Inc. and the ServiceTalk project authors
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -21,16 +21,19 @@
 import io.netty.channel.ChannelHandler;
 import io.netty.channel.ChannelHandlerContext;
 import io.netty.channel.ChannelInboundHandlerAdapter;
+import io.netty.handler.codec.http2.DefaultHttp2GoAwayFrame;
 import io.netty.handler.codec.http2.Http2FrameCodecBuilder;
 import io.netty.handler.codec.http2.Http2FrameLogger;
 import io.netty.handler.codec.http2.Http2MultiplexHandler;
 
 import java.util.function.BiPredicate;
 
+import static io.netty.handler.codec.http2.Http2Error.PROTOCOL_ERROR;
 import static io.netty.handler.codec.http2.Http2FrameCodecBuilder.forClient;
 import static io.netty.handler.logging.LogLevel.TRACE;
 
 final class H2ClientParentChannelInitializer implements ChannelInitializer {
+
     private final H2ProtocolConfig config;
 
     H2ClientParentChannelInitializer(final H2ProtocolConfig config) {
@@ -47,6 +50,9 @@ public void init(final Channel channel) {
                 // the user to apply their own timeout at the call site.
                 .gracefulShutdownTimeoutMillis(-1);
 
+        // Notify server that this client does not support server push and request it to be disabled.
+        multiplexCodecBuilder.initialSettings().pushEnabled(false).maxConcurrentStreams(0L);
+
         final BiPredicate<CharSequence, CharSequence> headersSensitivityDetector =
                 config.headersSensitivityDetector();
         multiplexCodecBuilder.headerSensitivityDetector(headersSensitivityDetector::test);
@@ -64,6 +70,7 @@ public void init(final Channel channel) {
 
     @ChannelHandler.Sharable
     private static final class H2PushStreamHandler extends ChannelInboundHandlerAdapter {
+
         static final ChannelInboundHandlerAdapter INSTANCE = new H2PushStreamHandler();
 
         private H2PushStreamHandler() {
@@ -72,7 +79,9 @@ private H2PushStreamHandler() {
 
         @Override
         public void channelRegistered(final ChannelHandlerContext ctx) {
-            ctx.close(); // push streams are not supported
+            // See SETTINGS_ENABLE_PUSH in https://tools.ietf.org/html/rfc7540#section-6.5.2
+            ctx.writeAndFlush(new DefaultHttp2GoAwayFrame(PROTOCOL_ERROR));
+            // Http2ConnectionHandler.processGoAwayWriteResult will close the connection after GO_AWAY is flushed
         }
     }
 }