Revert default HTTP/2 settings sent by the client (#2346)

Motivation:

#2341 changed default HTTP/2 settings that client sends to notify the
server it does not support Server Push.

Modification:

- Set `ENABLE_PUSH=0, MAX_CONCURRENT_STREAMS=0` in settings frame that
client sends to a server;
- Validate custom initial settings that users pass to `H2ProtocolConfig`
and throw if users try to override ENABLE_PUSH/MAX_CONCURRENT_STREAMS on
the client or set ENABLE_PUSH other than 0 for the server;
- Include content message in GO_AWAY frame if client receives a Push
Stream frame;

Result:

Default client behavior doesn't change, custom settings validated to
prevent programming mistakes.

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

@@ -27,6 +27,7 @@
 import io.netty.handler.codec.http2.Http2FrameCodecBuilder;
 import io.netty.handler.codec.http2.Http2MultiplexHandler;
 
+import static io.netty.buffer.ByteBufUtil.writeAscii;
 import static io.netty.handler.codec.http2.Http2Error.PROTOCOL_ERROR;
 import static io.servicetalk.http.netty.H2ServerParentChannelInitializer.initFrameLogger;
 import static io.servicetalk.http.netty.H2ServerParentChannelInitializer.toNettySettings;
@@ -37,8 +38,7 @@ final class H2ClientParentChannelInitializer implements ChannelInitializer {
 
     H2ClientParentChannelInitializer(final H2ProtocolConfig config) {
         this.config = config;
-        final io.servicetalk.http.api.Http2Settings h2Settings = config.initialSettings();
-        nettySettings = toNettySettings(h2Settings);
+        nettySettings = toNettySettings(config.initialSettings()).pushEnabled(false).maxConcurrentStreams(0L);
     }
 
     @Override
@@ -93,7 +93,8 @@ private H2PushStreamHandler() {
         @Override
         public void channelRegistered(final ChannelHandlerContext ctx) {
             // See SETTINGS_ENABLE_PUSH in https://tools.ietf.org/html/rfc7540#section-6.5.2
-            ctx.writeAndFlush(new DefaultHttp2GoAwayFrame(PROTOCOL_ERROR));
+            ctx.writeAndFlush(new DefaultHttp2GoAwayFrame(PROTOCOL_ERROR,
+                    writeAscii(ctx.alloc(), "Server Push is not supported")));
             // Http2ConnectionHandler.processGoAwayWriteResult will close the connection after GO_AWAY is flushed
         }
     }

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

@@ -15,13 +15,15 @@
  */
 package io.servicetalk.http.netty;
 
+import io.servicetalk.http.api.Http2Settings;
 import io.servicetalk.tcp.netty.internal.TcpClientConfig;
 import io.servicetalk.transport.api.ClientSslConfig;
 import io.servicetalk.transport.api.DelegatingClientSslConfig;
 
 import java.util.List;
 import javax.annotation.Nullable;
 
+import static io.netty.handler.codec.http2.Http2CodecUtil.SETTINGS_ENABLE_PUSH;
 import static io.servicetalk.http.netty.HttpServerConfig.httpAlpnProtocols;
 import static io.servicetalk.utils.internal.NetworkUtils.isValidIpV4Address;
 import static io.servicetalk.utils.internal.NetworkUtils.isValidIpV6Address;
@@ -41,7 +43,19 @@ final class HttpClientConfig {
 
     HttpClientConfig() {
         tcpConfig = new TcpClientConfig();
-        protocolConfigs = new HttpConfig();
+        protocolConfigs = new HttpConfig(h2Config -> {
+            final Http2Settings settings = h2Config.initialSettings();
+            final Long pushEnabled = settings.settingValue(SETTINGS_ENABLE_PUSH);
+            if (pushEnabled != null && pushEnabled != 0) {
+                throw new IllegalArgumentException("Server Push is not supported by the client, " +
+                        "expected SETTINGS_ENABLE_PUSH value is null or 0, settings=" + settings);
+            }
+            final Long maxConcurrentStreams = settings.maxConcurrentStreams();
+            if (maxConcurrentStreams != null && maxConcurrentStreams != 0) {
+                throw new IllegalArgumentException("Server Push is not supported by the client, " +
+                        "expected MAX_CONCURRENT_STREAMS value is null or 0, settings=" + settings);
+            }
+        });
     }
 
