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   /*
    * Copyright 2019 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.channel.ChannelFuture;
  import io.netty.channel.ChannelFutureListener;
  import io.netty.channel.ChannelHandlerContext;
  import io.netty.channel.ChannelPromise;
  import io.netty.util.internal.ObjectUtil;
  import io.netty.util.internal.logging.InternalLogger;
  import io.netty.util.internal.logging.InternalLoggerFactory;
  
  /**
   * {@link DecoratingHttp2ConnectionEncoder} which guards against a remote peer that will trigger a massive amount
   * of control frames but will not consume our responses to these.
   * This encoder will tear-down the connection once we reached the configured limit to reduce the risk of DDOS.
   */
  final class Http2ControlFrameLimitEncoder extends DecoratingHttp2ConnectionEncoder {
      private static final InternalLogger logger = InternalLoggerFactory.getInstance(Http2ControlFrameLimitEncoder.class);
  
      private final int maxOutstandingControlFrames;
      private final ChannelFutureListener outstandingControlFramesListener = new ChannelFutureListener() {
          @Override
          public void operationComplete(ChannelFuture future) {
              outstandingControlFrames--;
          }
      };
      private Http2LifecycleManager lifecycleManager;
      private int outstandingControlFrames;
      private boolean limitReached;
  
      Http2ControlFrameLimitEncoder(Http2ConnectionEncoder delegate, int maxOutstandingControlFrames) {
          super(delegate);
          this.maxOutstandingControlFrames = ObjectUtil.checkPositive(maxOutstandingControlFrames,
                  "maxOutstandingControlFrames");
      }
  
      @Override
      public void lifecycleManager(Http2LifecycleManager lifecycleManager) {
          this.lifecycleManager = lifecycleManager;
          super.lifecycleManager(lifecycleManager);
      }
  
      @Override
      public ChannelFuture writeSettingsAck(ChannelHandlerContext ctx, ChannelPromise promise) {
          ChannelPromise newPromise = handleOutstandingControlFrames(ctx, promise);
          if (newPromise == null) {
              return promise;
          }
          return super.writeSettingsAck(ctx, newPromise);
      }
  
      @Override
      public ChannelFuture writePing(ChannelHandlerContext ctx, boolean ack, long data, ChannelPromise promise) {
          // Only apply the limit to ping acks.
          if (ack) {
              ChannelPromise newPromise = handleOutstandingControlFrames(ctx, promise);
              if (newPromise == null) {
                  return promise;
              }
              return super.writePing(ctx, ack, data, newPromise);
          }
          return super.writePing(ctx, ack, data, promise);
      }
  
      @Override
      public ChannelFuture writeRstStream(
              ChannelHandlerContext ctx, int streamId, long errorCode, ChannelPromise promise) {
          ChannelPromise newPromise = handleOutstandingControlFrames(ctx, promise);
          if (newPromise == null) {
              return promise;
          }
          return super.writeRstStream(ctx, streamId, errorCode, newPromise);
      }
  
      private ChannelPromise handleOutstandingControlFrames(ChannelHandlerContext ctx, ChannelPromise promise) {
          if (!limitReached) {
              if (outstandingControlFrames == maxOutstandingControlFrames) {
                  // Let's try to flush once as we may be able to flush some of the control frames.
                  ctx.flush();
              }
              if (outstandingControlFrames == maxOutstandingControlFrames) {
                  limitReached = true;
                  Http2Exception exception = Http2Exception.connectionError(Http2Error.ENHANCE_YOUR_CALM,
                          "Maximum number %d of outstanding control frames reached", maxOutstandingControlFrames);
                  logger.info("Maximum number {} of outstanding control frames reached. Closing channel {}",
                          maxOutstandingControlFrames, ctx.channel(), exception);
 
                 // First notify the Http2LifecycleManager and then close the connection.
                 lifecycleManager.onError(ctx, true, exception);
                 ctx.close();
             }
             outstandingControlFrames++;
 
             // We did not reach the limit yet, add the listener to decrement the number of outstanding control frames
             // once the promise was completed
             return promise.unvoid().addListener(outstandingControlFramesListener);
         }
         return promise;
     }
 }