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   /*
    * Copyright 2017 The Netty Project
    *
    * The Netty Project licenses this file to you under the Apache License,
    * version 2.0 (the "License"); you may not use this file except in compliance
    * with the License. You may obtain a copy of the License at:
    *
    *   https://www.apache.org/licenses/LICENSE-2.0
    *
   * Unless required by applicable law or agreed to in writing, software
   * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
   * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
   * License for the specific language governing permissions and limitations
   * under the License.
   */
  package io.netty.handler.codec.http2;
  
  import io.netty.buffer.ByteBuf;
  import io.netty.buffer.ByteBufUtil;
  import io.netty.channel.ChannelHandler;
  import io.netty.channel.ChannelHandlerContext;
  import io.netty.handler.codec.ByteToMessageDecoder;
  import io.netty.handler.codec.http.HttpServerCodec;
  import io.netty.handler.codec.http.HttpServerUpgradeHandler;
  import io.netty.util.internal.UnstableApi;
  
  import java.util.List;
  
  import static io.netty.buffer.Unpooled.unreleasableBuffer;
  import static io.netty.handler.codec.http2.Http2CodecUtil.connectionPrefaceBuf;
  
  import static io.netty.util.internal.ObjectUtil.checkNotNull;
  
  /**
   * Performing cleartext upgrade, by h2c HTTP upgrade or Prior Knowledge.
   * This handler config pipeline for h2c upgrade when handler added.
   * And will update pipeline once it detect the connection is starting HTTP/2 by
   * prior knowledge or not.
   */
  @UnstableApi
  public final class CleartextHttp2ServerUpgradeHandler extends ByteToMessageDecoder {
      private static final ByteBuf CONNECTION_PREFACE = unreleasableBuffer(connectionPrefaceBuf()).asReadOnly();
  
      private final HttpServerCodec httpServerCodec;
      private final HttpServerUpgradeHandler httpServerUpgradeHandler;
      private final ChannelHandler http2ServerHandler;
  
      /**
       * Creates the channel handler provide cleartext HTTP/2 upgrade from HTTP
       * upgrade or prior knowledge
       *
       * @param httpServerCodec the http server codec
       * @param httpServerUpgradeHandler the http server upgrade handler for HTTP/2
       * @param http2ServerHandler the http2 server handler, will be added into pipeline
       *                           when starting HTTP/2 by prior knowledge
       */
      public CleartextHttp2ServerUpgradeHandler(HttpServerCodec httpServerCodec,
                                                HttpServerUpgradeHandler httpServerUpgradeHandler,
                                                ChannelHandler http2ServerHandler) {
          this.httpServerCodec = checkNotNull(httpServerCodec, "httpServerCodec");
          this.httpServerUpgradeHandler = checkNotNull(httpServerUpgradeHandler, "httpServerUpgradeHandler");
          this.http2ServerHandler = checkNotNull(http2ServerHandler, "http2ServerHandler");
      }
  
      @Override
      public void handlerAdded(ChannelHandlerContext ctx) throws Exception {
          ctx.pipeline()
                  .addAfter(ctx.name(), null, httpServerUpgradeHandler)
                  .addAfter(ctx.name(), null, httpServerCodec);
      }
  
      /**
       * Peek inbound message to determine current connection wants to start HTTP/2
       * by HTTP upgrade or prior knowledge
       */
      @Override
      protected void decode(ChannelHandlerContext ctx, ByteBuf in, List<Object> out) throws Exception {
          int prefaceLength = CONNECTION_PREFACE.readableBytes();
          int bytesRead = Math.min(in.readableBytes(), prefaceLength);
  
          if (!ByteBufUtil.equals(CONNECTION_PREFACE, CONNECTION_PREFACE.readerIndex(),
                  in, in.readerIndex(), bytesRead)) {
              ctx.pipeline().remove(this);
          } else if (bytesRead == prefaceLength) {
              // Full h2 preface match, removed source codec, using http2 codec to handle
              // following network traffic
              ctx.pipeline()
                      .remove(httpServerCodec)
                      .remove(httpServerUpgradeHandler);
  
              ctx.pipeline().addAfter(ctx.name(), null, http2ServerHandler);
              ctx.pipeline().remove(this);
  
              ctx.fireUserEventTriggered(PriorKnowledgeUpgradeEvent.INSTANCE);
          }
      }
  
      /**
       * User event that is fired to notify about HTTP/2 protocol is started.
      */
     public static final class PriorKnowledgeUpgradeEvent {
         private static final PriorKnowledgeUpgradeEvent INSTANCE = new PriorKnowledgeUpgradeEvent();
 
         private PriorKnowledgeUpgradeEvent() {
         }
     }
 }