     HttpClientConfig(final HttpClientConfig from) {

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

@@ -15,13 +15,12 @@
  */
 package io.servicetalk.http.netty;
 
-import io.servicetalk.http.api.Http2Settings;
 import io.servicetalk.http.api.HttpProtocolConfig;
 
 import java.util.List;
+import java.util.function.Consumer;
 import javax.annotation.Nullable;
 
-import static io.netty.handler.codec.http2.Http2CodecUtil.SETTINGS_ENABLE_PUSH;
 import static io.servicetalk.http.netty.HttpProtocolConfigs.h1Default;
 import static java.util.Arrays.asList;
 import static java.util.Collections.emptyList;
@@ -30,20 +29,23 @@
 import static java.util.Objects.requireNonNull;
 
 final class HttpConfig {
+    private final Consumer<H2ProtocolConfig> h2ConfigValidator;
     @Nullable
     private H1ProtocolConfig h1Config;
     @Nullable
     private H2ProtocolConfig h2Config;
     private List<String> supportedAlpnProtocols;
     private boolean allowDropTrailers;
 
-    HttpConfig() {
+    HttpConfig(final Consumer<H2ProtocolConfig> h2ConfigValidator) {
+        this.h2ConfigValidator = requireNonNull(h2ConfigValidator);
         h1Config = h1Default();
         h2Config = null;
         supportedAlpnProtocols = emptyList();
     }
 
     HttpConfig(final HttpConfig from) {
+        this.h2ConfigValidator = from.h2ConfigValidator;
         this.h1Config = from.h1Config;
         this.h2Config = from.h2Config;
         this.supportedAlpnProtocols = from.supportedAlpnProtocols;
@@ -107,12 +109,7 @@ private void h2Config(final H2ProtocolConfig h2Config) {
         if (this.h2Config != null) {
             throw new IllegalArgumentException("Duplicated configuration for HTTP/2 was found");
         }
-        final Http2Settings settings = h2Config.initialSettings();
-        final Long pushEnabled = settings.settingValue(SETTINGS_ENABLE_PUSH);
-        if (pushEnabled != null && pushEnabled != 0) {
-            throw new IllegalArgumentException("Http2Settings push is enabled but not supported. settings=" + settings);
-        }
-
+        h2ConfigValidator.accept(h2Config);
         this.h2Config = h2Config;
         supportedAlpnProtocols = h1Config == null ? singletonList(h2Config.alpnId()) :
                 unmodifiableList(asList(h1Config.alpnId(), h2Config.alpnId()));

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

@@ -15,6 +15,7 @@
  */
 package io.servicetalk.http.netty;
 
+import io.servicetalk.http.api.Http2Settings;
 import io.servicetalk.http.api.HttpLifecycleObserver;
 import io.servicetalk.tcp.netty.internal.TcpServerConfig;
 import io.servicetalk.transport.api.DelegatingServerSslConfig;
@@ -26,6 +27,7 @@
 import java.util.Map.Entry;
 import javax.annotation.Nullable;
 
+import static io.netty.handler.codec.http2.Http2CodecUtil.SETTINGS_ENABLE_PUSH;
 import static java.util.Objects.requireNonNull;
 
 final class HttpServerConfig {
@@ -37,7 +39,14 @@ final class HttpServerConfig {
 
     HttpServerConfig() {
         tcpConfig = new TcpServerConfig();
-        httpConfig = new HttpConfig();
+        httpConfig = new HttpConfig(h2Config -> {
+            final Http2Settings settings = h2Config.initialSettings();
+            final Long pushEnabled = settings.settingValue(SETTINGS_ENABLE_PUSH);
+            if (pushEnabled != null && pushEnabled != 0) {
+                throw new IllegalArgumentException(
+                        "Server cannot set SETTINGS_ENABLE_PUSH value other than 0, settings=" + settings);
+            }
+        });
     }
 
     TcpServerConfig tcpConfig() {